June 13, 2003
AMES, Iowa — Iowa corn fields need spring rains to produce high yields. But a recent study shows those spring rains also wash away some of the nutrients needed to attain high yields. The study conducted by Iowa State University researchers is published in the May-June issue of the Journal of Environmental Quality.
The study included a 10-year survey of nitrate concentrations in cornfields managed by farmers. Nitrate is the form of nitrogen that corn plants need from the soil.
More than 3,200 soil and cornstalk samples were collected by researchers from fields planted to corn and already fertilized by farmers using their normal practices. The sampling was done in the Des Moines River and the Iowa River watersheds.
The participating farmers provided information on how much nitrogen they applied in fields where samples were taken, and whether it was in the form of commercial fertilizer or manure. Researchers studied monthly precipitation totals and data on nitrate levels in rivers during the same time period.
Nitrate concentrations in fields were measured in early June. Soil samples were collected when corn plants were six to 12 inches tall. Researchers concluded that more spring rainfall resulted in less nitrate for plant growth. And nitrate levels found in the fields were inversely related to nitrate levels in rivers.
"These findings provide evidence that spring rainfall moves nitrate from cornfields to rivers before crops grow," said Alfred Blackmer, ISU agronomist and leader of the research team. "This movement of nitrate hurts farmers economically, because they pay for the fertilizer even when it does not remain in fields long enough to feed the crop."
Blackmer said the problem can be reduced by delaying fertilization until late May or early June which is when the plants need nitrogen, and is after most spring rainfall has occurred. "Such delays would enable most farmers to reduce average rates of fertilization while increasing average yields and profits," he said.
The effects of rainfall were confirmed in a second part of the research that involved taking cornstalk samples one to three weeks after corn plants matured, generally in mid-September through mid-October. "Because this test is taken at the end of the growing season, it evaluates fertilization practices for their ability to supply optimal amounts of nitrogen for plant growth late in the season," Blackmer said.
Blackmer said the late-spring test for soil nitrate and the end-of-season test for cornstalk nitrate offer novel ways to help crop producers join in efforts to reduce nitrate losses from fields.
Antonio Mallarino, another ISU agronomist, also was involved in the research project. The work was funded in part by grants from the Leopold Center for Sustainable Agriculture, USDA Sustainable Agriculture Research and Education Program, U.S. Environmental Protection Agency, the Northwest Area Foundation and the Iowa Corn Promotion Board.
The Journal of Environmental Quality is published by the American Society of Agronomy, Crop Science Society of America and Soil Science Society of America.