Soil Fertility

Kurtis D. Reitsma, David E. Clay, and C. Gregg Carlson

Soil supplies 14 essential plant nutrients (fig. 1). Of these nutrients, nitrogen (N) and

phosphorus (P) are most likely to limit crop yield in South Dakota. Other nutrients that

may limit crop yield in South Dakota are potassium (K), sulfur (S), zinc (Zn), chloride

(Cl), and iron (Fe). When developing a nutrient management strategy, keep in mind that

productivity may be compromised if one or more nutrients are limiting to the plant. The

amount of productivity reduction depends on both the nutrient and the degree to which

a deficiency affects photosynthesis and plant development.

Soil test results provide one of several approaches for developing a soil-fertility

program. Other approaches include calculating a nutrient mass balance, field scouting

for nutrient deficiency symptoms, or a combination of the above. However, if nutrient

deficiency symptoms appear during mid- to late

growing season, it is likely too late to correct the

deficiency for that crop and may result in yield

decline.

The mass balance approach may not provide an

accurate method for developing a nutrient strategy

but does provide information about nutrient export

values. This information can be useful in developing

P and K strategies but is not likely to help with

managing N. Targeted soil sampling is the best

approach, integrating scouting and nutrient import/

export information to determine if soil sampling is

necessary.

Soil test values for immobile nutrients such as P and

K are unlikely to change annually unless there are

significant imports or exports. Nitrogen sampling

becomes more important as the probability of

higher residual N is dependent on management

and environmental conditions. For example, if the

previous season was dry and/or the field had manure

applied, it is likely that residual soil N is high and

the field should be sampled and analyzed for N. If

the previous crop was a legume and no manure was

applied, one approach would be to assume a 50 lb N/

acre residual N and add the N credit from the legume

crop as per Gerwing and Gelderman (2005) (Reitsma

et al., 2007).

Soil test results are often mistaken as a measure of

the amount of nutrients in the soil at the time the

field was sampled. However, soil test methods in

South Dakota and other states measure a particular

portion of the nutrient, which is calibrated either to

a recommended application rate or to a yield goal.

 

Figure 1. Plant nutrients supplied by the soil. (Clay and Reitsma, 2009)

For example, recommended N rates are based on an amount of N required to achieve a

selected yield goal, subtracting soil test NO3--N(24” sample) and credits from legumes,

manure, and irrigation water. Keep in mind that soil test NO3- is only a portion of the N

in the soil and can change rapidly, depending on soil temperature and moisture content.

Soil test results for P are reported as either ‘Olsen P’, ‘Bray P’, or Mehlich III, which

will all report different values for the same sample but ultimately arrive at the same

recommendation (note: recommendations vary between states). Soil test methods for

P measure a portion of the P in the soil. Recommendations for fertilizer P result from

calibrations using field trial data comparing measured yield responses from additional P

at varying soil test levels.

What these values are not are measurements of the total P in the soil, which would

be of no value in production agronomy. In reviewing soil test results, the most crucial

question is, “What is the likelihood that the nutrient investment will have a positive

return?” Examining the concept of yield response curves can help to answer this

question.

Yield Response Curves and Economic Optimal RatesA yield response curve generally compares the soil nutrient supply with an expected

yield or yield goal. Figure 2 demonstrates the general concept of yield response curves.

In the large graph, yield

sharply increases as

soil nutrient supply

approaches medium

sufficiency. Yield

continues to increase

beyond medium

sufficiency, but the

response becomes

less as nutrient supply

approaches high

sufficiency. In Figure

2, the three charts to

the right demonstrate

expected yield response Figure 2. A conceptual model of corn response to soil nutrient supply. (Murrell and Bruulsema, 2008)

to additional nutrients when soil nutrients are low, medium, and high. If soil nutrient

supply is low, potential yield increases sharply, indicating that nutrient investments

have a high rate of return regardless of crop value or cost of nutrients. When soil

nutrient supply is at a medium level, investments in nutrients have a lower rate of

return. At this point, the cost of additional nutrients versus the value of the crop must

be evaluated. If costs of additional nutrients are low and crop value is high, additional

nutrients are likely economically justified. In an opposite situation, the cost of

additional nutrients needed to achieve higher yields may exceed the value of the crop.

When soil nutrient supply is high, there is no advantage to adding nutrients.

The principle of yield response curves can be put in practice when developing crop-

nutrient strategies. Some areas of the field will be more responsive to additional

nutrients than others. Yield monitors can help identify high- and low low-yielding

areas of the field; additional scouting and mapping can help to understand why certain

areas result in a higher response than others. Examples of low-yielding, less-responsive

areas are those with excessive pest pressure or areas with poorly drained, eroded, low-

organic-matter, or droughty soils.

High-yielding, less-responsive areas have opposite conditions of low-yielding areas.

Response to P and K fertilizers in these areas are likely to be limited due to medium to

high P and K levels. Nitrogen fertilizer response in these areas may be limited due to

finer-textured soils with higher organic-matter contents, higher levels of residual soil N,

or N contributions from the prior year’s legume crop.

Fertilizer use efficiency is a ratio of the amount of fertilizer needed to achieve

economically optimal yields and is variable within and among fields. High-yielding

areas are likely to be more efficient, while low-yielding areas will likely be less efficient.

For example, in high-yielding areas, 1.0 lb. of N per bushel may be required, while in

low-yielding areas, 1.4 lbs. of N per bushel may be needed. In situations where crop

value exceeds nutrient cost, it is likely that increasing rates in low-yielding areas or

high-yielding areas with medium soil-test P and K levels will result in increased profit.

When fertilizer prices are high and crop prices are low to moderate, reducing rates may

be an economically wiser choice. Improvements in precision-farming techniques and

technology provide the tools to identify zones of high-, moderate-, and low-yield in the

field. This information can improve decision making to achieve optimal profit.

Soil pH, Salts, and Nutrient AvailabilitySoil pH is a measurement of the amount of free hydrogen ion (H+) in the soil, reported

as the negative logarithm10

of the concentration of hydrogen ion in the soil (-log10

[H+]).

Increasing values of pH indicate lower levels of H+ and higher levels of the hydroxyl ion

(OH-), while lower pH values result from the opposite (h[H+]& i[OH-]). Why does this

matter? Results of research show that pH has varying effects on nutrient availability,

depending on the nutrient. Generally, optimal pH values for most agricultural soils are

6.5 to 7.0. Nutrients in soils with pH values in this range are likely to readily available to

the plant (see fig. 3). However, certain specialty crops may favor soil pH values outside

of this range.

