Cropland on soils that have severe or very severe

 limitations which require special conservation or

 management practices (Class III–VIII soils)

Cropland on soils that have slight or moderate

 limitations which require moderate conservation

 practices (Class I and II soils)

National Water-Quality Assessment Program

Environmental Factors That Influence the Location of 

Crop Agriculture in the Conterminous United States

Scientific Investigations Report 2011–5108

U.S. Department of the Interior

U.S. Geological Survey
Cover photos

Top foreground: Aerial shot of small farms with orchards, crops, and windbreaks. Leelanau County, 

Mich. Photo by Lynn Betts (2001), Natural Resources Conservation Service Photo Gallery, NRCSMI01043, 

http://photogallery.nrcs.gov/.

Background: A grassed backslope terrace in a soybean field in Story County, Iowa. Photo by Lynn Betts (2002), 

Natural Resources Conservation Service Photo Gallery, NRCSIA03005, http://photogallery.nrcs.usda.gov/.
Environmental Factors That Influence 

the Location of Crop Agriculture in the 

Conterminous United States

By Nancy T. Baker and Paul D. Capel

National Water-Quality Assessment Program

Scientific Investigations Report 2011–5108

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Baker, N.T., and Capel, P.D., 2011, Environmental factors that influence the location of crop agriculture in the conterminous United States: U.S. Geological Survey Scientific Investigations Report 2011–5108, 72 p.
iii

Foreword

The U.S. Geological Survey (USGS) is committed to providing the Nation with reliable scientific 

information that helps to enhance and protect the overall quality of life and that facilitates 

effective management of water, biological, energy, and mineral resources (http://www.usgs.

gov/). Information on the Nation’s water resources is critical to ensuring long-term availability 

of water that is safe for drinking and recreation and is suitable for industry, irrigation, and fish 

and wildlife. Population growth and increasing demands for water make the availability of 

that water, measured in terms of quantity and quality, even more essential to the long-term 

sustainability of our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program in 1991 

to support national, regional, State, and local information needs and decisions related to 

water-quality management and policy (http://water.usgs.gov/nawqa). The NAWQA Program 

is designed to answer: What is the quality of our Nation’s streams and groundwater? How are 

conditions changing over time? How do natural features and human activities affect the quality 

of streams and groundwater, and where are those effects most pronounced? By combining 

information on water chemistry, physical characteristics, stream habitat, and aquatic life, 

the NAWQA Program aims to provide science-based insights for current and emerging water 

issues and priorities. From 1991 to 2001, the NAWQA Program completed interdisciplinary 

assessments and established a baseline understanding of water-quality conditions in 51 of the 

Nation’s river basins and aquifers, referred to as Study Units (http://water.usgs.gov/nawqa/

studies/study_units.html).

National and regional assessments are ongoing in the second decade (2001–2012) of the 

NAWQA Program as 42 of the 51 Study Units are selectively reassessed. These assessments 

extend the findings in the Study Units by determining water-quality status and trends at sites 

that have been consistently monitored for more than a decade, and filling critical gaps in 

characterizing the quality of surface water and groundwater. For example, increased emphasis 

has been placed on assessing the quality of source water and finished water associated with 

many of the Nation’s largest community water systems. During the second decade, NAWQA is 

addressing five national priority topics that build an understanding of how natural features and 

human activities affect water quality, and establish links between sources of contaminants, 

the transport of those contaminants through the hydrologic system, and the potential effects 

of contaminants on humans and aquatic ecosystems. Included are studies on the fate of 

agricultural chemicals, effects of urbanization on stream ecosystems, bioaccumulation of 

mercury in stream ecosystems, effects of nutrient enrichment on aquatic ecosystems, and 

transport of contaminants to public-supply wells. In addition, national syntheses of information 

on pesticides, volatile organic compounds (VOCs), nutrients, trace elements, and aquatic ecology 

are continuing. 

The USGS aims to disseminate credible, timely, and relevant science information to address 

practical and effective water-resource management and strategies that protect and restore 

water quality. We hope this NAWQA publication will provide you with insights and information 

to meet your needs and will foster increased citizen awareness and involvement in the 

protection and restoration of our Nation’s waters. 
iv

The USGS recognizes that a national assessment by a single program cannot address all 

water-resource issues of interest. External coordination at all levels is critical for cost-effective 

management, regulation, and conservation of our Nation’s water resources. The NAWQA 

Program, therefore, depends on advice and information from other agencies—Federal, State, 

regional, interstate, Tribal, and local—as well as nongovernmental organizations, industry, 

academia, and other stakeholder groups. Your assistance and suggestions are greatly 

appreciated.

William H. Werkheiser

USGS Associate Director for Water
v

Acknowledgments

The authors thank Dr. Leonard Coop, Research professor, Integrated Plant Protection Center, 

Oregon State University, Corvallis, for providing the 1971–2000 Normal Growing Degree-Day 

geospatial data. 

The authors also express gratitude to Norman Bliss, U.S. Geological Survey, Earth Resources 

Observation and Science Center, for providing historical population and census of agricultural 

compilations and historical county-boundary geospatial data. The authors express gratitude to 

David M. Wolock, U.S. Geological Survey, Kansas Water Science Center, for providing updated 

average annual runoff (1970–2001) and recharge geospatial datasets. Finally, the authors thank 

Frank D. Voss and David C. Lampe, U.S. Geological Survey, for technical review of the material 

presented in this document. 

vii

Contents

Foreword ........................................................................................................................................................iii

Acknowledgments.........................................................................................................................................v

Abstract...........................................................................................................................................................1

Introduction: Why Is Crop Agriculture Located Where It Is? ................................................................2

Purpose and Scope ..............................................................................................................................2

Development of Croplands in the United States..............................................................................2

Environmental Factors That Influence the Location of Crop Agriculture.............................................8

Elevation and Slope............................................................................................................................12

Climate .................................................................................................................................................14

Soil ........................................................................................................................................................21

Soil Water.............................................................................................................................................26

Other Land Uses in Competition for Agricultural Lands .......................................................................32

Summary........................................................................................................................................................33

References Cited..........................................................................................................................................34

Appendix 1—Development of Croplands in the United States ............................................................38

Appendix 2—Environmental Factors That Influence the Location of Crop Agriculture ..................39

Appendix 3—Elevation and Slope ............................................................................................................48

Appendix 4—Climate...................................................................................................................................50

Appendix 5—Soil .........................................................................................................................................54

Appendix 6—Soil Water.............................................................................................................................56

Appendix 7—Other Land Uses in Competition for Agricultural Lands ..............................................61

Appendix 8—Summary of Data Sources and Online Links to Datasets ............................................65

References Cited in the Appendixes ........................................................................................................71

Figures

1. Potential natural vegetation of the conterminous United States .........................................3

2. Historical extent of settled area for people of primarily European descent, 

1790 and 1830.................................................................................................................................4

3. Extent of improved farmland for selected census years for 

the conterminous United States, 1850–2002 ............................................................................5

4. Trends in harvested cropland for the conterminous United States, 1879–2002.................6

5. Location of grassland and cropland, and total land area in each crop or 

crop group in the conterminous United States, 2009..............................................................9

6. Extent of cropland and total acreage grown for selected grain and 

specialty crops in the United States, 2007..............................................................................10

7. Location of cropland in each slope range and the percentage of 

cropland area for each slope range for selected crops in 

the conterminous United States...............................................................................................12

8. Location of crops on land with 3 percent or greater slope and location of 

other cropland in the conterminous United States...............................................................13
viii

9. Location of cropland and average growing degree-days for base 50°F 

(1971–2000) and the percentage of cropland area for each growing degree-day 

category for selected crops in the conterminous United States........................................15

10. Location of cropland and average annual precipitation (1971–2000) 

and the percentage of cropland area for each precipitation category 

for selected crops in the conterminous United States.........................................................16

11. Extent of favorable climate for growing selected crops based on 

average annual growing degree-days and precipitation (1970–2001) 

and location of selected crops for the conterminous United States .................................18

12. Extent of irrigated cropland from surface water and groundwater sources 

in the conterminous United States ..........................................................................................19

13. Extent of irrigated cropland and percentage of harvested cropland that is 

irrigated for selected crops in the conterminous United States, 2007 ..............................20

14. Areas of groundwater-level decline and the extent of cropland in areas 

that have groundwater-level decline in the conterminous United States ........................21

15. Distribution of cropland treated with commercial fertilizer, lime, and soil 

conditioners and land treated with manure; and percentage of all cropland 

(including hay) treated with commercial fertilizer, lime, and soil conditioners 

and percentage of land treated with nitrogen, phosphorus, and potash for 

selected crops in the conterminous United States, 1989–2007 ..........................................23

16. Location of cropland on each land capability class and the percentage of 

cropland area for each capability class for selected crops in the 

conterminous United States .....................................................................................................24

17. Location of cropland on soils that have severe or very severe limitations 

which require special conservation or management practices and 

cropland on soils that have slight or moderate limitations which require 

moderate conservation practices in the conterminous United States..............................25

18. Location of cropland on each soil hazard class and the percentage of 

cropland area for each soil hazard class for selected crops in the 

conterminous United States .....................................................................................................27

19. Location of cropland on soils in each hazard class (climate, erosion, soils, 

and water) in the conterminous United States......................................................................28

20. Cropland on soils where surface or subsurface artificial drainage is likely 

required to remove excess water in order to cultivate cropland in the 

conterminous United States .....................................................................................................29

21. Location of cropland and average annual runoff (1971–2000) and 

the percentage of cropland area for each runoff category for selected crops 

in the conterminous United States ..........................................................................................30

22. Location of cropland and average annual recharge (1971–2000) and 

the percentage of cropland area for each recharge category for 

selected crops in the conterminous United States...............................................................31

23. Location of cropland on potential natural vegetation and the percentage 

of cropland area in each natural vegetation type for selected crops 

in the conterminous United States ..........................................................................................32
ix

Appendix Tables

2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used 

in this report.................................................................................................................................40

2–2. Land area in each crop or crop group in the 2009 Cropland Data Layer 

for the conterminous United States.........................................................................................45

2–3. Area of land in each land cover group for the 2009 Cropland Data Layer 

for the conterminous United States.........................................................................................45

2–4. Comparison of land area in each land-cover group reported in “Major Uses of 

Land, 2002” and the 2009 Cropland Data Layer, and estimated land area for 

land-cover groups not differentiated in the 2009 Cropland Data Layer 

for the conterminous United States.........................................................................................46

2–5. Total acreage grown for selected grain and specialty crops for 

the conterminous United States ..............................................................................................47

3–1. Land area in each crop or crop group above 2,000 meters elevation in 

the 2009 Cropland Data Layer for the conterminous United States ...................................48

3–2. Land area in each crop or crop group for each slope range in 

the 2009 Cropland Data Layer for the conterminous United States ..................................49

4–1. Land area in each crop or crop group for each growing degree-day range 

in the 2009 Cropland Data Layer for the conterminous United States ..............................51

 4–2. Land area in each crop or crop group for precipitation category in 

the 2009 Cropland Data Layer for the conterminous United States ...................................52

4–3. Optimal and sub-optimal heat and water requirements for selected crops.....................53

4–4. Total irrigated acreage for selected crops for 2007 for the 

conterminous United States .....................................................................................................53

5–1. Land area in each crop or crop group for each land capability class in 

the 2009 Cropland Data Layer for the conterminous United States ...................................55

6–1. Land area in each crop or crop group for each land hazard class in 

the 2009 Cropland Data Layer for the conterminous United States ...................................57

6–2. Land area in each crop or crop group that is artificially drained in 

the 2009 Cropland Data Layer for the conterminous United States ...................................58

6–3. Land area in each crop or crop group for each runoff category in 

the 2009 Cropland Data Layer for the conterminous United States ...................................59

6–4. Land area in each crop or crop group for each recharge category in 

the 2009 Cropland Data Layer for the conterminous United States ...................................60

7–1. Land area in each potential natural vegetation type converted to cropland, 

and grassland and hay for the conterminous United States...............................................62

7–2. Land area in each crop or crop group for each potential natural vegetation 

type in the 2009 Cropland Data Layer for the conterminous United States ......................63

7–3. Land area for each potential natural vegetation class in cropland and 

in grassland. ................................................................................................................................64

8–1. Summary of data sources and online links to datasets .......................................................66
x

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

acre 0.4047 hectare (ha)

hectare (ha) 2.471 acre

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

hectare-meter (ha-m) 8.107 acre-foot (acre-ft) 

Precipitation rate

millimeter per year (mm/yr) 0.03937 inch per year (in/yr)

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) 

as follows:

°C=(°F-32)/1.8

.
Environmental Factors That Influence the Location of 

Crop Agriculture in the Conterminous United States

By Nancy T. Baker and Paul D. Capel

Abstract

This report presents and describes high-resolution geospatial data identifying the range of environmental conditions 

that influence the location of cropped agricultural lands in 

the conterminous United States. Also presented are estimates 

of the extent of land where environmental constraints limit 

agricultural production (marginal land) and the extents of land 

where modifications overcome environmental constraints. 

The report is the result of the compilation and manipulation 

of datasets from numerous sources; it consists of an explanatory text and a series of appendixes and associated tables that 

document the data sources and data-manipulation methods 

in detail.

Environmental factors that influence the extent of crop 

agriculture are terrain, climate, soil properties, and soil water. 

It is the combination of these four factors that allow specific crops to be grown in certain areas. Today, in order to 

maximize production, most of the cultivated croplands and 

grasslands for commercial agriculture are in areas where crops 

and livestock are well suited to local conditions. In the United 

States, cropland (row crops, closely sown crops (except hay), 

fruits, nuts, vegetables) occupies about 13 percent of the total 

land area. Grassland and rangeland occupy another 41 percent 

of the land area. 

Most crops are grown on land with shallow slope where 

the temperature, precipitation, and soils are favorable. In areas 

that are too steep, wet, or dry, landscapes have been modified 

to allow cultivation. Some of the limitations of the environmental factors that determine the location of agriculture can 

be overcome through modifications, but others cannot. On a 

larger-than-field scale, agricultural modifications commonly 

influence water availability through irrigation and (or) drainage and soil fertility and (or) organic-matter content through 

amendments such as manure, commercial fertilizer, and lime. 

In general, it is not feasible to modify the other environmental 

factors, soil texture, soil depth, soil mineralogy, temperature, 

and terrain at large scales. 
2 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Introduction: Why Is Crop Agriculture 

Located Where It Is? 

Environmental, economic, and societal factors have 

determined the location of crop agriculture. The environmental factors influencing the extent of arable land are terrain, 

climate, soil properties, and soil water. Crops need space to 

grow, sufficient light, warmth, and moisture. Soils must be of 

sufficient depth with sufficient drainage, texture, and chemical 

and fertility properties. Terrain must be neither too rugged nor 

at too high an elevation for accessibility, with slopes gentle 

enough to prevent soil and nutrient loss yet steep enough to 

prevent flooding. Of these environmental characteristics, only 

soil water and soil fertility can be significantly modified over 

large areas. Irrigation, drainage, and fertilization allow for 

crops to thrive in areas where environmental factors alone 

would not meet their basic needs.

On a subsistence level, land is modified for agriculture 

almost everywhere people settle. The exceptions are the most 

inhospitable areas of the world—extremely cold, dry, or rugged lands—where subsistence is derived from hunting and 

gathering food. Commercial agriculture, the production of 

crops and livestock intended for widespread distribution and 

consumption by others, requires a narrower and more optimal range of environmental conditions for productivity and 

profitability. Often, agricultural modifications are necessary to 

increase the productivity of agricultural lands. Beyond the normal tilling of the land, farmers use fertilizers and pesticides to 

increase soil fertility and crop yield, reshape the land surface 

to minimize erosion and loss of nutrients, drain wetlands, and 

irrigate dry lands to increase crop productivity in areas once 

considered marginal for agricultural use.

Purpose and Scope

This report presents and describes high-resolution geospatial data identifying the range of environmental conditions 

that influence the location of cropped agricultural lands in 

the conterminous United States. Also presented are estimates 

of the extent of land where environmental constraints limit 

agricultural production (marginal land) and the extents of land 

where modifications overcome environmental constraints. 

