Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 485

by watersheds in the Upper Mississippi River Basin and other parts of the United
States, are experiencing increasing frequency of high-intensity rainfall events (Karl
et al. 2009), strongly suggesting that elevated soil erosion rates have been occurring
and could be an increasing threat. An even greater complication involves limita-
tions of current soil erosion modeling technology. The universal soil loss equation
(USLE) and its derivative technologies and the WEPP model estimate only sheet
and rill erosion. Sediments lost in ephemeral gullies, small channels, and ditches
are not included in their estimates. Thus, erosion values given in numerous projects
that use these approaches for estimating soil erosion rates are likely very conserva-
tive. In considering the wide array of research methods used, interpretations made
of data presented, and seriousness of the issue, multiple authors have articulated
that globally, topsoil is thinning at an alarming rate (Pimentel 2006; Brown 1984;
Lal 1995). This condition poses serious threats to future food, feed, fiber, and fuel
feedstock security.
17.7 HISTORY OF TOLERABLE SOIL LOSS ( T )
The United States' national interest in soil erosion can be traced to the 1930s Dust
Bowl. Wind erosion generated nationwide attention and resulted in the formation
of the Soil Erosion Service in the Department of Interior in 1933 under the leader-
ship of Hugh Bennett as chief. In 1935, the Soil Erosion Service was transferred
to the Department of Agriculture and combined with other units to form the Soil
Conservation Service (SCS, now known as the NRCS).
Recognizing the damage caused by soil erosion was the first step in addressing an
existing and newly recognized serious national issue; however, quantifying the soil
loss occurring and damage was a prerequisite to developing a long-term strategy to
address the growing problem. Not only was there a need to quantify soil loss; quan-
tifying soil renewal rates was necessary to determine the net change in soil depth
and crop production potential. What ensued is arguably the largest historical human
conservation impact on soil resources, agricultural management, water quality,
and food/feed/fiber/fuel feedstock production known to man. The multiple-decade
research programs' goal was to determine the acceptable level of soil erosion and
what management practices on the country's farmed fields ensured that the identified
acceptable soil erosion level would not be exceeded.
The “ T factor,” as it has become known, is the soil loss tolerance (in mass of soil
per unit area). It is defined as the maximum amount of erosion at which the quality
of a soil medium for plant growth can be maintained. Maintaining soil quality has
three focus areas. It includes maintaining (1) the surface soil as a seedbed for plants;
(2) the atmosphere-soil interface to facilitate the entry of air and water into the soil
while protecting the underlying soil from wind and water erosion; and (3) the total
soil volume as a reservoir for water and plant nutrients preserved by minimizing soil
loss. The following discussion outlines the US developmental steps to determine T
for every soil and thus to nationally combat soil erosion based on quantitative sci-
ence. This same soil erosion issue plagues international soil resources and threatens
human capability to meet growing food demands, especially in the face of changing
climate (Soil and Water Conservation Society 2006).