The majority of soils in South

Dakota have ample or excessive

levels of calcium (Ca) and

magnesium (Mg) salts. This is

why most South Dakota soils

have near-neutral or basic pH

values. The majority of soils

in South Dakota are said to

be “highly buffered” (highly

buffered soils resist change in

pH to the point that it is nearly

impossible to change the pH).

Although uncommon and

localized, there are soils that

have lower-than-optimal pH

values. In some areas, lower pH values are the result from decades of application of

ammonia (NH3)-based fertilizers or manures with high levels of NH

3. Most of these soils

occur along the eastern boarder of the state and may have lower levels of Ca and Mg in

the upper profile. These soils can benefit from lime applications. Recommended rates

for lime can be found in Gerwing and Gelderman, 2005.

Depending on levels, calcium carbonates (CaCO3) or magnesium carbonates (MgCO

3)

can increase soil pH values to 7.8 and higher and have adverse effects on plant/soil

 

Figure 3. Soil pH and Nutrient Availability

water availability. Sodium (Na) is also a concern. While plants require minute amounts

of Na, excessive Na levels increase soil pH values above 7.8 and can severely affect soil

quality and the ability to produce a crop.

Most Na problems are the result of high-Na irrigation water, but there are also soils

in central and western South Dakota that naturally have elevated Na levels. Sodium

problems in some soils have become so severe that the affected soils have a limited

ability, or even an inability, to support plant life.

If Na problems are suspected, a soil test for sodium adsorption ratio (SAR) or

exchangeable sodium percent (ESP) is recommended. If values approach 8 to 9 (or

higher), remediation such as drainage or gypsum application is recommended.

Nutrient Cycling in Agronomic Production EnvironmentsImproving nutrient management requires a general understanding of how nutrients

cycle in and out of the system, becoming available for plant use. Crop production can

be compared to an assembly line, with the soil providing a portion of the raw materials

for plant use in producing grain, fiber, forage, and other consumable products. Various

microbial communities and chemical reactions work in the soil to provide the plant

with useable nutrients, but also are responsible for inefficiencies or losses in the

system. Individual nutrients have unique pathways for cycling in and out of the soil;

some pathways are influenced by management. The following is a general overview of

nutrients important to crop production in South Dakota.

NitrogenNitrogen (N) exists in nitrate (NO

3-), ammonium (NH

4+), and organic (compounds

containing C) forms in the soil. Of these three forms, only two— NO3- and NH

4+—are

utilized by the plant (fig. 4). For organic N to be utilized by the plant, the N must first

be mineralized or converted from an organic form to an inorganic form. For example,

one organic form of N, urea ((NH2)

2CO), is broken down by the urease enzyme to

produce NH3, water, and CO

2. The transformation from one form to another is generally

facilitated by the soil’s microbial population; because the transformations are microbial-

facilitated reactions, microbial inhibitors can be used to slow the conversion rates.

There are benefits of favoring one form of N over another. For example, if nitrification

(transformation of NH4+ to NO

3-) is slowed, leaching of NO

3- can be reduced. A

commercially available product that reduces nitrification is N-Serve®. This product has

been used to reduce denitrification in poorly drained areas and leaching in sandy well-

drained soils. Denitrification occurs when the soil microbial community is deprived of

O2 and converts NO

3- to N

2 gas making it unavailable to plants and usually lost to the

Figure 4. The Nitrogen Cycle. (Courtesy International Plant Nutrition Institute)

 Definition  of  Key  Terms  

• Volatilization – loss of N from the profile as ammonia (NH3) gas • Denitrification – loss of N from the soil as N2 gas • Leaching – movement and loss of N03from the root zone • Immobilization – microbial conversion of inorganic N (available) to organic N (unavailable) • Fixation – conversion of N2 from the atmosphere to ammonia form N • Mineralization – microbial conversion of organic N (unavailable) to inorganic N (available)

(Reitsma  et  al  .,  2008)  

 

atmosphere. Nitrogen may also be lost to the atmosphere as ammonia (NH3) gas via a

process known as volatilization. Urea- or ammonia-based fertilizers (or manure) applied

to the surface without incorporation or immediate rainfall or irrigation are examples of

situations where risk of N volatilization loss is significant.

Risk of volatilization losses are greatest during periods of high temperatures, slight

surface moisture (dew, traces of precipitation, or moist surface soil), and wind. Urea is

transformed to NH3 at the surface and is mixed in the atmosphere. Volatilization losses

can be avoided by incorporation, injection, or fertigation. Rainfall or irrigation levels of

½ inch or more will minimize volatilization losses.

Leaching losses occur when water moves NO3- out of the root zone. Soils of the Great

Plains have a net negative charge, repelling NO3- and allowing water to move NO

3-

through the profile. Light loamy and sandy soils are of greatest concern, but significant

losses can occur in heavier soils.

Additions of N to the system include fertilizers, manure, crop residues, and legume

crops. Minor amounts [~10 lbs./(A × Yr.)] of N are delivered in the rain or snow.

Immobilization is the microbial use of soil N during the breakdown of organic residues

and will out-compete plants for soil N. When soil organic carbon (SOC) levels are high

and N levels are limiting, immobilization can result in noticeable deficiency symptoms.

However, as SOC levels decline, microbial populations decline, releasing N back into the

soil. In some respects, immobilization could be considered storage.

Legume crops fix atmospheric nitrogen (N2) through a symbiotic relationship with

rhizobia bacteria. Rhizobia bacteria are said to be species specific, which means that

each legume plant will only form a relationship with a specific species of rhizobia.

Therefore, seed inoculums are sold for specific legume crops (e.g., soybeans, alfalfa,

clovers, etc.). Fields that have had recent (3 to 5 years) cropping history of a particular

crop may not need seed inoculum, as rhizobia bacteria can survive in the soil and will

infect host plants when present.

A well-developed N strategy considers how the cropping system will impact N cycling.

Management techniques that minimize losses and optimize plant availability have the

greatest potential to provide optimal economic return.