This report is the result of the compilation and manipulation of datasets from numerous sources. Specifically, it consists of an illustrated explanatory text plus a series of appendixes and associated tables that document the data sources and 

data-manipulation methods in detail. It is intended for scientists who study the effects of agricultural practices on water 

and air quality but do not normally study “why agriculture is 

located in a given area.” Throughout, we strive to highlight the 

relationship between environmental conditions that result in 

conversion of land from its natural state to agricultural use and 

the subsequent modifications to the landscape (disturbance of 

the soil, irrigation, artificial drainage, application of nutrients, and so forth) in order to promote better understanding 

of how those modifications affect environmental processes. 

In addition, analysis of spatial and temporal patterns for the 

location of agriculture may provide insight into future cropproduction potential. 

“Water scarcity, together with degradation of arable 

land, could become the most serious obstacle to 

future increases in food production.” (Fischer and 

others, 2002)

Development of Croplands in the United States

Before human settlement, the North American landscape 

consisted of forest, grasslands, and scrublands (fig. 1). Adequate precipitation to support tree growth generally defined 

the forest-grassland boundary. Areas with insufficient precipitation to support grass gave way to scrubland. Deciduous 

broadleaf forests once covered most of the East, the Ohio and 

lower Mississippi River Valleys, and the middle Great Lakes 

region (Küchler, 1964). Needleleaf forests covered much of 

the central and northern Pacific Coast, the higher elevations of 

the West, parts of the interior North, and a narrow belt in the 

Deep South. Grasslands covered much of the sub-humid interior lowlands of the Great Plains from Texas and New Mexico 

to the Canadian border. An eastward extension of the grasslands, the Prairie Wedge, reached across Illinois to the western 

edge of Indiana, where precipitation is clearly adequate to 

support tree growth (Birdsall and others, 1999). Scrublands 

were concentrated in the arid lowlands of the interior West. 

Vegetation varied from the cacti of the Southwest to the dense, 

brushy chaparral of southern California and the mesquite of 

Texas (Birdsall and others, 1999). 

Native Americans began farming the North American 

continent as early as 5,000 B.C.E. Nearly 1,000 years before 

European settlement, native farmers developed a productive 

agricultural system based on corn, beans, and squash (Hurt, 

1994). They successively adapted their crops to meet the environmental conditions of their regions—developing varieties 

that would grow in the cool regions along the Canadian border 

and other varieties that would grow in the hot, dry southwest. 

Agriculture for European settlers began in Jamestown 

in 1607 and in Plymouth in 1620 when the colonists learned 

from the Native Americans to plant corn (fig. 2). However, 

English settlers did not recognize or acknowledge much of 

Native American agricultural patterns already in place and 

fundamentally changed the landscape by importing cows, 

sheep, bees, and apple trees (Horn, 2005). Southern farmers 

began commercial production of tobacco as early as 1612. 

New England farmers commercially produced livestock as 

well as grain. Little attention was given to crop rotation, 

fertilization, or proper tillage practices, so these early farmers 

raised crops until the soils became depleted then migrated onto 

adjacent lands in search of more profitable and fertile land; for 

example, wheat production declined in New England because 

of depleted soils but became a staple of the middle colonies of 

New York, Pennsylvania, and New Jersey (Hurt, 1994). Farmers began raising rice as early as 1671 in South Carolina. 
Central and Eastern Grasslands

Central and Eastern Grasslands and Forest Combination

Eastern Broadleaf and Needleleaf Forests

Eastern Broadleaf Forests

Eastern Needleleaf Forests

Western Broadleaf and Needleleaf Forests

Western Broadleaf Forests

Western Grasslands

Western Needleleaf Forests

Western Shrub and Grassland Combinations

Western Shrubs

EXPLANATION

Introduction: Why Is Crop Agriculture Located Where It Is? 3

Figure 1. Potential natural vegetation of the conterminous United States (Missoula Fire Sciences Laboratory, 2001). 
4 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

During the 18th century, commercial agriculture spilled 

across the Appalachians and swept along the Gulf Coast 

(Hurt, 1994). At that time, rivers were the primary transportation routes; therefore, commercial agriculture thrived along 

river areas where farm produce could easily be shipped to 

market. Products were limited to nonperishable commodities 

such as tobacco, rice, wheat, corn, and salt meat (Hurt, 1994). 

In the early 1700s, the Spanish began to colonize the upper 

Rio Grande Valley. Agriculture was primarily cattle production, and crops were limited to subsistence farming. Grapes 

were introduced to California around 1780. Spanish missionaries introduced a variety of crops such as wheat, oats, barley, 

onions, peas, watermelons, muskmelons, peaches, apricots, 

and apples (Hurt, 1994). 

After the “Indian Removal Act” of the 1830s, European 

settlement extended rapidly westward. The “Homestead Act” 

of the 1860s (and the expanded act of the 1900s) was supposed to encourage agricultural settlement in the Great Plains 

(fig. 3). The act was often abused. In these arid areas, the plot 

of land allotted was generally too small for a viable farm (at 

least prior to major public investments in irrigation projects), 

so homesteads were instead used to control resources, especially water. The “Enlarged Homestead Act” of 1909 gave 

320 acres (1.3 km2) to farmers who accepted more marginal 

lands, which at the time could not be easily irrigated. A massive influx of new farmers eventually led to extensive land 

erosion and the Dust Bowl of the 1930s (fig. 3). 

Continued migration brought settlers and agriculture 

across the Great Plains to the Western States. Mormon settlers 

began producing potatoes in Idaho in the mid-1800s. Around 

the same time, the Willamette Valley became the destination of 

choice for emigrants on the Oregon Trail. Major agricultural 

products included a variety of berries and vegetables and, 

more recently, grape and wine production. By 1910, California 

had developed large-scale fruit and vegetable agriculture based 

on extensive lands, irrigation, railroad and steamer transportation, and strong marketing cooperatives (Hurt, 1994). Irrigation made the Central and Imperial Valleys of California one 

of the most productive agricultural areas in the world. 

1790 1830 

> 1 to 5

> 5 to 10

>10 to 20

>20 to 30

>30

EXPLANATION

1 

Population density, in people 

per square kilometer

Not enumerated or zero

Figure 2. Historical extent of settled area for people of primarily European descent, 1790 and 1830 (Waisanen and Bliss, 

2002). Population density is used as a surrogate for agricultural lands because agricultural census data are not available 

before 1850.
1850 1870 

1900 1930 

1949 1978 

2002 

>10 to 20

>20 to 40

>40 to 60

>60 to 80

>80

EXPLANATION

1 to 10

Proportion of total county area

in improved farmland, in percent

Not enumerated or zero

Introduction: Why Is Crop Agriculture Located Where It Is? 5

Figure 3. Extent of improved farmland for selected census years for the conterminous United States, 1850–2002 (Waisanen 

and Bliss, 2002; U.S. Department of Agriculture, 2005).
EXPLANATION

Year

0 to 3

0 to 10

0 to 30 

Range in harvested 

cropland, in million acres

(values of the y-axis on

state plots) 

1879 1899 1919 1939 1959 1979 1999

400

200

0

Harvested cropland, in million acres

United States

Harvested cropland, 

in million acres

0

10

0

10

WA

AL

10 MT

0

10 OR

0

10 ID

0

0

10 CA

0

10 CO

0

10 AR

0

10 LA

0

10 KY

0

10 MI

0

10 NY

0

10 PA

0

10

MS

0

10 TN

0

10 VA

0

10 NC

0

10 SC

0

3 WV

0

3 MD

0

3 DE

0

3 FL

0

3 RI

0

3 MA

0

3 ME

0

3

NH

0

3

VT

0

3

CT

0

3 NJ

0

3 AZ

0

3 NM

0

3 UT

0

3

NV 0

3 WY

0

30 GA

0

30 OH

0

30 IN

0

30 IL

0

30

TX

0

30 OK

0

30 KS

0

30 MO

0

30 NE

0

30 IA

0

30 SD

0

30 ND

0

30 MN

0

30

WI

6 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Figure 4. Trends in harvested cropland for the conterminous United States, 1879–2002 (Ramankutty and others, 2010).

Along with westward expansion, the extent of harvested 

cropland has changed in different parts of the country (fig. 4). 

In the East and South, total cropland area has declined in most 

states. The exception is Florida, where cropland area steadily 

increased through the 20th century. The midwestern Corn 

Belt increased in cropland area during early westward expansion and has remained fairly stable for the past century. The 

Northern and Southern Plains states show a dip in cropland 

area corresponding to the severe drought and Dust Bowl of the 

1930s. Cropland area increased in the Northern Plains after the 

1930s. New Mexico, Texas, and Oklahoma have declined in 

cropland area in the last 50 years. Most of the Western States 

have increase cropland area over the last century (fig. 4). 
Introduction: Why Is Crop Agriculture Located Where It Is? 7
8 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Environmental Factors That Influence 

the Location of Crop Agriculture

The range of environmental conditions conducive to 

the production of crops is wide, and particular combinations 

of these environmental conditions allow specific crops to be 

grown in certain areas. Obviously, in areas with rugged slopes, 

poor soils, and inhospitable climate, commercial agriculture 

would not be profitable or perhaps even possible. In other 

areas, commercial agriculture is very profitable, and environmental conditions are such that many types of high-yielding 

crops can be grown. However, most agricultural lands fall 

somewhere between the two ends of the spectrum. 

Agriculture has evolved over time. Early agriculture was 

primarily for subsistence, and farmers produced a diversity of 

crops and livestock in order to survive. In subsistence farming, 

the crops or livestock grown may not be particularly suited 

to the environmental conditions of the area. Today, most of 

the cultivated cropland and grassland for livestock is used for 

commercial purposes. Unlike subsistence farmers, commercial 

farmers tend to grow crops that are suited to the particular 

conditions of the areas where they farm in order to maximize 

profits for a given piece of land. 

Currently in the United States, cropland occupies about 

13 percent of the total land area (fig. 5) Croplands include 

row and closely sown crops (except hay), tree fruits and nuts, 

and vegetables and ground fruits. (Appendix 2, table 2–1, is a 

detailed list of land-cover categories used in this document.) 

Grassland (pasture and hay) occupy another 25 percent of the 

land area (appendix 2, tables 2–2 and 2–3). Pastures are those 

lands that are seeded and used primarily for the production of 

adapted, domesticated forage plants (for example, tall fescue or switchgrass) for livestock. Hay includes grasses and 

legumes—such as timothy and alfalfa—which are typically 

cut, dried, and stored for livestock fodder. Rangelands, which 

occupy another 16 percent of the land area, are those lands 

on which the native vegetation (climax or natural potential 

plant community) is predominantly grass, grasslike plants, 

and shrubs suitable for grazing or browsing (appendix 2, table 

2–4). Rangelands include natural grassland, savannas, many 

wetlands, some deserts, tundra, and certain forb and shrub 

communities. 

What combinations of terrain, climate, soils, and soil 

water determine the spatial distribution of specific crops? 

What environmental conditions result in the predominance of 

corn and soybeans in the Midwest and the predominance of 

rice along the Mississippi Valley (fig. 5)? Why is cotton clustered in the panhandle of Texas, the majority of wheat spread 

along the eastern edge of the Great Plains, and citrus primarily grown in selected areas of California, southern Texas, and 

Florida? Why is it that nearly all oilseeds, beans and peas, 

and sugar beets are grown in the Northern Plains, tobacco 

is primarily confined to Kentucky and North Carolina, and 

peanuts are grown in a southern band from South Carolina to 

Alabama (fig. 6; appendix 2, table 2–5)? The inherent characteristics of terrain, climate, soil, and water influence not only 

the location and types of crops grown in specific areas but also 

the agricultural modifications necessary to sustain profit and 

yield. Human-induced changes to the land such as accelerated 

erosion, accumulation of salts, and water removal or addition 

(artificial drainage, supplies of irrigation water) also influence 

the types of crops grown and the types of modifications implemented in certain areas. 
Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and ground fruit

Rice

Orchards and grapes

Citrus 

EXPLANATION

Grassland (includes pasture and hay)

Cropland combined (total of crops listed below)

Cropland

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Total land area, in 1,000 km2

Grassland

Cropland

combined

Cropland

0

50

100

150

200

250

300

350

0

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

Environmental Factors That Influence the Location of Crop Agriculture 9

Figure 5. Location of grassland and cropland, and total land area in each crop or crop group in the conterminous United States, 

2009 (U.S. Department of Agriculture, 2010b).
0

5

10

15

20

25

30

35

40

Total land area in each crop, 

in 100,000 acres

BarleyOatsRye

Sunflowers for oil Canola Sunflowers for seed

Dry edible beans Dry edible peas

Peanuts

Potatoes Tobacco

Sugarbeets

Sugarcane

Blueberries Strawberries

75 acres blueberries 

 300 acres rye 

50 acres strawberries 

2,000 acres barley 

1,000 acres oats 

 500 acres dry edible peas 

2,000 acres dry edible 

 beans 

 1,000 acres sunflowers for seeds 

1,000 acres sunflowers 

 for oil 

1,000 acres canola 

1,500 acres sugarbeets 

1,000 acres sugarcane 

 1,000 acres potatoes 

1,000 acres peanuts 

 500 acres tobacco 

10 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Figure 6. Extent of cropland and total acreage grown for selected grain and specialty crops in the United States, 2007 

(U.S. Department of Agriculture, 2009a).
Environmental Factors That Influence the Location of Crop Agriculture 11
12 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Elevation and Slope

Agricultural land used for commercial production is constrained by both elevation and slope. For agricultural production, high elevations have similar constraints as high latitudes, 

including decreased temperature, increased wind velocity, and 

poor soil. It is these secondary characteristics that constrain 

crop cultivation rather than high elevation itself (Singh and 

Dhillon, 2004). Terrain that is too rugged (steep slopes) is not 

readily accessible for mechanized agriculture. In addition, 

terrain indirectly effects soil formation, modifies climates, and 

affects water drainage and availability. Steep slopes are subject 

to soil and nutrient loss. In contrast, very flat terrain is prone 

to flooding and poorly drained soils. 

Only a few locations in the conterminous United States 

are constrained by too high elevation for commercial agriculture. About 1 percent of crops are grown at elevations higher 

than 2,000 m (appendix 3, table 3–1). Wheat, “other crops” 

(including grasses such as rye, oats, and barley), and vegetables and ground fruit make up most of the crops grown above 

2,000 m (appendix 3, table 3–1). 

0 to 1

>1 to 2

>2 to 3

>3 to 4

>4 

EXPLANATION

Slope range, in percent

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by slope range, 

in percent 

Figure 7. Location of cropland in each slope range and the percentage of cropland area for each slope range for selected 

crops in the conterminous United States (Falcone, 2003; U.S. Department of Agriculture, 2010b).
Environmental Factors That Influence the Location of Crop Agriculture 13

The slope of the landscape effects soil formation, climate, 

water drainage, and soil-water availability. Steep slopes are 

subject to erosion and soil loss. Steep slopes are also not readily accessible for farm machinery (Singh and Dillon, 2004). 

Areas that are nearly level (0 to 3 percent slope) are generally 

excellent for row-crop agriculture, although some of these 

areas are naturally wetlands. These nearly level slopes present 

no constraint for farm machinery. Gently rolling areas (3 to 

6 percent slopes) present no serious obstacles to cultivation; 

occasionally, substantial rainstorms can cause soil erosion, so 

terracing is implemented in some of these areas. Crop agriculture is limited in areas with slopes greater than 6 percent 

because of erosion and machinery constraints. Slopes that 

are too steep for row crops may still be suitable for orchards, 

vineyards, and animal grazing.

About 80 percent of all crops are grown on land with 

a slope of 3 percent or less (fig. 7) (appendix 3, table 3–2). 

Nearly all cotton, rice, and citrus are grown on land with a 

slope of 2 percent or less. About 10 percent of all crops are 

grown on slopes greater than 4 percent, and only a small 

percentage of all crops are grown on steeper slopes (5 to 

16 percent). The crops grown on steeper slopes are tree 

fruits, grapes, vegetables, and “other grains.” The areas of the 

country where crops are more likely to be grown on steeper 

slopes are along the border between Iowa and Nebraska, in 

western New York, along the Appalachian Mountains, and in 

southern Idaho and eastern Washington (fig. 8). A much larger 

percentage of grassland has steeper slopes than cropland; in 

fact, about 22 percent of grassland is grown on slopes steeper 

than 5 percent, because steeper slopes are generally unsuited 

to cultivation. 

