Sometimes you have to dig deep to find hidden treasure. That´s the case when testing agricultural soils for levels of valuable plant-available nitrogen, according to a Kansas State University agronomist.   
A profile nitrogen test, taken to a depth of 24 inches, to verify nitrogen credits can provide farmers with valuable information, says Dorivar Ruiz Diaz, Kansas State research and Extension nutrient management specialist.
"Most producers are unaware of the amount of nitrogen that may be present in their soils from the previous season," he notes. "Plant-available nitrogen can be present in the soil from fertilizer carryover, previous manure applications or legume plowdowns."
The nitrogen test can be especially useful in areas with relatively low rainfall and with reduced risk for nitrate losses by leaching or de-nitrification. In those situations, soil nitrate is likely to remain in place within the soil until plant roots take it up.
After a drought-induced crop failure, farmers may find that much of the nitrogen applied to a that crop remains in the soil and is available for the subsequent crop, Ruiz Diaz adds.
"Crop growth is normally extremely limited during a drought. As a result, the fertilizer nitrogen applied to that crop and mineralized soil nitrogen, is not fully utilized. This carryover nitrogen would be available for the next crop and in some cases, you can significantly reduce fertilizer nitrogen needed," he says. 
Proper soil sampling and testing is critical for an accurate assessment of residual soil nitrate. You need to take annual samples of each field for accurate residual-nitrogen estimates. 

"The key to good soil test is using the proper protocol. Each sample should contain 15 to 20 cores of soil from a reasonably uniform area of approximately 40 acres. Producers who want more detailed information can reduce the area represented by each sample. Large fields should be broken into sampling units based on crop, yield, and fertilizer histories," Ruiz Dias says. 

Late fall or early spring is a good time to sample for summer crops; and before planting for winter wheat. "Nitrate levels will fluctuate through the year, depending on soil temperatures and soil mineralization rates. The best time to take the sample is during cool periods after the previous crop has been harvested but before the soil warms up too much the following spring," he says.

These tests will offer a solid reading on how much nitrogen remains from the previous crop, before mineralization begins to increase nitrate levels.
More information is available in the Kansas-State publication MF-2586, "Soil Test Interpretations and Fertilizer Recommendations."