Urban Soil Primer

For homeowners and renters, local planning boards,

property managers, students, and educators

United States

Department of

Agriculture

Natural

Resources

Conservation

Service

1

Contents

Issued 2005

Preface ......................................................................................................................... 3

Chapter 1: Introduction ................................................................................................ 5

Chapter 2: Basic Soil Properties ................................................................................. 9

Chapter 3: Soils Regulate, Partition, and Filter Air and Water ................................... 19

Chapter 4: Soils Sustain Plant and Animal Diversity and Productivity ....................... 31

Chapter 5: Soils as Building Material and Structural Support .................................... 41

Chapter 6: Soils Preserve Natural and Cultural History ............................................ 53

Chapter 7: Soil Management for Recreation and Renewal ........................................ 57

Chapter 8: Help Is Nearby ......................................................................................... 61

References and Resources ........................................................................................ 69

Glossary ..................................................................................................................... 71
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The United States Department of Agriculture (USDA) prohibits discrimination in all

of its programs on the basis of race, color, national origin, gender, religion, age,

disability, political beliefs, sexual orientation, and marital or family status. (Not all

prohibited bases apply to all programs.) Persons with disabilities who require

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TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights,

Room 326W, Whitten Building, 14th and Independence Avenue SW, Washington, DC

20250-9410, or call 202-720-5964 (voice or TDD). USDA is an equal employment

provider and employer.

Suggested citation: Scheyer, J.M., and K.W. Hipple. 2005. Urban Soil Primer. United

States Department of Agriculture, Natural Resources Conservation Service, National

Soil Survey Center, Lincoln, Nebraska (http://soils.usda.gov/use).

Cover: Urban scenes and soil profiles from across the United States.
3

The Urban Soil Primer is intended to give planning officials and people who live in

urban areas an introduction to soils. It provides information important in planning and

managing land resources in a manner that helps to prevent or mitigate problems

associated with sedimentation, contamination, runoff, and structural failure. In

nontechnical language, this publication describes the basic processes and functions

common to all soils. Much of the complexity of soil science is simplified, and many

sensitive issues are discussed only in passing.

This primer lists many affordable resources available to people seeking information

about soils in urban areas. These resources include government agencies, such as

the Environmental Protection Agency (www.epa.gov), which provides information

about contamination, and the Natural Resources Conservation Service

(www.nrcs.usda.gov), which provides assistance with conservation planning and

implementation through local field offices. The Natural Resources Conservation

Service (NRCS) provides leadership in a partnership effort to help people conserve,

maintain, and improve our natural resources and environment. Other resources

include universities, private consultants, and nonprofit groups experienced with soils in

urban areas.

This primer provides the basic vocabulary and key concepts needed for further

explorations in urban soil survey and for the development of interpretive guidelines for

specific local uses by soil type. Many of the terms used in this publication are defined

in the Glossary. The primer was produced by staff at the NRCS National Soil Survey

Center with assistance from a cadre of NRCS field soil scientists. It is available as a

printed booklet and as a compact disc (CD), or it may be downloaded from the NRCS

Web site (http://soils.usda.gov/use).

Contact your State or local office of the Natural Resources Conservation Service for

more information. Visit http://offices.usda.gov/scripts/ndCGI.exe/oip_public/USA_map

to find the NRCS field office near you.

Welcome to the fascinating field of urban soils.

Preface
4

Photo Credits

Most of the photos included in this primer are courtesy of USDA, NRCS Photo

Gallery; USDA Image Gallery; USDA, ARS Photo Gallery; USDA, NRCS employees;

and USDA, NRCS Earth Team Volunteers. Exceptions are as follows:

Figure— Used by permission of—

2.1 and 4.5 Dr. Ann Kennedy, USDA, Agricultural

Research Service

3.1, 4.1, 5.1a,

5.1b, and 6.1 Laszlo Falvay, artist, Green Pac Program

4.8 Dr. Samantha Langley-Turnbaugh

6.6 USDI, United States Geologic Survey

Acknowledgements

A cadre of NRCS field soil scientists in urban areas helped to develop and review

this primer and provided some of the photos. Community members in various States

also provided assistance. Members of National Soil Interpretations Advisory Group

helped to review the document, and NRCS soil scientists at National Soil Survey

Center provided peer review. NRCS Earth Team Volunteers in Urban Soils at the

National Soil Survey Center contributed preliminary design, draft text, and photos.
5

Soil is an amazing, mostly natural material that covers nearly all of the land surface

of the earth. Soil, along with water and air, provides the basis for human existence. It

is the interface between the earth’s atmosphere and bedrock or ground water. It has

either formed in place or has been transported to its present location by wind, water,

ice, gravity, or humans. Soils may have been deposited thousands or millions of years

ago by volcanoes, glaciers, floods, or other processes or were delivered to the site by

truck or other mechanical device an hour ago, a week ago, or several months ago.

These facts illustrate why soils are very complex. Soil functions, as part of a natural

ecosystem, are also very complex and diverse. Basic knowledge about soil allows us

to use it wisely.

The primary goal of this publication is to help people who live in cities understand

soil and to help them know where and how to get information about soil. Knowing

about soil and its potentials and limitations helps urban planners and those living in

urban areas to make good decisions about using their soil as a basic and valuable

resource. Soil is the basic raw material and common link to all projects whether one

wants to build a park, a street, a golf course, or a large building, landscape a yard, or

just plant a backyard garden. Soil lies beneath each activity!

Chapter 1: Introduction

Figure 1.3: City skyline.

Figure 1.1: Urban garden. Figure 1.2: Playground.
6 Urban Soil Primer

Urban project managers and homeowners can predict a soil’s behavior under

similar situations when they know how soil responds to the same use in other

locations. Similar kinds of soil tend to behave in the same ways. There are more than

22,000 different soils identified and mapped in the United States. Some States

recognize more than 1,000 different kinds of soil. Knowing soil responses to specific

uses allows engineers and others to design projects that will not require high

maintenance costs, will last a long time, will not harm individuals, society, or

ecosystems, and will not fail and/or require expensive repair and/or removal costs.

Growth Trends in Urban Areas

Figure 1.5 demonstrates that urban areas are expanding at a rapid rate within the

United States. Urbanization is also a worldwide issue. Soils that are best suited to

other uses, such as providing food and fiber for our Nation, are commonly the easiest

to use as sites for homes and cities. Urban areas often expand into surrounding

forestland, rangeland, or agricultural land areas because these areas are adjacent to

existing urban areas. Prime farmland is vanishing at a alarming rate in certain regions

of the United States because of urban expansion and development (figure 1.6). We

must balance the increasing size of urban areas with our need for food and fiber.

Urban areas occur all across the USA, from coastal areas to areas high in the

mountains, and soils occur in such areas as parks, playgrounds, lawns, and gardens.

These soils are similar in some ways to soils in rural areas, but in other ways they are

very different. A basic understanding of urban soils will help you learn more about this

valuable resource. As urban areas grow and change, so must the management of

natural resources surrounding and within those areas.

We invite you to continue to explore the complex, fascinating, and fun science of

urban soils.

Figure 1.4: Urban development.
Chapter 1 7

Figure 1.5: Land converted to urban development from 1982 through 1997 (NRCS, NRI).

Figure 1.6: Annual rate of urban development from 1982 through 1997 (NRCS, NRI).

9

Soils in an urban area may share some properties with soils in forests, pastures,

cotton fields, or even other urban areas. There are large differences in soils as they

naturally occur in forests, farmed fields, and grazing land areas, and these differences

are changed when an area is converted to an urban area. Soil scientists have

developed conventions and language to communicate among themselves. It is

important that we share scientific information with everyone, not just other scientists

and professionals. Soil properties, such as soil texture and structure, particle-size

distribution, soil reaction, and bulk density, help us to understand and predict how soils

react and respond to different uses. Construction activities, compaction, and surface

sealing dramatically change soil properties and can sometimes result in a reduced

ability to perform the critical functions or activities of natural soil.

Topics in this chapter:

• Soil variation

• Soil components

• Soil-forming processes

• Soil horizons

• Measuring and monitoring soil properties

Soil Variation

What is soil and why is soil important to each of us? Traditionally, soil is defined as

a dynamic natural body that is made up solids, liquids, and gases, occurs on the

earth’s surface, contains living matter, and supports or is capable of supporting plants.

Bockheim (1974) defines urban soil as “soil material having a non-agricultural, manmade surface layer more than 50 cm (20 inches) thick that has been produced by

Chapter 2: Basic Soil Properties

Figure 2.1
10 Urban Soil Primer

mixing, filling, or by contamination of land surface in urban and suburban areas.” In

some important ways, soils of urban areas differ from soils of other areas.

Differences in urban soils have been observed and recorded by scientists,

engineers, equipment operators, and construction workers for a long time. Even within

urban areas, there is a multiplicity of soil conditions, ranging from “natural” soils that

are relatively undisturbed to soils in which the natural materials have been mixed or

truncated, to soils that formed in added materials, or fill, of varying thickness. Each of

these areas, in turn, can be subject to different types of use and management, which

can further affect their soil properties. Soils in urban areas can be divided into two

general types: natural soils, which formed in material naturally deposited by water,

wind, or ice or in material weathered from the underlying bedrock, and anthropogenic

soils, which formed in human-deposited material, or fill (table 2.1). Anthropogenic

soils are almost anywhere in the urban environment. The purpose of adding fill to an

area may be to alleviate undesirable soil properties or to modify the urban landscape

for specific activities.

Table 2.1: Examples of Fill Material in Urban Soils

• Natural soil materials that have been moved around by humans

• Construction debris

• Materials dredged from waterways

• Coal ash

• Municipal solid waste

• A combination of any or all of the above

Characteristics of soil in any urban area depend on many things. They depend on

how deep the site has been excavated during construction and if new materials were

brought in and mixed with the original soil materials. They depend on the properties of

the original natural soil and the past uses of the site. Many times topsoil is removed

from the site prior to construction and may or may not be returned to the site. After

excavation, subsoil may be placed as fill over topsoil. Changing the order of the soil

layers or mixing the topsoil and subsoil can alter soil properties. These variables make

predicting soil behavior difficult in urban areas.

Soil Components

All soil is made up of air, water, numerous kinds of living and/or dead organisms

(organic matter), and mineral matter (sand, silt, and clay). In the urban arena, it

includes many manmade materials. The amount of each of these soil components

varies from one place to another in the world or from one kind of soil to another. Soil

components can vary dramatically within distances of only a few feet on the same

landscape.

Soil composition can be dramatically changed by pedestrian or vehicular traffic,

especially when the soil is wet. The soil components most easily changed are the

amounts of soil air and water. Imagine the change in soil composition at construction

sites after large trucks and heavy construction equipment drive over a soil and

compact it. Imagine people walking and playing on wet soils in city parks and

recreation areas or yards. Note the differences in percent of soil air and soil water in

figures 2.2 and 2.3. Figure 2.2 illustrates the general composition of a natural soil.

Figure 2.3 illustrates the general composition of a soil that has been compacted by

heavy traffic. As soil particles are squeezed together, pores for air and water are

reduced in size and number (figure 2.4). The reduced pore space changes the way a

soil handles water intake and water movement throughout its layers, or horizons.
Chapter 2 11

Soil-Forming Processes

Soils form through a group of processes no matter where they are located or what

they are used for. All soils form because of four processes operating along with five

basic soil-forming factors. The four processes that operate on soil material are

additions, transformations, translocations, and losses (figure 2.5). We are able to map,

classify, and interpret soil because a given set of environmental factors produces a

predictable kind of soil.

The soil-forming factors are parent material, climate, living and dead organisms,

time, and landscape position. When all soil-forming factors are similar, a similar soil is

produced. If we change one or more of the soil-forming factors significantly, then a

different soil is produced.

Additions to soil generally include organic matter, fertilizer, pollutants, and deposits

of soil material. All of the additions change a soil and how it functions. In urban areas

new soil material is sometimes added on top of an existing soil. If thick enough, the

new layer or layers can change the way the soil develops. When a layer of concrete or

asphalt is added to the top of a soil in areas where streets, parking lots, or driveways

are built, additions to the soil are suddenly altered, restricted, or even stopped.