Phosphorus Cycling in the Agronomic EnvironmentPhosphorus (P) is a key nutrient in agronomic cropping systems. Levels of P in some

soils that have been in long-term crop and hay production have declined over the last

50 years, increasing the need for P fertilizers and manure. Losses of P from cropping

systems have raised water quality concerns, as excessive P levels in terrestrial waters

threatens commercial and recreational fisheries and tourism revenues. Environmental

concerns and rising cost of fertilizer and fuel has increased the importance of P-fertility

management.

Phosphorus has entirely different characteristics compared to N, as transformations to

plant-available forms depend more on chemical reactions than on microbial activity,

although both are important to P fertility. Plants use P in the form of ortho-phosphate

(HXPO

4) in the soil solution. Phosphorus enters the soil solution as minerals dissolve

or are released from the soil matrix. Mineralization is the conversion of organic P

from crop residues, manure, or other organic sources. It is a biological transformation

dependent on soil microbial activity.

Figure 5 is a simplified illustration of the P cycle in agricultural systems, depicting how

P moves between available (soil-solution P) and unavailable “pools.” The amount of P

in each pool is highly variable and dependent on soil physical and chemical properties.

Losses are largely due to runoff and erosion, with minor amounts that leach from the

profile. Significant losses from leaching are unlikely in South Dakota but can occur in

light, well-drained soils or situations where P levels exceed the soil’s capacity to hold it.

Although ortho-phosphate (HXPO

4-) is a negatively charged ion like NO

3-, unique

chemistries hold P in the soil. In fact, P moves very slowly through the soil. The

majority is held in unavailable pools that may take decades or centuries for it to become

plant available. As P moves slowly in the soil, it may accumulate at the surface in no-

tillage systems, and thus be unavailable for plant roots to reach it.

A well-designed P-fertility strategy strives to provide sufficient amounts for optimal

production in regions where plant roots can easily reach it. Mineralization is important

where P is supplied from organic sources such as manure and residues. Because

mineralization is dependent on soil bacteria, fungi, earthworms, and other soil flora and

fauna, water, temperature, and sources of organic carbon are critical components for

optimizing mineralization. In systems where P is supplied from fertilizer materials such

as rock phosphate or processed fertilizer, both the P-availability of the fertilizer itself

and soil-quality management become more critical.

PotassiumMost South Dakota soils have high to excessive levels of potassium (K) due to mineral

composition. There have been K deficiencies documented and instances of an economic

response to fertilizer K. However, a soil test is recommended to determine if a positive

yield response from K is likely.

Manure and crop residues, in addition to weathering of minerals in the soil, are a source

of K for the following crop. As K is a positively charged ion, it is held on exchange sites

in the soil and is not likely to be lost via leaching. Field runoff that causes erosion

causes more-significant K loss, but most K is lost by removing the crop from the field.

Figure 5. The Phosphorus Cycle (Courtesy International Plant Nutrition Institute)

 

Potassium-deficiency symptoms are notable in fields where the majority of the crop has

been harvested, such as corn silage, small grain with straw harvest, or hay production. A

positive response from additional K in these systems is more likely compared to systems

where residue is returned to the soil.

Secondary Nutrients and MicronutrientsIn most cropping systems, secondary nutrients (Ca, Mg, S) and micronutrients (B, Zn,

Fe, Cu, Mo, Mn, etc.) do not limit yields in South Dakota. There are exceptions, but the

exceptions are usually limited and localized in unique situations. However, S, Zn, Fe

deficiencies have been noted in corn, sunflower, soybeans, and wheat. Additions of S in

low-organic-matter soils have shown positive responses, but responses to Zn and Fe are

limited even in situations where crops exhibit deficiency symptoms.

Crops may show P-, Zn-, or Fe-deficiency symptoms early in the season, especially

in areas of high water tables. This is usually due to groundwater bringing Ca and Mg

salts closer to the surface, altering soil pH and rendering nutrients unavailable. Often,

when water tables drop and rainfall leaches the salts down below the root zone, these

nutrients become available for plant use.

Nutrient Imports and ExportsNutrient cycling in agronomic systems can be seen as a series of imports and exports.

Imports include applications of fertilizer and manure, and fixation of N through

symbiotic relationships between rhizobia bacteria and legume crops. Exports include

losses to the environment and harvest of grain and/or forage. The amount of nutrients

exported from a cropping system depends on the crop and the amount harvested.

Table 1 provides an estimate of the amount of selected nutrients contained in forage and

grains. Nutrient-export estimates can be calculated directly by multiplying the amount

of grain and forage harvested by the corresponding nutrient content.

Table 1. Nutrient content and removal rates of grains and forages.