Other cropland 

EXPLANATION

Crops on land with 3 percent or greater slope

Figure 8. Location of crops on land with 3 percent or greater slope and location of other cropland in the conterminous 

United States (Falcone, 2003; U.S. Department of Agriculture, 2010b).
14 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Climate 

Climate is an important factor for determining the location of crops, and it accounts for much of the regional differences in the types of crops grown across the Nation. The 

climate of a location is a function of precipitation and sunlight 

(solar radiation that determines light intensity and temperature). Climate is largely determined by latitude, altitude, and 

proximity to ice, snow cover, or water bodies.

The intensity, quantity, and duration of solar radiation 

that falls on the Earth at any given place determine local 

temperature and light. Light and heat are essential for the 

formation of chlorophyll and for the process of photosynthesis in plants. Different plants have different requirements for 

the amount of light and heat needed to reach maturity. For 

many plants, the growth rate from emergence to maturity 

depends upon the accumulation of specific quantities of heat. 

Cumulative growing degree-days (GDD) (or heat units), the 

metric used for the solar radiation requirements for crops, are 

the accumulated product of time and temperature above the 

minimum developmental thresholds for each day. Each plant 

has its own low-temperature threshold for development. One 

degree-day for a specific crop is one 24-hour period with an 

air temperature 1 degree (°F) above the lower developmental 

threshold. For instance, the low temperature threshold for corn 

is 50°F1 (Wiebold, 2000). When the air temperature remains 

52°F (or 2° above the threshold) for 24 hours, 2 degree-days 

are accumulated. Growth of many crops pauses when the air 

temperature exceeds a certain level; therefore, cumulative 

1

 50°F is the most common base GDD for crops in the United States. GDDs 

may be calculating using either Celsius or Fahrenheit (5GDDC = 9GDDF). 

GDDs do not capture the complete effect of temperature on 

growth. In addition, crops such as apples, grapes, and winter 

wheat require a period of cold dormancy before seeds or fruit 

can develop. Spring-seeded annual crops such as corn, soybeans, and rice do not require a period of cold dormancy. 

The spatial distribution of major crops compared to 

cumulative GDD is shown in figure 9. More than 90 percent 

of all crops are grown in areas with more than 2,000 cumulative GDD (fig. 9) (appendix 4, table 4–1). All cotton, rice, and 

citrus require more than 4,000 cumulative GDD. On the other 

hand, many vegetables and ground fruit are grown in areas 

with less than 3,000 cumulative GDD. 

Plants take water from the soil. In natural landscapes, soil 

water largely comes from precipitation that has infiltrated into 

the soil. The quantity of precipitation that falls on any given 

location has a strong influence on what kinds of plants thrive. 

Different plants have different requirements for the amount of 

water they need to develop to maturity. The amount of water 

that is needed for the same plant can be different because of 

air temperature and humidity throughout the growing season. 

The distribution of precipitation compared to cropland is 

shown in figure 10. It is evident from this figure that crop agriculture occurs in areas that have very low precipitation. Over 

60 percent of all crops are grown in areas that receive less than 

900 mm/yr (appendix 4, table 4–2). Rice and citrus tend to be 

grown in areas with more precipitation, whereas wheat, sorghum, vegetables and ground fruit, and orchard and vineyard 

crops are grown in areas with less precipitation (fig. 10).
0 to 2,000

>2,000 to 3,000

>3,000 to 4,000

>4,000 to 5,000

>5,000 

EXPLANATION

Growing degree-day, base 50°F

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by growing degreeday category, in percent 

Environmental Factors That Influence the Location of Crop Agriculture 15

Figure 9. Location of cropland and average growing degree-days for base 50°F (1971–2000) and the percentage of cropland 

area for each growing degree-day category for selected crops in the conterminous United States (Oregon State University, 

2010; U.S. Department of Agriculture, 2010b).
0 to 538

>538 to 913

>913 to 1,271

>1,271 to 2,032

>2,032 

EXPLANATION

Annual precipitation, in millimeters per year

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by precipitation 

category, in percent 

16 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Figure 10. Location of cropland and average annual precipitation (1971–2000) and the percentage of cropland area for each 

precipitation category for selected crops in the conterminous United States (Oregon State University, 2008; U.S. Department of 

Agriculture, 2010b).
Environmental Factors That Influence the Location of Crop Agriculture 17

The combined effect of the two climate factors, precipitation and temperature (solar radiation), on the favorable 

growing of various major crops can be seen in figure 11, 

which presents a range of favorable growing conditions, where 

optimal is the upper threshold temperature and precipitation 

requirement for most cultivars of each crop and sub-optimal

is the lower threshold requirement for each crop. Optimal and 

sub-optimal thresholds for selected crops are documented by 

Fischer and others (2008) for temperature and by Brouwer and 

Heibloem (1986) for precipitation and are given in appendix 4, table 4–3. Crops raised under sub-optimal conditions 

will grow, but yields may be lower than crops raised under 

optimal conditions. Wheat is a versatile crop and can be grown 

in a variety of climates. The heat and moisture requirement 

thresholds are lower than for other major crops. Sorghum 

can tolerate drier climates, but it is not as cold hardy as many 

wheat varieties. Sorghum is often dryland farmed and is grown 

in areas where corn—a more profitable crop—will not thrive. 

Corn and soybeans are well adapted to the climate of the 

Midwest. In contrast, citrus has very high heat and moisturerequirement thresholds and can be grown only in selected 

areas of the country. Rice and cotton can be grown at similar 

temperature ranges, but rice growing is constrained by higher 

water requirements. A wide range of cultivars is available for 

most crops, and many cultivars are developed specifically to 

produce good yields in less than ideal conditions, such as a 

short growing season or dry conditions. Crops grown in arid 

areas outside the optimal crop area usually require irrigation 

to thrive. 

Livestock are usually not as constrained by climate as 

plants are, and livestock are often grazed on lands that have 

too short a growing season or are too dry to profitably support 

cultivated crops. Grasses used for hay can be grown on these 

marginal lands, providing food for livestock during winter. 

Most of the rangelands in the United States extend westward 

from the wheat belt from Canada to Mexico. 

In many parts of the Nation, the amount and timing of 

precipitation is sufficient to meet crop needs. In some areas, 

the overall annual precipitation is sufficient to meet crop 

growth requirements, but rainfall does not occur at the time it 

is needed; therefore, irrigation is used to supply water that has 

accumulated during the year and is stored as groundwater or in 

reservoirs (fig. 12). 

Crops that are most likely to be irrigated are rice, nuts, 

citrus, other tree fruits, and grapes (fig. 13). All rice grown in 

the United States is produced in irrigated fields (U.S. Department of Agriculture, 2009c). Although about 15 percent of 

corn and 10 percent of soybeans are irrigated, these two crops 

account for 79,000 km2 of irrigated cropland. The extent of 

irrigated cropland for selected crops is shown in figure 13.

In areas where annual precipitation does not meet the 

crop water requirements, precipitation is supplemented by 

irrigation with water from another part of the same watershed 

(often stored as snow at high elevations), water from a different watershed, or mined groundwater. Mined groundwater is 

water that has accumulated in an aquifer over several years. 

Generally, the area where the aquifer is recharged is distant 

from its point of use for irrigation. 

For example, California’s Central Valley has ideal 

temperature and sunlight conditions for a wide variety of 

crops but has low annual precipitation in the valley itself. The 

mountainous regions to the east receive abundant precipitation 

in the form of snow; and, as the snow melts during the spring, 

it supplies water to the valley. In contrast, interbasin transfers 

of water are often used in the Northwestern States.

Irrigation is extensively used in the Great Plains. This 

water is more likely to be mined where more water is used 

on crops than is supplied by annual precipitation. The result 

is a lowering of the water table and a decrease in streamflow. 

Irrigation is also extensively used along the Mississippi River 

Valley. This area receives ample annual precipitation and, in 

the past, irrigation was a method of applying water to crops 

when needed by using water that was replenished every year. 

However, in recent years, intensive cropping practices have 

required more water than is replenished every year; the result 

has been a lowering of the water table in the area (fig. 14). 

Figure 14 shows areas of the Nation, including cropland, 

where groundwater levels have declined by more than 12 m 

(Reilly and others, 2008). 
10,000 acres of corn 10,000 acres of 

 soybeans 

10,000 acres of 

 wheat (includes 

 both winter and spring 

 wheat) 

5,000 acres of 

 cotton 

2,000 acres of 3,000 acres of rice 

 sorghum 

Optimal

Optimal; additonal extent for winter wheat 

Sub-optimal

Sub-optimal; additional extent for winter wheat

Climate is not favorable for selected crop

Minimum threshold for water and heat 

(growing degree days) requirements

EXPLANATION

18 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Figure 11. Extent of favorable climate for growing selected crops based on average annual growing degree-days and 

precipitation (1970–2001) and location of selected crops for the conterminous United States (Oregon State University, 2008 and 2010; 

U.S. Department of Agriculture, 2009a).
1 to 1,000

>1,000 to 5,000

>5,000 to 10,000

>10,000 

EXPLANATION

Withdrawals,

in hectare-meters

[multiply hectare-meter by 

8.107 to obtain acre-foot]

Surface-Water Sources

Groundwater Sources

Environmental Factors That Influence the Location of Crop Agriculture 19

Figure 12. Extent of irrigated cropland from surface-water and groundwater sources in the conterminous United 

States (Kenny and others, 2009; U.S. Department of Agriculture, 2010b).
2,000 acres corn 2,000 acres 

 soybeans 

1,000 acres wheat 2,000 acres 

 cotton 

1,000 acres 3,000 acres rice 

 sorghum 

1,000 acres 

 vegetables 

1,000 acres 

 orchards 

Nonirrigated

Irrigated 

Cropland area, in percent

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

incl. grapes

and citrus

 Cropland area, in percent

EXPLANATION

20 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Figure 13. Extent of irrigated cropland and percentage of harvested cropland that is irrigated for selected crops in 

the conterminous United States, 2007 (U.S. Department of Agriculture, 2009a).
Other cropland

Areas in excess of 1,000 square kilometers that have groundwater-level decline in excess of 12 meters 

 in at least one confined aquifer since predevelopment , or in excess of 7.5 meters of decline in 

 unconfined aquifers since predevelopment.

Wells in the U.S. Geological Survey National Water Information System database where the

 measured groundwater-level difference over time is greater than 12 meters.

Cropland in areas that have groundwater-level decline

Environmental Factors That Influence the Location of Crop Agriculture 21

Figure 14. Areas of groundwater-level decline (Reilly and others, 2008) and the extent of cropland 

in areas that have groundwater-level decline in the conterminous United States (U.S. Department of 

Agriculture, 2010b). The areas of cropland that have groundwater-level decline do not necessarily mean 

that irrigated agriculture is the cause of the decline but rather that crops which rely on water from 

sources in decline may be vulnerable.
22 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Soil 

Soils provide the physical base for crop roots and are the 

principal source of nutrients. Soils are formed by a combination of the weathering of rock (source of the mineral components) and the decomposition of vegetation (source of the 

organic-matter components). The degree to which vegetation 

can thrive in a soil is determined by a variety of soil properties: depth, texture (grain-size distribution), organic-matter 

content, fertility (nutrient level), mineralogy, and degree of 

weathering. Although soils in areas such as dunes, shifting 

sands, salt flats, rock debris, desert detritus, and glaciers and 

snow caps are either nonexistent or unsuitable for growing 

crops, most areas in the United States have soils where commercial agriculture is possible. Many soils require amendments to provide optimal growing conditions for plants. 

Soil depth is not a limitation for crops in most locations. 

Most crops need at least 100 centimeters of soil to grow, 

although some crops can be grown in shallower soils (Fischer 

and others, 2002). The availability of water may be more of a 

constraint in shallow soils than lack of media for roots. Sadras 

and Calviño (2001) found that grain yield for wheat, corn, and 

soybeans was reduced in soils less than 120 cm but that the 

limiting factor was water deficit due to the shallow soils. Shallow soils underlain by bedrock or hardpan are highly erodible. 

Most of these soils are on steep slopes. 

Soil texture is determined by the combined proportions 

of sand, silt, and clay. Texture, together with organic-matter 

content, influences the water-holding capacity and erodibility 

of a soil and allows aeration of plant roots. Sandy soils allow 

water to move more freely than through clay soils do, and they 

promote greater root aeration than clay soils do. However, clay 

soils have a greater water-holding capacity than sandy soils. 

Soils with a high percentage of silt and clay particles are more 

erodible than sandy soils under the same conditions. Differences in soil texture also affect organic-matter content; organic 

matter breaks down faster in sandy soils than in fine-textured 

soils (Berry and others, 2007). Although it is not practical to 

change soil texture, agricultural modifications can be implemented to make soils more favorable for growing crops. The 

organic-matter content can be increased by adding manure or 

by leaving crop residue on the surface. Soils with high waterholding capacities (high clay and (or) high organic-matter 

content) can be artificially drained to allow for proper root 

development. Terraces can be constructed on highly erodible 

soils to reduce soil erosion. 

“Soil fertility” refers to the potential capacity of the soil 

to support plant growth based on its content of nitrogen and 

other nutrients. Some essential nutrients, such as nitrogen, 

phosphorus and potassium, are needed in relatively large quantities by plants; other essential nutrients (micronutrients), such 

as selenium and boron, are needed in much smaller quantities. 

More than 65 percent of all cropland (including hay) is supplemented with commercial fertilizer, lime, and soil conditioners (fig. 15). Most agricultural crops in the United States are 

supplemented with nitrogen, phosphorus, and (or) potassium 

(via potash fertilizer) to improve soil fertility and, ultimately, 

crop yield. Less than 20 percent of soybeans are treated with 

commercial nitrogen products, whereas nearly 100 percent of 

corn and more than 80 percent of wheat and cotton are treated 

with commercial nitrogen products (fig. 15). 

The mineralogy of the soil particles, together with 

organic-matter content, determines soil chemical characteristics such as pH, salinity, and cation exchange capacity. The 

pH strongly influences the degree to which phosphorous is 

available for plant uptake and, thus, influences soil fertility. 

The pH in soils is often increased (that is, the soil is made 

less acidic) by applying lime (powdered limestone). Some 

minerals form soils that are highly saline and high in sodium 

or gypsum content, a combination that impedes water infiltration and retards or prohibits plant growth. Clay minerals in 

soils strongly absorb cations such as calcium, magnesium, and 

sodium. The absorption quality of soils strongly influences soil 

cohesion, crust formation at the soil surface, and rate at which 

water infiltrates the soil.
0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Cotton

 Cropland area, in percent

EXPLANATION

All commercial fertilizers, lime, and soil conditioners

Nitrogen 

Phosphate

Potash 

Bar graph

Fertilizer

25,000 acres

Manure

 5,000 acres

Environmental Factors That Influence the Location of Crop Agriculture 23

Figure 15. Distribution of cropland treated with commercial fertilizer, lime, and soil conditioners (266 million acres) and land 

treated with manure (22 million acres); and percentage of all cropland (including hay) treated with commercial fertilizer, lime, and 

soil conditioners and percentage of land treated with nitrogen, phosphorus, and potash for selected crops in the conterminous 

United States, 1989–2007 (U.S. Department of Agriculture, 2009b, 2010a).
24 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

The U.S. Department of Agriculture classifies soils on 

the basis of their potential suitability for cropland without 

deterioration of the soil over time (capability classes). The 

system uses eight classes based on landscape location, slope, 

soil depth, texture, and chemical properties (Helms, 1992). 

The spatial distribution of the soil capability classes shows 

that most soils in the East and the Great Plains are suitable 

for cropland (classes I–IV), whereas most of the soils in the 

interior West are more suited to grazing land (classes V–VIII) 

(fig. 16). About 7 percent of all crops are grown on soils with 

slight limitations (class I) with unrestricted use for agriculture (appendix 5, table 5–1). Forty-seven percent of crops 

are grown on soils that have moderate limitations (class II) 

and may require conservation measures, 29 percent of crops 

are grown on soils that have severe limitations (class III) 

and require special conservation practices, and 10 percent of 

crops are grown on soils with very severe limitations (class 

IV) that restrict the choice of plants and require very careful 

Undefined

Class I soils have slight limitations that restrict 

 their use.

Class II soils have moderate limitations that reduce 

 the choice of plants or require moderate 

 conservation practices.

Class III soils have severe limitations that reduce the 

 choice of plants or require special conservation 

 practices, or both.

Class IV soils have very severe limitations that restrict 

 the choice of plants or require very careful 

 management or both.

Class V, VI, VII, and VIII soils have very severe limitations 

 that make them unsuited to cultivation.