Transformations are changes that take place within a soil. In figure 2.5,

transformations are illustrated by the letters x and y and the arrows that connect them.

During transformation processes, material does not leave the soil but is simply

changed from one form to another or from one compound to another. Microorganisms and earthworms play an important role in soil transformations. Earthworms

eat soil and plant materials and transform them into organic material that provides

Figure 2.4: Soil pore space.

Figure 2.2: Composition of a natural soil, by

weight.

Figure 2.3: Composition of a compacted soil,

by weight.
12 Urban Soil Primer

food for plants and other organisms.

Chemical weathering changes parent

material, such as rocks and sand grains,

and creates new minerals and/or

smaller particles. Rocks are

transformed into sand grains, and sand

grains are transformed into silt and clay

particles over time. As iron particles

change form, they change soil colors

from gray to brown or to red and yellow.

Applying too much fertilizer of certain

kinds can transform a soil into one that

is too acidic for plants to grow.

Translocations are movements of

soil components from one place to

another in the soil. Translocations can

move materials from one soil layer to

another and can even move the

materials completely out of a soil. Water

moves through a soil profile and carries

clay particles, soluble salts, organic

matter, and chemical compounds

downward into the soil. Translocations

can also be upward or horizontal. As

soil dries and water evaporates from the

soil surface, minerals and salts may

move back toward the soil surface. In

dry areas translocations are restricted

because there is less water to carry

compounds and materials deep into the

soil. Compounds and minerals can

move only as deep as water moves into

a soil. Concentrations of soluble

material generally are closer to the

surface in dry areas than in other areas.

Windthrow and the activity of animals

(i.e., ants, termites, groundhogs, and

worms) also can move soil components

upward.

Losses occur when water moves

material through and out of a soil profile. If enough water is available, soluble

materials, such as sodium and calcium, are removed early in the process of soil

formation. Lawn and garden fertilizers are relatively soluble and may be removed from

a soil when too much water is applied. Ground-water pollution can occur if too much

water is added to a soil that contains contaminates. Erosion by wind or water

removes the soil particles and compounds needed for plant growth. Topsoil removed

through water erosion in a given area can improve the soil in the area where the

sediments are deposited.

Soil Horizons

Soils are made up of soil horizons, or layers, that form as the result of five soilforming factors. The six major kinds of soil horizons are designated as O, A, E, B, C,

and R (figure 2.6). All six of these horizons are not always evident in every soil profile.

Figure 2.5: Soil-forming processes.
Chapter 2 13

For example, most agricultural soils do

not have an O horizon because organic

horizons are usually mixed with A

horizons during tillage. Also, a soil has

an R horizon only if bedrock is close to

the surface.

O horizons are generally the

uppermost layers and form on top of

mineral horizons where they occur. They

are formed by the accumulation of fresh

and decaying plant parts, such as leaves,

grass, needles, and twigs. O horizons

are dark colored (mainly black or brown)

because decomposing plant and animal

materials produce humus. They are

generally in forested or wet areas.

A horizons are below O horizons and

are made up mostly of mineral material.

They are characterized by the loss of

iron, clay, and aluminum and the addition

of organic matter by soil organisms.

Hence, they are dark colored in most

areas, except for extremely dry areas. A

horizons are commonly referred to as

topsoil.

E horizons are commonly in forested

areas. The “E” stands for eluvial, which

means that clay, iron, organic matter, and

other minerals have been removed from this horizon. E horizons commonly appear

white or lighter in color than the horizons above and below them.

B horizons are below A or E horizons and are characterized by the accumulation

of iron, clay, aluminum, and other compounds. B horizons are commonly referred to

as subsoil.

C horizons are below B horizons and are commonly referred to as the substratum.

They are made up mainly of partially weathered or disintegrated parent material, but

soft bedrock can also occur. Because C horizons are deeper in the profile, the effects

of the soil-forming factors are less pronounced than the effects in the overlying A and

B horizons.

R horizons are made up of bedrock. The bedrock can be far below or just a few

inches below the surface.

Horizons in urban soils may not be fully related to the natural soil-forming factors

but instead may be manmade layers formed by the deposition of dredge, fill, and/or

mixed materials. Human artifacts, such as bricks, bottles, pieces of concrete, plastics,

glass, pesticides, petroleum products, pollutants, garbage, and disposable diapers, are

often components of urban soils. Manmade materials may be added to raise a

landscape to a higher level, backfill ditches or foundation walls, or construct berms. In

urban areas, human activity is often the predominant activity in making soil instead of

the action of the natural agents of wind, water, ice, gravity, and heat.

Urban soils differ from natural soils because they have been altered to some

degree. They have been excavated, compacted, disturbed, and mixed and may no

longer possess their natural soil properties and features. Many highly disturbed soils

in urban areas or on construction sites have not been in place long enough for soilforming factors to significantly change them and to form soil horizons. In areas where

Figure 2.6:Natural soil profile with major

horizons.
14 Urban Soil Primer

fill materials have been in place for a considerable time (e.g., 50 years or so), the

formation of A horizons and sometimes weakly expressed B horizons has been

documented. Figures 2.7 and 2.8 show soil horizons in urban and natural soil profiles.

Measuring and Monitoring Soil Properties

Soil properties are measured at specific sites or sampled for laboratory analysis.

The properties that can be described in the field include horizonation and layering,

color, texture, structure, consistence, depth to bedrock, and drainage class. The

properties that generally are measured in the laboratory include content of organic

matter, particle-size distribution, clay mineralogy, reaction, exchangeable cations, and

concentrations of contaminants. The soil characteristics that are estimated or

calculated from the measured properties include engineering classification and

erodibility.

Physical Soil Properties

Soil is a mixture of mineral matter, organic material, air, and water. The texture of a

mineral soil is based on the amounts of sand, silt, and clay in the soil. Sand, silt, and

clay are defined on the basis of the size of each individual soil particle. These size

A horizon

B horizon

C horizon

(human

artifacts)

Figure 2.7: Urban soil profile.
Chapter 2 15

relationships can be demonstrated by imagining that a sand particle is the size of a

basketball, a silt particle is the size of a baseball, and a clay is the size of an aspirin

tablet (figure 2.9).

Soil texture and other soil properties vary significantly within short distances on

urban or natural landscapes. This variation is caused by the movement and mixing of

soil materials during construction activities or changes in any of the soil-forming

A horizon

B horizon

C horizon

Figure 2.8: Natural soil profile.

Figure 2.9: Relative sizes of sand, silt, and clay particles.
16 Urban Soil Primer

Measures of Water Movement

Water movement in urban soils is described in three ways (figure 2.11):

• infiltration into the soil surface, especially from rainfall

• percolation within the soil drain lines from septic systems, which is especially

important in the soil below the drain line and above a restrictive layer

• permeability within the soil from the surface to a restrictive layer

Key terms in understanding water movement in soils are “restrictive layer” and

“water table.” Restrictive layers have high density (high weight in a given volume of

soil) and low porosity (limited space between particles), so that water cannot flow into

or through them. Restrictive layers at the surface can cause surface sealing and limit

factors. The combinations of different textures may improve or limit the soil for a

specific use.

Soil texture affects water and air movement through the soil as particles of different

sizes pack together and thus determine the size and spacing of pores and channels.

Sand particles have the largest pore spaces and allow water to drain through the

pores most freely. Silt particles have smaller pore spaces, so water moves through

them more slowly. Clay particles have very small pores, and so they tend to adsorb

and hold more water. The mixture of particle sizes affects water, nutrient, and

contaminant absorption. The specific type of mineral influences engineering

properties, such as shrink-swell potential and excavation difficulty, especially in

expanding clays (smectite), which behave like plastics.

The soil textural triangle (figure 2.10) can be used to determine soil texture from the

relative amounts of particles of any two sizes. For example, a clay percentage of 15

with a silt percentage of 70 gives a soil texture of silt loam.

Figure 2.10: Soil textural triangle.
Chapter 2 17

infiltration of water into the soil. Restrictive layers within the percolation zone reduce

the drainage rate of fluids in septic drain lines and can cause septic systems to back

up and fail. Compaction of soil materials can occur if heavy weight is on the surface

when the soil is wet, resulting in dense restrictive layers below the surface.

A “perched” water table occurs when a restrictive layer anywhere in the soil limits

waterflow deeper into the soil. Water drains down from the soil surface and builds up,

or “perches,” above the restrictive layer and above the expected water table depth. An

“apparent” water table is fed from below by ground water, streamflow, or subsurface

lateral flow as water moves across a restrictive layer below the soil surface.

Soil Color

Soil color differences in a profile reflect soil-forming processes and can be an

indicator of soil wetness. These differences help to distinguish fill from natural soil.

Important coloring agents in soil include parent (geologic) material, soil wetness,

extent of leaching, content of organic matter, and the chemical form and content of

iron.

Organic matter darkens the soil to a degree, depending on the content and the

extent of decomposition. Iron gives soil a brown, yellow, or red color. Shades of blue

or green may also appear, depending on iron amount, oxidation state, and hydration

state. When soil is saturated, iron can become soluble and can be removed, leaving

the soil with “mottled” brown and gray colors or completely gray colors, depending on

the extent of the wetness.

Soil Structure

Soil structure is the combination or arrangement of primary soil particles into

secondary units or aggregates. Organic materials and clay are important binding

agents. Wetting and drying cycles are important in creating structure. Soil structure

influences pore space and water movement in soils.

The principal forms of soil structure are—granular (roughly spherical); platy

(laminated); angular or subangular blocky (roughly cube shaped, with more-or-less flat

surfaces); prismatic (vertical axis of aggregates longer than horizontal); and columnar

(prisms with rounded tops). See figures 2.12 to 2.15.

Structureless soils are either single grained (each grain by itself, as in dune sand)

or massive (the particles adhering without any regular cleavage, as in many

hardpans).

Figure 2.11: Comparison of descriptive terms for water

movement in soils.
18

Figure 2.12: Granular structure. Figure 2.13: Blocky structure.

Figure 2.15: Columnar structure.

Figure 2.14: Prismatic structure.
19

Chapter 3: Soils Regulate, Partition, and Filter Air

and Water

Soils play a very important role in storing, regulating, and filtering both air and water

resources. As rainwater falls onto the soil surface, it may percolate into the soil or run

off the surface, depending on soil properties. Soil particles may hold chemicals and

nutrients, making them available for plant roots and keeping them from moving into

lakes and streams or entering the ground water. Soil pores that hold and transmit air

and water play an important role in the health of the soil environment. All living

organisms need both air and water. If all soil pores are filled with water or compacted,

then less air is available to plant roots. After site preparation or manipulation, the

properties of urban soils differ from those of natural soils and the soil air and water

react much differently.

Topics in this chapter:

• Soil functions

• Urban landscapes

• Soil and water interactions

• Soil temperature

• Stream corridors

• Storm water management

• Urban wind erosion

Soil Functions

The kinds of activities that soils perform are called soil functions. Soil functions help

us sort the extremely complex soil system into smaller parts that can be studied and

understood. We depend on soil for more than just producing food. Other soil functions

include a) providing building materials and support for structures; b) preserving natural

Figure 3.1
20 Urban Soil Primer

and cultural history; c) regulating, partitioning, and filtering air and water; d) sustaining

biological diversity and productivity; e) trapping pollutants; and f) providing sites for

recreation. Soils perform specific critical functions no matter where they are located,

and they perform more than one function at a time (table 3.1).

Table 3.1: Five Concurrent Soil Functions

• Soils act like sponges, soaking up rainwater and limiting runoff. Soils also impact

ground-water recharge and flood-control potentials in urban areas.

• Soils act like faucets, storing and releasing water and air for plants and animals to

use.

• Soils act like supermarkets, providing valuable nutrients and air and water to plants

and animals. Soils also store carbon and prevent its loss into the atmosphere.

• Soils act like strainers or filters, filtering and purifying water and air that flow through

them.

