Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 484

an issue as many suggest. Using these two evaluation approaches leads to vastly dif-
ferent conclusions regarding soil erosion impacts on sustainability.
Montgomery's (2007), or any, average global soil erosion value is likely a poor
indicator of soil erosion influences on land degradation. Soil loss rates averaged over
large areas simply give a distorted view of soil erosion impact on sustainability (Cox
et al. 2011). To exemplify on a smaller scale than that used by Montgomery, the lat-
est United States Department of Agriculture—National Resource Inventory (USDA-
NRI) report estimated average soil erosion rates for Iowa in 2007 to be 0.88 mm/
year, very close to the T value of 0.85 mm/year. However, Fenton (2010), using land
use data and the prime farmland definition given by the USDA and NRI, estimated
that the majority of the soil loss in Iowa was coming from about 27% of the crop-
land. Using this assumption, the soil loss from those areas would be over 3.14 mm/
year. These data suggest that relatively large areas in Iowa would be losing soil at an
unsustainable rate, even though based on the reported average, one would assume
that soil loss and T are balanced, leading to statewide sustained production poten-
tial. This soil loss trend is supported by data from the five most extensive soil series
(2,223,467 ha and all Mollisols) in Iowa that have slope and erosion phases (Fenton
2012). Soil map unit summaries from these soils show that on slope gradients of 5%
to 9% or greater, only 8 to 18 cm of surface soil remains on 75% to 100% of the slope
groups. Original depths were approximately 40 to 45 cm.
The Iowa Daily Erosion Project (IDEP) (Cruse et al. 2006), using the Water
Erosion Prediction Project (WEPP) soil erosion model, estimated average state-
wide 2007 soil erosion rates, based on spatial and temporal rainfall characteristics,
similar to those of the NRI; yet greater than 10.9 mm of erosion was estimated in
isolated areas of Iowa (Cox et al. 2011). This estimate is 12 times greater than the
estimated average annual erosion rate of 0.88 mm/year and also an order of mag-
nitude greater than the value of T used for most soils. The Losing Ground Report
(Cox et al. 2011) states that statewide erosion averages provide a poor representation
of actual erosion rates and that many areas are eroding faster than the “sustain-
able” rate. In fact, they estimated that approximately 20% of the state's agricultural
land was eroding at twice the T level (0.85 mm/year). If the soil renewal rates of
Wakatuski and Rasyidin (1992) and/or Alexander (1988) are considered the best
metric for determining allowable soil loss rates, then 20% of the state's agricultural
land eroded at more than an order of magnitude greater than what should be con-
sidered sustainable. In the more heavily impacted areas, erosion was two orders of
magnitude greater than what sustainability would dictate acceptable. Further, when
focusing on specific geographical areas, the combination of erosion rates and crop
production can generate relatively large impacts not factored in averaged yields
(Den Biggelaar et al. 2001).
Erosion rates may even be worse than current estimates when focusing on short-
duration, high-intensity rainfall events. Soil movement is most noticeable dur-
ing such rainfall events, and particularly when the soil is not protected by cover
(Larson et al. 1983). When studying individual storms and their spatial distribution,
more detailed and accurate estimates of soil erosion can be obtained than those
obtained by relying on an average annual estimate, especially over a large area, as
illustrated above. To further exacerbate the issue, areas of the world, exemplified