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Why Do Crop Yields Vary Across a Field?

By Hank Becker
October 13, 2000

Farmers nonplussed by variability in crop yields within typical agricultural fields may soon have an answer to their quandary.

Research has shown that while higher yields may be influenced partly by management practices and partly by topography at lowest field elevation, lower yields may be influenced by soil types plus topography. That’s the finding of Agricultural Research Service scientists at Ames, Iowa, working with Iowa State University-Ames researchers.

With the advent of satellite global positioning systems and yield monitors for combines, scientists and farmers have documented great variability in crop yields within typical agricultural fields. So the research team set out to determine what combination of soil, weather, and management factors accounts for this variability. Farmers could use such knowledge to increase yields and reduce costly expenditures for fertilizers and pesticides, while lowering the risk of contaminating the United States’ water resources with chemicals.

The team measured the yield variability of corn and soybeans within a 50-acre farm field and then related this variability to soil properties. In particular, they evaluated the Soil Tilth Index as an accurate predictor of observed crop yield variation. Developed by ARS soil experts at the National Soil Tilth Laboratory in Ames, the index measures soil health and ranks the soil's suitability as a seedbed.

The team found that the Soil Tilth Index was an accurate indicator of corn and soybean yield--but only for a limited portion of the field. For the remainder, factors other than tilth determined crop yield.

This information is valuable to farmers and researchers trying to determine the factors controlling crop yield and looking for management options that optimize yield while minimizing off-site risks. ARS is USDA’s chief research agency.

Scientific contact: Thomas S. Colvin, ARS National Soil Tilth Laboratory, Ames, Iowa; phone (515) 294-5724, fax (515) 294-8125,