• Soils buffer, degrade, immobilize, detoxify, and trap pollutants, such as oil,

pesticides, herbicides, and heavy metals, and keep them from entering ground-water

supplies. Soils also store nutrients for future use by plants and animals above

ground and by microbes within the soils.

Soil functions occur in spite of the land use. Rainwater must be dispersed or

regulated in urban areas, and landscaping plant roots must have air available for

growth. When areas are paved over, plans must be in place to handle rainwater.

Buildings constructed on fill material must still be supported by the materials on the

site. Soils perform the same or similar functions in all areas, including urban ones.

An important task is convincing people living in the urban environment to consider

soil information and data before urban projects begin. This information must be part of

the planning process for all urban projects. As soil properties change because of

construction or other disturbances, major changes occur in the capacity of a soil to

function, as predicted by engineering properties. The ability of a soil to support

buildings and other structures changes when the soil is disturbed and/or mixed with

other materials. Soil materials placed on top of garbage cannot support large

buildings and certain other structures. Thus, it is important to know ahead of time

what functional changes are expected to result from soil disturbance. Soil maps and

soil profile descriptions can help us to understand how the soil at the building site will

respond to project management.

Urban Landscapes

Landscapes in urban areas are controlled by underlying geologic landforms; by

human activities, such as excavation or other disturbances and removal of water, oil,

or minerals; and by microrelief in small areas. Soil movement can result from hazards,

such as the formation of sinkholes, soil settling, decomposition of buried trees or

landfills, and landslides. Some of these hazards are natural in the environment, and

others are caused by human activities, such as excavation and filling for building.

These impacts are secondary to the intended soil use. Old geologic formations, such

as lava flows and lava tubes, collapse and unexpectedly create large holes. Knowing

the underlying geologic formations before building can eliminate the need for costly

repairs.

Urban planning for landscape changes requires consideration of fill consistency, soil

porosity, internal water movement, surface drainage, and the increased water retention

as organic matter is added to the soil. Knowledge of landforms helps us to understand

water movement and storage whether the landforms were created by geologic forces

or human construction. Some human-constructed soil layers dramatically impact

water movement in soils. Geologic landforms lie beneath areas of urban development

and may not be visible on the landscape (figure 3.2).
Chapter 3 21

Table 3.2: Summary of Inputs Useful in Identifying Urban Soil-Landscape Units

• Infrastructure Storm drains, building heights, housing density, and road types

• Soil catenas Interrelated drainage, soil texture, soil depth, and geologic deposits

• Block diagrams Geologic material, relief, and spatial patterns of cuts and fills

• Site data Measured erosion, infiltration, streamflow, and waste filtration

• Soil science Chemical, physical, and biological interactions and discontinuities

• Vegetation Seasonal variation, opportunistic species, and adapted physiology

Soil and Water Interactions

Maps with contour lines, called topographic maps, show the direction of waterflow

from landforms (figure 3.3). The contour lines are drawn around landforms. Each line

represents the same elevation. Contour lines generally show 10- or 20-foot intervals.

They run side by side across a slope, and water moves perpendicularly (at a right

angle) to the lines to get downhill. The contour lines are closest together where the

slope, or downhill gradient, is steepest.

When contour lines form a V shape and elevation increases as you follow the point

of the V, the V points upstream. The lines for flat areas or gentle slopes are spaced

farther apart than the lines for steeper areas. A closed circle indicates a hilltop or

knoll. A closed circle with hatch lines inside indicates a closed depression or sinkhole

at the lowest point on a landscape. Map unit symbols on soil survey maps commonly

Figure 3.3: Topographic map detail.

Figure 3.2: Soil slope and underlying geology.
22 Urban Soil Primer

indicate the relative steepness of slope. They tie the map unit delineation in soil

survey reports to the name of the soil, the texture of its surface layer, and its slope. An

example is Ridgebury loam, 3 to 8 percent slopes.

Water tables are underground supplies of water that generally occur closer to the

surface during wet periods and are deeper during dry periods. Land use impacts

water tables and runoff. An area of wetland may occur where the land surface slopes

to an elevation below the water table. Where the underground water does not rise to

the surface, it is called an aquifer. Water tables can be identified by observing and

recording soil color and soil wetness in urban project excavations or in test holes.

The movement of water into a soil depends heavily on soil texture, soil structure,

slope, bulk density, compaction, surface loading, and vegetation. Figures 3.4 and 3.5

demonstrate that more water moves into the soil on natural landscapes than on

Figure 3.4: Water movement on a natural landscape with a plant cover.

This landscape is in a humid area. In the drier regions, the stream

level is higher than the surrounding land.

Figure 3.5: Water movement on a disturbed urban landscape with limited

vegetation and impervious surfaces. This landscape is in a humid

area. In the drier regions, the stream level is higher than the

surrounding land.
Chapter 3 23

disturbed landscapes, such as those in urban areas. More water evaporates into the

air on natural landscapes than in areas covered by streets, roads, homes, garages,

and other buildings. More water runs off urban areas because of the impervious

nature of pavement, compacted soil layers, and urban buildings. Water containing

sediment clogs lakes and reservoirs. Removing this sediment is costly (figure 3.6).

Oil, gas, lawn fertilizer, pesticides,

and other pollutants often run off from

urban areas and into lakes, streams,

or reservoirs and reduce water

quality. Some of the fertilizer,

pesticides, and herbicides can run

through the soil and into ground

water, also impacting urban water

quality.

Geologic formations, the kinds of

rocks that occur below soils, affect

water movement in soils and their

landscapes. An example of an

unstable geologic formation is a

shale bed, which is prone to slippage

and landslides (figures 3.7 and 3.8).

The weight of excess water in the

soil can reduce slope and soil

stability, especially in urban areas

where expensive urban projects are

built.

Soil Temperature

Soil temperature may be higher in urban areas than in the surrounding forests and

fields. Heat islands form where extensive pavement and large buildings absorb and

return heat and restrict airflow within a city. The water supply may be limited in the

heat islands as roof runoff and rainwater are piped to storm drains in the streets. Heat

stress can occur in plants in excessively dry soils. Soil water and microbial activity

within the soil have a significant impact on subsurface soil temperature (figure 3.9).

Figure 3.6: Removing sediment from a flood-control lake. A dam is in the background.

Figure 3.7: A home damaged by slippage of

shale beds.
24 Urban Soil Primer

Stream Corridors

Stream corridors provide opportunities for recreational green space, flood control,

and wildlife habitat in urban areas. Urban and suburban soil ecosystems are similar to

rural ones but have the added dynamics of heat islands, channeled storm water, and

transportation systems for urban residents. In some areas, streets interrupt normal

surface drainage and ponds or lakes form. Many urban projects restore streams that

were piped underground in the past and create riparian stream corridors. These areas

may connect with retrofitted parking lots of porous pavement, so that more rainwater

eventually returns to the ground water or surface rivers and lakes.

The filtering function of soils is critical in areas within stream corridors and under

parking areas. Soils require an active biological community for the chemical reactions

that draw toxic materials out of runoff water and hold them in less reactive forms within

the soils.

Because of a high population density and the resulting intensive land use, urban

soils commonly are disturbed. This disturbance can be small and involve only the soil

profile. Examples are mixing of soil horizons, removal of topsoil, and additions of soil

material for plant growth. Other disturbances, such as shaping and grading activities,

can be more dramatic and can change the shape of the landscape itself. Cutting and

Figure 3.8: Soil slump on a steep slope below a mall.

Figure 3.9: Increased microbial activity results in

elevated soil temperature in a landfill.
Chapter 3 25

filling activities change the surface characteristics that impact water movement into

and through soil, site erosion characteristics, and soil fertility. Shaping and grading

activities may improve a project site, but they may also change the direction and flow

of water, causing problems on adjacent sites. Planning is required prior to

construction to minimize the problems in adjacent areas and the impacts on ground

water, erosion, and sedimentation. Silt fences can keep sediment from reaching

streams and other water bodies (figure 3.10).

Watersheds in urban areas can be defined by the type of boundary between

landscape features that forces water to move in a different direction. We can

differentiate between an urban watershed, or “sewer shed” (defined by storm drains),

and a “natural” watershed, defined by topographic watershed divides.

Landscape disturbance may also have positive effects. It sometimes introduces

additional plant and animal species or helps to minimize the effects of the less

favorable traits of natural landscapes. Soil reconstruction can take advantage of

different soil textures and boundary conditions between soil layers to manage

waterflow, structural stability, and nutrient storage.

Storm Water Management

Construction activities can be major contributors to poor water quality from

sedimentation and dust in urban areas. Changes in water quality in adjacent streams

and wetlands commonly indicate poor management of urban soils. For example, a

lower abundance of organisms, such as crayfish and dragonflies, in streams can be an

indicator of poorly managed urban soils nearby.

Runoff is water that cannot infiltrate the soil and flows across the land surface,

picking up soil particles and any other objects that can be moved as sediment during

rainstorms and periods of flooding (figure 3.11). Sediment can clog streets and storm

drains with mud, and floodwater can carry excess phosphorus, nitrogen, and other

contaminants to streams or lakes. Excess nutrients, attached to soil particles in

sediment, may cause algae blooms and poor underwater visibility. Algae blooms are

sometimes health hazards and impact swimming and fishing. Algae blooms and

sedimentation also decrease water quality, usually by reducing the oxygen content.

Figure 3.10: Silt fences collect sediment and keep it from

reaching water bodies near construction sites.

Construction is just starting on this site.
26 Urban Soil Primer

 Erosion- and sediment-control plans are used where large land areas are to be

disturbed or where activity is expected to last through a number of rainfall or

windstorm events. Two major components of these plans are control of runoff and

windblown dust and maintenance of the flow rate and amount of water (hydrology) at

preconstruction levels. Preventative measures that slow the flow of sediment to

waterways include silt fences staked on contours across hillsides and hay bales

anchored at intervals within runoff ditches.

Some urban areas have rapid soil infiltration rates of approximately 2.5 inches per

hour. A negative effect of the high infiltration rate is that if fertilizer is applied

immediately before a severe thunderstorm, then a great deal of the fertilizer may be

leached through the soil into ground water or washed directly into the storm drains.

Soil scientists have called these nutrient-rich storm drains human-made wormholes.

Wormholes in the soil fulfill a similar function of carrying nutrients rapidly to distant

places in the soil.

More often, storm water management in highly developed areas is needed to

prevent flooding and emergency discharge of untreated sewage into rivers (figure

3.12). The amount and flow of storm water depend on how much rainfall can infiltrate

into the soil. The amount of rainfall varies greatly in urban areas across the Nation.

Construction practices that disturb the soil may differ from one State to another

because of local and State ordinances. Increased runoff resulting from decreased

water infiltration (from compaction or land shaping) is a high priority in urban planning.

“Urban Hydrology for Small Watersheds” (Technical Release 55, USDA, NRCS,

1986) is still widely used as a tool for planning and monitoring water movement,

especially in urban areas where soils have been disturbed. Water infiltration plays a

critical role in the calculation of the amount of water that will flow from a site in a

certain amount of time. Relative infiltration rates for different housing densities and

varying degrees of lawn vigor are expressed as runoff curve numbers (RCN) in TR-55

(table 3.3). The hydrologic soil group (HSG), which is an indicator of infiltration, is

predetermined for each soil. The letter A indicates rapid infiltration, and the letter D

indicates that rainwater generally runs off the surface. Soil management in urban

areas can focus on decreasing runoff (RCN) by increasing the area of “good” open

lawn, where more than 75 percent of the surface is covered with grass. Each addition

of pavement will increase the amount and speed of water leaving the site.

Figure 3.11: Street flooding submerges cars.
Chapter 3 27

Figure 3.12: A flooded parking lot and street.

