Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 487

Smith developed recommendations for maximum length and degree of slope
combinations for different conservation practices including strip cropping, contour-
ing, and terracing. In his summary, Smith pointed to the equation's development that
“provides for the effect of soil-climate-crop-treatment, length and degree of slope,
and mechanical conservation practices,” and “a soil loss of not more than 4 tons per
acre per year (9 Mg/ha) is suggested as a rate which would allow maintenance of
fertility with recommended cropping practices.”
Smith was an engineer, and he seemed to approach the design of conservation
systems as an engineer. In his paper, he stated, “Mathematical formulas have been
used in practically all design work. Early formulas were largely empirical, and as
additional knowledge was gained, they have been modified or replaced by theoreti-
cal relationships. Design formulas, even though empirical, should be of assistance to
soil conservationists, particularly to the inexperienced technician, in making more
correct field applications.”
Browning et al. (1947), published a paper on a method for determining the use
and limitations of rotation and conservation practices in the control of soil erosion in
Iowa. This paper gave permissible annual soil loss limitations for 13 important Iowa
soils. These rates ranged from 4.5 to 13.5 Mg/ha. Equation 17.3 and a permissible
annual soil loss of 11.2 Mg/ha were used to demonstrate how a table might be devel-
oped for various practices, a permissible soil loss, and a range of slopes. Equation
17.3 was also used to evaluate slope lengths for different permissible soil loss rates.
One of the most-quoted papers related to soil erosion prediction was published by
Musgrave (1947). The title of the paper was “The Qualitative Evaluation of Factors in
Water Erosion—A First Approximation.” This work was “the result of the findings
of a group of workers showing the relationship between the major casual factors and
the resulting rate of erosion.” Included in this group were D.D. Smith and George
Browning. This paper was a major work in the development of the USLE, which has
been widely used, along with allowable soil erosion, to select practices and manage-
ment strategies to keep average annual soil erosion below a tolerable limit. Yet, this
1947 paper does not mention tolerable or allowable soil loss limits.
Smith and Whitt (1947, 1948) presented a method of predicting soil loss they
referred to as the “factor system,” which drew heavily on the previous work of Smith
and Browning. In these papers, they discussed “allowable soil loss.” They indicated
that the ultimate objective of soil conservation is to maintain soil fertility, and hence
crop production, indefinitely. Smith and Whitt used organic matter as their criterion
of soil fertility and, based on plots of organic matter versus erosion rate, suggested
a limit of 9 Mg/ha for the Marshall and Shelby soils and 6.7 Mg/ha for the Putnam
soils. They recommended a limit of 6.7 Mg/ha for the claypan soils and 4.5 Mg/ha
for the Ozark Region soils. All soils were in Missouri.
The USLE was first published in 1961. It is clear that the major driving force
for the USLE development was the perceived need for a technology that could be
used to select conservation practices and cropping and management that would
limit soil losses from a particular field “to a level that will not reduce its future
production potential.” The USLE could be used “to select practices that will per-
mit a farmer to make the most profitable use of the field and still protect and
improve the soil.”