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Extension > Agriculture > Nutrient Management > Nitrogen > A soil nitrogen test option for N recommendations with corn

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A soil nitrogen test option for N recommendations with corn

M.A. Schmitt
G.W. Randall
G.W. Rehm

Overapplication of nitrogen (N) to corn is causing much concern due to the environmental consequences associated with unused N. There is also a significant economic advantage in applying the correct rate of N fertilizer. Excess, unused N can be lost from the soil system via denitrification and/or leaching, be tied up in the soil organic N pool, or stay in the crop's rooting zone as residual N.

Recent research has indicated that measuring residual, available N through a soil test can refine current N recommendations for corn. This soil N test involves collecting 0-2 ft. soil samples in the spring before planting and having the samples analyzed for nitrate-N. The quantity of nitrate-N found is then used to calculate a residual N credit.

Figure 1. Map of Minnesota indicating area of western Minnesota currently using a soil nitrate test (shaded area) and counties where new soil N test research was conducted (X's).

Existing N recommendation procedures

Soil N testing has been a management tool for western Minnesota (see Figure 1) for a number of years. It is recommended that a late fall or spring preplant N soil test be taken and the measured amount of nitrate-N be subtracted according to the formula in Equation 1.

NRec = (1.2)(YG)-STN(0-24in.)-NPC
NRec = amount of fertilizer N needed, lb./acre
YG = realistic yield goal, bu./acre
STN(0-24 in.) = amount of nitrate-nitrogen
(NO3 - N) measured by using the soil nitrate test, lb./acre
NPC = Nitrogen credits, if any, for previous crops in the rotation, lb./acre

Nitrogen recommendations for corn have been made using a much different approach in the remainder of the state. The approach used in western Minnesota is not appropriate for eastern and central Minnesota due to higher rainfall. In these regions, where higher rainfall amounts occur, N recommendations are made using Table 1, which accounts for organic matter levels, previous crop, and expected yield.

Table 1. Nitrogen recommendations for corn for situations where the soil nitrate test is not used.

Crop grown last year Organic* matter level Yield goal (bu./acre)
70-90 91-110 111-130 131-150 151-170 171-190 191+
N to apply (lb./acre)
alfalfa, (4 + plants/ft2), non-harvested sweetclover low 0 0 0 0 30 50 70
medium and high 0 0 0 0 0 20 40
soybeans, small grains,** alfalfa (1 or less plants/ft2) low 20*** 50*** 80*** 110 140 160 180
medium and high 0 30 60 80 110 130 150
edible beans,
field peas,
low 40 70 100 130 160 180 200
medium and high 20 50 80 100 130 150 170
Group 1 crops low 0 15 45 75 105 125 145
medium and high 0 0 15 45 75 95 115
Group 2 crops low 60 90 120 150 180 200 220
medium and high 40 70 100 120 150 170 190
* Low = less than 3.0% medium and high = 3.0% or more. The well-drained silt loam soils in southeastern Minnesota receive the N recommendations listed for soils with a medium and high organic matter level. All irrigated soils are included in the medium and high organic matter category. ** Use these credits for small grains only if stubble is tilled after harvest. If there was no tillage, use recommendations for crops in Group 2. *** These recommendations should be increased by 20 lb N/acre to compensate for less N credit on coarse-textured soils
Crops in Group 1 Crops in Group 2
alfalfa (2-3 plants/ft2 buckwheat grass pasture sorghum-sudan
alsike clover canola millet sugarbeets
birdsfoot trefoil corn mustard sunflowers
grass/legume hay flax potatoes sweet corn
grass/legume pasture grass hay vegetables
red clover

Recent research/new recommendations

The raised interest in economic and environmental concerns in the 1980s led to a revisiting of the concept of including a soil N test to refine fertilizer N recommendations for corn in the humid regions in Minnesota. A major research study conducted from 1989 through 1992 provided the data from 59 sites in 19 counties (see Figure 1) that has led to the development of a new soil N test throughout Minnesota.

