CHAMPAIGN, Ill. -- Studies on nitrogen fertilizer use have harvested two surprising finds:
-- Seed-corn growers in Illinois who use irrigated sandy soils, an increasingly common practice following the devastating drought of 1988, can increase their profits substantially by cutting back on their use of nitrogen. Such a reduction also protects the environment from nitrogen not used by the crops.
-- Nitrogen is accumulating in microorganisms in the soil -- not in the soil itself -- in fields where the chemical has been applied at higher-than-recommended rates, leading to corn crops failing to respond to nitrogen fertilizer when the microorganisms die and release nitrogen back into the soil.
The findings were released Nov. 3 in two presentations by W. Bart Stevens, a University of Illinois doctoral student in agronomy, in Indianapolis at the 1996 annual meetings of the American Society of Agronomy, Crop Science Society of America and Soil Science Society of America.
The studies were designed to re-evaluate nitrogen levels necessary for obtaining optimum growth of commercial corn and seed corn. Farmers often have used the same rates for both crops.
Two years of research on sandy soils in Whiteside County, near Prophetstown in northwest Illinois, found that seed corn reached maximum yields when 70-100 pounds of nitrogen were applied per acre rather than previously recommended 200 pounds per acre, Stevens said.
"This study showed us that you don't need as much as many producers have been using," he said. "If you do an overkill on a sandy soil, the most likely route of loss of the unused nitrogen is right into somebody's water table."
Cutting back on nitrogen use could mean a fertilizer savings of about $20 per acre and increased yields, said Robert G. Hoeft, a U. of I. professor of agronomy and faculty adviser on the research. Even lower recommendations on nitrogen use on sandy soils may be forthcoming, Hoeft added.
In the other study, done in three seasons near Monmouth in western Illinois, Stevens found that over-application of nitrogen fertilizer can lead to an accumulation of organic nitrogen in the bodies of soil microorganisms. Hoeft previously had found that commercial corn grown on 13 of 75 Illinois farms did not respond to nitrogen but still yielded 150-180 bushels per acre without it. Subsequent tests for accumulated nitrogen gave no answers, but they found high levels of phosphorous, indicating high fertilizer use in the past.
Based on these findings, researchers applied and tracked molecularly labeled nitrogen to the Monmouth farm, which had a history of nitrogen use in excess of recommended rates. After-harvest soil tests found that 40 percent of it went into plants while 40 percent had gone into an organic form.
The findings suggest farmers who have over-applied nitrogen may be able to effectively use up the surplus if they cut back, but, Hoeft said, soil scientists don't have a test yet for determining where these over-fertilized fields are located, and how long the accumulation lasts in the microorganisms.