The nation's concern about soil erosion has focused on tillage practices used to produce row crops. Because of this concern, legislation now requires grain producers to adopt soil conservation practices if they want to participate in many government programs. The 1985 Farm Bill's conservation provisions classify soils into different erodibility categories. Soils designated as "highly erodible" must have an approved conservation plan if the producer wants to plant crops on them as part of a government program's acreage base.
On what basis is a soil determined to be highly erodible? Experience and common sense point out that several factors such as rainfall intensity, soil properties, slope, crop and tillage system affect soil erodibility. After many years of research aimed at quantifying the combined effect of these factors, an equation was developed to help predict soil erosion losses. The Universal Soil Loss Equation (USLE), developed at the National Runoff and Soil Loss Data Center, incorporates environmental and topographic factors affecting soil erosion. It predicts how much erosion a given site will have in terms of tons/acre/year.

What Forms of Soil Erosion Does USLE Predict?
Like many other equations developed for prediction purposes, the USLE has its limitations. For example, the USLE only predicts sheet and rill erosion. Sheet erosion is the uniform removal of soil from an area without the development of conspicuous water channels. Rill erosion refers to the removal of sod through the cutting of numerous small but obvious water channels where runoff concentrates. Gully erosion on the other hand, is a more dramatic and visible form of soil erosion, but is not predicted by the USLE.
Users of USLE should also be aware that the soil loss values it generates show how much soil is subject to movement. Depending on a field's topography, rainfall often transports and deposits soil to other areas within the same field. Therefore, the "soil loss" as calculated by the USLE may refer to soil either removed from the field or simply soil relocated within the same field. The calculated "soil loss" does not necessarily mean that this amount of sediment will enter a stream.

How is the USLE Used?
The calculated soil loss is compared to the tolerable soil loss for a specific soil type. The tolerable soil loss (T value) is the maximum level of soil erosion that will still permit a high level of crop productivity to be sustained economically and indefinitely. The difference between the calculated soil loss and the tolerable soil loss indicates the degree to which an erosion problem exists and its potential impact on productivity. The tolerable soil loss of a particular soil type is therefore used as a base line value when you evaluate the effect that different cropping systems have on soil erosion losses.
The T values for individual soil types were derived by state and federal research leaders in soil science, agronomy, geology, and soil conservation at six regional workshops in 1961 and 1962. The factors considered when the researchers assigned numerical values to these tolerable limits included
•soil depth,
•physical properties and other characteristics affecting root development,
•gully prevention,
•on-field sediment problems,
•seeding losses,
•soil organic matter reduction,
•plant nutrient losses.

Detailed information for determining a T value is not available on every soil type. Therefore, estimates have been made by comparing these soils to those where research data is available.

What are the Components of the USLE?
The USLE is commonly expressed as

A = RKLSCP

A = The computed soil loss in tons/acre/year
R = Rainfall factor
K = Soil erodibility factor for a specific soil type
L = Slope length
S = Slope steepness
C = Crop-management factor
P = Conservation practices that reduce soil loss

R is the rainfall factor. Research indicates that soil loss from cultivated fields is directly related to the energy and intensity of each rainfall. Long term measurements of these rainfall parameters were used to develop specific rainfall factors for many areas of the U.S. Because rainfall is a cyclical pattern, the published R values were based on twenty-two year rainfall records. Planners using the USLE should determine your R values by referring to U.S. maps delineated for R values.
K is the soil erodibility factor for a specific soil type. It indicates how likely a soil is to erode based on its physical and chemical properties. These values were experimentally determined for several sod types on small plots referred to as standard or unit plots. A unit plot is 72.6 ft long with a uniform slope of 9% under continuous fallow and tilled up and down the slope. Continuous fallow for this purpose is land that has been tilled and kept free of vegetation for more than two years. This procedure eliminates the influence of cover and management on determining the inherent erosive nature of a soil.
Experimentally measuring the erodibility of every soil type would be practically impossible to do. Therefore, the K values for most of Kentucky's soil types have been estimated based on their similarity to soils on which K values have been measured.
LS is the topographic factor. The rate at which water erodes soil is related to both the land's length and its steepness. LS is an estimate of the soil loss from a field slope, based on research from a unit plot as defined previously. Slope length (L) is the distance from the surface flow's point of origin to the point where either the slope gradient decreases enough that deposition begins, or the runoff water enters a drainage way. Slope steepness is the slope gradient in percent. Determining the LS factor of a field is often difficult because of the topography that often occurs in Kentucky.
C is the cropping-management factor. The C factor in the USLE estimates the reduction of soil loss from land cropped under specified vegetative, residue, and management conditions as compared to clean-tilled, continuous fallow conditions. The C factor for a given cropping system and locality should consider

