Soil type, rainfall and temperature greatly affect the seasonal availability of N to plants. Nitrate leaching can occur at any soil temperature. The seasonal availability of nitrate N should be assessed each year and matched to crop needs. Soil nitrate testing is an excellent and inexpensive way of evaluating the available N status of your soil. Michigan State University research and demonstration studies have shown that many crop producers can reduce their N fertilizer application rate on corn without risk of reducing yields if they use the soil nitrate test.
Nitrate testing can also help to prevent overuse of N fertilizers.
If samples are taken in June, much of the ammonium and some of the organic N will have been converted to nitrate and will show up in the test. Although testing in early spring may still be helpful in assessing how much additional N is needed, samples taken just prior to sidedress time provide the greatest advantage in determining the appropriate rate of sidedress N. Manured fields and legume fields will likely contain the most nitrate. Early sampling of these fields will not result in the maximum N credit because ammonium N and easily decomposed organic N will not yet have beer converted to nitrate and will not be measured by the test. Therefore, only the PANT is recommended for these fields. Sandy soils, even though heavily fertilized the previous year, may not show much N carryover because nitrate N can be easily lost by leaching.