The typical spring soil nitrate tests do not account for nitrate loss or gain between soil sampling and planting, notes Kevin Bronson, associate professor of soil fertility and nutrient management with the Texas Agricultural Experiment Station. His study suggests in-season monitoring may lead to more accurate nitrogen fertilizer recommendations. Bronson led an interdisciplinary team of scientists to test in-season nitrogen using monitoring tools at two irrigated west Texas cotton sites in 2000 and 2001.
Based on spectroradiometer and chlorophyll meter readings, the team applied 30 pounds per acre of nitrogen in-season to their plots when indicated at early squaring, early bloom and peak bloom.
In 2000, they applied 30 to 90 pounds per acre less nitrogen than a soil-test recommendation of 120 lbs. per acre and achieved yields similar to plants receiving 120 pounds per acre. In 2001, cotton yields reached their goal of 2.5 bales per acre.
"In-season nitrogen sensing won't replace spring soil nitrate tests," Bronson says. "It can, however, reduce in-season nitrogen fertilizer rates in low-yielding seasons and it can help match soil test recommendations for yield potential in high-yielding seasons. We still recommend pre-plant soil testing to accurately gauge early season nitrogen needs when plants are too small to use monitoring instruments, and for determining nutrient needs besides nitrogen."
In addition to helping producers save dollars by reducing nitrogen applications, these monitoring tools can help protect ground and surface waters from nitrate contamination by reducing the amount of residual nitrate nitrogen in the soil at harvest, Bronson adds
Tess Chua of Texas A&M is the senior author on the paper. Co-authors include Jon Booker, Wayne Keeling, Jim Bordovsky and Robert Lascano, Texas A&M University; Cary Green and Eduardo Segarra, Texas Tech University; and Arvin Mosier, USDA-ARS.
Soil Science Society of America Journal (SSSAJ), http://soil.
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