•how the crop residues will be managed,
•the tillage used,
•the amount of residue remaining on the surface,
•the amount and distribution of rainfall,
•the vegetative cover's stage of growth and development when rain occurs.

Many possible combinations of these factors exist. To develop a table of C values inclusive enough for general field use, we computed expected C values for cover, residue systems. These values were not generated by research data.
The P factor in the USLE estimates how much less soil is lost when certain conservation practices are used, compared to how much is lost with up-and-down slope cultivation. These conservation practices include contour tillage and strip cropping on the contour.
The effectiveness of contouring is influenced by slope steepness. Contouring is most effective at a moderate slope. At both higher and lower slopes the effect of contouring on erosion control is less beneficial.

How Does the USLE Work?
The USLE, when used properly within its limitations, can be a useful tool in soil conservation planning. The soil loss equation allows a grower to compare the effect that crop management changes will have on soil loss. The following examples demonstrate how calculated predictions of soil loss allow a grower to evaluate different crop rotations and tillage systems on two different soil types.

Effect of Crop Rotation and Tillage System on Predicted Soil Loss on a Zanesville Silt Loam
Situation #1
Farm location -- Christian County, Kentucky
Soil type -- Zanesville silt loam
% Slope -- 5%
Slope length -- 150 ft
Tolerable soil loss (T) -- 3 tons/A/year
 

Common Crop Rotations	Predicted soil loss*
	Conv. Till1	Red. Till2	No-Till3	CT/NT4
	--------------------------------tons/A/yr--------------------------------
Continuous Corn	13.6	7.4	1.2	7.4
Corn-Soybeans	15.8	11.7	1.9	9.6
Corn-Wheat/Soybeans	15.7	7.7	1.3	--

*Predicted soil loss = Calculated using the USLE.
¹Conventional till = Moldboard plow plus 1 disking.
²Reduced till = Chisel plow (straight shanks) plus 1 disking.
³No till = Elimination of all tillage.
⁴CT/NT = Tillage rotation of one year conventional tillage, one year no-till.

Effect of Crop Rotation and Tillage System on Predicted Soil Loss on a Crider Silt Loam
Situation #1
Farm location -- Christian County, Kentucky
Soil type -- Crider silt loam
% Slope -- 5%
Slope length -- 150 ft
Tolerable soil loss (T) -- 5 tons/A/year
 

Common Crop Rotations	Predicted soil loss*
	Conv. Till1	Red. Till2	No-Till3	CT/NT4
	---------------------------------tons/A/yr--------------------------------
Continuous Corn	10.1	5.5	0.9	5.5
Corn-Soybeans	11.8	8.7	1.5	7.2
Corn-Wheat/Soybeans	11.7	5.7	0.9	--

*Predicted soil loss = Calculated using the USLE.
¹Conventional till = Moldboard plow plus 1 disking.
²Reduced till = Chisel plow (straight shanks) plus 1 disking.
³No till = Elimination of all tillage.
⁴CT/NT = Tillage rotation of one year conventional tillage, one year no-till.

To limit the annual soil loss to the tolerable limit of 3 tons/A/year in situation #1, a grain producer who plans to produce row crops should consider a total no-till system. In situation #2 on a well-drained Crider soil, a grower could conduct some form of reduced tillage and maintain soil loss near the 5 tons/A/year tolerable limit. The USLE could also be used to examine other rotations that involve pasture or meadow to reduce soil loss further.

Summary
The Universal Soil Loss Equation was developed to help land users select alternative cropping systems to control soil loss. Due to legislation, the USLE has indirectly become the primary tool for determining conservation compliance. Research continuing in Kentucky should improve our ability to predict soil erosion losses by improving the USLE's accuracy on Kentucky soils.