Pre-sidedress soil nitrate test will save money on N fertilizer

Date:06-07-2012

Pre-sidedress soil nitrate test (PSNT) is an excellent tool to determine the available soil nitrogen (N) on fields that have a high N mineralization potential. These are fields that have a previous history of manure or alfalfa. Studies have shown that farmers could reduce their N fertilizer application rate on corn without risking yields if the test is properly carried out. With the price of N fertilizer rising, growers may be able to save an average of $50/A based on this test.

Some prep work is necessary to achieve the desired results from the PSNT. Here are some points to remember:
The test is ideally suited for fields with high N mineralization potential, i.e., 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 the previous year and recently mineralized N from organic matter.

No broadcast or incorporated preplant N fertilizer should be applied. A modest amount of starter N up to 40lbs/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 to 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. Drying the sample is very important. Do not place soil in plastic bags.

The test will measure the nitrate concentration in parts per million (ppm). 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.