A Summary of University Presidedress Nitrogen Test (PSNT) for Corn

The PSNT is a soil test for nitrate-nitrogen (NO₃-N) developed for use at the 4 to 6 leaf stage of corn to help in making more accurate N fertilizer recommendations at sidedressing time. The test, designed by Dr. Fred Magdoff at the University of Vermont, was initially developed to help in estimating the amount of N available where manure or other organic wastes have been applied, or corn is grown in rotation with a forage legume. The PSNT was initially developed to identify fields that would not be expected to respond to additional N. The test uses the NO₃-N content of the top foot of soil as an estimate of the amount of N available to the crop. Research in a number of states has confirmed that this test can be useful in managing N on corn. Work in Massachusetts, California and Florida has also shown the PSNT to be useful for some vegetable crops. By far, the major use of PSNT is in the production of field corn, which is the focus of this paper.

As the PSNT has been adopted by agriculture, there has been a great demand to make calibrated N recommendations based on the test results. In other words, agronomists have been asked to make N recommendations that are adjusted for, or inversely proportional to the amount of residual NO₃-N found in the soil. Given that the original purpose of the PSNT was simply to identify soils that were non-responsive to N, we may be asking a lot of this test by using it to accurately calibrate soil N status levels or N recommendations. However, some progress in this area has occurred. There is agreement on the general range of soil NO₃-N that is considered critical for adequate corn growth without additional N applications. This range is from 21 to 30 ppm NO₃-N in the top 12 inches of the soil when the corn is in the 4 to 6 leaf stage. Many Universities consider the critical NO₃-N value to be 25 ppm.

There is less agreement on how much supplemental N to recommend when the PSNT result is between zero and the critical level. In other words, how do we calibrate N recommendations to PSNT results? Most of us understand that agronomists often disagree on fertilizer recommendations, and the PSNT is no exception. There is some research that supports higher N recommendations when the PSNT result is low and lower recommendations when the result is closer to the critical level. However, many factors of weather, soil conditions, corn hybrid, crop management, and others may change crop response to a particular PSNT level. It is not within the scope of this paper to thoroughly discuss all of the factors that can influence the reliability of N recommendations based on PSNT results. However, like all soil testing, the PSNT should be used as a guide, not a guarantee. Neither the PSNT result nor the N recommendation should be over-simplified to a simple recipe. The PSNT is simply one part of the complicated process of crop management.

Another occasional point of confusion occurs when we try to reconcile the ppm of NO₃-N with the amount of N required by a corn crop, or the amount of N applied prior to taking the PSNT sample. The PSNT result is an “index” of available N, not a calculation factor. You should not try to perform calculations to reconcile the amount of N applied or crop uptake with PSNT results, because they will not likely “add up”. We must keep in mind that a crop typically utilizes N from below the 12 inch sampling depth; it will receive N from mineralizing soil organic matter; it may lose access to N due to denitrification or leaching later in the season; or other factors may change the N available to the crop, either before or after the PSNT is taken.

To use this test effectively, soil samples must be collected when corn is in the 4 to 6 leaf stage, or 6 to 12 inches tall. The cores should be collected to a depth of 12 inches.

In situations where little or no N has been applied to the field prior to taking the PSNT sample, you should take at least 10, and preferably 15 cores per sample. Authorities suggest that where manure or fertilizer has been broadcast, you should take a composite soil sample of 20-25 soil cores at random throughout the sampling area. The large number of cores is important due to the variability in N content of manure, and non-uniformity in spreading. If manure or fertilizer has been banded, 10 to 15 sets of cores, each set consisting of three cores, one 3 inches to the right of the corn row, one 15 inches from the row (row middle for 30 inch row corn) and one 3 inches to the left of the next corn row, should be collected for the composite sample. In all situations, you should place the cores in a bucket, thoroughly mix the cores, and then place a sub-sample of the soil into the soil bag for shipment.

Over the years, a number of customers have reminded us that sometimes you cannot push a sampling probe 12 inches deep into the soil. What do we do in this case? If a gravel or fractured shale layer is the problem, we would expect that N and roots will penetrate it, and the crop will have access to this deeper N. In this case, a shallow sample could underestimate the N available to the crop. If the problem is a dense clay layer, we would expect that the N might “perch” on the clay layer. In this case, a shallow sample might represent the N available to the crop. However, a soil with a dense clay layer less than 12 inches deep would likely have other significant problems such as being easily saturated, leading to potential losses from denitrification. These are only a couple of examples illustrating the difficulty in predicting N availability from shallow sampling. Unfortunately, there is no way to make adjustments or corrections for a shallow sample.

Most, if not all, University research and recommendations are based on the nitrate (NO₃-N) found by the test. They have found that the soil ammonium (NH₄-N) level is not well correlated to crop N response. However, our own research showed that when a nitrification inhibitor is used, the NH₄-N can be quite high at sampling time. In these cases, an N recommendation based only on the NO₃-N level would be much higher than needed. Therefore, we often suggest that you request both the NO₃-N and NH₄-N tests, especially where nitrification inhibitors are used.

The following are examples of how some Universities use the PSNT.

!lock

Critical PSNT NO₃-N level is 30 ppm for Northern Alabama soils. No calibrated recommendations are available.