Table 3.3: TR-55 Runoff Curve Numbers by Housing Density and Vigor of Cover

Increasing runoff (RCN) by

Cover type decreasing infiltration (HSG) Soil condition

A BCD

Paved driveway 98 98 98 98 impervious

Commercial district 89 92 94 95 85% impervious

Newly graded area 77 86 91 94 no vegetation

Housing lot <1/8 acre 77 85 90 92 65% impervious

Housing lot 1/4 acre 61 75 83 87 38% impervious

Housing lot 1/2 acre 54 70 80 85 25% impervious

Housing lot 2 acres 46 65 77 82 12% impervious

“Poor” open lawn 68 79 86 89 <50% grassed

“Good” open lawn 39 61 74 80 >75% grassed

 Excerpt from table 2.2a in TR-55, “Urban Hydrology for Small Watersheds.” RCN is the runoff curve

number (30-98). The number 30 indicates the least runoff. HSG is hydrologic soil group (A-D). Group A

consists of soils characterized by rapid infiltration.
28 Urban Soil Primer

Figure 3.13: A cut on a construction site. This site is subject

to wnd erosion.

Figure 3.14: A dust cloud along a highway. Windblown

particles are measured by the meter in the foreground.

Urban Wind Erosion

Wind erosion is the movement of soil particles by wind. It occurs when land

surfaces lack vegetation and the soil dries out. Windspeeds must reach a certain

velocity (in most cases more than 12 miles per hour at 1 foot above the land surface)

to move soil particles, depending on the size of the particles. The smaller soil particles

(silt and clay) require lower windspeeds, and individual particles of organic matter

move most easily and with the lowest windspeeds because of their low weight. “PM2.5” dust refers to soil particles less than 2.5 microns in size. It can enter human lungs

and cause respiratory problems. These small particles form when construction

vehicles pulverize soil under dry and windy conditions. There is a high potential for

dust blowing on large construction sites and in other disturbed areas (figures 3.13 and

3.14).
Chapter 3 29

Figure 3.15: Process of wind erosion.

There are three kinds of wind erosion based on particle size and weight (figure

3.15). “Soil creep” occurs when very high wind moves coarse sand particles by rolling

them along the soil surface. “Saltation” occurs when wind moves soil particles by

bouncing them along the soil surface. Medium-sized sand particles usually are moved

by this process. A “dust storm” occurs when wind detaches small soil particles from

the land surface and suspends them in the air. Wind erosion is most visible during the

suspension stage of dust storms.

Flat areas in dry climates are likely to have serious wind erosion problems. Certain

areas of the United States are more prone to high-velocity winds than others.

Construction activities usually disturb the land surface by removing plants and

pulverizing soil aggregates, making the site more likely to dry out. Reducing traffic

over the land surface, keeping the surface rough by maintaining soil clods or

aggregates, watering construction site surfaces, applying mulch to disturbed sites, and

maintaining windbreaks or barriers reduce the risk of urban wind erosion.

Establishing grasses and other plants as soon as possible after construction is

completed also helps to control wind erosion.

31

Chapter 4: Soils Sustain Plant and Animal

Diversity and Productivity

Figure 4.1

Whether they are in urban or natural areas, soils provide living space and supply

air, water, and nutrients for micro-organisms, plants, animals, and humans. In most

areas, soil properties determine which plants and animals can live in and on the soil.

Urban soils that have been disturbed and mixed may no longer possess the natural

characteristics needed to support life. Soil amendments may be required to

reestablish plants. In many urban areas, the remaining soil materials must be modified

before they can support plant and animal life.

Topics in this chapter:

• Soil fertility and plant nutrition

• Soil acidity

• Soil organisms and biochemistry

• Soil as a filter and buffer for waste

• Identifying problem sites from historical records

• Identifying problem sites by visual clues

• Precautions for community gardens, playgrounds, and parks

• Historical tidbits on waste management

Soil Fertility and Plant Nutrition

Management of urban soils for productive gardens requires a basic understanding

of physical and chemical soil properties. Local sampling and testing can help

gardeners to determine the suitability of urban soils for certain plants and the need for

fertilizer, or plant food (table 4.1).
32 Urban Soil Primer

Table 4.1: Examples of the Factors That May Affect the Productivity of Urban Soil

• Little or no addition of organic matter

• Artifacts that disrupt water movement

• Elevated salt content

• Interrupted nutrient cycling and modified activity of micro-organisms

• High soil temperatures that increase the rate of chemical reactions

• Generally higher pH values resulting from additions of cement, plaster, and road

salts

• Lateral (sideways) subsurface waterflow resulting from compacted layers

Meeting the nutritional needs of urban plants requires consideration of soil moisture

and temperature as well as the chemicals and biological organisms needed to convert

fertilizers into useful nutrients. Plant selection may vary according to the grower’s

nutritional needs, cultural traditions, soil conditions, and the space available. Plants

common in different ethnic diets can be successfully grown in urban areas (figures 4.2

and 4.3). Attention must be paid to different plant tolerances for metals and to

drainage, the growing season, and weed control.

Figure 4.2: Produce from a Vietnamese home garden.

Figure 4.3: Intensive Vietnamese home garden in an urban

area.
Chapter 4 33

Plant growth and nutrition are closely linked to soil properties. The ability of soil

particles to hold and release nutrients for plants and micro-organisms to use is called

the cation-exchange capacity (CEC). This capacity determines which nutrients stay in

solution and are available for uptake by plant roots and which nutrients are moved

through the soil and thus are not available for plant and microbe use. Cations in the

soil are positively charged nutrients, such as nitrogen, sodium, calcium, and

potassium. Different plants and microbes require different kinds and amounts of

nutrients. Trace metals also are nutrients in the soil. They generally are used in very

small amounts. Such trace metals as iron and manganese are necessary for plant

growth. Also, they help plants to fight diseases. Metal mobility and potential toxicity in

soil occur at the lower pH levels and depend on metal binding through cation

exchange.

Various kinds of clay in the soil

attract and hold cations onto negatively

charged parts of their surfaces.

Certain clays internally bind some

chemicals very tightly. As a result, it is

difficult for plants to obtain the

necessary nutrients from the soil

solution. In areas of these highly

active clays, we often add lime

(calcium carbonate) to reduce the

acidity of the soils and facilitate release

of the nutrients from the clays into soil

solution.

Organic matter has many active

sites that bind chemicals in a manner

similar to the way clay particles bind

the chemicals in the soil. Organic

matter is often visible in a thick, dark

surface layer, in which plants begin to

grow and take up nutrients. Clays and

other soil materials are mixed with the

organic matter in each soil layer to

form a chemical system. Intensive

vegetable gardening over many years

during which unused plant materials

and organic waste are returned to the

soil can produce a thick, dark surface layer of organic matter. The color of the

resulting dark surface layer may contrast with the color of the underlying soil, as is

shown in figure 4.4, which pictures a 100-year-old continuous vegetable garden.

Soil Acidity

An acid is a substance that has a positive charge and usually yields hydrogen ions

when dissolved in water. Hydrogen ions are positively charged. The stronger the acid,

the better it dissolves in water. The pH scale (1-14) is a common measure of soil

reaction. The lower the number, the greater the acidity. The midpoint of the pH scale

is neutral (7.0), a good level for the growth of most plants.

Changes in soil reaction, as measured by pH, have significant effects on metals in

soil. Metal toxicity to plants and animals increases in strongly acid soils with a low pH

(3.5). Metals in these soils are released from negative sites back into soil solution. At

a higher pH (8.5), the metals often are sequestered in the soil. The term

“sequestered” indicates that the positively charged metal ions are bound tightly to

Figure 4.4 Soil profile in a long-term garden.
34 Urban Soil Primer

negatively charged sites in the soil. These sites may be on clays, mineral compounds,

or organic matter, including the surfaces of some micro-organisms. These strong, tight

bonds restrict the availability of metals for plant uptake and reduce the risk of animal

consumption or human skin contact.

Soil Organisms and Biochemistry

Soil is made up of mineral particles and organic matter, the decomposed remains of

living things. Bacteria, fungi, and other micro-organisms are largely responsible for

breaking down dead plants and animals in the soil. Small organisms (microbes) have

negatively charged sites where soil nutrients and metals can bind to form soil

aggregates and compounds. Earthworms and larger animals eat and digest organic

materials and minerals, transform them into soil aggregates, and deposit them as

waste. Soil aggregates are loose groupings of many different soil components in a

structure allowing water and air movement as well as biochemical reactions for energy

production and nutrient cycling (figure 4.5).

Soil as a Filter and Buffer for Waste

Managing compost and organic waste is important for plant nutrition and for the

biological degradation and conversion of contaminants into inactive forms in the soil.

Two key ways to manage waste are filtering and buffering. Waste is filtered when it

flows through the soil and is slowly trapped and bound to soil particles. Soil buffering

traps waste particles and transforms them into inactive forms.

Composting and using septic systems are examples of waste management in urban

soils. Organic materials are needed to hold water and nutrients in the soil for plant

growth. In urban parks and community gardens, as well as suburban home gardens

and yards, composting can recycle most of the leaves and grass clippings (figure 4.6).

This management alternative provides inexpensive soil conditioner that increases

porosity and improves the rooting environment for plants.

The major considerations in applying yard and garden waste after composting are

plant nutritional needs and the potential of the compost to contain weed seeds or

contaminants. Existing resources from the Cooperative Extension Service provide

guidelines for managing compost in a manner that maximizes the nutrient content and

minimizes the transfer of diseases or contaminants. The same practices work for

Figure 4.5 : Soil components at a microbial scale.
Chapter 4 35

organic waste whether from urban or agricultural sources, and the economic benefits

of recycling apply to both.

Understanding the role of soils in septic systems helps residents of small towns or

remote housing developments to manage the return of some nutrients to the soil. The

liquid septic effluent can provide nitrogen and phosphorus for use by the roots of lawn

grasses. Lawn areas receiving liquid drainage from poorly designed or failing septic

systems may appear darker green and have thicker grass than surrounding lawn

areas. Lakes surrounded by intensive development using septic systems may have

water-quality problems, such as algae blooms or high phosphorus levels, if the

systems become overloaded.

Conversion of summer cottages to year-round homes may lead to septic system

failure or excessive drainage of nutrient-rich septic effluent to lakes or streams.

Upgrades, cleanouts, and enlargements of septic systems are needed to

accommodate the amount of human waste produced and to make sure that the waste

does not pollute surface water or the ground-water supply for wells. Soil properties

affecting septic system design and installation include slope, depth to bedrock,

permeability, depth to the water table, plasticity of the soil (possible expansion when

the soil is wetted and then dried), soil texture and structure, and potential for corrosion

of steel or concrete pipe.

Figure 4.6: Composting barrels or traditional fenced piles fit different management intensities in

home gardens.
36 Urban Soil Primer

Identifying Problem Sites From Historical Records

Metals in soils come from various sources. They may have been present in the

geologic rock, or they may occur as atmospheric additions of copper, mercury, lead,

and zinc. Metals also may have been deposited by past industrial activities, such as

battery production, brass and steel manufacturing, mining, and many different

processes involving nickel, cadmium, copper, and lead. Lead is especially evident

near roadways because of automobile emissions before the availability of unleaded

gasoline, and automobile demolition areas may contain a variety of metals that were

commonly used in older cars. As lead paints and some window blinds and soldered

pipes used in houses before 1978 wear out and deteriorate, they add lead to nearby

soils.

Other ongoing sources of metals and organic waste material are landfills and dump

areas that are poorly maintained or unregulated. Landfill materials eventually

decompose and form a highly variable type of urban soil. The volunteer vegetation

may be dominated by phragmites, as is shown in figure 4.7. These sites can be

reclaimed for limited recreational or industrial use.

Areas affected by city fires may have concentrated metals buried in the soils. These

concentrations are discovered only by referring to historical records or by digging into

the soils (figure 4.8). Major fires may leave surface residue high in contaminants. A

variety of plants may still grow well, but careful evaluation of each site is needed to

determine the risk to human health.

Marine sediments may be dredged and used as fill in low-lying urban areas.

Contaminants in the dredged material may be moved onto a site. Other problems with

water movement and root resistance may result from compaction of a subsurface layer

of very fine sand.

Figure 4.7: An older landfill with phragmites.
Chapter 4 37

Figure 4.8: Soil profile with a buried layer of ash and refuse.

This site was burned by a city fire.

