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Planning For A Pre-Sidedress Soil Nitrate Test

By George Silva, Michigan State University Extension
Some basic requirements for corn growers to achieve the desired results from a pre-sidedress soil nitrate test (PSNT).
Studies have shown that corn producers could reduce their nitrogen (N) fertilizer rates utilizing the pre-sidedress soil nitrate test (PSNT) in the proper situations. The PSNT season is upon us – late May to early June. Some preparation work and planning are necessary to identify the proper fields and achieve the desired results. Michigan State University Extension has some basics to remember.
  • This test was developed for fields with high N mineralization potential. For example, fields with previous manure or alfalfa/clover backgrounds. Some of these fields may be capable of providing over 100 pounds of available N per acre. This test is not economical for fields that do not meet this requirement. The test measures both residual nitrate N from previous year (if any) and recently mineralized N from organic matter.
  • No broadcast or incorporated pre-plant N fertilizer should be applied on the PSNT fields. A modest amount of starter N up to 40 pounds per acre could be band applied near the seed.
  • The greatest amount of available soil nitrate usually occurs once the soil has warmed up and about three to four weeks after corn emergence. This is the most appropriate time to take soil samples, when corn is about 6-12 inches tall and V4-V6 growth stage. Since N release from mineralization is an ongoing process, collecting PSNT soil sample as close as possible to N sidedress time gives the best information about the appropriate N credit. It may take five to seven days to process the samples.
  • 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.
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. The turnaround time is 48 hours with a fee of $10 a sample. The results will be emailed or faxed. Many private soil testing laboratories offer the same test.
The test will report the nitrate concentration in parts per million (ppm). The critical level is 25 ppm. If the nitrate concentration is above 25 ppm, no sidedress N fertilizer is needed. When the concentration is below 25 ppm, the sidedress N fertilizer rate is adjusted accordingly.

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