EXPLANATION

Land capability class

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by capability class,

 in percent 

Figure 16. Location of cropland on each land capability class and the percentage of cropland area for each capability class 

for selected crops in the conterminous United States (U.S. Department of Agriculture, 2006, 2010b).
Environmental Factors That Influence the Location of Crop Agriculture 25

management (fig. 16). About 11 percent of crops are grown on 

soils considered unsuitable for cultivation (classes V–VIII). 

In some instances when class V–VIII soils are irrigated, the 

limitations for agricultural use are reduced. Still, many soils in 

the arid West are characterized by limitations such as shallowness, stoniness, low moisture-holding capacity, low fertility, 

and salinity that are difficult to correct. Over half of all tree 

nuts and fruits, as well as grapes, are grown on class IV or less 

suitable soils. These crops do not require continued cultivation 

and are more likely to thrive in these soils than annual crops. 

Wheat, cotton, vegetables, rice and citrus are more likely to 

be grown on poorer soils than corn and soybeans. Figure 17 

shows the location of cropland on soils that have severe or 

very severe limitations and may require special conservation 

or management practices. 

EXPLANATION

Cropland on soils that have severe or very severe

 limitations which require special conservation or

 management practices (Class III–VIII soils)

Cropland on soils that have slight or moderate

 limitations which require moderate conservation

 practices (Class I and II soils)

or management practices (Class III–VIII soils) and cropland on soils that have slight or moderate limitations which require 

moderate conservation practices (Class I and II soils) in the conterminous United States (U.S. Department of Agriculture, 2006, 

2010b).

Figure 17. Location of cropland on soils that have severe or very severe limitations which require special conservation 
26 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Soil Water

The amount of water in soil that is available to plants is 

controlled by terrain, climate, and soil characteristics. Soilwater availability is the limiting factor for the location of crop 

agriculture in many areas. The water in soil comes largely 

from precipitation. In some locations, soil water can also come 

via groundwater or surface-water flowpaths. Crop agriculture 

is highly dependent upon the amount and seasonal patterns 

of available soil water from precipitation and, in many areas, 

irrigation. Animal agriculture is not directly dependent on 

soil water, but it is dependent on the quantity and quality of 

groundwater and (or) surface water that is available. 

In addition to its soil capability classification system, 

the U.S. Department of Agriculture has a soil classification 

based on the dominant hazard that affects use of the soil for 

agriculture (fig. 18) (Helms, 1992). The dominant hazard can 

be climate, soil, erosion, or excess water. Susceptibility to soil 

erosion from water runoff (or wind) is the dominant hazard 

that affects the use of most soils in the Nation. Fifty-one percent of crops are grown on soils where erosion susceptibility 

and past erosion damage are the major factors affecting these 

soils (appendix 6, table 6–1). Twenty-five percent of crops are 

grown on soils where poor drainage, wetness, high water table, 

or overland flow is the dominant hazard. Most of the rice and 

citrus grown in this country are grown on soils where water is 

the dominant hazard. Many of the soils along the Gulf Coast 

and East Coast and along the river valleys are limited by their 

excess water content. These soils can be artificially drained to 

overcome such limitations. Figure 19 shows the location of 

cropland on soils in each hazard class (climate, erosion, soils, 

and water).

It is important that excessive soil water does not occur 

at times detrimental to plant growth and harvest. The occurrence of moisture stress during flowering, pollination, and 

grain-filling is harmful to most crops and particularly to corn, 

soybeans, and wheat. Excessive soil water interferes with plant 

nutrient flow, increases the risk of plant disease and insect 

infestation, and delays to planting or harvesting. In addition, 

the necessity and feasibility of irrigation are determined by the 

degree of evapotranspiration and the distribution of precipitation through the growing season. 
Undefined.

Climate class is made up of soils for which the climate— 

 the temperature or lack of moisture—is the major hazard 

 or limitation.

Erosion class is made up of soils for which the susceptibility 

 to erosion is the dominant problem or hazard affecting their 

 use. Erosion susceptibility and past erosion damage are 

 the major soil factors that affect soils in this subclass.

Soils class is made up of soils that have soil limitations within 

 the rooting zone, such as shallowness of the rooting zone, 

 stones, low moisture-holding capacity, low fertility that is 

 difficult to correct, and salinity or sodium content.

Water class is made up of soils for which excess water is the 

 dominant hazard or limitation affecting their use. Poor soil 

 drainage, wetness, a high water table, and overflow are 

 factors that affect these soils.

EXPLANATION

Soil hazard class

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by hazard class,

 in percent 

Environmental Factors That Influence the Location of Crop Agriculture 27

Figure 18. Location of cropland on each soil hazard class and the percentage of cropland area for each soil hazard class for 

selected crops in the conterminous United States (U.S. Department of Agriculture, 2006, 2010b).
28 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Some of the most fertile cropland is in areas with too 

much water. Much of the Midwest and Eastern Piedmont 

were previously wetlands and are underlain by poorly drained 

soils (Dahl, 1990). The landscape has shallow slopes, and 

annual precipitation exceeds evapotranspiration. Much of 

the cropland in this area is drained so that fields are accessible for planting and roots are not damaged by too much 

water. It is estimated that 87 percent of wetland losses from 

the mid-1950s to mid-1970s were due to the installation of 

artificial drainage for conversion to agricultural land (Frayer 

and others, 1983). Artificial drainage includes both surface and 

subsurface modifications that remove excess water away from 

crops. Surface artificial drainage usually involves constructing ditches or straightening and deepening natural channels, 

and subsurface artificial drainage includes the installation of 

drainage pipes (often called tile drains) at some depth (1–2 m) 

below the land surface. Crops most likely to be grown in 

areas that are drained are corn, soybeans, and citrus (fig. 20) 

(appendix 6, table 6–2). Most of the rice grown in the United 

States is grown on poorly drained soils. In contrast to other 

crop environments, rice fields are allowed to remain flooded 

until harvest. 

Climate Erosion

Soils Water

EXPLANATION

Cropland on soils in selected hazard class

Other cropland

Figure 19. Location of cropland on soils in each hazard class (Climate, Erosion, Soils, and Water) in the conterminous United States 

(U.S. Department of Agriculture, 2006, 2010b).
0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area, in percent

EXPLANATION

Cropland on soils requiring artificial drainage

Other cropland

Subsurface

Surface

Environmental Factors That Influence the Location of Crop Agriculture 29

Figure 20. Cropland on soils where surface or subsurface artificial drainage is likely required to remove excess 

water in order to cultivate cropland in the conterminous United States (U.S. Department of Agriculture, 2006, 2010b; 

Michael E. Wieczorek, U.S. Geological Survey, written commun., 2010). Maps show areas where either surface or 

subsurface artificial drainage is predominant.
30 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Area runoff and recharge are the amount of water available for streamflow and seepage to groundwater. The sustained 

supply of water from surface and underground resources 

influences which crops can be grown in specific areas. Average annual runoff and recharge depth for a watershed is the 

combined runoff and recharge for a year uniformly distributed 

over the entire watershed and is an indication of the wetness 

or dryness of an area. Much of the wheat, cotton, sorghum, 

and citrus produced in the United States is grown in areas 

with less than 214 mm of annual runoff or recharge (figs. 21 

and 22; appendix 6, tables 6–3 and 6–4). With the exception 

of citrus, most of these crops are grown in Great Plains States 

characterized by low to moderate slopes, evapotranspiration 

often exceeding precipitation, and soils with moderate or 

0 to 214

>214 to 628

>628 to1,355

>1,355 to 2,976

>2,976 

EXPLANATION

Water depth, in millimeters per year

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by water depth 

category, in percent 

Figure 21. Location of cropland and average annual runoff (1971–2000) and the percentage of cropland area for each runoff 

category for selected crops in the conterminous United States (David M. Wolock, U.S. Geological Survey, written commun., 

2009; U.S. Department of Agriculture, 2010b).
Environmental Factors That Influence the Location of Crop Agriculture 31

severe limitations for cultivation. Many tree fruits are grown 

in areas with more than 628 mm annual runoff and recharge. 

The areas with the highest runoff and recharge, the Pacific 

Northwest, northern Michigan, and the Northeast, tend to have 

high precipitation, low evapotranspiration, steeper slopes, and 

sandy soils. These areas are particularly suited to tree fruit 

production. Areas with more moderate runoff and recharge 

are the Midwest and Southern Gulf States. Corn and soybeans are the predominant crops in these areas characterized 

by low slopes, adequate precipitation for evapotranspiration 

requirements, and soils with slight or moderate limitations for 

cultivation. 

0 to 214

>214 to 628

>628 to1,355

>1,355 to 2,976

>2,976 

EXPLANATION

Water depth, in millimeters per year

Cropland

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area by water depth 

category, in percent 

Figure 22. Location of cropland and average annual recharge (1971–2000) and the percentage of cropland area for each 

recharge category for selected crops in the conterminous United States (David M. Wolock, U.S. Geological Survey, written 

commun., 2009; U.S. Department of Agriculture, 2010b).
32 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Other Land Uses in Competition for 

Agricultural Lands 

Crops need space to grow. Since the beginning of 

agriculture over 10,000 years ago, land has been cleared for 

the production of crops. Much of the Nation’s forests, grasslands, and wetlands have been converted to agricultural land 

(fig. 23) (appendix 7, tables 7–1, 7–2 and 7–3). Nearly 25 

percent of wetlands have been converted to cropland. A large 

percentage of land now devoted to rice (88 percent) and citrus 

(67 percent) was once natural wetlands (table 7—2). About 

80 percent of land now devoted to wheat was once natural 

grasslands. In contrast, only a small percentage of needleleaf 

forests have been converted to cropland because needleleaf 

forests commonly grow in soils and on terrain less suitable for 

cropland.

Increasing population and land development during the 

last 100 years have increased the extent of urban use competing for agricultural land. Continuing population growth will 

cause further increases in urbanization, land development, and 

demand for agricultural products—the latter paradoxically 

resulting in an even greater need of space for agricultural land. 

The annual conversion of prime agricultural land to developed 

land in the United States amounts to about 2,600 km2

, nearly 

30 percent of this conversion occurring in just four states: 

Texas (271 km2), Ohio (170 km2), North Carolina (138 km2) 

and Illinois (129 km2) (Abdalla, 2005).

0

20

40

60

80

100

All crops

Corn

Soybeans

Wheat

Other crops

Cotton

Sorghum

Vegetables and

ground fruit Rice

Orchards

and grapes

Citrus

 Cropland area in each potential 

natural vegetation type, in percent

Wetlands

Broadleaf forests

Needleleaf forests

Grasslands

Scrublands

EXPLANATION

Potential natural vegetation type

Cropland

Figure 23. Location of cropland on potential natural vegetation, and the percentage of cropland area in each natural vegetation 

type for selected crops in the conterminous United States (Missoula Fire Sciences Laboratory, 2001; U.S. Department of 

Agriculture, 2010b).
Summary 33

Summary

Environmental, economic, and societal factors have 

determined the location of crop agriculture. The environmental 

factors influencing the extent of crop agriculture are terrain, 

climate, soil properties, and soil water. It is the combination of 

these four environmental factors that create suitable conditions for specific crops to be grown in certain areas. In some 

locations, conditions are nearly ideal for commercial agriculture. In other locations—places with rugged slopes, poor 

soil, lack of soil water, excess soil water and (or) inhospitable 

climate—commercial agriculture is unprofitable, perhaps even 

impossible. However, most agricultural lands fall somewhere 

between the two ends of the spectrum. Today, in order to 

maximize production, most of the cultivated croplands and 

grasslands for commercial agriculture are in areas where 

crops and livestock are well suited to local conditions. In the 

United States, cropland (row crops, closely sown crops, fruits, 

nuts, and vegetables) occupies about 13 percent of the total 

land area. Grassland (including hay) and rangeland occupy 

another 41 percent of the land area. 

In areas that are too steep, too wet, or too dry, modifications of the landscape have been implemented to make 

conditions suitable for crops to grow and thrive. Some of the 

limitations of the environmental influences can be overcome 

through agricultural modifications, but others cannot. On a 

larger-than-field scale, agricultural modifications commonly 

influence water availability through irrigation and (or) drainage and soil fertility and (or) organic-matter content through 

amendments such as manure, commercial fertilizer and lime. 

In general, it is not feasible to modify the other environmental 

influences—soil texture, soil depth, soil mineralogy, temperature, and terrain—at large scales. 

About 1 percent of crops in the conterminous United 

States are grown at elevations higher than 2,000 m; wheat, 

“other crops” (including grasses such as rye, oats, and barley), 

and vegetables and ground fruit make up most of the crops 

grown at these high altitudes. 

About 80 percent of all crops in the conterminous 

United States are grown on land with a slope of 3 percent 

or less. Nearly all cotton, rice, and citrus are grown on land 

with a slope of 2 percent or less. About 10 percent of all crops 

are grown on slopes greater than 4 percent, and only a small 

percentage of all crops are grown on steeper slopes (5 to 

16 percent). The crops grown on steeper slopes are tree fruits, 

grapes, vegetables, and other grains. A much larger percentage of grassland has steeper slopes than cropland; in fact, 

about 22 percent of grassland is grown on slopes steeper than 

5 percent, because steeper slopes are generally unsuited to 

cultivation.

Cumulative growing degree-days (GDD) (or heat units), 

the metric used for the solar radiation requirements for crops, 

are the accumulated product of time and temperature above 

the minimum developmental thresholds for each day. More 

than 90 percent of all crops in the conterminous United States 

are grown in areas with more than 2,000 cumulative GDD. 

All cotton, rice, and citrus require more than 4,000 cumulative GDD, but many vegetables and ground fruits are grown in 

areas with less than 3,000 cumulative GDD.

More than 60 percent of all crops in the conterminous 

United States are grown in areas that receive less than 900 

mm/yr. Rice and citrus tend to be grown in areas with more 

precipitation, whereas wheat, sorghum, vegetables and ground 

fruit, and orchard and vineyard crops are grown in areas with 

less precipitation. Crops that are most likely to be irrigated are 

rice, nuts, citrus, other tree fruits, and grapes. All rice grown 

in the United States is produced in irrigated fields. Although 

about 15 percent of corn and 10 percent of soybeans are 

irrigated, these two crops account for 79,000 km2 of irrigated 

cropland.

With respect to U.S. Department of Agriculture soil capability classes, about 7 percent of all crops in the conterminous 

United States are grown on soils with slight limitations (class 

I) with unrestricted use for agriculture 47 percent of crops 

are grown on soils that have moderate limitations (class II) 

and may require conservation measures, 29 percent of crops 

are grown on soils that have severe limitations (class III) and 

require special conservation practices, and 10 percent of crops 

are grown on soils with very severe limitations (class IV) that 

restrict the choice of plants and require very careful management. About 11 percent of crops are grown on soils considered 

unsuitable for cultivation (classes V–VIII).

Increasing population and land development during the 

last 100 years have increased the extent of urban use competing for agricultural land. Continuing population growth will 

cause further increases in urbanization, land development, and 

demand for agricultural products—the latter paradoxically 

resulting in an even greater need of space for agricultural land. 
34 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

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 37

Appendixes 1–8

Appendix 1—Development of Croplands in the United States ............................................................37

Appendix 2—Environmental Factors That Influence the Location of Crop Agriculture ..................38

Appendix 3—Elevation and Slope ............................................................................................................39

Appendix 4—Climate...................................................................................................................................48

Appendix 5—Soil .........................................................................................................................................53

Appendix 6—Soil Water.............................................................................................................................56

Appendix 7—Other Land Uses in Competition for Agricultural Lands ..............................................61

Appendix 8—Summary of Data Sources and Online Links to Datasets ............................................65

References Cited in the Appendixes ........................................................................................................71
38 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Appendix 1—Development of 

Croplands in the United States

Data used to show development of crop agricultural lands 

include the geospatial dataset “Potential Natural Vegetation” 

and compilations of the U.S. censuses of population and agriculture. A compilation of the agricultural censuses was also 

used to show state-level trends in harvested cropland.

The Potential Natural Vegetation (PNV) map in figure 1 

is used as a surrogate for presettlement land cover in the 

United States. PNV is the “climax” vegetation that will occupy 

a site without disturbance or climatic change and is an expression of environmental factors such as terrain, soils, and climate across an area. The original PNV map was developed by 

Küchler (1964). The version used in this report is an updated 

digital geospatial dataset developed by the Missoula Fire Sciences Laboratory (2001).