Identifying Problem Sites by Visual Clues

Metal contamination on a site may be evidenced by plant growth, animal behavior,

or paint flecks containing lead from older buildings. Many plants simply cannot grow

where the level of certain metals is high. Other plants grow well in contaminated soil

but fail to set seed or do not grow as well as expected. Absence of any plant growth is

a warning sign that a site may be severely contaminated. Caution during sampling is

needed.

Metals may be present at a site but not be a high risk for gardening or recreation,

depending on the soil properties, drainage, and vegetation at the site. A human health

risk from mosquitoes can occur not only in areas of standing water but also in any

areas near homes or on city streets with stagnant water. Compaction is often the main

problem causing water to pool on the surface without infiltrating into the soil. Mixing

the soil when it is just a little moist can increase the porosity (air space between

particles) and allow water to soak in. Other options are to divert the water away from

low spots and to create channels for storm water to flow around the site or in specific

streams or ditches across the site.
38 Urban Soil Primer

Table 4.2: Human Health Risks

potential health risks

◆ dust inhaled

◆ soluble lead for plant intake

◆ mud puddles that attract children and increase skin contact

soil chemical properties influencing relative risks 

◆ strongly bound and insoluble forms of contaminants 

◆ prevalence of active clay surfaces for binding

◆ organic carbon in various active forms for binding

◆ other cations, electrical conductivity, pH, and salts

soil physical properties influencing relative risk 

◆ drainage 

◆ infiltration and permeability 

◆ erosion potential for runoff and sediment loss

◆ particle sizes and water in soil pore space

Precautions for Community Gardens, Playgrounds, and Parks

Outdoor recreation and gardening are popular activities on urban soils. The risk to

human health varies among the sites used for these activities and even between the

soils on the same site. A careful study of the area and consideration of key soil

properties are needed (table 4.2)

Community and home gardens on contaminated soils may not be a health risk if the

garden vegetables supply a very small proportion of the vegetables in the overall

human diet. Caution is advised, however, when produce grown in contaminated soils

is eaten. Often, the garden supplements the produce bought at grocery stores and for

most of the year the nutritional needs of the growers are met elsewhere. Buying

vegetables at farmer’s markets or school fundraising gardens is another way to dilute

the dietary intake of contaminated plants by any one person

Caution is needed in areas of bare ground or leaking water near past industrial

sites, dumps, or older homes. Gloves should be worn during soil sampling. Dust from

contaminated sites may be dangerous if inhaled by humans or animals. Extended skin

contact or hand-to-mouth activities may allow metals to enter children’s bodies and

interfere with growth and mental development. Pets may collect contaminated dust or

mud and carry it into the home.

Prolonged skin contact with contaminated gardens can endanger young children.

Raised bed gardens built with a liner on the soil surface and carefully selected fill

materials provide a relatively safe and productive alternative. For many residents of

urban areas, a community garden is a desirable opportunity for physical exercise,

visiting with neighbors, supplementing vegetables, and relaxation.
Chapter 4 39

Historical Tidbits on Waste Management

Night soil was a traditional material in monasteries and in urban areas without

sewer systems. Human waste was recycled into gardens and agricultural areas as

fertilizer. It was usually emptied from storage pots each morning. Thus, it was called

“night soil.”

Kitchen gardens were located near cooking areas to provide vegetables and herbs

convenient for food preparation. These often were fertilized with night soil. Many

historical discard areas are uncovered by the efforts of archaeology and give us

insight into the foods people ate, the cooking vessels they used, and the ways they

recycled waste materials.

41

Chapter 5: Soils as Building Material and

Structural Support

Figure 5.1a Figure 5.1b

Soil is used as raw material in urban construction activities. Sand, gravel, and clay

are mined and used as important materials for many purposes, such as constructing

foundations and roadbeds, leveling and filling building sites, and lining ponds and lakes

where porous materials require sealing. Soil provides structural support for the

houses, schools, shopping malls, churches, industrial parks, and parks and recreation

fields that are part of urban areas. Not all kinds of soil are suitable for the many urban

uses that are required. Sandy soils are better drained than clayey soils, and some soil

layers that are exposed during construction have low strength and are easily

compacted. When exposed, bedrock and hardpans are difficult to manage.

Knowledge of the soils on the site is needed prior to construction.

Topics in this chapter:

• Site preparation

• What happens when soil is disturbed?

• Management after disturbance

• Special care of plants in islands within paved areas

• Sinkholes

• Artificial landscapes

• Building material

• Materials that allow water to infiltrate

• Open space in planned developments

• Reclaiming contaminated sites and vacant lots

Site Preparation

One goal of construction activities in urban areas is to provide material that

supports buildings, streets and roads, ballfields, tennis courts, golf courses, parks, and

gardens. Another goal is to use soil and other land resources wisely. Each

construction site has different needs from the standpoint of supporting structures to be
42 Urban Soil Primer

built and then providing materials for landscaping the site after construction is

completed. Some sites are left in their natural condition, but many are leveled,

drained, shaped, and compacted. These activities help to overcome the engineering

and construction limitations affecting building foundations or concrete slabs for building

floors, parking lots, athletic fields, or other uses. Soil or even bedrock must be moved

or removed when a construction site is leveled or graded. When bedrock or hardpans

are involved, drilling and blasting may be needed to loosen the materials. Large

machinery, such as an earthmover (figure 5.2), is used to move soil material from one

place to another on the site.

Soil materials that are moved from one construction site to another or to a different

location on the same site must be compacted if they are to support the weight of

buildings. Machinery is used to reduce the number and size of soil pores and increase

soil strength (figure 5.3). When soils are not compacted or when sites are unstable,

project failures occur. Figure 5.4 shows a house foundation that is unstable. The

electric meter was torn from the house as the soil next to the foundation settled. In

figure 5.5 settling around a house has torn the outside step away from the patio doors.

The red line on the foundation is the original level of the patio. Figure 5.6 shows

damage to a road built on an unstable soil.

Soil compaction often occurs in areas where sidewalks were not built along the

preferred footpaths (figure 5.7). This compaction is unintentionally caused by people

after construction is completed. Because of soil sealing at the surface, vegetation

cannot grow in compacted areas. Compaction below the surface may be evidenced by

puddles on the surface or trees that are blown over by heavy winds because of

shallow root systems. Compaction often is caused by the heavy machinery used by

builders and contractors (figure 5.8). The use of large machinery to move materials

around on a building site when the soil is wet compacts surface and subsurface

horizons in the soil. These compacted horizons, which are characterized by reduced

pore space and increased density, alter soil drainage, root penetration, and even

microbial communities on the site.

Unintentional soil compaction is a symptom of soil mismanagement and can be a

cause of excessive runoff, with or without sedimentation. Compaction occurs when

soil particles are packed tightly together as heavy forces (including vehicles, foot

traffic, or even glaciers) are applied to wet soils. Compaction is reflected in decreased

Figure 5.2: An example of construction machinery.
Chapter 5 43

Figure 5.4: An electrical service box torn from a home

because of soil settling.

Figure 5.3: Sheepsfoot rollers compact soil.
44 Urban Soil Primer

Figure 5.6: A road constructed on an unstable

Figure 5.5: A home damaged by soil settling. soil.

Figure 5.7: Compaction in a footpath.
Chapter 5 45

water infiltration, limited internal water movement, and the inability of plant roots to

grow through a restrictive soil layer.

After soils are modified and used for urban projects, the landscape must still

function as a natural system. In other words, the soils must still regulate, partition, and

filter air and water; sustain biological diversity and productivity; and support structures.

This is the challenge. Soils in densely populated urban areas are dramatically different

from soils naturally occurring in forests, on rangeland, in agricultural areas, or at the

urban fringe. The functions of urban soils often are modified (figure 5.9), sometimes in

a positive way and sometimes in a negative way.

Urban soils range from slightly disturbed to completely manmade. Natural soils can

occur in urban areas where site preparation has not been extensive. Urban soils

present unique challenges to landscape architects, horticulturalists, engineers, and

urban planners. The general types of soil disturbance in urban areas include

intentional cutting and filling; vehicular or foot traffic, which can cause compaction;

introduction of manufactured soils for raised bed gardens and containerized plantings;

and special preparation of sites for parks, gardens (figure 5.10), athletic fields, and golf

courses (figure 5.11). Large-scale soil disturbance includes leveling through cutting

Figure 5.9: A modified urban landscape.

Figure 5.8: Compaction caused by the use of heavy

machinery during wet periods.
46 Urban Soil Primer

and filling or through grading in certain areas, such as sites for buildings or athletic

fields; filling of wet areas or areas that have undesirable soil characteristics; and filling

of unused or abandoned areas in preparation for waste disposal.

Disturbed soils differ from soils in natural areas because their horizons have been

mixed, destroyed, or removed; natural soil structure has been destroyed; compaction

has occurred because of heavy machinery use; water transmission rates have

probably been reduced because of soil compaction and loss of soil structure; and

runoff and soil erosion rates typically have been increased.

What Happens When Soil is Disturbed?

Humans are probably the most important organisms of the soil-forming factors in

urban areas. Urban soils have been disturbed by human activity in some manner and

to some degree. This disturbance has changed the properties of the soils, and the

soils should now be managed in a different manner. Mixing different parts of more

than one soil can result in a new soil that may be better suited to a certain use than

the original soils. Some soils have to be altered before they are suitable for certain

uses. An example is a soil in an area where preparing a roadbed requires mixing and

Figure 5.10: A garden near a home foundation.

Figure 5.11: Golf course.
Chapter 5 47

deliberately compacting soil material. Topsoil commonly is piled up and then spread

on top of the altered soil after construction. In some strongly sloping areas, soil may

be moved from one area to another to fill low-lying areas and level the site for

construction. In some areas soil material is created on the site by mixing manmade

and/or natural materials from various sources. The materials may be mixed as they

are moved by heavy construction equipment.

Soils in urban areas are used for many purposes even if they are compacted and/or

contaminated. Contaminated soil material is sometimes buried because of the need to

protect those who work or play on the site. This contaminated material may be

exposed during construction activities. Old dump sites for petroleum and chemicals

are now being exposed as urban redevelopment occurs in many cities. The buried

materials have leached into other soil layers or even into the ground water, and

cleanup coasts are extremely high. In some areas cleanup may be impossible. Some

soils are unintentionally compacted prior to their use. This compaction causes

problems after construction. Soils that have been compacted or contaminated create

special problems for certain uses or for the people who live or work on them. A soil

scientist and specialists in other disciplines can provide valuable information to help

people use soils properly and to address existing problems in urban areas. It is

important to get advice and help from soil scientists before projects are started.

These facts help us begin to understand that urban soils are very different from

soils in natural areas. Even in undisturbed areas, no two soils are exactly alike. Thus,

it is important to know all one can about a soil before it is used for any purpose,

including urban projects. Most soil-related limitations can be overcome if enough

money is available to correctly design, install, and maintain a project. Costs to

overcome project errors are often higher than the original project costs.

Management After Disturbance

After disturbance, the surface layer of urban soils should have the characteristics

needed for good plant growth. Management includes overcoming physical and

chemical root restrictions, providing nutrients by managing soil fertility and acidity

(pH), and reducing the likelihood of contamination or disease problems. In areas

where the climate is dry, a water supply for the site also is needed.

Special Care of Plants in Islands Within Paved Areas

Urban plants may grow on small islands within disturbed or paved areas. Overland

waterflow to the vegetation is limited on these islands. Large soil pores that connect to

the surface are critical if water is to move to the deep roots of the plants. The soil in

the island of vegetation should extend below the pavement and should provide enough

volume for root growth in proportion to the above-ground height of the trees and

shrubs. Trees and flowerbeds along streets may require more frequent applications of

fertilizer and water than backyards or gardens.

Sinkholes

Sinkholes severely limit urban uses. They form where water has been pumped from

underlying geologic formations, leaving the surface soil without support; where

limestone bedrock is dissolved in water and removed below the soil during geologic

weathering; and where underground volcanic lava tubes collapse after geologic

weathering (figure 5.12).
48 Urban Soil Primer

Building Material

Soils in urban areas are more often the recipients of excavated and dredged

materials than the source of those materials. Some areas do have large deposits of

sand or gravel that can easily mined and transported to other areas. The pits left

behind when quarries in urban areas are closed are sometimes used for water-based

recreation or are refilled with other material, such as garbage or road debris.

