Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 488

A series of regional workshops were held in 1961 to introduce the equation to
SCS personnel. The agenda for the regional workshop in Little Rock, AR, for SCS
personnel in Mississippi, Louisiana, Arkansas, Texas, and Oklahoma included the
history of soil loss prediction by D. D. Smith; the rainfall erosion index and the crop-
ping and management factor by W. H. Wischmeier; soil erodibility values by T. C.
Olson; and slope length and steepness and conservation practice factors by D. D.
Smith. A session on soil factor and soil loss tolerance and a 2-h session on actual
soil factors and soil loss tolerances were held. At an evening session, state soil sci-
entists assigned soil factor values to important soils based on soil factor values that
were derived from plot studies on major US soils. While no workshop objective was
explicitly stated, the agenda clarifies the expectation that attendees were in a position
to apply soil loss prediction to the land in those states. Attendees were led through
soil loss computations using the USLE. There was also a discussion of implementing
soil loss prediction in the SCS field programs (Laflen, Agenda—Personal communi-
cation from D. D. Smith, evening session based on personal attendance).
In 1965, Agricultural Handbook 282 , the first complete publication of the USLE
(Wischmeier and Smith 1965) became available. While it was well known that the
erosion prediction equations were for sheet and rill erosion, this was explicitly stated
in Agricultural Handbook 282 , and there was discussion of estimating sediment
delivery from watersheds. There was a separate section on soil loss tolerance, which
was defined as the “maximum rate of soil erosion that will permit a high level of crop
productivity to be sustained economically and indefinitely.”
In 1978, Agricultural Handbook 537 was released; it presented a revision of
the USLE, expanding Agricultural Handbook 282 to include new tillage systems,
improved yields, and improved science. It also included an enlarged treatment of
soil loss tolerances—including a discussion of considerations such as water quality
standards and sediment delivery.
In 1997, the revised universal soil loss equation (RUSLE) was released (Renard et
al. 1996), which also discussed soil loss tolerance. As in earlier works, T values were
recommended to remain as originally defined and intended. If issues of water qual-
ity, economics, and policy were to be addressed for erosion control, T values were
recommended to be designated as T wq , T ep , and T pol .
The concept of soil loss tolerance emerged when it became apparent that
one could predict sheet and rill erosion on agricultural lands using relationships
between slope gradient, length, and factors representing cropping and management.
The primary focus, in terms of soil loss, since Zingg first published Equation 17.1
expressing the effect of slope and length on soil loss, has been to develop better
technology with a wider applicability for predicting soil loss, rather than on soil
tolerance values.
17.8 EROSIONAL IMPACT ON PRODUCTIVITY
The negative effects of erosion on soil properties have been well documented by
researchers (Follett and Stewart 1985; Lal and Stewart 1990; Pimentel 1993;
Cleveland 1995; Loch and Silburn 1997): (1) reduction in soil depth and potential
rooting depth; (2) reduction in soil organic matter content; (3) reduction in nutrient