University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

June 15, 2006

Using soil tests to determine manure availability

One of the unknowns about manure use as a fertilizer is how quickly nitrogen is released from the organic material in manure. This nitrogen is not found in fall, winter, or early spring soil tests.

Soil temperature, moisture and aeration influence how fast nitrogen is released from manure. Usually most nitrogen is released from manure in late spring when conditions favor release. Given the variability around the state, mineralized nitrogen release predictions can not be generalized for the entire state. For those who found high levels of nitrates in fall and winter soil samples, this is a way to verify that nitrogen levels are still above normal.

Nitrogen fertilizer recommendations for manured soils* and corn after alfalfa. (From ISU Cooperative Extension publication, Nitrogen Fertilizer Recommendations for Corn in Iowa).

Grain and
fertilizer prices

Soil test nitrate
Recommended N rate
Excess** Rainfall
Normal Rainfall
 
(ppm N)
(lb. N/acre)
Unfavorable
0-10
90
90
(1 bu buys 7 lbs. of N)
11-15
0
60
16-20
0
0***
 
> 20
0
0
Favorable
0-10
90
90
(1 bu buys 11-15 60 60 15 lbs. of N)
11-15
60
60
16-25
0
30
 
> 25
0
0

* A field should be considered manured if animal manures were applied with a reasonable degree of uniformity since harvest of the previous crop or in two of the last four years.
** Rainfall should be considered excess if rainfall in May exceeded five inches [in Iowa].
*** Adding 30 lbs. N/acre may have no detectable effects on profits, but producers could reasonably elect to apply this rate.

The Pre-Sidedress Nitrate Test (PSNT) for corn was developed to test soil for nitrate nitrogen that becomes available in late April and May. The University of Nebraska has not yet published specific recommendations for the PSNT, but guidelines developed by Iowa State University may be used. Their guidelines are available in Nitrogen Fertilizer Recommendations for Corn in Iowa, ISU Cooperative Extension publication Pm-1714.

Sample corn when it is 6 to 12 inches tall or in June. Sample from field areas that are similar and 10 to 20 acres in size. Sample from the surface to one foot deep. Have the sample tested for nitrates. The results will be reported in ppm (parts per million). This test works best if previous fertilizer application bands are avoided. Try to avoid starter and anhydrous ammonia bands. Take 20 cores per sample.

Interpreting the results -- the Iowa method

Iowa has the most specific recommendations. They are based on the price of corn and nitrogen, spring rain, and the PSNT results. When corn is about $2.00/bu and nitrogen is over $0.25/lb, use the upper section of the table. This table should be used only for fields with manure or alfalfa history.

For fields in continuous corn or corn following soybeans: ISU recommends subtracting the soil nitrates (ppm) from 25 and multiplying the difference by 8. For example: with a soil test of 18 ppm nitrate-N the nitrogen recommendation would be: 25-18=7 x 8= 56 lbs N/acre.

Recommendations

Sample corn fields with a high likelihood of having high nitrate concentrations such as: manured fields and those with alfalfa last year. If soil tests come back over 23 ppm nitrates, there is probably no need for additional nitrogen. The NU corn nitrogen recommendation formula was developed for preplant soil samples to 24 or more inches. It is not intended to be used with the PSNT numbers in the table below. However, the formula does give eight pounds of nitrogen credit for each ppm nitrate. If previous fertilizer bands were avoided in the sampling, and more nitrogen is recommended, then reduce the additional nitrogen needed by the amount of the band application. Broadcast nitrogen applications will become part of the soil nitrates; so do not subtract broadcast applications from the predicted nitrogen needs.

Charles Shapiro
Extension Soils Specialist

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Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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