Artificial Landscapes

Many urban parks were once deep ravines. These ravines were filled with

construction materials and refuse during the process of land leveling. Examining a pit

dug into the soil under the vegetation helps soil scientists to determine the soil

properties controlling the way water moves through the soil and the way nutrients are

released to plants (figure 5.13).

Figure 5.12: A sinkhole where a lava tube has collapsed.

Figure 5.13: Soil scientists examining urban soils.
Chapter 5 49

Fill material can be natural soil (derived locally or moved onsite), waste (e.g., coal

ash, dredged spoil, and construction debris), or a mixture of both (figure 5.14). Soils in

urban areas may have cultural artifacts (garbage), construction debris, and various

waste products.

Figure 5.14: A filled area excavated for a home foundation.

Soil material is sometimes made on a site by mixing manufactured materials with

natural materials from various sources as both of the materials are moved around by

heavy construction equipment. This process may result in hardpans or compacted

layers that impede foundation drainage under extensive fill material added for home

construction. The potential problems associated with disturbed urban soils include a

scarcity of organic material for plant nutrition and biological soil-building reactions, the

presence of artifacts that damage construction equipment or release contaminants,

and significant variability between soil layers. The variability between the soil layers

affects water movement and the stability of the soil under weight.

Materials That Allow Water To Infiltrate

Urban renewal may include removing old culverts and pipes and thus restoring a

natural stream, as in a park with trees and picnic areas. Restoration of stream

corridors for recreation, flood control, and wildlife may require rebuilding of

streambanks with such material as rock or gabions (rock baskets, as shown in figure

5.15) that can withstand the erosive force of floodwater (FISRWG, 1998). Pavement

may be removed from parking lots near the reconstructed stream so that more water

can infiltrate into the soil and thus raise the stream level. Porous building materials

may be added to the surface of streets (figure 5.16), parking lots, or sloping areas that

require greater stability. The porous materials permit water to move into the soil

instead of running off the site. Also, plants growing through the materials help to

control erosion (figure 5.17).
50 Urban Soil Primer

Figure 5.17: Soil surfaces may be covered with products that

provide strength for slope stability and still allow water

to infiltrate and plants to grow.

Figure 5.15: Rock baskets.

Figure 5.16: Brick street in an historic district.
Urban Primer 51

Open Space in Planned Developments

Planting beds may be constructed in parking lots, around playgrounds, and near

nature trails to allow a wider variety of plants to flourish with frequent watering and

intensive fertilizing. Urban planning can combine housing and open space needs for a

community through consideration of the soil resources of the larger urban area

(figures 5.18 and 5.19).

Figure 5.18: Aerial view of planned development.

Figure 5.19: A subdivision.
52

Building suburban houses on slight or moderate slopes and establishing lawns

minimize erosion. The houses generally have gutters that channel runoff from roofs

into storm drains. Very little space is left between houses, but larger lawn areas are in

the backyards, where mowing and fertilizing can be coordinated among suburban

owners. The soils around the houses should be characterized by good infiltration.

They should slope away from the foundation, so that basements are not flooded. A

level soil with moderate infiltration and good drainage is desirable in areas near

swimming pools and play structures.

Reclaiming Contaminated Sites and Vacant Lots

In the more densely populated urban areas, a vacant lot may be the only site

available for recreation and gardening. This restriction forces residents to use soils

that may be compacted and contaminated or filled with unknown materials. Reclaimed

mine areas provide land for urban housing and recreation after soil reconstruction

measures are applied for water movement, structural stability, and plant growth.

Raised bed gardens built with composted leaves and grass provide a plant rooting

space of minimal risk to children and adults in many urban areas (figure 5.20).

Figure 5.20: A raised bed garden.
53

Chapter 6: Soils Preserve Natural and Cultural

History

Figure 6.1

Soils can tell us much about our past. They are derived from a variety of parent

materials and are transported by wind, water, ice, gravity, or humans. Soil material

has been transported to its present location over very long to very short periods of

time. What occurs in or under a soil can indicate how old a buried soil layer is and

even how it got to its present location. For example, glaciers from the Canadian ice

sheets moved down into the northern part of the United States about 12,000 to 15,000

years ago. Rocks otherwise found only in Canada are within the soils impacted by

these glacial events. Soils can be dated by the plant pollen or artifacts within them.

Layers of garbage or dredge material also can indicate the age of the soils. In

addition, they help to preserve artifacts of culture and history.

Topics in this chapter:

• Urban site preparation

• Intentional burial

• Catastrophic events

Urban Site Preparation

When most sites for urban projects are prepared, soil excavation is required to allow

the construction of building foundations, roads, and streets and to shape the lot for

construction activities. Excavations vary in depth and size, depending on the building

project. Very deep excavations are required when skyscrapers or other large buildings

are constructed or when roads and streets are built on strongly sloping land. As soil

material is removed by excavation, objects that have been buried, either by natural
54 Urban Soil Primer

processes or by humans, are sometimes uncovered and must be dealt with before

construction begins.

The objects that are discovered in the soils during excavation may have been

accidentally lost, intentionally discarded or buried, or buried by catastrophic events.

Coins and jewelry are examples of the buried items that are discovered. Old dumps,

hazardous materials, fossils, or artifacts of past civilizations also may be exposed

during site preparation.

Intentional Burial

Landfills and garbage dumps are examples of intentional burial sites (figure 6.2).

Construction debris, garbage, and other items are often placed in dumps or low-lying

areas and then covered by soil or dredge materials. Operators of most landfills are

required to cover each layer of garbage with a layer of soil on a daily basis. When old

landfills are discovered during project excavations, the content may appear to be a

stack of debris and soil in layers. In the past, old dumpsites and burial areas were not

marked on maps. Radioactive materials or carcinogens may have been buried in

some areas before landfill regulations were in place. These areas can be avoided if

their location is known, but the location is not always known. The buried materials can

be very dangerous when exposed during construction activities. If areas such as

these are discovered on a homesite or building lot, the discovery should be reported to

the proper authorities.

Liquid materials, such as oil and oil products, pesticides, and other pollutants also

have been placed in landfills and garbage dumps. These materials are in various

kinds of containers that eventually corrode, degrade, and begin to leak (figure 6.3).

The pollutants may leak into the ground water or out onto the surface in areas

downslope from the landfill or dumpsite. In our Nation’s past, dumpsites in many rural

areas were unregulated. Toxic or hazardous waste material was deposited on these

sites along with household garbage. As towns and cities grow and expand, these sites

are often uncovered and must be cleaned up before use.

Small towns or a few buildings are sometimes intentionally covered by water

impounded by large dams. Over time, sediment that settles out of the water buries the

buildings and streets. The dam may be filled with sediment and abandoned. If the

site is later used for another purpose, the buildings and debris material may be

exposed during site preparation.

Figure 6.2: Bulldozers working in a landfill.
Chapter 6 55

Current technology helps us to design safe dumps and landfills. Liners made of

impervious materials are now used under garbage and waste materials in dumps to

contain the waste materials and keep the by-products of decomposition from leaving

the site and leaking into ground water. Cover material placed on top of the garbage

can keep odors and hazardous gases from escaping into the atmosphere. Most

dumps now require permits. Strict monitoring helps to ensure that hazardous material

is not deposited in the dumps. When dumps and landfills are closed, monitoring plans

require that the sites be checked on a regular basis.

Catastrophic Events

Catastrophic events, such as

earthquakes, volcanoes, landslides,

dust storms, and large floods, bury

objects unintentionally (figures 6.4, 6.5,

and 6.6). An object discovered during

urban development is sometimes a

surprise. The exposed objects may be

skeletons of prehistoric plants and

animals, old buildings, human artifacts

from earlier civilizations, footprints, leaf

imprints, or even pollutants and

contaminants that were buried during

past disposal activities. Some project

sites must be altered when pollutants or

contaminants are exposed during

excavation. On other sites, changes in project and/or construction plans are needed

because of artifacts or hazardous materials uncovered during construction.

Soil horizons are important time markers and are used to help date items

discovered when sites for urban projects are excavated. Newspaper articles tell of

human and animal bones that were disturbed during excavations in urban areas. In

some areas excavation is stopped until the bones can be identified so that the remains

of members of ancient civilizations are not disturbed prior to study and cataloging.

When the conditions and soil properties are right, buried artifacts and remains are well

preserved for long periods of time. Not all kinds of soil, however, preserve artifacts

and plant and animal remains. Some soils are so acid that they dissolve the buried

materials. In some areas of wet soils, organisms living in the water consume the

buried materials as food.

Figure 6.3: Barrels containing hazardous material.

Figure 6.4: Human artifacts buried by

windblown dust.
56

Locating existing information about a site before site preparation begins can save

the owner a great deal of time and expense. Soil survey reports commonly provide

information about historic sites and past land use activities as well as information

about soils. Soil survey information regarding slope stability, restrictive layers in the

soils, and water transmission rates can help builders and landowners to avoid

problems and save money. The costs of repairing, rebuilding, or relocating structures

can be avoided, and the life expectancy of projects can be enhanced.

Figure 6.5: Animal skeletons buried by volcanic ash.

Figure 6.6: 1994 landslide (USGS photo).
57

Chapter 7: Soil Management for Recreation and Renewal

Many cities and towns maintain botanical gardens or arboretums for use by their

citizens and visitors. Some of these gardens and arboretums are very well known and

draw people from all over the United States and the world. Some are used for large

gatherings of people celebrating special community or holiday events. Unmanaged

use of parks, gardens, and arboretums can cause damage to plants and severe soil

compaction, which restricts the movement of water into the soil. The result is

increased soil erosion, poor plant vigor and growth, increased runoff and offsite

sedimentation, and renovation costs.

The most common kinds of urban parks and gardens are meditation gardens,

rooftop gardens, rain gardens, butterfly gardens, outdoor classrooms, desert gardens,

kitchen gardens, victory gardens, pocket parks, riverfront parks, green space, open

space, medicine circles, labyrinths, and peace parks (figures 7.1, 7.2, 7.3, 7.4, and

7.5). The kinds of plants in these areas vary because of differences in soil properties.

Additions of special types of nutrients (plant foods or soil conditioners) to the soil or

containers in which the plants can grow may be needed.

Schoolyards are ideal spots for building butterfly gardens and outdoor classrooms.

These areas can also serve as small neighborhood parks. The soil and site history in

the areas can show whether contamination is likely and whether protective measures

are needed to minimize exposure. Soil testing is needed before the projects begin.

Applications of basic environmental science in urban areas can quickly become

complicated and may require special contributions from scientists in many fields. A

garden site surrounded by buildings may be hotter and drier than expected because

rainwater is channeled in roof gutters and street drains away from the site, nearby

concrete walls and streets stay hot longer, and the buildings cut off breezes. In some

parts of the country, the added heat can extend the short growing season for

Figure 7.1: Butterfly garden at an elementary school.
58 Urban Soil Primer

Figure 7.2: An urban pocket park under development.

Figure 7.3: A developed urban pocket park.

Figure 7.4: An urban riverfront park.
Chapter 7 59

vegetables, but in the warmer regions, it can make a garden site undesirable for some

plants.

Careful planning of lawns and picnic areas is needed because of the possibility of

heavy foot traffic during wet periods. Urban parks commonly have a combination of

paved areas and manufactured soils that can withstand trampling.

Conversion of a vacant lot into a ballfield or picnic area may be delayed if the site

was used in the past as a garbage dump. Soil contamination, bad smells, uneven

ground surfaces, and other problems may be caused by buried garbage that is slowly

decomposing. Some former landfills provide space for parks and ballfields, but they

must be carefully planned and managed for intensive use by many people in variable

weather.

