Why Do Crop Yields Vary Across a
By Hank Becker
October 13, 2000
Farmers nonplussed by variability in
crop yields within typical agricultural fields may soon have an answer to their
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. Thats 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
USDAs chief research agency.
Scientific contact: Thomas S. Colvin, ARS National Soil Tilth
Laboratory, Ames, Iowa; phone (515) 294-5724, fax (515) 294-8125,