The Universal Soil Loss Equation was developed to predict the average soil loss from sheet and rill erosion on any specific site.
A farmer can use the equation to decide what conservation practices are needed to keep soil losses within tolerable levels. The USLE also helps the farmer to adopt the most economic soil preservation measure.
The USLE is based on a standard test plot, which represents an average eroded site. This plot has a 9% slope 2.6 feet long. The slope is kept in clean-tilled fallow, using conventional tillage up and down the slope. The equation works by comparing a specific spot to this test plot. The equation reads as follows:
A = R K L S C P
“A” is the tons of soil lost per acre each year. Obviously, “A” should be less than “T”. To solve for A, values are inserted for the six variables and are multiplied. The variables are:
R – Rainfall and runoff factor. R is based on the total erosive power of storms during an average year. R depends on local weather conditions. The isoerodent map shows R values for the country or region.
K – Soil erodability factor. K depends on texture, structure, and organic matter content. Soil survey reports give the value of K for mapped soils. They may also be calculated.
Read Also: Soil Reaction: Meaning, Acidity, and Liming
LS – Slope factor. L compares the slope length and S compares the grade with the standard plot. L and S are separate factors, but they can be treated as one variable, “LS”. LS values can be determined from the chart.
C – Cover and management factor. C compares cropping practices, residue management, and soil cover to the standard clean-fallow plot. C values are calculated from detailed tables and are valid only within the area for which they are calculated.
Many SC’s offices prepare simplified tables for use in the field, and some have computerized the computations.
P – Support practice factor. P compares the effect of contour tillage, contour strip-cropping, and terracing with the test plot. The LS factor accounts for terraces primarily, but they are included here because they are contoured.
Sample Solution: For a sample solution of the USLE, use mapping unit 158C. The slope is 2% to 6% and about 100 feet long. (Ask yourself: How do I know the grade?) According to the soil survey for this area, K = 0.18 and 7=5.0. Assume the grower uses conventional tillage to grow continuous corn.
Read Also: The Systems of Soil Classification