The historical expansion of agriculture is shown by 

extracting county census of population for years prior to the 

agricultural census (fig. 2) and area of improved farmland 

from the agricultural census for 1850–2002 (fig. 3). In the conterminous United States, a census of population has been taken 

every 10 years since 1790, and information on agricultural 

land use has been collected since 1850 at 10-year or 5-year 

intervals. Figure 2 shows the extent of settled area for 1790 

and 1830, compiled by Waisanen and Bliss (2002) from the 

census of population, to show the probable extent of agriculture prior to the first agricultural census. Waisanen and Bliss 

used spatial historical county-boundary data and matched 

them to the tabular census data. 

County-level census of agriculture area of improved 

farmland for 1850–1997 compiled by Waisanen and Bliss 

(2002) was used to show historical extent of agriculture 

(fig. 3). The tabular datasets, which are no longer available 

online, were provided by Norman Bliss, U.S. Geological 

Survey (written commun., 2011). The Waisanen and Bliss 

compilation is available through the 1997 agricultural census. Harvested cropland (improved farmland) for the 2002 

census was obtained from the 2002 Census of Agriculture 

(U.S. Department of Agriculture, 2005). 

Trends in harvested cropland for 1879–1997 (fig. 4) 

generated from state-level historical cropland area were compiled by Ramankutty and others (2010) from the agricultural 

censuses. Ramankutty and others provide a tabular dataset. 
Appendix 2—Environmental Factors That Influence the Location of Crop Agriculture 39

Appendix 2—Environmental Factors 

That Influence the Location of 

Crop Agriculture

The extent and amount of cropland in the United States 

are available from several sources. The most recent spatial datasets of cropland are in the Cropland Data Layer 

(U.S. Department of Agriculture, 2010), a raster, georeferenced, crop-specific land-cover data layer with a ground 

resolution of 56 m. It is produced from satellite imagery collected during the growing season. The overall accuracy of the 

crop-specific portion of the data is 94 percent (U.S. Department of Agriculture, 2010). The 2009 Cropland Data Layer 

was used for most of the illustrations in this report. This 

dataset shows the spatial extent of each land cover, including 

individual crops, at a 56-m-cell-size resolution. Several crops 

were grouped together for the purposes of this report and were 

used to generate the map in figure 5. Vegetables and ground 

fruit were grouped together, tree fruits (with the exception of 

citrus) and nuts were grouped together, and minor crops (in 

terms of total land area occupied) were grouped into the “other 

crops” category. These groups are given in table 2–1 and were 

used throughout the report. Breakdowns of the amount of land 

in each crop and crop group, shown in the bar graph in figure 

5, are summarized in table 2–2. The area of land in each landcover group for the 2009 Cropland Data Layer is summarized 

in table 2–3.

There is no reliable way in the Cropland Data Layer to 

differentiate between grassland and hay or between grassland 

and range used for pasture or livestock. The spectral signature 

for grassland and hay on satellite imagery is similar and difficult to distinguish. In addition, there is no way to determine 

whether grassy fields or range (shrubland) are used to graze 

livestock or simply left fallow. To derive these estimates for 

the report, supplemental information from the Economic 

Research Service (ERS) national compilation “Major Uses 

of Land in the United States, 2002” (Lubowski and others, 

2006) was used to differentiate the national total land area 

used for hay, fallow, idle and cropland pasture, grassland, and 

range. The total area for each matching general land cover was 

similar for the 2002 and 2009 datasets (table 2–4). Land-cover 

percentages for hay and total grassland obtained from the 2002 

data were applied to the 2009 data to obtain relative land area 

for grassland and range (table 2–4). The distinction between 

pasture, rangelands, and other grazing lands is available online 

at http://www.epa.gov/oecaagct/anprgidx.html.

Dot density maps for the extent of cropland and total 

cropland area for specialty crops are available from the 2007 

Census of Agriculture (U.S. Department of Agriculture, 

2009a). These maps are not available as digital datasets and 

were used in this report simply to illustrate the primary locations for some economically significant specialty crops that 

occupy relatively small land areas and are not readily visible 

on the national scale Cropland Data Layer maps (fig. 6). Total 

acreage for each specialty crop is given on the online maps. 

These values, along with the Map Number used to generate 

figure 6, are given in table 2–5.
40 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used in this report (U.S. Department of Agriculture, 

2010b). —Continued

[Ma, million acres; kkm2, 1,000 square kilometers; NLCD, National Land Cover Database]

Cropland Data 

Layer landcover codeCropland Data Layer land-cover name Land-use group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total land 

131 NLCD - Barren Barren 19.89 80.51 1.05

Barren Total 19.89 80.51 1.05

72 Citrus Citrus 0.17 0.67 0.01

212 Oranges Citrus 1.06 4.31 0.06

Citrus Total 1.23 4.98 0.07

1 Corn Corn 80.41 325.44 4.25

225 Dbl. Crop WinWht/Corn Corn 0.03 0.14 0.00

226 Dbl. Crop Oats/Corn Corn 0.00 0.01 0.00

237 Dbl. Crop Barley/Corn Corn 0.02 0.10 0.00

241 Dbl. Crop Corn/Soybeans Corn 0.00 0.01 0.00

Corn Total 80.48 325.69 4.25

2 Cotton Cotton 9.37 37.93 0.50

232 Dbl. Crop Lettuce/Upland Cotton Cotton 0.01 0.03 0.00

238 Dbl. Crop WinWht/Cotton Cotton 0.01 0.03 0.00

Cotton Total 9.39 37.99 0.50

63 Woodland Forest 0.98 3.98 0.05

141 NLCD - Deciduous Forest Forest 245.55 993.72 12.98

142 NLCD - Evergreen Forest Forest 250.07 1012.04 13.22

143 NLCD - Mixed Forest Forest 28.55 115.54 1.51

Forest Total 525.15 2125.28 27.76

36 Alfalfa Grassland 10.86 43.96 0.57

37 Other Hays Grassland 33.78 136.72 1.79

60 Switchgrass Grassland 0.02 0.08 0.00

61 Fallow/Idle Cropland Grassland 22.22 89.94 1.17

62 Pasture/Grass Grassland 43.23 174.95 2.29

171 NLCD - Grassland Herbaceous Grassland 262.60 1062.74 13.88

181 NLCD - Pasture/Hay Grassland 109.52 443.23 5.79

Grassland Total 482.24 1951.63 25.49

66 Cherry Orchard Orchards and grapes 0.09 0.37 0.00

67 Peaches Orchards and grapes 0.05 0.19 0.00

68 Apples Orchards and grapes 0.16 0.66 0.01

69 Grapes Orchards and grapes 0.45 1.82 0.02
Appendix 1—Development of Croplands in the United States 41

Table 2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used in this report (U.S. Department of Agriculture, 

2010b). —Continued

[Ma, million acres; kkm2, 1,000 square kilometers; NLCD, National Land Cover Database]

Cropland Data 

Layer landcover codeCropland Data Layer land-cover name Land-use group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total land 

71 Other Tree Nuts Orchards and grapes 0.26 1.04 0.01

73 Other Tree Fruits Orchards and grapes 0.02 0.07 0.00

74 Pecans Orchards and grapes 0.06 0.25 0.00

75 Almonds Orchards and grapes 0.89 3.60 0.05

76 Walnuts Orchards and grapes 0.32 1.30 0.02

77 Pear Orchards and grapes 0.01 0.04 0.00

204 Pistachios Orchards and grapes 0.11 0.46 0.01

210 Prunes Orchards and grapes 0.05 0.19 0.00

211 Olives Orchards and grapes 0.09 0.35 0.00

217 Pomegranates Orchards and grapes 0.01 0.05 0.00

218 Nectarine Orchards and grapes 0.00 0.00 0.00

220 Plums Orchards and grapes 0.01 0.03 0.00

223 Apricots Orchards and grapes 0.01 0.05 0.00

Orchards and grapes Total 2.58 10.46 0.14

6 Sunflowers Other crops 1.52 6.15 0.08

10 Peanuts Other crops 1.20 4.86 0.06

11 Tobacco Other crops 0.01 0.05 0.00

21 Barley Other crops 2.27 9.17 0.12

25 Other Small Grains Other crops 0.07 0.29 0.00

27 Rye Other crops 0.33 1.33 0.02

28 Oats Other crops 1.50 6.06 0.08

29 Millet Other crops 0.38 1.53 0.02

30 Spelt Other crops 0.00 0.01 0.00

31 Canola Other crops 0.86 3.48 0.05

32 Flaxseed Other crops 0.21 0.86 0.01

33 Safflower Other crops 0.12 0.47 0.01

34 Rape Seed Other crops 0.00 0.00 0.00

35 Mustard Other crops 0.02 0.09 0.00

38 Camelina Other crops 0.01 0.05 0.00

41 Sugarbeets Other crops 0.97 3.92 0.05

44 Other Crops Other crops 0.14 0.55 0.01

45 Sugarcane Other crops 0.84 3.40 0.04
42 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used in this report (U.S. Department of Agriculture, 

2010b). —Continued

[Ma, million acres; kkm2, 1,000 square kilometers; NLCD, National Land Cover Database]

Cropland Data 

Layer landcover codeCropland Data Layer land-cover name Land-use group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total land 

56 Hops Other crops 0.02 0.07 0.00

57 Herbs Other crops 0.06 0.25 0.00

58 Clover/Wildflowers Other crops 0.13 0.54 0.01

59 Seed/Sod Grass Other crops 0.97 3.91 0.05

70 Christmas Trees Other crops 0.06 0.25 0.00

92 Aquaculture Other crops 0.17 0.67 0.01

205 Triticale Other crops 0.06 0.22 0.00

224 Vetch Other crops 0.00 0.01 0.00

Other crops Total 11.91 48.19 0.63

3 Rice Rice 3.35 13.54 0.18

Rice Total 3.35 13.54 0.18

152 NLCD - Shrubland Shrubland 416.72 1686.46 22.03

Shrubland Total 416.72 1686.46 22.03

4 Sorghum Sorghum 6.28 25.41 0.33

234 Dbl. Crop Durum Wht/Sorghum Sorghum 0.01 0.02 0.00

235 Dbl. Crop Barley/Sorghum Sorghum 0.01 0.02 0.00

236 Dbl. Crop WinWht/Sorghum Sorghum 0.03 0.12 0.00

Sorghum Total 6.32 25.57 0.33

5 Soybeans Soybeans 70.57 285.61 3.73

26 W. Wht./Soy. Dbl. Crop Soybeans 4.24 17.16 0.22

240 Dbl. Crop Soybeans/Oats Soybeans 0.01 0.03 0.00

254 Dbl. Crop Barley/Soybeans Soybeans 0.07 0.29 0.00

Soybeans Total 74.89 303.09 3.96

121 NLCD - Developed/Open Space Urban 72.27 292.48 3.82

122 NLCD - Developed/Low Intensity Urban 27.73 112.22 1.47

123 NLCD - Developed/Medium Intensity Urban 10.02 40.55 0.53

124 NLCD - Developed/High Intensity Urban 3.31 13.38 0.17

Urban Total 113.33 458.63 5.99

12 Sweet Corn Vegetables and ground fruit 0.28 1.13 0.01

13 Pop. or Orn. Corn Vegetables and ground fruit 0.11 0.44 0.01

14 Mint Vegetables and ground fruit 0.02 0.07 0.00

42 Dry Beans Vegetables and ground fruit 1.28 5.20 0.07
Appendix 1—Development of Croplands in the United States 43

Table 2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used in this report (U.S. Department of Agriculture, 

2010b). —Continued

[Ma, million acres; kkm2, 1,000 square kilometers; NLCD, National Land Cover Database]

Cropland Data 

Layer landcover codeCropland Data Layer land-cover name Land-use group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total land 

43 Potatoes Vegetables and ground fruit 0.99 4.00 0.05

46 Sweet Potatoes Vegetables and ground fruit 0.03 0.12 0.00

47 Misc. Vegs. & Fruits Vegetables and ground fruit 0.17 0.70 0.01

48 Watermelon Vegetables and ground fruit 0.03 0.10 0.00

49 Onions Vegetables and ground fruit 0.08 0.32 0.00

52 Lentils Vegetables and ground fruit 0.36 1.46 0.02

53 Peas Vegetables and ground fruit 0.91 3.70 0.05

54 Tomatoes Vegetables and ground fruit 0.15 0.60 0.01

55 Caneberry Vegetables and ground fruit 0.01 0.04 0.00

206 Carrots Vegetables and ground fruit 0.00 0.01 0.00

207 Asparagus Vegetables and ground fruit 0.00 0.01 0.00

208 Garlic Vegetables and ground fruit 0.01 0.04 0.00

209 Cantaloupe Vegetables and ground fruit 0.02 0.07 0.00

214 Broccoli Vegetables and ground fruit 0.01 0.04 0.00

216 Peppers Vegetables and ground fruit 0.01 0.05 0.00

219 Greens Vegetables and ground fruit 0.01 0.02 0.00

221 Strawberries Vegetables and ground fruit 0.02 0.09 0.00

222 Squash Vegetables and ground fruit 0.01 0.06 0.00

227 Lettuce Vegetables and ground fruit 0.04 0.15 0.00

229 Cucumber Vegetables and ground fruit 0.00 0.01 0.00

230 Pumpkin Vegetables and ground fruit 0.04 0.18 0.00

231 Dbl. Crop Lettuce/Cantaloupe Vegetables and ground fruit 0.00 0.01 0.00

242 Blueberry Vegetables and ground fruit 0.10 0.41 0.01

243 Cabbage Vegetables and ground fruit 0.01 0.05 0.00

244 Cauliflower Vegetables and ground fruit 0.00 0.01 0.00

245 Celery Vegetables and ground fruit 0.00 0.01 0.00

246 Radish Vegetables and ground fruit 0.00 0.01 0.00

247 Turnip Vegetables and ground fruit 0.00 0.00 0.00

250 Cranberry Vegetables and ground fruit 0.01 0.03 0.00

Vegetables and ground fruit Total 4.73 19.16 0.25

87 Wetlands Wetlands 0.09 0.35 0.00

190 NLCD - Woody Wetlands Wetlands 67.32 272.45 3.56
44 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 2–1. 2009 Cropland Data Layer land-cover codes and land-cover groups used in this report (U.S. Department of Agriculture, 

2010b). —Continued

[Ma, million acres; kkm2, 1,000 square kilometers; NLCD, National Land Cover Database]

Cropland Data 

Layer landcover codeCropland Data Layer land-cover name Land-use group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total land 

195 NLCD - Herbaceous Wetlands Wetlands 19.75 79.92 1.04

Wetlands Total 87.16 352.72 4.61

22 Durum Wheat Wheat 2.23 9.01 0.12

23 Spring Wheat Wheat 13.85 56.05 0.73

24 Winter Wheat Wheat 36.19 146.45 1.91

Wheat Total 52.26 211.51 2.76

Total Land Cover 1891.63 7655.41 100.00

111 NLCD - Open Water Water 32.69 132.31 1.70

Water Total 32.69 132.31 1.70

112 NLCD - Perennial Ice/Snow Perennial Ice/Snow 0.32 1.31 0.02

Perennial Ice/Snow Total 0.32 1.31 0.02
Appendix 1—Development of Croplands in the United States 45

Table 2–2. Land area in each crop or crop group in the 2009 Cropland Data Layer for the conterminous United States 

(U.S. Department of Agriculture, 2010b). 

[Ma, million acres; kkm2, thousand square kilometers]

Crop or crop group Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent of 

total combined cropland

Corn 80.48 325.69 32.56

Soybeans 74.89 303.09 30.30

Wheat 52.26 211.51 21.15

Other crops 11.91 48.19 4.82

Cotton 9.39 37.99 3.80

Sorghum 6.32 25.57 2.56

Vegetables and ground fruit 4.73 19.16 1.92

Rice 3.35 13.54 1.35

Orchards and grapes 2.58 10.46 1.04

Citrus 1.23 4.98 0.50

All crops combined 247.14 1,000.18 100.00

Grassland and pasture; includes hay 482.24 1,951.63

Table 2–3. Area of land in each land cover group for the 2009 Cropland Data Layer for the conterminous United States 

(U.S. Department of Agriculture, 2010b).

