Too Much Fertilizer Ineffective, Data From Historic Farm Plots Indicate

October 31, 1997

CHAMPAIGN, Ill. -- The nation's oldest crop-research plots are speaking out: Soil stewardship pays. Yields in fertilized continuous corn plots are one-third lower than in similarly fertilized plots where long rotations have been used.

The Morrow Plots, planted at the University of Illinois in 1876 to find out how exploitable Illinois soil is, have shrunk over the years, but their link with how Illinois land has been farmed remains strong.

"Despite the fact that soil organic matter has been decreasing, yields have increased," said Susanne Aref, an agricultural statistician in the U. of I. department of crop sciences. "During the past decade, yields have been level, though changes to stand density may boost yield to a higher level."

Yields have risen as a result of technological advances, such as the introduction of hybrids, pesticides and commercial fertilizers, but the use of fertilizers at 1 1/2 times recommended rates, as used by many farmers and as an experiment in the Morrow Plots, has not provided consistent benefits.

"Based on the last 30 years of experiments, we have learned that too much fertilizer does not work," Aref said. "It does increase yields in some years, but it makes production unstable." During the 1988 drought, for instance, crop yields in overfertilized plots were dramatically smaller than those from untreated soils, where corn yields were essentially unaffected.

Lessons learned from the Morrow Plots, which cover about 3/5 of an acre, were described by U. of I. soil scientist Michelle M. Wander at the joint annual meetings of the American Society of Agronomy, Crop Science Society of America and the Soil Science Society of America Oct. 26-31 in Anaheim, Calif.

A comprehensive review of soil changes in the Morrow Plots -- named for George E. Morrow, the first dean of the U. of I. College of Agriculture -- will be published next year in "Advances in Agronomy, Volume 62." The plots were recognized as a National Historic Landmark in 1968.

The first experiment began with three different crop rotations involving corn, oats and hay and three fertility treatments. As changes in agriculture occurred, so changed the experiments. The long-term data that have been harvested, researchers say, "provide us with the only reasonable empirical basis upon which we can evaluate agricultural sustainability."

The plots' history was divided into four phases, based on activity. Phase five begins next spring, when only the recommended rates of fertilizer will be used, plant density will increase from a rate of 24,000 plants per acre to 28,000, and chisel plowing will replace traditional mold-board plowing. The use of longer rotations in future years also is being considered, Aref said.

Corn-oats-hay rotation on manure-treated plots has led to greater increases in organic matter and nutrients to the soil than that attained by corn-soybean rotation in fertilized soil. "Rotation also seems to work by increasing yields, but it may be that we should consider longer rotations," Aref said."If you don't treat the soil well," she added, "you are cheating yourself."

University of Illinois at Urbana-Champaign

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