A few basic soil management practices can help to maintain existing parks or

reclaimed recreation areas over time. A soil-based site management plan might

include ways to balance water content and movement. Compost leaves and grass add

organic materials and thus increase the supply of plant nutrients. Also, they help to

maintain soil porosity and thus reduce the likelihood of compaction. In general,

gardens and parks in urban areas require careful initial site selection followed by

intensive maintenance of the vegetation in lawns and landscaped areas. A soil

scientist may need to dig a pit so that the soil properties that affect management can

be investigated (figures 7.6 and 7.7).

Figure 7.5: Intensively managed ballfields after soil

reconstruction.

Figure 7.6: A backhoe used for digging a soil pit.
60

Figure 7.7: Human artifacts buried in a soil pit.

Schools, parks and recreation departments, and garden centers often employ

people with training in soils and landscaping. Many recreation areas and parks are

heavily used for more than one purpose throughout the year (figure 7.8). Maintenance

plans are needed in these areas. These plans can be used as templates for planning

smaller pocket parks and gardens that comply with local regulations.

Figure 7.8: High school playing fields are used for sports

and for band and music competitions or performances.
61

Chapter 8: Help Is Nearby

Many of the conservation practices used in agricultural and forested areas can be

adapted to work in urban areas or small towns. The Backyard Conservation Program

of the Natural Resources Conservation Service (NRCS) helps urban people to plan,

establish, and maintain simple waterways and plantings around homes, schools, and

small parks. Team-building skills across disciplines, among generations, and among

cultural groups are essential for successful urban conservation programs. These skills

have been combined with the science behind NRCS field-office conservation planning.

The Backyard Conservation Program has improved the ability of NRCS to serve

customers using ecosystem concepts in urban areas.

Topics in this chapter:

• Soil surveys in urban areas

• Naming soil series and soil map units

• Help in evaluating your soil and urban site

Soil Surveys in Urban Areas

The program responsible for mapping all soils in the United States is called the

National Cooperative Soil Survey (NCSS). It has been ongoing for more than 100

years. The NCSS is a partnership that combines the resources of Federal agencies,

including the Natural Resources Conservation Service (NRCS), the U.S. Forest

Service (USFS), the National Park Service (NPS), and the Bureau of Land

Management (BLM), and the resources of State and local agencies and universities.

In the earliest years of the soil survey program, soil surveys were focused on lands

that were or could be important for agriculture. At that time, urban areas were broadly

mapped or sometimes even ignored as soil surveys were completed.

The NCSS program has nearly completed the initial mapping of all nonurban land

within the Nation and is now turning its attention to updating and maintaining existing

soil survey data, to mapping lands managed by certain Federal and State agencies,

and to mapping urban areas. Some urban areas have already been mapped, while

others are just beginning soil surveys. Table 8.1 lists several completed and published

Table 8.1: Examples of Soil Surveys in Urban Areas

Soil Survey of the San Diego Area, California, 1973

Soil Survey of Suffolk County (Long Island), New York, 1975

Soil Survey of Washington, District of Columbia, 1976

Soil Survey of St. Louis County and St. Louis City, Missouri, 1982

Soil Survey of Charlotte County, Florida, 1984

Soil Survey of Cumberland and Hoke Counties, North Carolina, 1984

Soil Survey of Montgomery County, Maryland, 1985

Soil Survey of Nassau County (Long Island), New York, 1987

Soil Survey of South LaTourette Park, Staten Island, New York,1997

Soil Survey of Baltimore City, Maryland, 1998
62 Urban Soil Primer

soil surveys of urban areas. Soil survey activities currently are either planned or

underway in several other urban areas.

Mapping urban areas can be difficult because the present land use restricts the

access to sites needed for observation of soil profiles and because land use can

quickly change. Observing the soils in these areas is difficult because impervious

layers of concrete and asphalt are commonly on the surface and because most people

do not want holes dug in their yards or gardens.

An urban soil classification scheme based mostly on the use or intended use of the

soils could be developed. Urban soils can be investigated for commercial, residential,

industrial, and greenbelt uses. For a satisfactory design of each use, there are key

soil properties that must be known and understood. The NCSS program maps and

classifies soils according to National standards and collects soil property data to be

used in developing soil interpretations helpful in wise land use planning.

The emphasis on mapping urban areas results from the increasing size and

density of the urban areas, a more environmentally aware clientele, the fact that

nearly all rural areas have already been mapped once, and the need for more and

better land resource data within and adjacent to urban areas.

NCSS soil survey information is useful for general planning activities. Because of

the map scale, however, this information is not adequate for most site-specific

activities. Most activities in urban areas are site specific, apply to areas of land

smaller than those generally shown on soil maps, and involve highly intensive uses.

Site-specific examinations are needed to provide the accurate and detailed data

needed for most urban projects.

Naming Soil Series and Soil Map Units

Table 8.2 lists some soil series in urban areas and the type of material in the upper

part of the soils and in the underlying substratum. Each soil that is mapped in the

NCSS program is given a name that distinguishes it from other soils. Each has a set

of chemical and physical soil properties that are different from those of other soils.

Groups of soils having profiles that are almost alike, except for differences in the

Table 8.2: Examples of Urban Soil Series

Series Survey area Depth and type of surface fill Substratum

Harvester St. Louis, MO 12-40" fill or reworked loess < 20% glass,

brick

Fishpot St. Louis, MO up to 48" fill or reworked

loess < 20% glass,

brick

Matlacha Charlotte Co., FL 35" dredge spoil natural sand

St. Augustine Charlotte Co., FL 30" dredge spoil natural sand

Bragg Cumberland and 20-81" sandy clay loam fill buried loamy

Hoke Cos., NC sand

Greatkills Staten Island, NY 7-24" mod. coarse texture garbage

Canarsie Staten Island, NY 10-39" mod. coarse texture dense glacial

till

Foresthills Staten Island, NY 10-39" mod. coarse texture buried soil

Greenbelt Staten Island, NY 40-80" mod. coarse texture same fill

Centralpark Staten Island, NY 40-80" very gravelly material same fill
Chapter 8 63

texture of the surface layer, are called soil series. For example, the soils of one series

may have bedrock or a layer of garbage at a depth of 24 inches, while the soils of

another series may have bedrock or a layer of garbage at a depth of 75 inches. Other

soil series are separated on the basis of chemical properties or, more often, on the

basis of a combination of physical and chemical properties. Soils usually are named

by the soil scientists who map them. Most are named after landmarks, geographic

areas, rivers, mountain peaks, cities, or towns. Examples are the Centralpark series

(named after Central Park in New York City) and the Seattle series (named after

Seattle, Washington). In a few instances, the names are simply made up.

Urban soils form in different types of human-deposited material, including a) loamy

fill over natural sand, b) dredged spoil, c) coal ash, and d) construction debris (figures

8.1a, 8.1b, 8.1c, and 8.1d).

Nonsoil areas are given such names as Rock outcrop, Urban land, Dumps, Water,

and Rubble land. Urban land is defined as areas with a specific percentage of

pavement, driveways, and buildings (impervious cover). Collectively, nonsoil areas are

classified as miscellaneous areas.

Soils are mapped either as a single soil series or a group of soil series and/or

miscellaneous areas. For example, one group could be mapped as “Urban landCharlton complex, 2 to 8 percent slopes.” A single soil map unit could be mapped as

“Seattle loam, 1 to 3 percent slopes.” The end of this soil name (“2 to 8 percent

slopes”) is an example of additional information called “soil phase” criteria. Soil phase

criteria may be related to such characteristics as slope, texture of the surface layer

(e.g., loam), and flooding.

Figure 8.1a: Profile of a Verrazano soil, which

formed in loamy fill over natural sand.

Figure 8.1b: Profile of a Big Apple soil, which

formed in dredged spoil.
64 Urban Soil Primer

Figure 8.1c: Profile of a Riker soil, which

formed in coal ash.

Figure 8.1d: Profile of an Inwood soil, which

formed in construction debris.

Soil maps are usually made with aerial photographs as the background. Figure 8.2

is an example of a soil map of a dominantly urban area. By carefully examining the

background aerial photograph, one can observe the streets and buildings of the urban

area. The light blue areas are lakes or reservoirs. The soils delineated on the map

have been separated from one another by black lines. Every delineation on the map

has a set of letters that identifies the map unit. Examples of these map symbols are

Ur, Urb, UpC, and CrC. The list of map unit symbols and map unit names is called a

legend. The legend is very important because it can be used as a link to information

or data about the map units. Those who need soil information must have the soil map,

the legend, and the soil data or interpretations to make effective use of the soil

information in planning onsite investigations for specific projects at the proper scale.

Help Evaluating in Your Soil and Urban Site

Local resource information can be obtained from soil scientists, professional

gardeners or garden clubs, science teachers, city planners and planning boards,

Master Gardeners, landscape contractors, private soil consultants, and local people

with expertise gained through work on schools, outdoor worship areas, and parks. A

field tour that involves a combination of these experts can be an effective learning

experience (figure 8.3).

Local resource specialists can better assist you when you know some of the

vocabulary that applies to urban soils, such as the terms used in this primer.

Questions about site planning, landscaping, composting, and urban waste

management may lead to a search of city laws and regulations including scientific

terms. Management intended to reduce the risk to human health may require microengineering for urban sites and selection of certain plants that can grow well on the

sites.
Chapter 8 65

Figure 8.2: A map of dominantly urban soils.
66 Urban Soil Primer

The local Cooperative Extension Service provides brochures explaining how you

can collect samples for soil testing and can help you to locate the nearest public

university offering soil fertility tests at a low cost. The Cooperative Extension Service

also trains Master Gardeners, local volunteers who can help you to manage garden

plants and related insects and diseases (figure 8.4).

The Urban Soil Quality Card includes some basic soil tests that you can perform

before you plant a garden or vegetation in a park. The card was developed in

Connecticut by local community groups with assistance from NRCS soil scientists and

community planners. It is designed to be printed and used as written or with changes

that you make to help you evaluate your specific site. It is available on the Internet

(http://soils.usda.gov/sqi/soil_quality/assessment/cardguide.html).

Other sources of local soil expertise and assistance are youth program leaders and

handbooks from organizations, such as Girl Scouts, Boy Scouts, 4H Clubs, Envirothon

teams, the Science Bowl, school science fairs, the National Science Teacher’s

Association, the Future Farmers of America, and Vocational Agriculture Curriculum

(figures 8.5, 8.6, and 8.7). Many of these resources are listed under the heading

“References and Resources” in this primer.

Figure 8.3: A stop on a field tour of soils in Central Park, New

York City.

Figure 8.4: Adult working in a community garden plot.
Chapter 8 67

Figure 8.5: A boy with a soil color book.

Figure 8.6: Boys and girls testing soil.
68

Figure 8.7: Preliminary examination of soil in a backyard.
69

References and Resources

Bockheim, J.G. 1974. Nature and Properties of Highly Disturbed Urban Soils.

Philadelphia, Pennsylvania. Paper presented before Div. S-5, Soil Science Society of

America, Chicago, Illinois.

Broderson, William D. 1991. From the Surface Down: An Introduction to Soil Surveys

for Agronomic Use. Revised 2000. United States Department of Agriculture, Soil

Conservation Service, National Employee Development Staff, Fort Worth, Texas.

http://soils.usda.gov/education/resources/k_12

FISRWG. 1998. Stream Corridor Restoration: Principles, Processes, and Practices. By

the Federal Interagency Stream Restoration Working Group (FISRWG; 15 Federal

agencies of the U.S. Government).

http://www.nrcs.usda.gov/technical/stream_restoration/

GLOBE Program. 2003. Teacher’s Guide: Soil Investigation Chapter. 1800 G Street

NW, Suite 800, Washington, DC 20006-4403.

http://archive.globe.gov/tctg/globetg.jsp

Huddleston, J.H., and G.F. Kling. 1996. Manual for Judging Oregon Soils. Manual 6.

Reprinted November 1996. Oregon State University Extension Service, Corvallis,

Oregon.

Kennedy, Carolyn L. 1996. Outdoor Education in Girl Scouting. Girl Scouts of the

United States of America. 420 Fifth Avenue, New York, NY 10018-2798.

http://www.girlscouts.org/program/program_opportunities/environment/

National Academy of Sciences. 1996. Soils: Teacher’s Guide and Student Notebook.