Calculate the sidedress nitrogen recommendation (SDNR) using the equation:

SDNR (Lbs N/acre) = BNR - SNAP - MC

1. Select the Basic N Rate (BNR)

2. Determine if starter N was applied. If yes, SNAP = starter N applied (lbs N/ac). If no starter is used, SNAP = 0.

3. Determine if a manure credit (MC) is needed. If less than 3 tons/acre of manure was applied, no credit is needed and MC=0. If 3 or more tons/ac was applied, use the PSNT value to select the appropriate adjustment from Table 2. MC = adjustment x manure application rate.

Table 2 Credits for Manure Applications ≥ 3 tons/acre

Source: ST-01 Advances in Nitrogen Management for Corn in Delaware, University of Delaware

Illinois is not supporting the use of the PSNT. However, they state in a recent newsletter that “If the values are high–greater than 25 ppm–then the odds are good that no additional N will be needed for the 2002 crop”.

Source: AY-314-W, The Presidedress Soil Nitrate Test for Improving N Management in Corn, Purdue University

Soybean-Corn and Corn-Corn Rotations:

Recommended N rate (lb. N/A) = (Critical N conc.¹ - NO₃-N ppm) x 8

Manured² Soils and Alfalfa-Corn:

Grain & Fertilizer Prices: Unfavorable (where 1 bu corn buys 7 lbs N)

Grain & Fertilizer Prices: Favorable (where 1 bu corn buys 15 lbs N)

Source: Pm-1714, Nitrogen Fertilizer Recommendations for Corn in Iowa, Iowa State University

¹ A critical concentration of 25 ppm NO₃-N is appropriate in absence of additional information. Reduce the critical concentration by 3 to 5 ppm if rainfall is more than 20% above normal between April 1 and time of soil sampling.

² 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 2 of the past 4 years.

³ Rainfall should be considered excess if rainfall in May exceeded 5 inches.

⁴ Addition of 30 lb. N/A may have no detectable effects on profits, but producers could reasonably elect to apply this rate.

If PSNT concentrations are 15-21 ppm, then apply 0-40 lbs N/acre sidedress.

If PSNT concentrations are <15 ppm and alfalfa has been grown on this soil within the last 6 months, then apply 0-40 lbs N/acre sidedress.

If PSNT concentrations are <15 ppm and alfalfa has not been grown on the soil within 6 months, then apply 40-80 lbs N/acre sidedress.

Source: SFM-2 “Making Decisions for Nitrogen Management of Corn Using the Pre-Sidedress Soil Nitrate Test (PSNT)” April 2010.

Sidedress N (lb. N/A) = Standard N Rate - (NO₃-N x 6)

Source: “Michigan's Soil Nitrate Test for Corn”, FCAT Alert, May 14, 2009.

Nebraska uses Iowa data and recommendations.

Sidedress N Recommendations Based on the PSNT Soil Test Level

When PSNT levels are greater than 25 ppm there is no response to additional N and none is recommended.

* When 100 lb. or more of sidedress N are recommended on very light sandy soils, apply half of the sidedress when the corn is 12 inches tall and half when the corn is 18 to 24 inches tall.

Source: FS174 Presidedress Soil Nitrate Test (PSNT) Recommendations for Field Corn, Rutgers Cooperative Extension.

While the Ohio State University does not use the PSNT in making routine N recommendations, it does provide some guidance to using the PSNT.

Source: C.O.R.N. Newsletter 2007-04

Source: EM 8978, Silage Corn Nutrient Management Guide, Oregon State University

N Recommendation¹ (lb./A) = (Expected Yield x 1.1) - (History Factor² x NO₃-N ppm)

¹ If the soil nitrate level is >20 ppm NO₃-N, the N recommendation is 0.

Source: Agronomy Facts 17, Pre-sidedress Soil Nitrate Test for Corn, Pennsylvania State University

Source: SERA-IEG-682, Use of Soil Tests for Nitrate-Nitrogen in the South.

*If previous crop was a well-managed stand of grass, legumes, or mixed forage, subtract 30 lb N/acre from above N rates.

Source: http://pss.uvm.edu/vtcrops/articles/VT_Nutrient_Rec_Field_Crops_1390.pdf University of Vermont

“The PSNT is primarily designed to be conducted on soils that have received no more N than a starter N fertilizer application (25-30 lbs/Acre); however, the test has been shown to be accurate in some situations where high rates of pre-plant fertilizer N have been applied. Fields that have received manure can and should be tested prior to making any supplemental N fertilizer applications at sidedress time.”

“Nitrogen recommendations for corn should be calculated from the Virginia Agronomic Land Use Evaluation System (VALUES) (Simpson, et al. 1993) with adjustments from the results of the PSNT as outlined below. These recommendations should not substitute for common sense and an understanding of the effects of soil properties and management practices on N availability to corn.”

Source: Pub 418-016, Nitrogen Soil Testing for Corn in Virginia, Virginia Poly-Tech

See Oregon

¹ To determine a soil's yield potential, consult the UWEX publication A2809, Soil Test Recommendations for Field, Vegetable and Fruit Crops.

² No adjustment made to corn N recommendations.

Source: UWEX A3630, Wisconsin's Pre-sidedress Soil Nitrate Test, University of Wisconsin

Given the different approaches to interpreting PSNT results and making N recommendations, it can be difficult to understand or compare the effects of these differences on recommendations. The following table contains a comparison of recommendations based on the information available from the previously listed Universities and our best attempt to represent that information. The historical information that is included was required by one or more of the methods, but not necessarily all. In some cases, the yield goals of 150 and 175 bu./acre were either not a factor in recommendations, or were higher than those included in the particular system. Some corn producing areas of the Great Plains were omitted because they use a 2 ft. deep sample for soil N testing.