Fertilization in Dryland Cropping Systems: a brief overview

This is a brief summary of the amount of nitrogen (N) and phosphorous (P) required to maximize

dryland crop yields. The values given are based on USDA-ARS research at Akron, CO, Mandan, ND and

Land Grant college research in CO (CSU), KS (KSU), and NE (UNL).

The nutrients that are found to be most limiting in our region of the Central Great Plains are N and

P. Occasionally, on very sandy soils with low organic matter, sulfur (S) and/or potassium (K) may be

deficient. However, in a survey of farm soils tested from Western Kansas, Western Nebraska and Eastern

Colorado less than 1% were low in K.

Most of the soils in our region of the Great Plains are calcareous. These soils particularly on eroded

hill tops will test low for available zinc (Zn) and iron (Fe). The deficiency is really a problem of high

pH and shallow eroded soil. Attempts to correct the deficiency with soil applied Zn and Fe fertilizers are

often unsuccessful because high soil pH in the eroded area chemically causes the added fertilizers to form

insoluble iron and zinc minerals. Heavy applications of composted manure may be the best remedy but

shallow soils may still limit crop rooting depths and subsequentially crop yields.

We suspect that oilseed crops (sunflowers, canola, safflower) which have a higher S requirement than

cereal grains (wheat, barley, corn, sorghum) may be S deficient in some soils. However, S deficiency has

not yet been officially documented in Eastern Colorado. Table 1 shows the N and P required per bushel,

or per 100 lb of grain for some typical dryland crops.

Table 1. N and P requirements of selected dryland crops grown in the Central Great Plains.

Crop N required*

Wheat

corn

Sorghum

Millet §

Sunflower

- confection

- oil

Canola

Forage Millet

1.8 lb per bushel

1.3 lb per bushel

1.2 lb per bushel

1.5 lb N per bushel

7.0 lb/100 lb of grain

6.5 lb/100 lb of grain

7.0 lb/100 lb of grain

50 lb per ton of hay

1.0 lb per bushel

0.6 lb per bushel

0.6 lb per bushel

0.6 lb per bushel

1.0 lb per 100 lb of grain

1.0 lb per 100 lb of grain

0.8 lb per 100 lb of grain

10 lb per ton of hay

* N requirements were developed from experimental data collected by USDA-ARS at Akron, CO, Lincoln Ne, and

Mandan, ND, or University research from Kansas State Univ. (KSU), Univ. of Nebraska at Lincoln (UNL) and

Colorado State Univ. (CSU).

** P requirements are taken from CSU research and the Western Fertilizer Handbook.

§ The N and P requirements for proso-millet (hershey) are our best guesses based on limited data.

Conservation Tillage Fact sheet #4-95. Published by USDA-ARS and USDA-NRCS

Merle F. Vigil (Soil Scientist) USDA-ARS, Central Great Plains Research Station, Akron, Colorado
How can the data in table 1 be used? The use of this data is best explained by an example. The

following example is based on soil test information for available N as NO3--N and a “realistic” yield goal.

A “realistic” yield goal can be calculated by taking the average yield of the last 5 crops grown on a field

plus 5%. A 5 crop average is used to minimize the effects of extremely bad years, or unusually good years

that are more the exception than the rule for a given field.

Example:

*

*

*

*

*

*

Let’s assume a soil test indicates that there are 30 lbs of available N per acre, in the surface 2 ft of

soil, in the field of a farmer who wishes to grow wheat. Also assume the farmer’s soil contains 1%

organic matter.

If the farmer’s average yield for that field, over the last 5 crop years, was 45 bushel then the farmer

could calculate a “realistic” yield goal by taking that average and adding 5% as follows:

45 bu +(45bu x 0.05) = 47.25 or 47.3 bu

where bu=bushels

If we assume (from

85 lbs of N.

Table 1.) that wheat needs 1.8 lb of N per bushel then his crop will require about

1.8 x 47.3 =85.1 lb of N required

CSU soil testing laboratory assumes 30 lb of N will become available for every 1% of native soil

organic matter. Recent work in Nebraska indicates the value may actually be between 20 and 60 lb

of N per acre for every 1 % soil organic matter (OM), but for simplicity let’s stick with the 30 lb

assumption.

The amount of N needed by the crop as fertilizer is then:

85 lb -30(from 1% soil OM) -30(from residual NO3-N) = 25 lb of N as fertilizer

Most laboratories that make fertilizer recommendations, based on soil tests, incorporate an efficiency

factor. The idea is that not all of the N applied is recovered and used by the crop. A good all around

efficiency factor is about 55%. This value of 55% is debatable and will change with management

(methods of fertilizer placement, fertilizer source, soil type, crop fertilized, etc.). However, if we use

the 55% efficiency value, which probably isn’t too far out of line, then:

251b N/0.55 = 45.7 lb of N to apply as fertilizer

Sources of N and methods of placement? In general the source of N is not as important as the cost of

the source and its ease of use. For example, anhydrous-ammonia (NH3 gas) is the cheapest form of N on

a per pound of nutrient basis. Even at $300/ton anhydrous ammonia is still cheaper per pound lb of N than

urea-ammonium-nitrate (UAN, 28-32% N solution) or dry urea (46-0-0). On the other hand UAN-solution

and dry urea are much easier to apply than anhydrous.

