By George Silva and Eric Anderson
The pre-sidedress soil nitrate test (PSNT) for corn was developed by Fred Magdoff in Vermont in 1984. When done correctly, the PSNT is an excellent tool to determine available nitrate-N from organic matter mineralization. It will enable farmers to reduce their N fertilizer rates without risking yields.
Identifying the right fields based on the field’s history and some preparation work are critical to the success of the PSNT. The following are some points to remember when taking a PSNT.
In Michigan, the test needs to be conducted on fields with high N mineralization potential, such as fields having a history of manure or forage legumes like alfalfa and clover. The PSNT was developed for these kinds of situations. It is an index of N mineralization potential and its best use is to identify corn fields that will not respond to N fertilizer.
Michigan State University Extension recommends fields having repeated applications of livestock manure, biosolids or recent forage legume crops as good choices for PSNT. Other fields that may show high soil nitrate-N include medium- to fine-textured soils that have been heavily fertilized in the previous year. Fields that do not have these backgrounds will not show sufficient N credit to justify the time and cost of doing the PSNT.
Once the fields are selected, no broadcast, incorporated pre-plant or plant N fertilizer should be applied. A modest amount of starter N up to 40 pounds per acre could be band-applied near the seed. This band-applied N should not interfere with the soil samples that are taken right between the corn rows. If N fertilizers were broadcast before the test, the PSNT soil samples may reveal hot spots and the test results may not be reliable.
Whenever possible, the PSNT soil samples are best taken shortly before sidedressing when corn is between 8-12 inches tall. Soil samples taken earlier will not give a full measure of N mineralization; 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 15-20 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 wet soil in plastic bags.
The test will measure the nitrate-N concentration in parts per million (ppm). The critical level is 25 ppm and above, which no N fertilizer is recommended for corn. When the concentration is below 25 ppm, the N fertilizer recommendation is adjusted accordingly. To determine the N credit, refer to “Michigan’s Soil Nitrate Test for Corn” by MSU Soil and Plant Nutrient Laboratory.
Dry soil samples should be sent for testing as soon as possible to a soil lab, such as the MSU Soil and Plant Nutrient Laboratory, 1066 Bogue St. Room A81, East Lansing, MI 48824-1325. It takes a few days to collect and get the samples analyzed. You can contact the lab at 517-355-0218. Some county MSU Extension offices may have arranged some dates to deliver samples to MSU, so contact your local MSU Extension office. The turnaround time is 48 hours and the fee is $10 a sample. The results will be emailed or faxed. Some private soil test labs in your area may be offering the same test.