This research was conducted on a variety of soil types, primarily on farmers' fields. A host of sampling depths, times, and analysis parameters were evaluated in countless combinations. Grain yield response to N was best indicated by soil nitrate-N measured from a two-foot sampling depth. The direct correlation between yield response to N and soil N test was not as strong as desired, but subsequent calculations led to a strong correlation between soil nitrate-N at preplant time and "unaccounted N" using the traditional, non-soil test recommendations. The "unaccounted N" was deemed to be residual N. This relationship can be seen for one set of data in Figure 2, which is the basis for the residual N credit in Figure 3.

Figure 2. Linear relationship between 0-2 ft. nitrate-N concentrations measured before planting and residual N credit for a subset of dataset.

Figure 3. Soil N credit amounts as a delineated, adjusted function of the preplant nitrate-N concentrations. At 19 ppm or greater nitrate-N, no fertilizer N would be recommended.

Where/when to use this test

Although the western tier of Minnesota has an existing process for including soil N test information into corn N recommendations, the new procedure described in this bulletin can also be used in western Minnesota. Slight differences in N recommendations may appear in comparing the two procedures for a given site due to mathematical rounding differences inherent in each procedure.

Evaluation of N recommendations made with and without the preplant soil N test in this research project identified some conditions where the new 0-2 ft. preplant soil nitrate-N test has the greatest utility and impact. The user should first evaluate whether conditions exist for residual N to accumulate. Factors such as previous crop, soil texture, and preceding rainfall have significant effects on accumulation of residual N.

A crop rotation that has corn following corn generally provides the greatest potential for significant residual N accumulation. This is probably due to the amount of fertilizer N added to the previous corn crop, the fact that corn doesn't scavenge excess N from the soil–in contrast to a legume–and manure additions are frequently made to continuous corn fields. In contrast, when soybean is the previous crop, much less residual N has been measured, as soybeans will use residual N in the soil in addition to the N that it symbiotically fixes within its nodules.

The new soil N test works best on medium- and fine-textured soils derived from loess or glacial till. The use of the proposed soil N test on coarse-textured soils derived from glacial outwash does not appear to be promising. Outwash sites consistently had low soil N test values in the study and one would not expect to measure significant residual N in these sandy soils.

The amount of residual N in the soil is also dependent on the rainfall received the previous year. In a year following a widespread drought, 1989 for example, a majority of fields will have significant residual N. However, following relatively wet years, such as the early 1990s, little residual N can be expected.

Previous manure applications to a field generally result in increased residual N in the soil. While it is perceived that the most recent manure applications (year of or year after application) would best be reflected by the soil N test, research experience indicates that the residual effects–2, 3, or 4 years after application–are best quantified by this new test.

In summary, the new soil N testing option, which accounts for residual N, will not be appropriate for all fields and conditions. Figure 4 can be used to help decide which fields may need to be tested for residual nitrate-N. This flowchart uses such factors as previous crop, manure history, and a knowledge of previous rainfall.

Figure 4. Flow chart decision-aid for determining the probability of having significant residual N in the soil.

How to use the test

Nitrogen fertilizer recommendations for corn can be made with or without the new soil N test. Because the soil N test is used to estimate residual, available N, the University of Minnesota's previous N recommendations, which assume minimal residual N, are still the starting point for all recommendations. A five-step process is suggested for making corn N recommendations when the soil N test is considered.

  1. Determine N recommendation using Table 1 (Table 1 in FO-3790, Fertilizing Corn in Minnesota ) using yield goal, previous crop, and organic matter content for the specific field. The prescribed rate recommendation assumes that best management practices will be followed for the specific conditions.
  2. Determine whether conditions are such that residual N may be appreciable. Figure 4, which includes factors such as previous crop, manure history, and previous fall rainfall can provide insight as to the applicability of testing for N. If conditions are such that the probability of residual N is small and soil testing for N is not recommended, use the N recommendation derived in Step 1.
  3. If conditions suggest that a soil N test is warranted, collect a preplant, 0-2 ft. soil sample–taking enough soil cores from a field so that the sample is representative of the entire field. The sample should be sent to a laboratory and analyzed for nitrate-N.
  4. Determine residual N credit based on the measured soil nitrate-N concentrations. Use Figure 3 to determine this credit.
  5. Calculate the final N recommendation by subtracting the residual N credit (Step 4) from the previously determined N recommendation (Step 1). This fertilizer N amount can then be applied either preplant and/or as a sidedress application.