[Ma, million acres; kkm2, thousand square kilometers]

Land-cover category 2009 Cropland Data Layer, 

in Ma

2009 Cropland Data Layer, 

in kkm2 Percent land cover

Cropland 247 1,000 13

Grassland and pasture; includes hay 482 1,952 25

Forest 525 2,125 28

Urban 113 459 6

Shrublands, wetlands, and barren 525 2,120 28

Total 1,892 7,656 100
46 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 2–4. Comparison of land area in each land-cover group reported in “Major Uses of Land, 2002” (Lubowski and others, 2006) and 

the 2009 Cropland Data Layer (U.S. Department of Agriculture, 2010b), and estimated land area for land-cover groups not differentiated in 

the 2009 Cropland Data Layer for the conterminous United States. 

[Ma, million acres; kkm2, thousand square kilometers]

Land-cover category

“Major Uses of 

Land, 2002,” 

in Ma

“Major Uses of 

Land, 2002,” 

in kkm2

Percent land 

cover for 

“Major Uses 

of Land, 2002”

2009 Cropland 

Data Layer, 

in Ma

2009 Cropland 

Data Layer, 

in kkm2

Percent land 

cover for the 

Cropland Data 

Layer

Harvested cropland without hay 242 979 13 247 1,000 13

Hay 65 263 3 661 267 3

Crop failure and cultivated summer fallow 33 133 2 1282 518 7

Idle cropland 40 162 2

Cropland pasture 62 251 3

Total cropland 441 1,788 23 4413 1,785 23

Grassland and other pasture

Not differentiated

2884 1,165 15

Range 16

Total grassland pasture and range 584 2,363 31 5836 2,359 31

Total forest 559 2,262 30 525 2,125 28

Total urban 59 239 3 113 459 6

Total miscellaneous and special uses 250 1,012 13 2297 927 12

Total land cover 1,893 7,664 100 1,892 7,655 100

1

 Hay based on 21 percent of harvested cropland per “Major Uses of Land, 2002.”

2

 Crop failure and cultivated summer fallow equal the difference between total cropland (based on 23 percent total land use per “Major Uses of Land, 2002”) and 

the sum of hay plus harvested cropland. The 128 Ma value was subtracted from the 482 Ma value for grassland and pasture in the Cropland Data Layer.

3

 Total cropland based on 23 percent total land use per “Major Uses of Land, 2002.”

4

 Grassland and other pasture based on 482 Ma from 2009 Cropland Data Layer, minus the sum of Hay (66 Ma) plus other cropland (128 Ma).

5

 Range is the difference between the Total grassland pasture and range (583 Ma) and Grassland and other pasture (288 Ma). 

6

 Total grassland pasture and range based on 31 percent of Total land cover per “Major Uses of Land, 2002.” 

7

 Total miscellaneous based on difference between Total land cover (1,892 Ma) and the sum of all other categories (1,662 Ma).
Appendix 1—Development of Croplands in the United States 47

Table 2–5. Total acreage grown for selected grain and specialty crops for the 

conterminous United States (U.S. Department of Agriculture, 2009a). 

[km2, square kilometers; USDA, U.S. Department of Agriculture]

Crop Area, in acres Area, in km2 USDA Map Number

Barley 3,521,957 14,253 07–M176

Oats 1,509,149 6,107 07–M178

Rye 267,361 1,082 07–M180

Sunflowers for oil 1,710,057 6,920 07–M182

Canola 1,149,682 4,653 07–M184

Sunflowers for seed 290,096 1,174 07–M183

Dry edible beans 1,455,549 5,890 07–M197

Dry edible peas 848,874 3,435 07–M198

Peanuts 12,000,564 48,565 07–M203

Potatoes 1,131,963 4,581 07–M199

Tobacco 359,846 1,456 07–M191

Sugarbeets 1,253,817 5,074 07–M201

Sugarcane 846,666 3,426 07–M202

Strawberries 60,353 244 07–M232

Blueberries 55,601 225 07–M231
48 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Appendix 3—Elevation and Slope

The National Elevation Dataset (NED) can be used 

to identify those areas that are constrained for agricultural 

production by elevation and slope. The NED is composed of 

raster elevation data at a resolution of 1 arc-second (approximately 30 m) for the conterminous United States (Gesch, 

2007; Gesch and others, 2002). The NED 30-m data are available in 1,390 separate raster data files and are too large, when 

combined, to be useful for national-scale analyses. To make 

the 30-m NED usable for national analyses, 30-m elevation 

raster data were bilinearly resampled to 100 m (NED 100 m) 

and assembled to 1-degree grids (Falcone, 2003). To determine 

the amount of cropland at higher elevations, the NED 100 m 

was converted to 56 m and combined with the 2009 Cropland 

Data Layer to calculate the percentage of land area in each 

crop group for elevations higher than 2,000 m (table 3–1).

Percent slope was generated from the NED 100-m raster 

dataset by calculating a slope surface using the ARCInfo 

Spatial Analyst>Surface Analysis>Slope tool. To determine 

the amount of cropland on each slope category (fig. 7) the 

NED 100 m was converted to 56 m and was combined with 

the 2009 Cropland Data Layer to calculate the percentage of 

land area in each crop group for each slope category (1, 2, 3, 

and greater than 4 percent) (table 3–2). The combined raster 

dataset was used to generate a map showing the location of 

crops on land with 3 percent or greater slope (fig. 8).

Table 3–1. Land area in each crop or crop group above 2,000 meters elevation in the 2009 

Cropland Data Layer for the conterminous United States (Falcone, 2003; U.S. Department of 

Agriculture, 2010b).

[m, meters; Ma, million acres; kkm2, thousand square kilometers]

Crop or crop group Land area, 

in Ma

Land area, 

in kkm2

Land area, in percent 

of total combined 

cropland above 

2,000 m

Corn 0.0 0.01 0.07

Soybeans 0.0 0.0 0.0

Wheat 0.83 3.37 29.31

Other crops 1.12 4.55 39.53

Cotton 0.0 0.0 0.04

Sorghum 0.0 0.01 0.1

Vegetables and ground fruit 0.88 0.36 30.92

Rice 0.0 0.0 0.0

Orchards and grapes 0.0 0.0 0.0

Citrus 0.0 0.0 0.0

All crops above 2,000 m 2.84 11.51 100.00

All crops 247.14 1,000.18 1.15
Table 3–2. Land area in each crop or crop group for each slope range in the 2009 Cropland Data Layer for the conterminous United States (Falcone, Agriculture, 2010b). [Ma, million acres; kkm2, thousand square kilometers]

Slope range, in percent 0 to 1 > 1 to 2 > 2 to 3 > 3 to 4 

Crop or crop group Land area, in Ma Land area, in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total f

or crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

La

Corn 19.08 77.23 23.71 32.13 130.02 39.92 12.69 51.36 15.77 6.78 27.43 8.42 Soybeans 22.02 89.11 29.40 29.52 119.47 39.42 10.94 44.29 14.61 5.52 22.36 7.38 Wheat 10.24 41.44 19.59 22.14 89.59 42.36 8.95 36.21 17.12 4.50 18.22 8.61 Other crops 3.44 13.92 28.89 4.50 18.19 37.75 1.75 7.07 14.68 0.91 3.66 7.60 Cotton 3.68 14.89 39.19 4.43 17.94 47.23 0.82 3.33 8.76 0.29 1.16 3.06 Sorghum 1.80 7.30 28.56 2.79 11.28 44.11 0.97 3.92 15.32 0.44 1.78 6.97 Vegetables and ground fruit 1.24 5.02 26.18 1.77 7.18 37.49 0.70 2.83 14.76 0.36 1.44 7.53 Rice 2.51 10.13 74.80 0.80 3.25 24.03 0.03 0.11 0.84 0.01 0.02 0.17 Orchards and grapes 1.13 4.60 43.99 0.88 3.55 33.94 0.21 0.84 8.08 0.12 0.47 4.49 Citrus 0.76 3.08 61.77 0.32 1.28 25.79 0.07 0.30 5.95 0.03 0.13 2.64 All crops 65.91 266.73 26.67 99.29 401.81 40.17 37.13 150.25 15.02 18.95 76.67 7.67 
50 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Appendix 4—Climate

Cumulative growing degree-days and precipitation 

were used to identify favorable climates for the production 

of selected crops and crop groups. The 30-year average for 

1971–2000 (normal) cumulative growing degree-day geospatial dataset (0.010281-degree cell size) was provided by 

Leonard Coop, Integrated Plant Protection Center, Oregon 

State University (written commun., 2009). Degree-day maps 

generated by Coop take into account elevation, terrain, and 

local effects to show degree-day accumulations over a given 

time period (Oregon State University, 2010). “Normal” refers 

to the 30-year annual average of climatological data (temperature, precipitation, growing degree-days). At the time of 

data compilation and writing for this report, the 1981–2010 

normals were not available. 

The original dataset was provided in geographic coordinates that were reprojected into Albers Equal Area, resulting in 

a raster dataset with an approximately 1,013 m cell size. The 

raster dataset was then resampled (Nearest method) to 56 m 

cell size so that it could be combined with the Cropland Data 

Layer. To determine the amount of cropland for each range of 

degree-days (fig. 9) the 56-m degree-day raster dataset was 

combined with the 2009 Cropland Data Layer to calculate the 

percentage of land area in each crop group for each degree-day 

range (table 4–1). 

Average annual 1971–2000 (normal) precipitation spatial 

data were obtained from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) climate mapping system, Oregon State University (2008). Distribution of 

precipitation point measurements to a spatial raster dataset 

was accomplished by using the PRISM model, developed and 

applied by Chris Daly of the PRISM Climate Group at Oregon 

State University (2008). The precipitation raster dataset was 

then resampled (Nearest method) to 56 m cell size so that it 

could be combined with the 2009 Cropland Data Layer. To 

determine the amount of cropland for each precipitation range 

(fig. 10) the 56-m precipitation raster dataset was combined 

with the 2009 Cropland Data Layer to calculate the percentage 

of land area in each crop group for each precipitation range 

(table 4–2).

Generation of favorable water and heat (growing degreeday) boundaries for selected crops was done by combining the 

degree-day and precipitation raster datasets for selected crops. 

The most comprehensive list of growing degree-day thresholds was optioned from Fischer and others, 2002 (appendix 

IV). Water-requirement thresholds are available from Brouwer 

and Heibloem (1986). Thresholds used for the crops shown 

in figure 11 were obtained from these sources and are listed 

in table 4–3. For each crop, a separate raster dataset was 

extracted from the degree-day and precipitation raster dataset 

for the sub-optimal and optimal heat and water thresholds, 

resulting in four separate raster datasets for each crop. Using 

corn as an example, the sub-optimal heat requirement is 

1,900 degree-days, so a raster dataset was generated by selecting all values greater than 1,900. The sub-optimal degree-day 

and precipitation raster dataset for each crop was combined 

to result in a raster dataset showing the extent of sub-optimal 

conditions for that crop. The same process was used to generate the optimal raster dataset for each crop. The optimal and 

sub-optimal raster datasets were overlain to show the extent 

of favorable climate for selected crops, and the extent of each 

crop was subsequently overlain on the raster dataset maps. 

These maps are intended to be used as a general indication of 

the extent of favorable climate for specific crops. Many cultivars for each crop have been developed to extend the range in 

which they can be grown. In addition, irrigation can be used to 

extend the range into areas that do not receive enough precipitation to support the crop. 

The extent of irrigated cropland from surface-water and 

groundwater sources was determined from “Estimated Use 

of Water in the United States, 2005” (Kenney and others, 

2009). Irrigation water use includes water that is applied by 

an irrigation system to sustain plant growth in all agricultural 

and horticultural practices (Kenney and others, 2009). This 

source provides withdrawals of water for irrigation in million 

gallons per day. Also provided is total amount of irrigated land 

in acres. The extent of irrigated cropland from surface-water 

and groundwater sources (fig. 12) was generated by converting 

county-level withdrawal estimates for 2005 to withdrawals in 

thousand acre-feet per year and joining the converted withdrawals to a geospatial raster dataset generated by combining 

county boundaries and the 2009 Cropland Data Layer so that 

areas of irrigated cropland could be mapped. This method 

spreads the irrigation use across all the cropland in the county. 

In reality, only selected fields within each county are irrigated. 

More detailed information about the location of irrigated fields 

was not available at the time this report was written. 

To supplement the extent of irrigated cropland, dot 

density maps are used to show the extent of irrigated cropland 

for selected crops (fig. 13). These maps are available from the 

2007 Census of Agriculture (U.S. Department of Agriculture, 

2009a). These maps are not available in this report as digital 

datasets and were used simply to illustrate the primary locations for irrigated cropland. Total irrigated acreage for each 

crop shown in figure 13 is given on the online maps. These 

values, along with the Map Number used to generate the figure, are given in table 4–4.

Areas of groundwater decline in the United States were 

obtained from Reilly and others (2008). The areas delineated 

on the upper map in figure 14 were generated from a combination of wells in the U.S. Geological Survey National 

Water Information System (NWIS) database where measured 

groundwater-level difference over time is greater than 12 m, 

and areas in excess of 1,000 km2

 that have groundwater-level 

decline in excess of 12 m in at least one confined aquifer since 

predevelopment or in excess of 7.5 m of decline in unconfined 

aquifers since predevelopment. The dataset of NWIS wells 

and the polygon areas shown in the map were provided by 

William Cunningham, U.S. Geological Survey (written commun., 2011) Well locations were converted to a 10-km2

 raster 

dataset so that cells that had one or more wells in decline 
Table 4–1. Land area in each crop or crop group for each growing degree-day range in the 2009 Cropland Data Layer for the conterminous United SUniversity, 2010; U.S. Department of Agriculture, 2010b). [Ma, million acres; kkm2, thousand square kilometers]

Category 0 to 2,000 > 2,000 to 3,000 > 3,000 to 4,000 > 4,000 to 5,000 

Crop or crop group Land area, in Ma Land area, in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

La

Corn 0.79 3.19 0.98 35.89 145.23 44.59 32.94 133.31 40.93 6.13 24.82 7.62 Soybeans 1.01 4.07 1.34 28.85 116.74 38.52 26.14 105.80 34.91 11.67 47.25 15.59 Wheat 6.63 26.82 12.68 19.87 80.44 38.03 9.14 36.97 17.48 10.04 40.64 19.21 Other crops 2.54 10.28 21.34 5.10 20.62 42.79 0.53 2.13 4.41 0.82 3.31 6.87 Cotton 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.04 0.11 3.83 15.48 40.75 Sorghum 0.00 0.02 0.08 0.15 0.59 2.32 2.06 8.34 32.63 1.71 6.94 27.13 Vegetables and ground fruit 1.07 4.32 22.53 2.73 11.07 57.78 0.50 2.02 10.53 0.15 0.62 3.23 Rice 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 1.57 6.34 46.84 Orchards and grapes 0.03 0.12 1.12 0.24 0.98 9.35 0.14 0.55 5.27 0.89 3.60 34.40 Citrus 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.12 0.00 0.01 0.12 All crops 12.04 48.75 4.87 92.79 375.56 37.55 71.45 289.20 28.91 36.83 149.07 14.90 
Table 4–2. Land area in each crop or crop group for precipitation category in the 2009 Cropland Data Layer for the conterminous United States (Falcof Agriculture, 2010b). [mm/yr, millimeters per year; Ma, million acres; kkm2, thousand square kilometers] Precipitation category, in mm/yr 0 to 538 > 538 to 913 > 913 to 1,271 > 1,271 to 2,032 

Crop or crop group Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop La

Corn 8.87 35.89 11.02 39.97 161.76 49.67 27.83 112.61 34.58 3.81 15.42 4.73 Soybeans 5.24 21.21 7.00 30.83 124.76 41.16 30.04 121.59 40.12 8.78 35.53 11.72 Wheat 31.42 127.18 60.13 18.24 73.82 34.90 2.33 9.45 4.47 0.26 1.06 0.50 Other crops 6.78 27.44 56.95 1.70 6.86 14.24 1.66 6.74 13.98 1.76 7.14 14.81 Cotton 4.13 16.69 43.95 1.78 7.22 19.01 1.76 7.11 18.72 1.72 6.96 18.33 Sorghum 1.64 6.63 25.92 4.03 16.29 63.70 0.57 2.30 9.00 0.09 0.35 1.39 Vegetables and ground fruit 3.04 12.31 64.23 1.02 4.14 21.59 0.56 2.27 11.84 0.11 0.45 2.34 Rice 0.41 1.65 12.15 0.18 0.73 5.36 1.26 5.08 37.51 1.51 6.09 44.97 Orchards and grapes 1.84 7.47 71.40 0.38 1.53 14.64 0.25 1.00 9.55 0.11 0.46 4.39 Citrus 0.15 0.60 12.10 0.00 0.01 0.22 0.52 2.10 42.07 0.56 2.27 45.60 All crops 63.50 256.99 25.69 98.14 397.19 39.71 66.77 270.26 27.02 18.71 75.73 7.57 
Appendix 4—Climate 53

could be tagged as an area in decline. The polygon areas of 

groundwater-level decline also were converted to a raster dataset. The Well raster and Area raster datasets were combined to 

generate a map showing areas of groundwater-level decline. 

