Last week’s column focused on the types of soil water erosion. While government help in understanding and controlling wind erosion was the focus in the Great Plains after the Dust Bowl, the focus in the eastern U.S. was water erosion. A series of research sites were set up with some still conducting research into the 2000s. The main focus was initially to determine what factors determine the degree of soil erosion and then how to control it. From this work, the Universal Soil Loss Equation or USLE was developed and it is probably the easiest, most concise way, to understand soil water erosion factors. There are new and modified USLE models but the original suits our purpose here.
The USLE is A = R x K x LS x C x P where:
• A – This is the predicted annual soil loss from water erosion based upon the factors described below. The key word here is predicted. As seen below, actual conditions will determine actual soil loss.
• R – This is the rainfall erosivity factor. This is not simply how much rainfall occurs but the intensity, how much rain falls in a given period of time, and the distribution, when it falls. Here, we think of hard, intense downpours when we think of rain, however, most of the rains we receive are 0.25 inches or less and light. Rainfall the equivalent of several inches per hour is more erosive. Distribution denotes when rain falls – winter, spring, summer or fall. Rainfall when vegetation is out is less erosive then say late fall or winter.
• K – This factor describes how susceptible the soil is to water erosion. A sandy soil erodes more easily than a silt loam or clay soil. Lower organic matter levels also lead to greater susceptibility to erosion. The ability of the soil to take in water, the infiltration capacity, and soil structure also matter here. This factor is determined using a 9 percent slope.
• LS – LS is the length and steepness of slope. As the percent slope increases so does the water erosion potential. The length of the slope is just that, how far along the slope can water flow uninterrupted. A steep slope that is 100 feet in length has a lower erosion potential then a less steep slope that is 1000 feet long. When you see terraces on a cropped hillside, it is decreasing the length the water can flow.
• C – This factor takes into account vegetative cover and cropping systems. A bare soil has a high C approaching one, cover and management factor, while a well-established alfalfa field a low number heading towards zero. Is the surface covered during critical periods for runoff? Simply this is the soil loss under actual conditions divided by the soil loss with bare soil.
• P – Finally we have P, the supporting practice factor. What is being done augment C? These are practices to supplement the cover and cropping system when that isn’t enough to control erosion.
Please remember this is a very brief description of the USLE. First a question to consider. What happens to erosion when any one factor goes to zero? Next week, using this equation to manage soil erosion.

Dr. Victor L. Martin is the agriculture instructor/coordinator for Barton Community College. He can be reached at 620-792-9207, ext. 207.