The method of placement is more important than the source of N. If anhydrous is knifed or bladed

in at a very high rate (> 150 Ibs actual N/acre), in a dry soil, at a very shallow depth (less than 6 inches)

there will likely be some volatile loss of the N. In the same manner, if UAN solutions are applied

broadcast to no-till stubble and the fertilizer is not incorporated by at least a third of an inch of rain, or

by tillage and if this application is done during the warm days of summer volatile losses of the fertilizer

will occur. The point is, regardless of the source, if the N is not applied carefully, volatile losses can
occur. If applied correctly} N losses are minimal with either source.

Research (labeled N studies) in Kansas, Nebraska, Iowa and Colorado tends to show higher N

recovery when the N is applied in “concentrated zones” as compared to broadcast applications. Nitrogen

knifed below the soil surface, banded in a 2 by 2 placement from the seed, or dribbled on the soil surface

is N applied in a concentrated zone.

Why are broadcast applications not as efficient? When N is applied broadcast it is mixed evenly with

the soil and crop residues. Broadcast applications provide a greater opportunity for microbial tie-up of the

fertilizer. Most folks don’t realize that soil microbes will effectively compete with the crop for the N. If

the N is applied in a concentrated zone only those microbes immediately adjacent to the zone of

application will compete with the crop, Whereas, the crop’s roots will grow to the concentrated zone of

application and will even proliferate in that zone and therefore effectively recover the fertilizer N. The

advantage a crop has over soil-microorganisms is its root system. Crop roots will effectively explore a soil

and translocate needed nutrient back to the plant. Microorganisms can’t do that as well as roots.

What about Phosphorous? Unfortunately, the “Yield goal” scheme used for N in the example above

doesn’t work quite as well for P. However, soil testing can help eliminate some of the guess work. In

most cases, for soils that test greater than 8 ppm extractable P (using the AB-DTPA extraction method

developed at CSU) P fertilizers will not be of benefit. That is, the addition of fertilizer P will probably

not increase yields enough to pay for cost of the fertilizer. Soils that test between 4 and 8 ppm extractable

P are considered marginally deficient. Fertilizer additions of 15 to 20 lbs of P as PZO, are usually worth

the cost of fertilizer. Soils that test less than 4 ppm extractable P are deficient. These soils should be

fertilized with 30 to 40 lbs of P as PzO5, and in most cases a yield increase from the added P fertilizer

can be expected.

Sources of P and methods of placement? In general, on a per lb of actual nutrient 11-52-O is as good

as 10-34-O-solution or 18-46-O. The big advantage is that 11-52-O is cheaper per lb of actual P than the

other sources. However, as with UAN-scdutions, 10-34-O can be easier to handle then the dry sources. As

with N, greater P recovery has been observed when P is applied in bands than when applied broadcast.

If phosphorous is to be banded with the seed (which incidently is a good practice) then never apply more

than 15 lbs of actual N as 11-52-O, 10-34-0 ect. or seedling injury could result. Remember that 15 lbs of

N as 11-52-O is 71 Ibs of P as PzO5 which for most crops is more than enough P.

Some examples of N response: The yield increase from added N for several dryland crops grown on the

same land (Weld silt loam), over several years, in a dryland-annual-cropping study was measured by

Ardell Halvorson and Curt Ruele (Fig 1). That data indicates the optimum N rate is dependant on the crop

grown, rainfall received, residual soil N levels, and with the previous crop’s level of production. In 1984,

initial soil test levels of residual soil N were high (120 lbs of available N in the top 4 ft of soil) and so

the response to added fertilizer N was low (optimum N rate of about 30 lb). Consider the wheat yields

and N response measured in 1988 (Fig 1). In 1988, 2500 lb of grain (42 bushel) was raised without any

N fertilizer. The 1988 wheat crop was grown on corn ground that was hailed out in 1987. Since in 1987

no crop was removed from the field, much of the N in the hailed corn crop was returned to the soil (was

left behind) for subsequent use in 1988. However, in 198850 lb of N fertilizer produced an additional

14 bushels of wheat. Even with $3 wheat the cost of the additional fertilizer was worth it. On average

the corn crops (1985 and 1989) have a higher optimum N rate (about 80 lb) than the small grain crops.

In 1990, a dry year, combined with an assault by aphids greatly reduced yields. These graphs provide

real estimates of N response in a dryland cropping system. They should not be used to make fertilizer

recommendations for other fields, but provide an indication of what can be expected from N fertilizer for

some of the crops, soils and climate of our region.

USDA-ARS and USDA-NRCS are equal opportunity~ employers and all agency services are available without discrimination
1985

N rate Ibs / acre

1986

1989

o 30 60 9“ 120 150

Fig. 1.

N rate, ibs/acre

N rate, lbs/acre

N rate Ibs /acre

“O 30 60 90 120 150

N rate, lbs/acre

Yield response from N fertilizer measured in a dryland-annual-cropping study conducted at

Akron, Colorado by USDA-ARS researchers Ardell Halvorson and Curt Reule. The symbols in

each graph are the average grain yields for 4 replications. The dark line indicates the general trend

of yield verses N application. The vertical dashed line indicates the N rate where yield was

maximized.