Severe soil erosion in a wheat field in Washington.
and rushing snowmelt had long washed soil from the steep potato field. Uprooted
potatoes lay scattered about after downpours. Fresh deposits of soil clogged
road ditches bordering the field.
The farmer and local soil conservationist
were walking the field to plan a remedy. When they stopped, the conservationist
held up a little workbook and said, "Here's a simple tool that'll pinpoint
some practical solutions to your erosion problems. It's the Universal Soil Loss
EquationUSLE for short."
The conservationist explained that after
plugging in six factors, the USLE would predict the long-term yearly erosion
rate for the field under specified cropping and management conditions. The
planning goal was to reduce erosion to a level that permitted the field's
productivity to be sustained indefinitely in this case, 3 tons per acre
"Nature itself largely controls three
of the six equation factors," the conservationist said. "Those are
rainfall energy and intensity, erodibility of the particular soil, and
steepness of the slope. Length of slope can be partially modified. But every
farmer fully controls two factors: cover crop and soil-management practices,
and erosion-control practices."
Explanations over, the conservationist got
to work determining soil type, percent of slope, and length of slope. Then he
referred to tables in the workbook and punched a calculator.
Studies for the Long Haul
continuous potato rotation, planted up and down hill, loses almost 15 tons of
soil per acre every year," he concluded. "Now let's check some
In quick order, the conservationist applied
USLE to gauging soil losses under several practices. He suggested growing a
rotation of potatoes-oats-hay and installing contour strips. That alternative
would hold soil losses well below tolerable levels. The farmer vetoed it,
however, because it would reduce his income by cutting into his potato cash
The solution ultimately selected was to
reduce the slope length by installing a diversion ditch 400 feet below the
field's crest to control runoff. Next, the diversion would be combined with a
potato-oats rotation grown across the slope. Then, by incorporating
conservation tillage practices that hold soil disturbance to a minimum and
leave most crop residues on the field, the farmer could keep soil losses under
2 tons per acre.
Although the conservationist applied USLE in
minutes to save the potato grower's soil, the equation rests on over half a
century of research in many States. Similar equations were put to use in the
Corn Belt and the Northeast during the 1940's, but the USLE is the first to be
applicable nationally. Its broad-based usefulness stems from the painstaking
analysis of precipitation data from over 65,000 individual rainstorms and
runoff and soil loss data from 49 field plots.
Major credit for developing USLE which
was released in 1960belongs to ARS agricultural engineer Dwight L. Smith
and research statistician Walter H. Wischmeir. In recent years the equation has
been improved and refined by other ARS researchers, particularly soil scientist
William C. Moldenhauer and hydrologic engineer George R. Foster. Today, USLE is
a major weapon in combating the annual loss of 2 billion tons of soil that wash
from the Nation's croplands.