Produced by the National Science Resources Center, Smithsonian Institution,

Washington, DC. Published by Carolina Biological Supply, 1-800-334-5551.

National Association of Conservation Districts (NACD). Backyard Conservation. A

Partnership of NACD, NRCS, and the Wildlife Habitat Council.

http://www.nacdnet.org/pubaff/backyard.htm

Oregon State University. 2003. Soil Sampling for Home Gardens and Small Acreages.

Extension Service Bulletin Reprinted April 2003.

http://wwwagcomm.ads.orst.edu/AgComWebFile/EdMat/EC628.pdf

Soil and Water Conservation Society. 2000. Soil Biology Primer. Revised edition. Soil

and Water Conservation Society (SWCS), Ankeny, Iowa.

http://soils.usda.gov/sqi/soil_quality/soil_biology/soil_biology_primer.html
70

United States Department of Agriculture, Natural Resources Conservation Service.

2004. Soil Potential Ratings for Low Density Development in the Unorganized Area of

Maine. Accessible online in the eFOTG, Section II, Cumberland County.

http://www.nrcs.usda.gov/technical/efotg/

United States Department of Agriculture, Natural Resources Conservation Service.

Helping People Understand Soils—Tools for Educators.

http://soils.usda.gov/education/resources/k_12

United States Department of Agriculture, Natural Resources Conservation Service.

Resource Inventory Maps.

http://www.nrcs.usda.gov/technical/land/urban.html

United States Department of Agriculture, Natural Resources Conservation Service.

Soil Quality and Site Assessment Card for Connecticut Community Gardeners.

http://soils.usda.gov/sqi/soil_quality/assessment/cardguide.html

United States Department of Agriculture, Soil Conservation Service. 1986. Urban

Hydrology for Small Watersheds (TR-55).

http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html
71

Glossary

Aggregate, soil. Many fine particles held in a single mass or cluster. Natural soil

aggregates, such as granules, blocks, or prisms, are called peds. Clods are

aggregates produced by tillage or logging.

Bedrock. The solid rock that underlies the soil and other unconsolidated material or

that is exposed at the surface.

Catena. A sequence, or “chain,” of soils on a landscape that formed in similar kinds of

parent material but have different characteristics as a result of differences in relief

and drainage.

Cation. An ion carrying a positive charge of electricity. The common soil cations are

calcium, potassium, magnesium, sodium, and hydrogen.

Cation-exchange capacity. The total amount of exchangeable cations that can be

held by the soil, expressed in terms of milliequivalents per 100 grams of soil at

neutrality (pH 7.0) or at some other stated pH value.

Clay. As a soil separate, the mineral soil particles less than 0.002 millimeter in

diameter. As a soil textural class, soil material that is 40 percent or more clay, less

than 45 percent sand, and less than 40 percent silt.

Compaction. Creation of dense soil layers when the soil is subject to the heavy

weight of machinery or foot traffic, especially during wet periods.

Composting. Managing the decomposition of organic materials, such as leaves,

grass, and garden waste.

Container gardens. Gardens planted in pots, concrete boxes, brick or stone basins,

or other isolated rooting areas within paved areas.

Contaminated soil. A soil that has high concentrations of trace metals or organic

waste that is toxic or a high risk to people or animals.

Drainage class (natural). Refers to the frequency and duration of wet periods under

conditions similar to those under which the soil formed. Alterations of the water

regime by human activities are not a consideration unless they have significantly

changed the morphology of the soil. Seven classes of natural soil drainage are

recognized—excessively drained, somewhat excessively drained, well drained,

moderately well drained, somewhat poorly drained, poorly drained, and very

poorly drained.

Drainage, surface. Runoff, or surface flow of water, from an area.

Erosion. The wearing away of the land surface by water, wind, ice, or other geologic

agents and by such processes as gravitational creep.

Erosion (geologic). Erosion caused by geologic processes acting over long

geologic periods and resulting in the wearing away of mountains and the building

up of such landscape features as flood plains and coastal plains. Synonym:

natural erosion.

Erosion (accelerated). Erosion much more rapid than geologic erosion, mainly as

a result of human or animal activities or of a catastrophe in nature, such as a fire,

that exposes the surface.

Fertility, soil. The quality that enables a soil to provide plant nutrients, in adequate

amounts and in proper balance, for the growth of specified plants when light,

moisture, temperature, tilth, and other growth factors are favorable.
72 Urban Soil Primer

Gravel. Rounded or angular fragments of rock as much as 3 inches (2 millimeters to

7.6 centimeters) in diameter. An individual piece is a pebble.

Hard bedrock. Bedrock that cannot be excavated except by blasting or by the use of

special equipment that is not commonly used in construction.

Heat islands. Small areas of artificially drained urban soils surrounded by tall

buildings that change soil temperature and moisture patterns. May also refer to an

entire city with an artificial microclimate.

Horizon, soil. A layer of soil, approximately parallel to the surface, having distinct

characteristics produced by soil-forming processes. In the identification of soil

horizons, an uppercase letter represents the major horizons. Numbers or

lowercase letters that follow represent subdivisions of the major horizons. The

major horizons of mineral soil are as follows:

O horizon.—An organic layer of fresh and decaying plant residue.

A horizon.—The mineral horizon at or near the surface in which an accumulation

of humified organic matter is mixed with the mineral material. Also, a plowed

surface horizon, most of which was originally part of a B horizon.

E horizon.—The mineral horizon in which the main feature is loss of silicate clay,

iron, aluminum, or some combination of these.

B horizon.—The mineral horizon below an A horizon. The B horizon is in part a

layer of transition from the overlying A to the underlying C horizon. The B horizon

also has distinctive characteristics, such as (1) accumulation of clay, sesquioxides,

humus, or a combination of these; (2) prismatic or blocky structure; (3) redder or

browner colors than those in the A horizon; or (4) a combination of these.

C horizon.—The mineral horizon or layer, excluding indurated bedrock, that is little

affected by soil-forming processes and does not have the properties typical of the

overlying soil material. The material of a C horizon may be either like or unlike that

in which the solum formed. If the material is known to differ from that in the solum,

an Arabic numeral, commonly a 2, precedes the letter C.

Cr horizon.—Soft, consolidated bedrock beneath the soil.

R layer.—Consolidated bedrock beneath the soil. The bedrock commonly

underlies a C horizon, but it can be directly below an A or a B horizon.

Humus. The well decomposed, more or less stable part of the organic matter in

mineral soils.

Hydrologic soil groups. Refers to soils grouped according to their runoff potential.

The soil properties that influence this potential are those that affect the minimum

rate of water infiltration on a bare soil during periods after prolonged wetting when

the soil is not frozen. These properties are depth to a seasonal high water table,

the infiltration rate and permeability after prolonged wetting, and depth to a very

slowly permeable layer. The slope and the kind of plant cover are not considered

but are separate factors in predicting runoff.

Hydrologic unit or watershed. In urban areas, a catchment area with an outlet in or

affecting a densely populated area.

Impervious soil. A soil through which water, air, or roots penetrate slowly or not at all.

No soil is absolutely impervious to air and water all the time.

Infiltration. The downward entry of water into the immediate surface of soil or other

material, as contrasted with percolation, which is movement of water through soil

layers or material.

Landslide. The rapid downhill movement of a mass of soil and loose rock, generally

when wet or saturated. The speed and distance of movement, as well as the

amount of soil and rock material, vary greatly.

Leaching. The removal of soluble material from soil or other material by percolating

water.

Loam. Soil material that is 7 to 27 percent clay particles, 28 to 50 percent silt

particles, and less than 52 percent sand particles.
Glossary 73

Low strength. The soil is not strong enough to support loads.

Nutrient, plant. Any element taken in by a plant essential to its growth. Plant

nutrients are mainly nitrogen, phosphorus, potassium, calcium, magnesium, sulfur,

iron, manganese, copper, boron, and zinc obtained from the soil and carbon,

hydrogen, and oxygen obtained from the air and water.

Organic matter. Plant and animal residue in the soil in various stages of

decomposition.

Parent material. The unconsolidated organic and mineral material in which soil forms.

Percolation. The movement of water through the soil.

Permeability. The quality of the soil that enables water or air to move downward

through the profile. The rate at which a saturated soil transmits water is accepted

as a measure of this quality. In soil physics, the rate is referred to as “saturated

hydraulic conductivity.”

pH value. A numerical designation of acidity and alkalinity in soil. (See Reaction,

soil.)

Pocket park. A relatively small area reserved for recreation or gardening and

surrounded by streets or buildings.

Profile, soil. A vertical section of the soil extending through all its horizons and into

the parent material.

Raised bed gardens. Gardens that are planted in boxes made of wood or other

materials and have the rooting area above the ground surface. The boxes may be

filled with composted materials mixed with uncontaminated soil.

Reaction, soil. A measure of acidity or alkalinity of a soil, expressed in pH values. A

soil that tests to pH 7.0 is described as precisely neutral in reaction because it is

neither acid nor alkaline.

Relief. The elevations or inequalities of a land surface, considered collectively.

Restrictive layer. A compact, dense layer in a soil that impedes the movement of

water and the growth of roots.

Runoff. The precipitation discharged into stream channels from an area. The water

that flows off the surface of the land without sinking into the soil is called surface

runoff. Water that enters the soil before reaching surface streams is called

ground-water runoff or seepage flow from ground water.

Sand. As a soil separate, individual rock or mineral fragments from 0.05 millimeter to

2.0 millimeters in diameter. Most sand grains consist of quartz. As a soil textural

class, a soil that is 85 percent or more sand and not more than 10 percent clay.

Sealed soil. Soil that is covered with buildings, pavement, asphalt, or other material.

Water and air do not enter the soil from the surface.

Series, soil. A group of soils that have profiles that are almost alike, except for

differences in texture of the surface layer. All the soils of a series have horizons

that are similar in composition, thickness, and arrangement.

Shale. Sedimentary rock formed by the hardening of a clay deposit.

Shrink-swell potential. The potential for volume change in a soil with a loss or gain

in moisture. Volume change occurs mainly because of the interaction of clay

minerals with water and varies with the amount and type of clay minerals in the

soil. The size of the load on the soil and the magnitude of the change in soil

moisture content influence the amount of swelling of soils in place. Shrinking and

swelling can damage roads, dams, building foundations, and other structures. It

can also damage plant roots.

Silt. As a soil separate, individual mineral particles that range in diameter from the

upper limit of clay (0.002 millimeter) to the lower limit of very fine sand (0.05

millimeter). As a soil textural class, soil that is 80 percent or more silt and less than

12 percent clay.

Sinkhole. A depression in the landscape where limestone has been dissolved or lava

tubes have collapsed.
74

Slope. The inclination of the land surface from the horizontal. Percentage of slope is

the vertical distance divided by horizontal distance, then multiplied by 100. Thus,

a slope of 20 percent is a drop of 20 feet in 100 feet of horizontal distance.

Soft bedrock. Bedrock that can be excavated with trenching machines, backhoes,

small rippers, and other equipment commonly used in construction.

Soil-forming factors. Five factors responsible for the formation of the soil from the

unconsolidated parent material. The factors are time, climate, parent material,

living organisms (including humans), and relief.

Structure, soil. The arrangement of primary soil particles into compound particles or

aggregates. The principal forms of soil structure are platy, prismatic, columnar,

blocky, and granular. Structureless soils are either single grained or massive.

Texture, soil. The relative proportions of sand, silt, and clay particles in a mass of

soil. The basic textural classes, in order of increasing proportion of fine particles,

are sand, loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay

loam, silty clay loam, sandy clay, silty clay, and clay. The sand, loamy sand, and

sandy loam classes may be further divided by specifying “coarse,” “fine,” or “very

fine.”

Topographic maps (USGS). Maps that show terrain, ridges, waterways, contours,

elevations, and geographic locations. Also may show roads and buildings.

Trace elements. Chemical elements, for example, zinc, cobalt, manganese, copper,

and iron, in soils in extremely small amounts. They are essential to plant growth.
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