171UTILIZING THE LATE SPRING (PSNT)SOIL TEST TO OPTIMIZENITROGEN MANAGEMENT FOR CORNReasons for Using the Soil TestNitrogen fertilization is essential for profitable corn production, but it also is a major cost of production and cancontribute to degradation of the environment.Who Should Use the TestProducers who do not sidedress can use this test in fields already fertilized to obtain feedback on their nitrogenmanagement.Producers who normally sidedress can use the soil test to adjust nitrogen rates for all factors affecting soil nitrateconcentrations in late spring.The use of this test requires higher levels of management, but the additional management usually yields significanteconomic and environmental benefits.Time of SamplingCollect soil samples when corn plants are 6 to 12 inches tall (measured from the ground surface to the center of thewhorl). The test is called the "late-spring" soil test or Pre-Sidedress Nitrate Test (PSNT) to emphasize that the time ofsampling makes this test different than other soil nitrate tests.Depth of SamplingSamples collected for the late-spring soil test must be representative of the top foot of soil. Samples taken at adifferent depth or timing could alter the interpretation of the data.Methods of Sample CollectionSoil samples analyzed for this test should be derived from at least 16 to 24 cores collected within an area notexceeding 10 acres. Areas with different soil types or management histories should be sampled separately.Care should be taken to ensure that the soil samples are collected in a manner that is not biased by the presence ofcorn rows or bands of fertilizer. At least 24 cores should be collected in fields having more than 50 poundsnitrogen/acre applied as anhydrous ammonia.Sampling in patterns relative to the corn row can help improve the accuracy and eliminate potential effects from bandapplications. Move to the first sampling location and take a core 3 inches to the left. The cores from the secondlocation should be taken 6 inches to the left. Repeat this process moving an additional 3 inches each time until theadjacent row is reached.Handling and Shipping Soil SamplesProtect moist soil samples from temperatures above 75˚F and refrigerate the samples if they cannot be analyzed withintwo days. Mailing usually poses no problem if the samples spend no more than two days without refrigeration.Nitrogen Applications Before or Near Planting
171If sidedressing is not planned, the producers normal rate should be applied unless previous use of the test hasdemonstrated need for change.If sidedressing is planned, apply only modest amounts of nitrogen before or at planting. This means no more than 70percent of the total amount normally applied in a year. At least 30 to 50 percent of the amount normally applied isrecommended for unmanured fields in continuous corn. See attached table on page 3.If sidedressing is possible, no more than a normal starter rate of nitrogen should be applied to manured soils, first-yearcorn after alfalfa, or second-year corn after alfalfa. These soils often need little or no additional nitrogen to maximizeyields, so application of high rates of nitrogen before or at planting reduces the potential benefits of using the test.Producers having fields that frequently receive animal manure are strongly encouraged to apply fertilizer nitrogenonly as a sidedressing or a topdressing after testing the field in late spring.Where Caution is RequiredThe soil test may underestimate amounts of plant available N when (1) nitrification inhibitors or urease inhibitors areapplied with fertilizers, (2) more than 150 lb. N/acre are applied as anhydrous ammonia, and (3) more than 150 lb.N/acre are applied as injected manure.Midwest Laboratories can provide you with quick, accurate results for these late spring tests. Results can beemailed, faxed or phoned back within 24-48 hours upon receipt of the samples. The fee for this service is $5.50without recommendations and $6.00 with recommendations.PSNT Iowa State University
171manured or corn on alflalfa corn on corn or corn on beansexcess precip normal precip average yield high yieldNitrate ppm N lbs/ac N lbs/ac N lbs/ac N lbs/ac1 90 90 192 2322 90 90 184 2243 90 90 176 2164 90 90 168 2085 90 90 160 2006 90 90 152 1927 90 90 144 1848 90 90 136 1769 90 90 128 16810 90 90 120 16011 60 60 112 15212 60 60 104 14413 60 60 96 13614 60 60 88 12815 60 60 80 12016 0 30 72 11217 0 30 64 10418 0 30 56 9619 0 30 48 8820 0 30 40 8021 0 30 32 7222 0 30 24 6423 0 30 16 5624 0 30 8 4825 0 30 0 4026 0 0 0 3227 0 0 0 2428 0 0 0 1629 0 0 0 830 0 0 0 0Higher than normal rainfall can reduce the recommended rate by 20-40 lbs/acThere may need to be some adjustment to the N recommendations for cornons whenhigh than average yield goals are desired on either manured ground orcorn following alfalfa.