Nutrient Removal, lb/unit

Crop Unit N P2O

5K

2O Mg S

Alfalfa ton 51 12 49 5.4 5.4

Alsike clover ton 41 11 54 7 3

Barley grain bu 0.99 0.4 0.32 0.06 0.09

Barley straw bu 0.4 0.16 1.2 0.08 0.1

Barley straw ton 13 5.1 39 3 3

Beans, dry bu 3 0.79 0.92 0.06 0.52

Birdsfoot trefoil ton 45 11 42 -- --

Bluegrass ton 30 12 46 3.5 5.0

Bromegrass ton 32 10 46 -- 5.0

Buckwheat bu 0.83 0.25 0.22 -- --

Canola bu 1.9 1.2 2.0 -- 0.34

Corn grain bu 0.90 0.38 0.27 0.09 0.08

Corn stover bu 0.45 0.16 1.1 0.14 0.07

Corn stover ton 16 5.8 40 5.0 3

Corn silage bu 1.6 0.51 1.2 0.33 0.18

Corn silage ton 9.7 3.1 7.3 2.0 1.1

Fescue ton 37 12 54 3.7 5.7

Flax grain bu 2.5 0.7 0.6 -- 0.19

Flax straw bu 0.7 0.16 2.2 -- 0.15

Millet bu 1.4 0.4 0.4 0.08 0.08

Mint lb oil 1.9 1.1 4.5 -- --

Oat grain bu 0.77 0.28 0.19 0.04 0.07

Oat straw bu 0.31 0.16 0.94 0.1 0.11

Oat straw ton 12 6.3 37 4 4.5

Oat silage ton 9.0 11 45 -- --

Orchardgrass ton 36 13 54 4.2 5.8

Potato tuber cwt 0.32 0.12 0.55 0.03 0.03

Potato vine cwt 0.2 0.05 0.3 0.04 0.02

Red clover ton 45 12 42 7 3

Reed canarygrass ton 28 9.7 44 -- --

Rye grain bu 1.4 0.46 0.31 0.1 0.1

Rye straw bu 0.8 0.21 1.5 0.07 0.14

Rye straw ton 12 3.0 22 1 2.0

Ryegrass ton 43 12 43 8 --

Sorghum grain bu 0.66 0.39 0.27 0.06 0.06

Sorghum stover bu 0.56 0.16 0.83 0.12 0.12

Sorghum stover ton 28 8.3 42 6.1 5.9

Sorghum-sudan ton 30 9.5 34 6.8 5.8

Soybean grain bu 3.8 0.84 1.3 0.21 0.18

Soybean stover bu 1.1 0.24 1.0 0.22 0.17

Soybean stover ton 40 8.8 37 8.1 6.2

Soybean hay ton 45 11 25 9 5

Sugarbeet root ton 3.7 2.2 7.3 0.95 0.45

Sugarbeet top ton 7.4 4.0 20 1.1 0.40

Sunflower grain cwt 2.7 0.97 0.90 0.25 0.25

Sunflower stover cwt 2.8 0.24 4.1 1.6 0.6

Sunflower stover ton 23 2.0 34 13 5

Switchgrass ton 22 12 58 -- --

Timothy ton 25 11 42 2 2

Tobacco (leaves) cwt 3.6 0.90 5.7 0.45 0.6

Vetch ton 57 15 49 -- --

Wheat grain bu 1.5 0.60 0.34 0.15 0.1

Wheat straw bu 0.7 0.16 1.2 0.1 0.14

Wheat straw ton 14 3.3 24 2 2.8

A fundamental component of a nutrient management strategy is a nutrient budget.

Using the data from Table 1, a 200 bu/A corn crop would export 180, 76, and 54 lbs./A

of N, P2O

5, and K

2O, respectively, if only the grain were harvested. Suppose that total

imports were 240, 150, and 100 lbs./A of N, P2O

5, and K

2O, respectively. Subtracting

exports from imports shows that 60, 74, and 46 lbs./A of was the net import of N, P2O

5,

and K2O, respectively.

It is important to recognize that a nutrient budget is only one component of a nutrient

strategy: selected fertilizer material, timing, method of application, crop rotation, and

companion, green manure, or cover crops are also critical components. Soil test values

may be used directly to calculate nutrient applications rates, but these rates can be

adjusted considering nutrient budgets, management techniques, and probability of

economic yield response.

The most efficient and sustainable approach is to prevent mining soil nutrients,

minimizing net export so that nutrient deficiencies do not limit crop production.

Carlson et al. (2010) provides an example of this in proposing that nutrient export is

minimal in a corn system where the grain and 60% of the stover is harvested from a

field within an integrated ethanol-beef production system; the grain then being used for

ethanol, with the distillers grain blended with corn stover to compose a feed ration for

beef production; and nutrients returned to the field as beef manure. This value-added

sustainable system has minimal nutrient export with optimal productivity.

Developing Crop Nutrient StrategiesA one-size-fits-all approach to nutrient management is near impossible, considering

variability among crops, soils, climates, and farming systems. However, the principles

involved are common among nearly all systems.

The principle components of a nutrient-management strategy:

1. Estimate crop need.

2. Estimate nutrients in the soil.

3. Estimate credits.

4. Estimate additional nutrient need.

5. Utilize optimal method and timing of application.

Estimating Crop Need – Yield GoalsIn South Dakota, fertilizer recommendations for N, P, and K are based on expected

yield or “yield goal.” (Current fertilizer and nutrient recommendations are available

in South Dakota Cooperative Extension Service Extension Circular 750, “Fertilizer

Recommendations Guide.”)

Calculating yield goals is complicated by improved genetics, which are gradually

increasing crop yields. Further complications are introduced by rotations, which reduce

the amount of available information for specific crops. An assumption with these

calculations is that the crop response to N is similar in high- and low-yielding soils. (See

Table 2.)

General guidelines for calculating yield goals include the following:

• Itisnotrecommendedtoconsidermorethan10years.

• Abnormallyhighorabnormallylowyieldvaluesshouldnotbeincludedin the calculation.

• Some attempt should be made to account for increasing yield potentials. For

example, corn yields in South Dakota have been increasing at an annual rate of ≈2.0 bu/acre over the past 20 years (Clay and Reitsma, 2009). A field with a yield of 140 bu/acre 10 years ago may now produce a yield of 160 bu/acre. Managing for an optimistic, yet realistic, yield goal is important, underestimating yield goals can lead to gradual yield declines.

• Potentialandactualyieldsarevariablebetweenandwithinfieldsandgrowing seasons.

• Achieving full yield potential depends on management, climate, and soil.

Additional information regarding yield goals

is available in Clay and Reitsma

(2009) and Reitsma et al. (2008).

Nutrient recommendations for South

Dakota are published in Cooperative

Extension Service Extension Circular

750, “Fertilizer Recommendations Guide”

(Gerwing and Gelderman, 2005). These

recommendations are yield-goal based

and may be used for nutrients applied as

commercial fertilizer, organic fertilizer, and

manure. Table 3 is excerpted from EC750

and provides guidance for estimating N, P,

and K for corn.

For example, recommendations for a corn

crop with a 160 bu/A yield goal to be grown

in a field with medium P and K levels

are 192, 45, and 70 lbs/A of N, P2O

5, and

K2O, respectively. These values are the

basis for developing nutrient rates and an

overall strategy. These calculations do not

consider differential nutrient responses

in high- and low-yielding areas and do not

consider the purchase price of the fertilizer

and selling price of the crop. However, they

do provide a starting point for developing

nutrient application rates.

Table  2.  Estimating  a  yield  goal  from  multiple  years  of  data  

Field  records  

Year  #Standardized  yield  

(bu/A)   Conditions  

1   136   Average  2   133   Average  3   126   Average  4   128   Average  5   126   Average  6   145   Average  7   *171   Excellent  8   163   Excellent  9   *112   Poor  10   129   Average  

Base  yield  goal  =  136      

#Standardized  yield  considering  average  annual  increase  of  2  bu/A/yr.  *Outliers  were  removed  to  calculate  average  yield.  The  yield  goal  +  10%  recommendation  would  be:              136  •  1.10  =  150  bushel.  Yield  goal  +  moisture  recommendation  would  be:        Full  soil  profile  at  planting            136  •  1.10  =  150  bushels        Average  soil  profile  at  planting            136  bushels        Poor  moisture  conditions  at  planting            136  –  0.10  •  136  =  123  bushels  Producers  should  be  prepared  to  apply  additional  fertilizer  N  as  an  in-­‐season  side-­‐dress  if  needed  when  using  the  soil  moisture  approach.  (Clay  and  Reitsma,  2009)    

 

Estimating Nutrients in the SoilA soil test is the best way to estimate 1) how much of each nutrient is in the soil to

develop a recommended rate and 2) how much N to credit or subtract from the N

requirement. In lieu of a soil test, 50 lbs/A N may be used as an estimate of soil N in

row-crop systems. The time that samples are collected in the field will affect soil test N

values, depending on soil temperature and moisture conditions.