The combined dataset was then combined with the 2009 Cropland Data Layer so that cropland in areas of groundwater-level 

decline could be identified (fig. 14, lower map).

Table 4–3. Optimal and sub-optimal heat and water requirements for selected crops. 

[Sources: Fischer and others, 2002, Appendix IV, Temperature regime requirements of crops/LUTs; Brouwer and Heibloem, 1986, chapter 3, Crop water 

needs, 3.3.4, Indicative values of crop water needs. mm/yr, millimeters per year]

Crop

Sub-optimal heat 

requirements, 

in accumulated 

growing degree-days

Optimal heat 

requirements, 

in accumulated 

growing degree-days

Sub-optimal water 

requirements, 

in mm/yr

Optimal water 

requirements, 

in mm/yr

Corn (Maize sub-tropics) 1,900 2,400 500 800

Soybeans 1,700 2,000 450 700

Wheat (winter) 1,200 1,300 450 650

Wheat (spring) 1,400 1,800 450 650

Cotton 2,700 3,000 700 1,300

Sorghum (lowland sorghum) 2,200 2,500 450 650

Rice (Indica, wetland—southern U.S.) 2,400 3,000 450 700

Rice (Japonica, wetland—California ) 1,800 2,200 450 700

Table 4–4. Total irrigated acreage for selected crops for 2007 for the conterminous United States (U.S. Department of 

Agriculture, 2009a). 

[km2, square kilometers; USDA, U.S. Department of Agriculture]

Crop Total area, 

in acres

Total area, 

in km2

Irrigated area, 

in acres

Irrigated area, 

in km2

Irrigated area, 

in percent

USDA 

Map Number

Corn 86,248,542 349,035 13,156,769 53,244 15 07–M164

Soybeans 63,915,821 258,658 5,237,075 21,194 8 07–M194

Wheat 50,932,969 206,118 3,364,079 13,614 7 07–M173

Cotton 10,493,238 42,465 4,035,610 16,332 38 07–M186

Sorghum 6,769,834 27,397 845,214 3,420 12 07–M169

Rice 2,758,792 11,164 2,758,792 11,164 100 07–M180

Vegetables 4,682,588 18,950 3,068,485 12,418 66 07–M218

Orchards 5,039,476 20,394 3,981,316 16,112 79 07–M233
54 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Appendix 5—Soil

Dot density maps are used to show the distribution of 

cropland treated with commercial fertilizer, lime, and soil 

conditioners, as well as cropland treated with manure (fig. 15). 

These maps (Map Numbers 07–M103 and 07–M105) are 

available from the 2007 Census of Agriculture (U.S. Department of Agriculture, 2009b). These maps are not available as 

digital datasets in this report and were used simply to illustrate 

the primary extent of treated cropland.

The percentage of cropland acres treated with nitrogen, 

phosphate, and potash for selected crops (fig. 15) was obtained 

from fertilizer use data estimated by the Economic Research 

Service (U.S. Department of Agriculture, 2010a). The average 

annual percentage of cropland acres treated was calculated for 

1989–2007. 

The soils capability class and hazard class data were 

obtained from the U.S. Department of Agriculture (2006) 

U.S. General Soils Map (STATSGO) dataset. The dataset 

consists of tabular data in Microsoft Access format and vector spatial data. The spatial data consists of soils-association 

polygons identified by a unique map unit (MUKEY) code. 

The MUKEY is used to relate the spatial data to the tabular 

data. Each soils association consists of two or more major soil 

components that occur together in a similar pattern so that 

the patterns and proportions of major soils are alike within a 

single association. The Component table includes soil properties for all the soil components in the United States and can be 

linked to the soils associations through the MUKEY. 

Capability class and hazard class are soil properties 

available in the Component table. The major soil component 

(the largest areal extent within a soils association) was used 

to assign a capability class and hazard class for each soilsassociation polygon. The definitions for land capability class 

and hazard class are described by Helms (1992) and are listed 

in table 5–1. To determine the amount of cropland in each 

class (fig. 16), the soils capability class was assigned to each 

soils-association polygon for the United States, converted to 

a 56-m-cell-size raster dataset, and combined with the 2009 

Cropland Data Layer to calculate the percentage of land area 

in each crop group for each land capability class (I, II, III, 

IV, and V–VIII) (table 5–1). The combined raster dataset was 

used to generate a map showing the location of cropland on 

soils with severe or very severe limitations that require special 

conservation or management practices (fig. 17).
Table 5–1. Land area in each crop or crop group for each land capability class in the 2009 Cropland Data Layer for the conterminous United States (Agriculture, 2006, 2010b). [Ma, million acres; kkm2, thousand square kilometers]

Land capability class I II III IV 

Crop or crop group Land area, in Ma Land area, in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

La

Corn 7.27 29.42 9.03 42.87 173.48 53.26 20.26 81.97 25.17 5.30 21.46 6.59 Soybeans 6.65 26.91 8.88 40.32 163.19 53.84 19.23 77.81 25.67 4.80 19.41 6.40 Wheat 1.24 5.00 2.36 20.57 83.26 39.37 18.02 72.91 34.47 7.27 29.42 13.91 Other crops 0.56 2.25 4.66 4.59 18.56 38.51 3.25 13.15 27.28 1.59 6.45 13.38 Cotton 0.70 2.82 7.41 2.30 9.32 24.52 4.09 16.54 43.54 1.16 4.70 12.37 Sorghum 0.17 0.68 2.67 3.06 12.38 48.43 2.23 9.02 35.29 0.45 1.82 7.12 Vegetables and ground fruit 0.08 0.32 1.69 1.68 6.79 35.42 0.93 3.76 19.63 0.71 2.89 15.07 Rice 0.07 0.28 2.04 0.69 2.79 20.63 1.86 7.50 55.38 0.64 2.57 18.98 Orchards and grapes 0.04 0.17 1.61 0.21 0.83 7.95 0.68 2.75 26.30 1.04 4.20 40.12 Citrus 0.00 0.00 0.00 0.00 0.01 0.14 0.36 1.46 29.27 0.77 3.12 62.71 All crops 16.77 67.87 6.79 116.28 470.63 47.05 70.89 286.91 28.69 23.72 95.99 9.60 

Class I soils have slight limitations that restrict their use.Class II soils have moderate limitations that reduce the choice of plants or require moderate conservation practices.Class III soils have severe limitations that reduce the choice of plants or require special conservation practices or both.Class IV soils have very severe limitations that restrict the choice of plants or require very careful management or both.Class V, VI, VII, and VIII soils have very severe limitations that make them unsuited to cultivation.
56 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Appendix 6—Soil Water

Soil hazard class (also called soil sub-class) was used 

to show soils where the dominant hazard is from climate, 

soil limitations, erosion, or excess water. The methods for 

determining soil hazard class for cropland is the same as the 

method for determining soils capability class as explained 

in appendix 5. To determine the amount of cropland in each 

hazard class (fig. 18), the class was assigned to each soilsassociation polygon, converted to a 56-m-cell-size raster 

dataset, and combined with the 2009 Cropland Data Layer to 

calculate the percentage of land area in each crop group for 

each land capability hazard class (Climate, Soil, Erosion, and 

Water) (table 6–1). The combined raster dataset was used to 

generate a map showing the location of cropland on soils in 

each hazard class (fig. 19).

Soils hazard Water was used to determine areas where 

artificial drainage is likely needed to support agriculture. 

Artificial drainage includes both surface drainage (manmade 

ditches and canals) and subsurface drainage (tile drains). Specific information about the location of ditches and tile drains 

was not available at the time of this writing. Jaynes and James 

(2007) found that using this soil hazard class with row crops 

is a reasonable estimate of where artificial drainage will be. 

In reality, drainage may be placed in areas that are not in the 

soil hazard class Water and, conversely, not all cultivated land 

in the soil hazard class Water is drained. The illustrations and 

charts in figure 20 are used to show the areas likely requiring 

drainage to remove excess water in order to cultivate cropland. 

Further differentiation of hazard class Water for soils 

likely using either surface or subsurface drainage was determined by overlaying artificial drainage raster dataset with the 

1992 Natural Resources Inventory county-level conservation 

practice raster dataset (c606—subsurface drainage and c607—

surface drainage, field ditches) (Michael Wieczorek, U.S. Geological Survey, written commun., 2010). This was done to generate the surface- and subsurface-drainage maps in figure 20 

so that the reader can get a general idea of the locations where 

each type of artificial drainage is predominant. The value of 

each cell in the raster dataset represents the estimated percentage of the 1-km cell that is covered by or subject to a 

particular agricultural practice, on agricultural land by county. 

Federal Lands are excluded from the set and are designated as 

NODATA. (Data from 1992 are available at http://water.usgs.

gov/GIS/metadata/usgswrd/XML/ofr041189606.xml.) 

To determine the amount of cropland that is likely either 

artificially drained or not artificially drained (fig. 20, bar 

graph), the soils hazard class raster dataset for Water was converted to a 56-m-cell-size raster dataset and combined with the 

2009 Cropland Data Layer to calculate the percentage of land 

area in each crop group that is artificially drained and all other 

cropland not artificially drained (table 6–2). The National 

Resources Inventory dataset was not at high enough resolution 

to differentiate the amount of cropland artificially drained by 

surface or subsurface methods. 

Average annual runoff and recharge (1971–2000) are 

spatial datasets provided by David Wolock (U.S. Geological 

Survey, written commun., 2010). The 1951–1980 data were 

previously published for runoff (Gebert and others, 1987) and 

for recharge (Wolock, 2003). Methods used to generate the 

1951–1980 datasets are described in the metadata of the dataset and are the same methods used to generate the 1971–2000 

datasets. 

The recharge map (fig. 22) is an index of long-term average natural groundwater recharge, and it was created by multiplying a raster of base-flow index values by a raster of average 

annual runoff values derived from a 1971–2000 average 

annual runoff contour map. Average annual runoff is long-term 

average streamflow expressed on a per-unit-area basis. The 

concept used to construct the dataset is based on two assumptions: (1) long-term average natural groundwater recharge is 

equal to long-term average natural groundwater discharge to 

streams, and (2) the base-flow index reasonably represents, 

over the long term, the percentage of natural ground-water 

discharge in streamflow (Wolock, 2003).

To determine the amount of cropland for each runoff 

and recharge range (figs. 21 and 22) the spatial data were 

converted to a 56-m raster dataset and combined with the 

2009 Cropland Data Layer to calculate the percentage of land 

area in each crop group for each runoff and recharge range 

(tables 6–3 and 6–4).
Table 6–1. Land area in each crop or crop group for each land hazard class in the 2009 Cropland Data Layer for the conterminous United States (U.S2006, 2010b).[Ma, million acres; kkm2, thousand square kilometers]

Land hazard class Undefined Climate Erosion Soils 

Crop or crop group Land area, in Ma Land area, in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

La

Corn 7.48 30.29 9.30 4.09 16.53 5.08 42.07 170.24 52.27 5.65 22.85 7.01 Soybeans 6.91 27.96 9.23 2.21 8.93 2.95 34.07 137.89 45.49 4.08 16.50 5.44 Wheat 1.33 5.39 2.55 8.56 34.63 16.37 33.96 137.44 64.98 4.41 17.84 8.43 Other crops 0.65 2.63 5.46 1.79 7.24 15.02 5.70 23.08 47.90 1.42 5.76 11.96 Cotton 0.70 2.85 7.49 0.55 2.23 5.86 5.41 21.87 57.56 0.99 4.00 10.53 Sorghum 0.18 0.72 2.82 1.39 5.64 22.05 3.41 13.79 53.94 0.85 3.44 13.47 Vegetables and ground fruit 0.12 0.48 2.50 0.72 2.90 15.13 2.34 9.46 49.39 0.74 3.00 15.64 Rice 0.07 0.30 2.19 0.03 0.11 0.81 0.20 0.80 5.92 0.13 0.51 3.73 Orchards and grapes 0.05 0.19 1.85 0.83 3.37 32.18 0.65 2.63 25.12 0.80 3.24 30.93 Citrus 0.00 0.01 0.14 0.05 0.21 4.18 0.05 0.20 4.11 0.29 1.18 23.68 All crops 17.39 70.38 7.04 20.07 81.23 8.12 127.02 514.11 51.40 19.22 77.78 7.78 

Climate class is made up of soils for which the climate—the temperature or lack of moisture—is the major hazard or limitation.Erosion class is made up of soils for which the susceptibility to erosion is the dominant problem or hazard affecting their use. Erosion susceptibility and past erosion damage 

affect soils in this subclass.

Soils class is made up of soils that have soil limitations within the rooting zone, such as shallowness of the rooting zone, stones, low moisture-holding capacity, low fertility t

salinity or sodium content.

Water class is made up of soils for which excess water is the dominant hazard or limitation affecting their use. Poor soil drainage, wetness, a high water table, and overflow ar
58 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 6–2. Land area in each crop or crop group that is artificially drained in the 2009 Cropland Data Layer 

for the conterminous United States (U.S. Department of Agriculture, 2006,2010b; Michael Wieczorek, U.S. 

Geological Survey, written commun., 2010).

[Ma, million acres; kkm2, thousand square kilometers]

Crop or crop group

Not artificially drained Artificially drained

Land area , 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area , 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Corn 59.19 239.54 73.55 21.29 86.15 26.45

Soybeans 47.23 191.15 63.07 27.66 111.94 36.93

Wheat 48.27 195.37 92.37 3.99 16.14 7.63

Other crops 9.56 38.69 80.29 2.35 9.50 19.71

Cotton 7.65 30.95 81.47 1.74 7.04 18.53

Sorghum 5.82 23.54 92.07 0.50 2.03 7.93

Vegetables and ground fruit 3.91 15.83 82.64 0.82 3.33 17.36

Rice 0.41 1.64 12.12 2.94 11.90 87.88

Orchards and grapes 2.32 9.42 90.01 0.26 1.04 9.99

Citrus 0.41 1.64 32.99 0.82 3.34 67.01

All crops 184.76 747.79 74.77 62.36 252.39 25.23
Table 6–3. Land area in each crop or crop group for each runoff category in the 2009 Cropland Data Layer for the conterminous United States (DaviGeological Survey, written commun. 2009; U.S. Department of Agriculture, 2010b).[mm/y, millimeters per year, Ma, million acres; kkm2, thousand square kilometers]

Runoff category, in mm/yr 0 to 214 > 214 to 628 > 628 to 1,355 > 1,255 to 2,976 

Crop or crop group Land area, in Ma Land area, in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total 

for crop

La

Corn 29.61 119.84 36.80 27.88 112.84 34.65 18.15 73.44 22.55 4.74 19.16 5.88 Soybeans 27.21 110.13 36.34 27.75 112.32 37.06 16.74 67.73 22.35 3.14 12.71 4.19 Wheat 44.25 179.08 84.67 5.19 21.01 9.93 2.21 8.94 4.23 0.53 2.15 1.02 Other crops 6.55 26.49 54.96 2.13 8.63 17.91 1.45 5.85 12.15 1.65 6.66 13.81 Cotton 6.39 25.86 68.07 1.05 4.25 11.18 1.53 6.21 16.34 0.40 1.60 4.22 Sorghum 6.05 24.48 95.72 0.20 0.80 3.14 0.04 0.17 0.67 0.03 0.11 0.45 Vegetables and ground fruit 2.42 9.79 51.10 0.89 3.61 18.87 0.90 3.66 19.12 0.48 1.93 10.06 Rice 2.28 9.22 68.07 0.37 1.51 11.18 0.55 2.21 16.34 0.14 0.57 4.22 Orchards and grapes 1.19 4.82 46.08 0.48 1.94 18.52 0.59 2.38 22.79 0.26 1.06 10.14 Citrus 1.00 4.05 81.29 0.08 0.31 6.19 0.12 0.48 9.68 0.03 0.14 2.84 All crops 125.28 507.04 50.70 66.89 270.74 27.07 43.02 174.14 17.41 11.43 46.25 4.62 
Table 6–4. Land area in each crop or crop group for each recharge category in the 2009 Cropland Data Layer for the conterminous United States (DaGeological Survey, written commun., 2009; U.S. Department of Agriculture, 2010b). [mm/y, millimeters per year, Ma, million acres; kkm2, thousand square kilometers] Recharge category, in mm/yr 0 to 214 > 214 to 628 > 628 to 1,355 > 1,255 to 2,976 

