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Scientist reviews value of soil profile nitrogen testing

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, a Kansas State University agronomist said.

Using a profile nitrogen test, taken to a depth of 24 inches, to verify nitrogen credits can provide valuable information to producers, said Dorivar Ruiz Diaz, K-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," Ruiz Diaz said. "Plant available nitrogen can be present in the soil from fertilizer carryover, previous manure applications, or legume plowdowns."

This test can be especially useful in areas with relatively low rainfall and with reduced risk for nitrate losses by leaching or denitrification. In those situations, soil nitrate is likely to remain in place within the soil until it is taken up by plant roots.

After a crop failure due to drought conditions, producers may find that much of the nitrogen applied to that crop remains in the soil and is available for the subsequent crop, he added.

"Crop growth is normally extremely limited during a drought. As a result, the fertilizer nitrogen applied to that crop, as well as mineralized soil nitrogen, is typically not fully utilized. This carryover nitrogen would be available for the next crop and in some cases, fertilizer nitrogen needs can be significantly reduced," Ruiz Diaz said.

Proper soil sampling and testing is very important for a good assessment of residual soil nitrate, he said. Annual sampling of each field is necessary for accurate residual nitrogen estimations.

"When sampling, the key to good soil test results 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 may want to reduce the area represented by each sample. Large fields should be broken into sampling units based on crop, yield, and fertilizer histories," he said.

When taking samples for nitrate analysis, late fall or early spring is a good time to sample for the summer crops, and before planting for winter wheat, the agronomist said.

"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.

This will give producers a good reading on how much nitrogen remains from the previous crop, before mineralization begins to increase nitrate levels," he concluded.

More information is available in the K-State publication MF-2586, "Soil Test Interpretations and Fertilizer Recommendations," at www.oznet.ksu.edu/library/crpsl2/mf2586.pdf.

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