More information for collecting soil samples is provided in Gelderman et al. (2005) and

Clay et al. (2002).

Nitrogen will be the nutrient of interest, as rates of P and K are calibrated against

soil-test values. The principles of the N cycle and of nutrient imports and exports

assist in understanding N credits and the rate adjustments that may be necessary.

The probability of significant soil residual NO3- - N level is influenced by manure

application, legume crop production the previous year, and precipitation level. Figure 6

demonstrates how these three variables influence soil residual NO3- - N and can be used

as a guide when deciding if soil sampling for N is necessary.

Table 3. Nutrient Recommendations for Corn (Gerwing & Gelderman,

Estimating N CreditsAdditional nitrogen (N) is

contributed to the soil system

from a legume crop grown

in the previous season, from

NO3- in irrigation water,

and from manure. (Manure

will be discussed in the

following section.) Legumes

and rhizobia bacteria form

symbiotic relationships, where

the bacterial fix nitrogen gas

(N2) from the atmosphere,

supplying the plant with N

needs, and the remaining fixed

N is contributed to the soil

for the next year’s crop. An

estimate of the amount of N

to credit for a legume crop is

provided in Table 4.

The amount of N contributed

from forage legumes is

estimated based on plant

population, while row crop

credits are based on yield.

Legumes are often used in

cover-crop mixtures planted

following small-grain harvest,

but it is not recommended to

use values in Table 4 for cover-

crop N credits.

Currently, the amount of

N credits from cover crops

are unknown, but research

 

Table 4. Nitrogen Credits from Previous Legume Crop  

Crop

Population (Plants/ft2)

1,2N Credit (lbs N/Acre)

<1 0 1-2 50 3-5 100

Alfalfa or

3Legume Green Manure >5 150

Soybeans, Edible Beans, Peas, Lentils and Other Annual Legumes

40

1 No-till corn into alfalfa or green manure crops, use ½ credit first year. 2 For second year following alfalfa and green manure crops, use half credit. 3 Includes sweet clover, red clover, and other similar legumes.

(Adapted from Gerwing and Gelderman, 2005)

 

Table  5.  Estimate  of  Irrigation  Water  N  Credit  (IC)  

IC = NO3--N IW x IA x 0.23

Where: IC = Irrigation Water N Credit (lbs/Acre) NO3

--N IW = Irrigation Water NO3--N Lab

Result (PPM) IA = Amount of Irrigation Applied (in/Acre)

(Clay and Reitsma, 2009)

FIGURE 0. PROBABILITY OF SIGNIFICANT RESIDUAL SOIL NO3- - N LEVELS

Figure 6. Probability of Significant Residual Soil NO3--N Levels

continues at SDSU to determine if cover crops contribute N to the soil system.

Therefore, it is recommended not to assume any N credit for cover crops.Irrigation

water can contribute significant amounts of N, depending on how much water is applied

and on the concentration of NO3- in the irrigation water. Estimates of N credit for

irrigation water can be calculated using the equation in Table 5.

Livestock ManurePrior to the introduction of commercial fertilizer, crop producers had few options for

adding nutrients to the soil; the options were animal manure, raw minerals such as rock

phosphate, animal remains, or composted organic materials. What sets animal manure

apart from the other non-commercial

fertilizer materials is that nutrients

in manure are immediately available,

or shortly become available, for plant

use. In some ways, livestock can

be viewed as “fertilizer factories,”

as digestion of feed and mineral

additives in the gut tract break down

complex materials into simpler,

more soluble forms. Manure provides

additions of organic matter to the

soil and contains large organic acids

that serve to break down complex

materials to soluble forms.

Livestock manure contains nearly

every plant nutrient that is supplied

by the soil. The disadvantage of

manure is that the amount of

each nutrient in manure is highly

variable—even with laboratory

analysis of manure samples, the

amount of nutrients applied is

a best-guess effort. In addition,

manure typically has much

lower concentrations of nutrients

Table 6. Estimated nitrogen content of liquid and solid manure

Liquid Manure Solid Manure Nitrogen (N) lbs/1000 gal

Nitrogen (N) lbs/ton

Type of

Livestock NORGANIC NINORGANIC NORGANIC NINORGANIC

Swine Farrowing 7 8 11 3 Nursery 11 14 8 5 Grow-Finish - - 10 6 Grow-Finish(deep pit) 17 33 - - Grow-Finish(wet/dry feeder) 21 39 - - Grow-Finish(earthen pit) 8 24 - - Breeding-Gestation 13 12 4 5 Farrow-Finish 12 16 8 6 Farrow-Feeder 10 11 5 5

Dairy Cow 25 6 8 2 Heifer 26 6 8 2 Calf 22 5 8 2 Veal calf 26 21 4 5 Herd 25 6 7 2

Beef Beef cows 13 7 4 3 Feeder calves 19 8 6 3 Finishing cattle 21 8 7 4

Poultry Broilers 50 13 34 12 Pullets 48 12 39 9 Layers 20 37 22 12 Tom turkeys 37 16 32 8 Hen turkeys 40 20 32 8 Ducks 17 5 13 4

(Adapted from Lorimor and Powers 2004) These values should not be used in place of a regular manure analysis as true nutrient content varies drastically depending on feeding and manure storage and handling practices. Use only for planning purposes.

and requires higher application rates than

commercial fertilizer.

Livestock manure can also be a vector for weed

seed that can be controlled by composting, but

much of the N in manure will be lost during the

composting process. However, the benefits of

manure far outweigh the limitations and can be

overcome with management.

