Prep work for PSNT

Some prep work necessary to achieve the desired results from the Pre-sidedress Soil Nitrate Test (PSNT) for corn are highlighted.

Soil nitrate testing is an excellent way to determine the available nitrogen (N) status of the soil. Studies have shown that farmers could reduce their N fertilizer application rate on corn without risking yields if they used the soil nitrate test. Pre-sidedress Soil Nitrate Test (PSNT) season will be upon us soon (June). After identifying fields for PSNT, some prep work and planning are necessary to achieve the desired results. Here are some points to remember:

  • The test is ideally suited for fields with high N mineralization potential, for example, fields with high organic matter, manure or legumes. Other fields that will have high nitrate levels are loam, clay loam and clay soils that have been heavily fertilized in previous years. The test measures both residual nitrate N from previous year and recently mineralized N from organic matter.
  • No broadcast or incorporated pre-plant N fertilizer should be applied. A modest amount of starter N up to 40 lb./A could be band applied near the seed.
  • The greatest amount of available soil nitrate usually occurs in the soil once the soil has warmed up and about three to four weeks after corn emergence (V6-V8 growth stage). At this stage, corn begins to take up N quite rapidly. This is the most appropriate time to apply sidedress N. Since N release from mineralization is an ongoing process, collecting PSNT soil sample as close to sidedress date gives the best information about the appropriate N credit. It takes a few days to collect the samples and get them analyzed.
  • Soil samples taken earlier will not give a full measure of mineralizable N; therefore, the amount of N credit will be smaller.
  • Soil cores should be taken midway between the corn rows, avoiding the starter fertilizer band. The sampling depth is 12 inches. Each sample should be a composite of 15 to 20 soil cores and represent no more than 20 acres.
  • Air dry the sample in paper bags near a fan or heated air vent. Do not place soil in plastic bags.
  • The test will measure the nitrate concentration in parts per million (ppm) in the sample and use a multiplication factor to estimate the N available to a depth of 24 inches. The critical level is 25 ppm above which no N fertilizer is recommended. When the concentration is below 25 ppm, the N fertilizer recommendation is adjusted accordingly. To determine the N credit, refer to MSU’s Michigan’s Soil Nitrate Test for Corn web site.
  • Dry soil samples should be sent for testing as soon as possible. The address is MSU Soil and Plant Nutrient Laboratory, East Lansing, MI 48824-1325. Contact number is 517-355-0218. Some county Extension offices may have arranged certain dates to deliver samples to MSU, so contact your local MSU Extension office. The turnaround time is 48 hours with a fee of $10 a sample. The results will be emailed or faxed. Many private soil test labs offer the same test.

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