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soils, the A horizons have significantly decreased in thickness or been removed by
erosion, leaving soils in the B horizon that are unfavorable for plant growth (Larson
et al. 1983).
The importance of organic matter in soils has long been recognized. Stevenson
(1972) summarized the important contributions of soil organic matter as it impacts
nutrients, nutrient holding capacity, microbial growth, water holding capacity, buff-
ering capacity, rainfall energy absorption, cementing agent for soil structural units,
and possible plant growth stimulation. Verity and Anderson (1990) reported that
5 cm of organic matter-rich topsoil added to severely eroded knolls increased grain
yields by more than 50%, which is in sharp contrast to the results of Bakker et al.
(2007) cited above. Fenton et al. (2005) studied the impact of erosion on organic mat-
ter content and productivity of some Iowa soils. They concluded that organic matter
content was significantly correlated with erosion phase.
Organic matter content for well- and moderately drained till-derived Mollisols
with clay contents ranging from 18% to 40% was 3.65%, 2.46%, and 1.82% for slight,
moderately, and severely eroded phases, respectively. For loess-derived soils with
similar parameters, organic matter contents were 3.7%, 2.9%, and 2.0%. Models
developed in the same study showed that for till-derived soils, corn yields decreased
by 1.37 Mg/ha from slightly eroded to severely eroded soil phases. For loess-derived
soils, corn yields decreased by 0.67 Mg/ha for the same erosion phase change.
Decreased organic matter content and associated change in properties were major
factors in the decreased productivity of the eroded soils.
Field studies and simulation models confirm large variations in soil degradation
impacts to soil properties and quality (Maetzold and Alt 1986). It may not directly be
the decrease in the A horizon depth that impacts yields but rather the corresponding
change to other soil properties. Hoag (1998) concluded that “depth is not generally
an adequate measure of productivity and that the most profitable management of a
soil will depend on the quality and distribution of soil layers.” Comparisons are also
difficult to make among sites because of differences in soil properties and weather
conditions (Schumacher et al. 1994). During degradation, some soils experience con-
sistent yield reductions, while others suffer no impact until a critical soil loss level is
reached. Once this level is reached, significant yield losses occur with further erosion
(Hoag 1998).
Numerous studies have aimed at quantifying the relationship between soil erosion
and productivity through relating yield declines to loss in Topsoil Depth (TSD). The
relationship is typically nonlinear with greater yield declines at thinner TSD, but also
with diminishing increased returns in relation to increasing TSD in areas of eroded
sediment deposition (Walker and Young 1983). This relationship was also studied by
Fenton et al. (2005) for medium-textured loess-derived and till-derived soils in Iowa,
with corn yield expressed as a function of the A horizon thickness (
Fig u r e 17.1
).
The slope of yield curves increases as soil transitions from slightly to moderately to
severely eroded, indicating increased yield sensitivity for more severely eroded soils.
This relationship was further developed through yield response curves to increased
nitrogen input (
Fig u r e 17.2
). The slope of the curves decreases with increased nitro-
gen fertilizer input but also indicates that the decrease in yield was not corrected
totally by the additional fertilizer (Fenton et al. 2005).
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