The amount of N in manure depends on the

livestock species of origination and feeding

practices. Table 6 provides an estimate of

inorganic and organic N concentrations of

selected livestock species’ manure. Manure can

act as a “slow-release” product, as the organic

N in manure must be mineralized before it is

plant-available. Estimates for first and second

N credits can be calculated using the worksheet

in Table 7. Note that significant N losses occur

when manure is surface-applied without

incorporation. Rainfall or irrigation events of

½” or more following application will act to

move N into the soil (similar to incorporation).

Table 8 shows the estimated phosphorus

content of manure depending upon the

type of livestock.

The total amount of P and K contained in

manure can be credited to the first year’s crop

following application. Incorporation of manure

is recommended to avoid losses from runoff

and erosion events.

  Table  7    Estimate  of  Manure  Nitrogen  Credit  

A Ninorganic Lab Test or Estimate x 1RF Lbs#

B Norganic Lab Test or Estimate x 2Mkx

Lbs#

C Manure N Credit (A+B) Lbs#

# Liquid Manure Units = Lbs-N/1,000 gal # Dry Manure Units = Lbs-N/ton

2If manure was applied the previous growing season, replace Mk1 with Mk2 to estimate N credit for Norganic. Poultry Manure All Other Manure

Mk1 = 0.60 Mk1 = 0.35 Mk2 = 0.85 Mk2 = 0.50

1RF = Retention Factor Injected 0.98 Incorporate < 24 Hrs 0.90 Incorporate > 5 Days 0.20

(Reitsma  et  al.,  2008)  

 

Table  8.  Estimated  phosphorus  content  of  manure  

P2O5    Type  of  

Livestock  Liquid  

(Lbs/1,000  gal.)  Solid  

(Lbs/ton)  

Swine      Farrowing   12   6  Nursery   19   8  Grow-­‐Finish(deep  pit  or  

solid)  42   9  

Grow-­‐Finish(wet/dry  

feeder)  44   -­‐  

Grow-­‐Finish(earthen  pit)   22   -­‐  Breeding-­‐Gestation   25   7  Farrow-­‐Finish   24   8  Farrow-­‐Feeder   18   7  Dairy      Cow   15   3  Heifer   14   3  Calf   14   3  Veal  calf   22   3  Herd   15   4  Beef      Beef  cows   16   4  Feeder  calves   18   4  Finishing  cattle   18   7  Poultry      Broilers   40   53  Pullets   35   35  Layers   52   51  Tom  turkeys   40   50  Hen  turkeys   38   50  Ducks   15   21  

(Adapted from Lorimor and Powers 2004) These values vary drastically depending on feeding and manure storage and handling practices and are not likely representative of actual nutrient content of the manure. Use only for planning purposes. These values should not be used in place of a regular manure analysis.

Fertilizer MaterialsSelection of fertilizer materials will depend on the type of available equipment and the

farming system. No-tillage systems may be better suited to liquid products, as liquid

products can be injected using low-disturbance applicators.

Dry products may also be used, but may result in high concentrations of P and K near

the surface and loss of N via volatilization if not properly managed. The most restrictive

farming system is organic, because public and private certification standards restrict the

use of many fertilizer materials, limiting product selection.

Commercial fertilizer products are formulated to be immediately or rapidly available to

plants. Most commercial N products are made from ammonia (NH3), using N

2 fixed from

the atmosphere and converting to NH3, using the Haber-Bosch method. Liquid products

and dry products are formulated from NH3. Anhydrous-ammonia is a liquid stored under

pressure but immediately becomes a gas when applied.

Organic fertilizer materials are often made from manures, waste products, minerals,

or a combination thereof. Manures provide nutrients that are rapidly available, due to

the chemical form of the nutrients and organic acids in the manure. Waste products

may have medium-to-low availability, depending on the product, and most mineral

products have low availability. Many mineral products approved for organic use are used

in manufacturing commercial fertilizers (e.g., rock phosphate or apatite is mixed with

phosphoric acid to make the raw material for various P-fertilizer formulations).

Table 9 compares nutrient analysis and relative availability of conventional commercial

and organic fertilizers. In comparing these two groups of materials, the greatest

differences are nutrient analysis and relative availability. Nutrient availability of

a product is important when selecting a product, because products that are “slowly

available” may require several years to become available and may be unable to supply

sufficient amounts of nutrients to the crop.

With products that are slowly available, an option would be to apply enough product to

satisfy the N need for ~1 to 3 cropping seasons and P and K needs for ~5 to 7 cropping

seasons. The previous statement should only be used as a guide, as mineralization rates

will be highly variable; annual soil sampling to monitor nutrient levels is recommended.

Another

consideration is

how the product

will react in the

soil environments.

Some products—

such as blood

meal—are

mineralized by soil

microbes, similar

to manure. Mineral

products become

available through

chemical reactions

in the soil. An example is rock phosphate, which requires acid to become available.

Rock phosphate is essentially ineffective in soils with pH levels above 6.0 (Sanchez,

2009); with most soils in South Dakota, rock phosphate will remain in mineral form,

unavailable for plant use.

Commercial fertilizers are typically made by blending rock phosphate with phosphoric

acid; these materials can be used on soils with regardless of pH, although soil levels

above 6.0 can impact availability.

The nutrient content of commercial fertilizers is significantly higher than organic or

unconventional products. Conventional fertilizers with high nutrient content require

less fertilizer product to meet nutrient needs and therefore are more efficient to

transport and apply. Low-analysis fertilizers require higher rates and may lead to

higher costs on a nutrient basis. It is recommended to always calculate fertilizer cost

on a nutrient cost basis and consider application costs. Actual nutrient cost can be

calculated using Equation 1.

Nutrient Application Methods and PlacementEquipment used or the cropping system may limit the choices available to apply

nutrients. For example, broadcast with disk incorporation is not an option in a no-till

system. Injection of manure and fertilizers has become possible in no-till and strip-till

Table 9. Relative Availability of Conventional and Organic Fertilizer Materials

Nutrient Analysis (%) Material N P2O5 K2O

Relative Availability

Conventional Fertilizer Materials Anhydrous NH3 82.0 0 0 Rapid Urea 46.0 0 0 Rapid Concentrated Super Phosphate 0 45.0 0 Rapid Mono-Ammonium Phosphate 11 52.0 0 Rapid Di-Ammonium Phosphate 18.0 46.0 0 Rapid Ammonium Poly-Phosphate 10.0 34.0 0 Rapid Urea Ammonium Nitrate 20.0 0 0 Rapid

1Allowable Organic Fertilizer Materials Blood Meal 12.0 1.5 0.57 Medium – Rapid Bone Meal (Raw) 2.0 – 6.0 15.0 – 27.0 0 Slow Bone Meal (Steamed) 0.7 – 4 18.0 – 34.0 0 Slow - Medium Fish Emulsion 5.0 2.0 2.0 Rapid Compost (Unfortified) 1.5 -3.5 0.5 – 1 1.0 – 2.0 Slow Colloidal Phosphate 0 25.0 0 Slow Rock Phosphate 0 20.0 – 32.0 0 Very Slow Granite Meal 0 0 3.0 – 5.0 Very Slow 1Always consult with certifying agency before using any fertilizer material.