This soil N test should not be used when commercial fertilizer was applied in the previous fall. The variability in the degree of N conversion to nitrate-N before the spring make this test meaningless in these situations.

Manure Additions and Soil N Testing

When a soil N test is not used, manure N should be credited by calculating the amount of N that will be available (manure rate x manure N content x N availability) and subtracting this amount from the N recommendation determined based on yield, organic matter and previous crop.

Manure additions to soil can significantly increase soil nitrate-N concentrations; however, the amount of nitrate-N at any given time (i.e. spring preplant) is a function of the time of manure application and method of application. The following scenarios may help you understand how to most effectively use the new soil N test in combination with manure management to determine N recommendations.

If manure is to be applied in the spring before corn planting, collect the soil samples for N measurement before manure application. Determine the N recommendation and then separately calculate the manure N credit and subtract this amount from the previous N recommendation derived from the use of the soil N test.

If manure was applied during the winter months, it is assumed that the majority of the inorganic N would be lost via volatilization. Thus, the soil N test can be used to determine an N recommendation. However, the manure N credit (although less than from spring-applied manure) should be subtracted from this amount. Keep in mind that the spring sampling can occur anytime after the frost is out of the ground.

If manure was applied between October 1 and December 1 the previous fall, both the soil N test and the manure N worksheet credit system cannot be used. Not taking a soil N test, and just using the standard manure N crediting system, may result in high fertilizer recommendations if significant residual N was present before the manure was applied. In contrast, using only the spring soil N test to credit fall-applied manure will underestimate the credit due to the manure because all of the available manure N will not have converted to nitrate-N by this time. It is recommended that where manure had been applied in this time frame and the previous crop was soybeans, the N recommendation should be made based on the standard manure crediting system. However, if corn was the previous crop, the N recommendation should be based on the soil N test, knowing that the soil test may underestimate the available N coming from the manure.

If manure was applied before October 1 the previous fall, the soil N test will probably pick up most of the N from manure that will be available during the growing season. The amount of N released from manure's organic N fraction after taking the preplant sample will be minor compared to the accuracy of the N recommendation provided by the soil testing option.

The test does an excellent job of accounting for residual N where manure had been applied eleven months or more before the preplant soil N test samples are collected. This test should work particularly well in those situations with a long history of manure applications.


The preplant, 0-2-ft soil nitrate-N test can be used to provide a soil N credit that will improve N recommendations for corn. Current University of Minnesota N guidelines provide the initial N recommendation. The soil nitrate test will then be used to determine how much, if any, soil N credit should be subtracted from the initial recommendation.

A soil N test to estimate available residual N is not necessary for all acres to be planted to corn in Minnesota. Factors such as previous crop, soil texture, manure history, and previous rainfall should be used to determine which fields are most likely to benefit from the N test.

A five-step process is used to determine N recommendations.

  1. Determine N recommendation using yield goal, previous crop, and organic matter content.
  2. Determine whether conditions are such that residual N may be appreciable.
  3. If soil N testing is recommended, collect a preplant, 0-2 ft. soil sample and send to a laboratory for nitrate-N analysis.
  4. Determine residual N credit based on the measured soil nitrate-N concentrations.
  5. Calculate the final N recommendation by subtracting the residual N credit (Step 5) from the previously determined N recommendation (Step 1).

M.A. Schmitt and G.W. Rehm are Extension Soil Scientists with the Department of Soil, Water and Climate in St. Paul. G.W.Randall is a Soil Scientist at the Southern Experiment Station in Waseca.


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