Michigan State University Extension provides these recommendations for season-long nitrate testing:

Early spring. These soil samples primarily measure residual nitrate; the amount of N credit will be smaller. Soil nitrate content increases as soils warm. Testing for ammonium where manure has been applied provides a preliminary indication of available N release.

Spring. The greatest amount of soil nitrate usually is available three to four weeks after corn emergence (V6-V8 stage). Sidedress N when corn begins to take up N rapidly. Samples taken before sidedress can determine the appropriate N rate and measure both residual nitrate from the previous year and recently mineralized N from ammonium and organic matter.

A soil nitrate test provides the best information about available soil N when only small amounts of N have been broadcast preplant. When large amounts have been knifed in or broadcast and incorporated prior to planting, getting a good indication of available N is difficult.

Early summer. Samples taken in June from fields where N was broadcast before planting can guide N additions through irrigation or for planning next year’s application.

Fall. Samples taken in the fall help to evaluate how much N is left at the end of the season. Farmers with excess soil N in June or at harvest should consider reducing the next year’s fertilizer rate or using a PSNT to determine the appropriate rate.