Soil Renewal and Sustainability - Principles of Sustainable Soil Management in Agroecosystems - page 489

availability; (4) nonuniform removal of topsoil within a field; (5) exposure of, and/or
mixing of topsoil with, subsoil of poorer physical, biological, and chemical proper-
ties; (6) changes in soil physical properties (such as changes in bulk density, water
infiltration, water holding capacity, texture, or structure); or (7) some combination of
the above factors (Den Biggelaar et al. 2001).
The detrimental effects of soil erosion associated with agricultural production
have also been demonstrated through numerous experiments conducted where ero-
sion was simulated either by artificial desurfacing (Mbagwu et al. 1984; Dormaar
et al. 1986; Gollany et al. 1992; Malhi et al. 1994; Tanaka 1995; Larney et al. 2000)
or by comparing yield on strongly eroded areas with yield on less eroded areas
(Bramble-Brodahl et al. 1984; Busacca et al. 1984; White et al. 1984; Mielke and
Schepers 1986; Olson and Carmer 1990; Kosmas et al. 2001; Bakker et al. 2007).
Study results suggest that yield reductions at the field scale are approximately 4%
for each 0.1 m of soil loss, where yield reductions could generally be attributed to a
reduction in rooting depth (Bakker et al. 2007).
Erosion rates alone are not necessarily good indicators of damage to productiv-
ity. There is little establishment of the direct cause-effect relationship, and most
studies of erosional processes are lacking in quantification of structural attributes
(Lal 1998). Further, there is a need for more credible data on soil erosion rates, soil
formation rate, soil loss tolerance, and impact of erosion on productivity (Lal 1998).
To better estimate the effects of erosion on productivity, the development of more
sophisticated and reliable crop simulation models may be required (Schumacher et
al. 1994).
The relationship between soil erosion and productivity is complex. Although
soil erosion is thought to be occurring at unsustainable rates, portions of the United
States have experienced unprecedented increases in crop yields in recent years. The
complexity in predicting impact on yield may occur because soil properties are
not the only factors influencing crop yields. It is well documented that technology
has greatly impacted yields. The increased use of higher-yielding corn hybrids and
increased nitrogen fertilizer inputs has masked the effects of erosion on yields, and
in many cases, yields have increased over time (Fenton et al. 2005). Climatic factors
such as changes in growing season, precipitation, and temperature further compli-
cate this relationship (Schumacher et al. 1994).
Many believe that the reliance on technology is masking the long-term damage
being done to soil resources. While soil fertility can be improved through tech-
nological advances, the impacts of erosion on soil texture and function cannot be
readily corrected by technology (Craft et. al 1992). Soil degradation effects can be
grouped into two categories: (1) reversible, with such components as nutrient levels,
pH, organic matter, and biological activity, and (2) irreversible, such as occurs with
changes in rooting depth, water holding capacity, structure, and texture. The revers-
ibility or irreversibility of soil degradation is a function not only of available technol-
ogy but also of economic return (Den Biggelaar et al. 2001).
C, or the A horizon, can be considered an irreversible impact of soil erosion
based on current estimates of soil formation. The A horizon plays a critical role in
soil fertility. Plant roots and available nutrients are concentrated in this layer, and
it is critical for nutrient retention and water holding capacity. In many cultivated