Just as wheat growers take a breather from this year's wheat harvest, the time is drawing near for them to take soil samples from fields that will be planted to wheat this fall.
In addition to a six-inch sample for a routine test for phosphorus, potassium, organic matter, and pH, it would be a good idea to take a 24-inch sample for a profile nitrogen test, said Dale Leikam, K-State Research and Extension nutrient management specialist.
"In eastern and central Kansas, many fields had very low wheat yields this year. These fields could have significant amounts of residual nitrate-N remaining in the soil, which would reduce the rates of N fertilizer needed for next year's wheat crop," he explained.
But don't assume residual soil nitrogen levels are high, Leikam said. "Nitrogen taken up by the wheat earlier this spring and remaining in plant residues this summer will be in an organic form -- and unavailable for plant uptake until mineralized by soil microbes."
Also, with the unusually wet weather this spring and summer, part of that unused soil nitrogen may have been lost to denitrification on medium- to fine-textured soils, or leaching on sandy soils, he said.
Weed growth is another factor.
"Broadleaf and grassy weeds require significant amounts of nitrogen, just like crops. Where fields are dense with weeds and grasses, nitrogen uptake by weeds will be high and residual soil nitrogen levels may be very low," the agronomist said. "Nitrogen utilized by weeds will be in an organic form, and unavailable for plant uptake until mineralized by soil microbes.
"In western Kansas, where wheat yields were unusually good this year, soil nitrogen levels are likely depleted more than normal," he added. In those cases, lower residual soil nitrogen will result in higher-than-normal required rates of fertilizer nitrogen for the wheat crop this fall.
The only way to know for sure is to take a profile-nitrogen soil test, Leikam said.
"This should be done about two to four weeks prior to wheat
planting," he advised. "If fields have a dense stand of weeds, producers should make sure the weeds have been controlled before sampling the soils. As long as weeds are still alive and growing, they will continue to take up nitrogen."
Source: Kansas State Research and Extension release