Crop or crop group Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop La

Corn 59.48 240.70 73.91 16.15 65.34 20.06 4.46 18.04 5.54 0.34 1.39 0.43 Soybeans 49.81 201.60 66.51 19.14 77.45 25.55 5.64 22.81 7.52 0.27 1.11 0.37 Wheat 37.03 149.86 70.85 12.57 50.88 24.05 2.55 10.33 4.88 0.11 0.44 0.21 Other crops 10.75 43.48 90.22 0.89 3.60 7.47 0.24 0.97 2.01 0.02 0.09 0.18 Cotton 5.97 24.15 63.58 2.02 8.19 21.55 1.15 4.66 12.27 0.24 0.96 2.53 Sorghum 4.91 19.86 77.66 1.02 4.14 16.18 0.33 1.32 5.16 0.06 0.26 1.01 Vegetables and ground fruit 4.66 18.89 98.61 0.05 0.22 1.15 0.01 0.04 0.21 0.00 0.01 0.03 Rice 2.02 8.16 60.26 0.84 3.39 25.00 0.43 1.74 12.87 0.05 0.20 1.50 Orchards and grapes 1.73 7.01 67.03 0.79 3.19 30.54 0.06 0.25 2.43 0.00 0.00 0.00 Citrus 0.78 3.16 63.46 0.29 1.18 23.71 0.12 0.48 9.58 0.02 0.08 1.59 All crops 102.72 415.75 41.57 11.93 48.28 4.83 3.18 12.89 1.29 0.00 0.00 0.00 
Appendix 7—Other Land Uses in Competition for Agricultural Lands 61

Appendix 7—Other Land Uses in 

Competition for Agricultural Lands 

The Potential Natural Vegetation (PNV) map was 

generalized by grouping PNV classes into Broadleaf Forests, 

Needleleaf Forests, Grasslands, and Scrublands (Missoula 

Fire Sciences Laboratory, 2001). Wetland areas are not a category on the PNV map and were added by combining Water 

hazard class soils (see Appendix 6, table 6–1) to the PNV 

(U.S. Department of Agriculture, 2006). 

To determine the amount of cropland that has been 

converted from the “natural” state, the PNV polygons were 

converted to a 56-m raster dataset and first combined with the 

Water-hazard-class soils. The resulting raster dataset was then 

combined with the 2009 Cropland Data Layer to calculate the 

percentage of “natural” land area that has converted to each 

crop (tables 7–1, 7–2, and 7–3; fig. 23). 
Table 7–1. Land area in each potential natural vegetation type converted to cropland, and grassland and hay for the conterminous United States (MLaboratory, 2001; U.S. Department of Agriculture, 2006). [Ma, million acres; kkm2, thousand square kilometers] Potential natural vegetation type Wetlands Broadleaf forests Needleleaf forests Grasslands 

Crop or crop group Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop Land area, in Ma Land area, in kkm2 Land area, in percent of total for crop La

Total potential natural vegetation land area 246 995 13 511 2067 27 284 1148 15 549 2220 29 Potential natural vegetation converted to cropland 62 250 25 47 190 19 0 0 0 129 520 52 Potential natural vegetation converted to grassland and hay 43 176 9 106 429 22 29 117 6 265 1073 55 
Table 7–2. Land area in each crop or crop group for each potential natural vegetation type in the 2009 Cropland Data Layer for the conterminous United SSciences Laboratory, 2001; U.S. Department of Agriculture, 2010b).[Ma, million acres; kkm2, thousand square kilometers]

Potential 

natural 

vegetation 

type

Wetlands Broadleaf forests Needleleaf forests Grasslands Shrublands 

Crop or 

crop group

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total

for crop

Land area, 

in Ma

Land area, 

in kkm2

Land area, 

in percent 

of total

for crop

Corn 20.92 84.68 26.00 19.32 78.17 24.00 0.80 3.26 1.00 38.63 156.33 48.00 0.80 3.26 1.00 Soybeans 27.71 112.14 37.00 17.97 72.74 24.00 0.00 0.00 0.00 29.21 118.21 39.00 0.00 0.00 0.00 Wheat 4.18 16.92 8.00 2.61 10.58 5.00 0.52 2.12 1.00 42.33 171.32 81.00 2.61 10.58 5.00 Other crops 2.38 9.64 20.00 2.38 9.64 20.00 0.24 0.96 2.00 5.60 22.65 47.00 1.31 5.30 11.00 Cotton 1.78 7.22 19.00 1.88 7.60 20.00 0.00 0.00 0.00 5.16 20.89 55.00 0.56 2.28 6.00 Sorghum 0.51 2.05 8.00 0.32 1.28 5.00 0.00 0.00 0.00 4.80 19.43 76.00 0.70 2.81 11.00 Vegetables 

and ground fruit

0.80 3.26 17.00 0.66 2.68 14.00 0.19 0.77 4.00 1.99 8.05 42.00 1.09 4.41 23.00 

Rice 2.95 11.92 88.00 0.23 0.95 7.00 0.00 0.00 0.00 0.17 0.68 5.00 0.00 0.00 0.00 Orchards and 

grapes

0.26 1.05 10.00 0.57 2.30 22.00 0.05 0.21 2.00 1.37 5.54 53.00 0.34 1.36 13.00 

Citrus 0.82 3.34 67.00 0.31 1.25 25.00 0.00 0.00 0.00 0.09 0.35 7.00 0.01 0.05 1.00 All crops 61.78 250.05 25.00 46.95 190.03 19.00 2.47 10.00 1.00 128.50 520.09 52.00 7.41 30.01 3.00 Grassland 

and hay

43.38 176.68 9.00 106.4 429.44 22.00 28.92 117.12 6.00 265.1 1,073.6 55.00 38.56 156.16 8.00 
64 Environmental Factors That Influence the Location of Crop Agriculture in the Conterminous United States

Table 7–3. Land area for each potential natural vegetation class in cropland and in grassland.

[kkm2, thousand square kilometers]

Natural vegetation 

class

Total land area 

in class 

(kkm2)

Total cropland area 

in class 

(kkm2)

a

Total grassland 

and hay area 

in class

(kkm2)

b

Percent of land 

in class converted 

to cropland 

(%)

Percent of land 

in class converted 

to grassland and hay 

(%)

Percent of 

total agricultral 

land 

(%)

Broadleaf Forests 2,092 190 429 9.1 20.5 21.0

Needleleaf Forests 1,163 10 117 0.9 10.1 4.3

Grassland 2,208 520 1,074 23.6 48.6 54.0

Wetland 1,014 250 177 24.7 17.5 14.5

Shrublands 1,271 30 156 2.4 12.3 6.3

All land 7,748 1,000 1,953

a

 Not including grasslands and hay.

b

 Grasslands converted to grassland; may or may not include lands that have been reseeded and otherwise disturbed.
Appendix 8—Summary of Data Sources and Online Links to Datasets 65

Appendix 8—Summary of Data 

Sources and Online Links to Datasets 

The data sources used to generate this report and the 

associated online links to selected datasets are summarized in 

table 8–1. An effort was made to use readily available data. In 

some cases, the data were not yet published and thus are cited 

as written communications.
Table 8–1. Summary of data sources and online links to datasets.—Continued [m, meters] Index letter Dataset name Type of data Resolution (cell size) in square meters Scale Figures Tables Reference 

A Potential Natural Vegetation Geospatial polygon dataset Original map refined to match 500-m National Eleva- tion Dataset National 1 Missoula Fire Sci- ences Laboratory, 2001 http://www.fB 1790 and 1830 Pop- ulation Census Report (tabular data from Norman Bliss, U.S. Geological Survey, written com- mun., 2011) County 2 Waisanen and Bliss, 2002 http://www.a

crossref/2

C 1850–1997 Census of Agriculture (improved farm- land) Report (tabular data from Norman Bliss, U.S. Geological Survey, written com- mun., 2011) County 3 Waisanen and Bliss, 2002 http://www.a

crossref/2

D 2002 Census of Agriculture (county-level total harvested cropland) Tabular dataset County 3 U.S. Department of Agriculture, 2005 http://www.a

Publicatio

E 1879–1997 Census of Agriculture (state-level crop- land area) Report and tabular dataset State 4 Ramankutty and others, 2010 http://www.i

erface~dbab=multim

F 2009 Cropland Data Layer Geospatial raster dataset 56 m Local 5 Appendix 2, tables 2–1, 2–2, 2–3, and 2–4 U.S. Department of Agriculture, 2010b http://www.n

Cropland/

G Major Uses of Land in the United States, 2002 Report National Appendix 2, table 2–4 Lubowski and oth- ers, 2006 http://www.e

EIB14/ H Dot density grain 

and specialty crops (2007)

Map report National 6 Appendix 2, 

table 2–5

U.S. Department 

of Agriculture, 2009a

http://www.a

PublicatioAg_Atlas_index.asp

I National Elevation 

Data resampled to 100 m

Geospatial raster 

dataset

100 m, resampled 

from 30-m dataset

Local 7 Falcone, 2003 ftp://ftpext.u

NAWQA_
Table 8–1. Summary of data sources and online links to datasets.—Continued[m, meters]

Index 

letter Dataset name Type of data Resolution (cell size) in square meters Scale Figures Tables Reference 

J Combined slope 

and cropland bar graph (fig. 7) and map (fig. 8)

Geospatial raster data 

generated from F and I

100 m resampled to 

56 m

Local 7, 8 Appendix 3, 

tables 3–1 and 3–2

Generated for this 

report

K Average growing 

degree-day

Map, (geospatial raster 

dataset from Leonard Coop, Oregon State University, written commun., 2009) 

1,000 m Local 9 Oregon State University, 2010 http://uspest

L Combined growing 

degree-day and cropland

Geospatial raster data 

generated from F and K

1,000 m resampled 

to 56 m

Local 9 Appendix 4, 

table 4–1

Oregon State University, 2008

M Average precipitation Geospatial raster dataset 1,000 m Local 10 Generated for this report http://www.pN Combined precipitation and cropland Geospatial raster data generated from F 

and M

1,000 m resampled 

to 56 m

Local 10 Appendix 4, 

table 4–2

Generated for this 

report

O Threshold for growing degree days 

for selected crops

Report Global 11 Appendix 4, 

table 4–3

Fischer and others, 

2002 

http://www.i

GAEZ/ind

P Threshold for water 

requirements for selected crops

Report Global 11 Appendix 4, 

table 4–3

Brouwer and 

Heibloem, 1986;

http://www.f

s2022e07.

Q Dot density maps 

of selected crops, 2007

Map report National 11 Appendix 4, 

table 4–4

U.S. Department 

of Agriculture, 2009a

http://www.a

PublicatioAg_Atlas_index.asp

R Favorable climate 

for selected crops

Generated from L and 

N based on thresholds O and P, and dot density maps of crops Q

National 11 Generated for this 

report

S Water withdrawals 

for irrigation

Tabular dataset County 12 Kenney and others, 

2009

http://water.
Table 8–1. Summary of data sources and online links to datasets.—Continued [m, meters] Index letter Dataset name Type of data Resolution (cell size) in square meters Scale Figures Tables Reference 

T Extent of irrigated cropland Geospatial raster data generated from F and S 56 m resampled from 1,000-m county boundar- ies County 12 Generated for this report U Dot density maps of irrigated crop- land, 2007 Map report National 13 U.S. Department of Agriculture, 2009a http://www.

PublicatiAg_Atlasindex.asp V Areas of groundwa- ter-level decline Geospatial polygon dataset Generalized poly- gons (resolution unknown) National 14 Report: Reilly and others, 2008; Geospatial data- set from William Cunningham, U.S. Geological Survey, written commun., 2011 http://pubs.

W Wells with measured groundwater-level decline Geospatial point dataset Variable accuracy— documented in the U.S. Geo- logical Survey, National Water Information Sys- tem database Local 14 Report: Reilly and others, 2008; List of wells used in the report were provided by William Cunning- ham, U.S. Geo- logical Survey, written commun., 2011 http://pubs.

http://wa

X Areas with 1 or more wells that have measured groundwater-level decline Geospatial raster data generated from W 10,000 m Regional 14 Generated for this reportY Cropland in 

areas that have groundwater-level decline

Geospatial raster data 

generated from F, V, and X

56 m resampled 

from 10,000 m

Regional 14 Generated for this 

report
Table 8–1. Summary of data sources and online links to datasets.—Continued[m, meters]

Index 

letter Dataset name Type of data Resolution (cell size) in square meters Scale Figures Tables Reference 

Z Dot density maps 

for fertilzer and manure

Map report National 15 U.S. Department 

of Agriculture, 2009a

http://www.

PublicatiAg_Atlas

AA Percentage of cropland treated with 

fertilizer

Tabular data National 15 U.S. Department 

of Agriculture, 2010a

http://www.

Fertilizer

AB Land capability 

class (from U.S. General Soils Map)

Geospatial vector and 

tabular dataset

Approximately 

6,000 m

Regional 16 U.S. Department of 

Agriculture, 2006

http://soilda

USDGSM

AC Cropland on each 

land capability class

Geospatial raster data 

generated from F and AB

56 m resampled 

from 6,000 m

Regional 16 Appendix 5, 

table 5–1

Generated for this 

report

AD Cropland on soils 

that have limitations

Geospatial raster data 

generated from F and AB

56 m resampled 

from 6,000 m

Regional 17 Generated for this 

report

AE Soil hazard class 

(from U.S. General Soils Map)

Geospatial vector and 

tabular dataset

Approximately 

6,000 m

Regional 18 U.S. Department of 

Agriculture, 2006

http://soilda

USDGSM

AF Cropland on soils in 

each hazard class

Geospatial raster data 

generated from F and AE

56 m resampled 

from 6,000 m

Regional 18 Appendix 6, 

table 6–1

Generated for this 

report

AG Cropland on soils in 

each hazard class (separated by hazard)

Geospatial raster data 

generated from F and AE

56 m resampled 

from 6,000 m

Regional 19 Generated for this 

report

AH Cropland on soils 

where artificial drainage is likely

Geospatial raster 

generated from AE (Water hazard class)

56 m resampled 

from 6,000 m

Regional 20 Appendix 6, 

table 6–2

Generated for this 

report
Table 8–1. Summary of data sources and online links to datasets.—Continued [m, meters] Index letter Dataset name Type of data Resolution (cell size) in square meters Scale Figures Tables Reference 

AI Areas where surface or subsurface drainage is likely Geospatial raster da- taset based on 1992 Natural Resources Inventory county- level agricultural practices (c606— subsurface drain- age and C607) and 1992 National Land Cover 1,000 m County 20 Michael Wieczorek, U.S. Geological Survey, digital dataset, 2002 http://water.

usgswrd/X

AJ Average annual run- off (1971–2000) Geospatial raster dataset 1,000 m Regional (source data 1:2,000,000) 21 David M. Wolock, U.S. Geological Survey, written commun., 2009 Older datase

water.usgsXML/runo

AK Cropland on annual runoff Geospatial raster data generated from F and AJ 56 m resampled from 1,000 m Regional 21 Appendix 6, table 6–3 Generated for this report AL Average annual re- charge (1971– 2000) Geospatial raster dataset 1,000 m Regional (source data 1:2,000,000) 22 David M. Wolock, U.S. Geological Survey, written commun., 2009 Older datase

water.usgsXML/rech

AM Cropland on annual recharge Geospatial raster data generated from F and AL 56 m resampled from 1,000 m Regional 22 Appendix 6, table 6–4 Generated for this report AN Potential Natural Vegetation with wetlands Geospatial raster data generated from A and AE (Water hazard class) 56 m resampled from 6,000 m Regional 23 Generated for this reportAO Cropland on 

potential natural vegetation and wetlands

Geospatial raster data 

generated from F and AN

56 m resampled 

from 6,000 m

Regional 23 Appendix 7, 

tables 7–1, 7–2, and 7–3

Generated for this 

report
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