Adapted from Sanchez, 2009

Example. Dry urea sells for $505/ton, what is the cost of a lb of N? $505 / 2,000 / 0.46 = $0.55

Many fertilizers contain multiple nutrients. To calculate the cost of these nutrients, the cost of one fertilizer must be first subtracted from its selling price. For example, if DAP (18-46-0) is selling for $1,000/ton and N fertilizer is selling for ~ $0.50/lb of N, then the cost of the P can be determined fol-lowing these steps.

First, determine total N and subtract the value of N from the cost of the fertilizer.

2000lbs . 0.18lbN . $0.50 = $180 ton 1 pound 1 lb N ton

Second, subtract N from DAP cost: $1000 - $180 = $780

Third, recalculate P2O

5 cost.

ton . lb . $780 = $0.848 2000lbs 0.46lb P

2O

5 ton lb P

2O

5

This procedure can be used to calculate the nutrient cost of any fertilizer. However, the cost of at least one nutrient mucst be known or assumed. More information for calculating fertilizer costs is available in Clay et al., 20102.

systems with the development of low-disturbance applicators; these implements allow

nutrients to be placed below the surface, minimizing nutrient losses via volatilization,

runoff, and erosion. Injection places nutrients within the rooting zone, where they are

more available to plants. Low-disturbance applicators create some soil disturbance,

but the disturbance is minimal. No-till producers will need to decide if the level of

disturbance is acceptable or if surface application with zero disturbance is a more

favorable method.

Fertigation delivers nutrients via irrigation water. Fertigation is an efficient and effective

method for applying nutrients such as manure or fertilizer in liquid forms. Investments

in additional equipment are required, as are local permits, but there are additional

savings in time and fuel.

Equation 1. Calculating nutrient cost in fertilizer materials

Most modern planters have the option of adding fertilizer attachments that allow

fertilizer to be placed with, or in close proximity to, the seed. This practice is often

referred to as “starter fertilizer.” If placing fertilizer with the seed, do not use urea

fertilizers. Seed germination can be reduced from salt damage if the N plus K2O in the

fertilizer exceeds 10 lbs/acre (Gerwing and Gelderman, 2005). The more desirable

method for application of starter fertilizer is to place it in a band, 2 inches to the side

and 2 inches below the seed (also called a “2x2 placement”). This method eliminates

the risk of seed salt damage and allows for high application rates.

The most common and oldest method of nutrient application is to “broadcast” the

nutrient. With the exception of anhydrous ammonia, any form of manure or fertilizer

can be broadcast-applied. The greatest drawback with broadcast applications is that

losses can be great if not incorporated, which requires another trip across the field,

using fuel and increasing the potential for compaction.

In some systems, optimal results can be realized by dividing the total nutrient need

among two or more applications performed at different stages in the growing season.

This is most critical for N because it is most volatile in the environment, but splitting

the application of P and K between a

broadcast and starter application can

result in more efficient nutrient use

and positive yield responses. Corn has

been shown to respond to in-season N

applications. A 6-inch soil sample taken

immediately before the side-dress is to be

applied is the best method for determining

rate of application. Table 10 provides

guidance for rate recommendations based

on an in-season soil test.

Fall nutrient application can help free up time in the spring, when labor and equipment

is dedicated to planting. However, the risk of nutrient loss is significant and may result

in reduced yields and increased environmental risk. Fall application of fertilizer and

manure should be avoided unless the fertilizer is incorporated or injected and the risk

of leaching is minimal.

Table  10.  Side-­Dress  N  Rate  Recommendation  

Soil Test NO3

-N (ppm) Side-Dress N

(lbs/Acre) 0 – 10 80 – 120

11 – 15 50 – 90 16 – 20 30 – 60 21 – 25 0 – 40

> 25 0 (Reitsma et al., 2008)

 

Fertilizer Additives and Soil EnhancersThere are many products in the marketplace with a wide range of claims that are

designed to be added to fertilizers and manure. The most common products are urease

inhibitors and nitrification inhibitors, designed to slow the release of N fertilizers and

the urea component of manure. These products will perform as claimed, but have highly

variable results with respect to yield response. The reason for the variability in yield

response is the unpredictability of when the applied N will be available to the plant, as

climate and soil conditions influence the rate at which these products decay and allow

N to become available. However, fertilizer additives can be effective in reducing leaching

losses in sandy soils.

Other products claim to increase nutrient availability through a variety of chemical

and biological mechanisms. Although it is true that fertilizers have varying degrees

of availability, these product should be used with caution and should be avoided if

the manufacturer does not have unbiased scientific research to back its claims. Some

products are marketed as “microbial” solutions, claiming to enhance root growth and

increase nutrient availability. Although soil microbial communities have been studied

for decades, there are many unknowns. However, it is likely that crop rotation and

management will have a greater impact on soil microbial communities than biological

amendments (Larkin, 2007).

When considering any of these products, a good rule of thumb is, “If it is too good to be

true, it probably is.” Always request research results and references of other growers

who use the product, and consult with the South Dakota Department of Agriculture to

see if it is registered for sale in the state.

The Role of Crop Rotation in Nutrient ManagementContinuous cropping was once thought to be a sign of poor management. In the 1950s,

the increased supply of fertilizer, especially N, encouraged continuous cropping practices

on soils where erosion was not a problem. However, production declines from continuous

cropping practices have led producers to return to strategic crop rotation management.

Currently, largely due to economics or to environmental limitations, continuous

cropping is still practiced in some areas. Although the reasons for production advantages

due to crop rotation are not fully understood, it is evident that a well-managed rotation

provides benefits such as increased soil quality, fertility, and pest control.

More information about crop rotation is presented in the “Crop Rotation” section of this

publication.

Green Manure and Cover CropsGreen manure and cover crops are likely

to be most successful in the Northern

Plains when planted following small-grain

harvest. In the Northern Plains, small

grains are typically harvested in late

summer; this allows time for cover crop

establishment before fall freeze. However,

water limitations at this time can present

challenges for cover crop establishment.

The terms “green manure” and “cover

crops” are often used interchangeably.

Green manure crops are planted to provide

nutrients or organic matter to the soil, retrieve nutrients from the lower root zone,

suppress weeds, and/or reduce erosion. Typically green manure crops are planted in the

spring and incorporated into the soil while still green or soon after flowering.

Cover crops are cultivated for a multitude of purposes, mainly to provide ground cover

to reduce erosion and increase water infiltration. However, they can also be a source

of forage for grazing livestock. Cover crops become green manure crops when they

are killed and incorporated into the soil. Fall-seeded grasses and legumes such as rye,

ryegrass, and vetches are used for cover crops in the Midwest region of the United

States. The advantages of using a grass such as ryegrass include low seed costs, rapid

establishment, vigorous growth, recovery of residual N in the soil, and good winter

survival rates. However, grass residues have a high carbon to nitrogen ratio (C:N), which

can lead to early season N deficiencies in non-leguminous crops such as corn, because

the N is immobilized by microbial activity associated with decay of the crop.

Dense-grass cover crops can also delay spring fieldwork because the soil dries and

warms slower. Careful consideration must also be given to the subsequent crop,

as significant yield declines have been noted where grass cover and green manure

crops precede crops such as corn and sorghum. These declines are may attributed to

 

Benefits of Green Manure and Cover Crops

• Fix atmospheric nitrogen (legumes)

• Stabilize nutrients

• Add organic matter to soil

• Reduce erosion and runoff

• Increase water infiltration

• Use excess soil water in fall

• Provide fall and winter grazing

allelopathic effects (release of toxic substances) that inhibit the growth of the following

grass crop. This effect is not observed when a legume crop follows a grass cover crop.

Therefore, it is not recommended to follow a grass cover crop with another grass crop

such as corn.

Legume cover and green manure crops present a variety of benefits that grass cover

crops do not; primarily, the ability to fix atmospheric N. Legume crops commonly

seeded for cover and green manure crops include alfalfa, sweet clover, clovers, peas, or

medics. Because residues from most legume crops have a low C:N ratio, immobilization

of N is not an issue. Legume cover crops may be beneficial in mitigating early season

N deficiencies in corn, sorghum, or other crops that are impaired from early season N

deficiencies. Furthermore, allelopathic effects are not an issue, as legumes do not affect

grass crops.

Legume crops are often slower to establish than grass crops. Planting a legume cover/

green manure crop after a full-season crop is not recommended; the crop has little

time to develop, resulting in low winter survival, limited N fixation, and little cover or

dry-matter contribution. Deep-rooted cover and green manure crops such as sweet

clover and buckwheat have the ability to remove P from the lower root zone. The decay

of residues of these crops leads to release of P into the soil, available for use by the

following crop.

Tuber crops such as radish, turnip, and sugar beet have become popular cover crops in

the Dakotas. These crops provide a source of forage for fall and winter grazing and may

be beneficial in removing excess water in the top 12 inches of soil as well as in breaking

compaction.

ReferencesClay, D.E., C.G. Carlson, S.A. Clay, and T.S. Murrell. 2010. Nutrient Removal and Nutrient Budgets. In Mathematics and Calculations for Agronomists and Soil Scientists. Pp 35-40. International Plant Nutrition Institute, Norcross, GA, and South Dakota State University, Brookings, SD.

Clay, D.E., C.G. Carlson, S.A. Clay, and T.S. Murrell. 20102. Unit Conversions Involving Fertilizers. In Mathematics and Calculations for Agronomists and Soil Scientists. Pp 27-33. International Plant Nutrition Institute, Norcross, GA, and South Dakota State University, Brookings, SD.

Carlson, C.G., D.E. Clay, C. Wright, and K.D. Reitsma, 2010. The Impact of Linking Ethanol and Beef Production on Economics, Carbon, and Nutrient Budgets. ExEx8165. South Dakota State University, College of Agriculture and Biological Sciences and South Dakota Cooperative Extension Service, Brookings, SD.

Clay, D.E., N. Kitchen, C.G. Carlson, J.C. Kleinjan and W.A. Tjeutland. 2002. Collecting representative soil samples for N and P fertilizer recommendations. Crop Management doi:10.1094/CM-2002-1216-01-MA.

Clay, D.E., and K.D. Reitsma. 2009. Soil Fertility. Pp. 39-48. In Clay, D.E., K.D. Reitsma, and S.A, Clay (eds). Best Management Practices for Corn Production in South Dakota. EC929. South Dakota State University, South Dakota Cooperative Extension Service, Brookings, SD.

Gelderman R., Gerwing J. and K.D. Reitsma, 2005. Recommended soil sampling methods for South Dakota. FS 935. College of Agriculture and Biological Sciences, South Dakota State University, South Dakota Cooperative Extension Service, South Dakota Department of Agriculture, Brookings, SD.

Gerwing J. and R. Gelderman, 2005. Fertilizer recommendations guide. EC 750. South Dakota Cooperative Extension Service, South Dakota State University, United States Department of Agriculture, Brookings, SD.

Larkin, R.P. 2008. Relative effects of biological amendments and crop rotations on soil microbial communities and soilborne diseases of potato. Soil Biology & Biochemistry. 40:1341-1351.

Lorimor, J. and W. Powers, 2004. Manure characteristics – manure management systems series, Midwest Plan Service – 18, Sec. 1, 2nd ed. Ames, IA.

Murrell, T.S. and T.W. Bruulsema. 2008. Principles of Allocating Funds across Nutrients. Better Crops 92:3-5. International Plant Nutrition Institute, Norcross, GA.

Reitsma, K.D., R. Gelderman, P. Skiles, K. Alverson, J. Hemenway, H.J. Woodard, T.E. Schumacher, D. Malo, and D.E. Clay. 2008. Nitrogen Best Management Practices for Corn in South Dakota. FS 941. South Dakota State University, South Dakota Cooperative Extension Service, Brookings, SD.

Sanchez, E.S. 2009. Using Organic Nutrient Sources. UJ256. Pennsylvania State University, College of Agricultural Sciences, Agricultural Research and Cooperative Extension, University Park, PA.