Agriculture Reference
In-Depth Information
alleviation of compaction involves soil management practices that lead to an increase
in the organic matter content of the soil and incorporation of deep-rooted crops into
the rotation to restore structure to the lower soil profile.
2.3.2 c hemical D egraDation
Chemical degradation is a result of changes in the chemical processes within the
soil, which will, in turn, affect plant growth. A reduction in pH causing soil acidifi-
cation occurs after leaching of bases through the soil profile or the continual addition
of acid-producing fertilizers. Depletion of nutrients by removing plants or leaching
without resupplying nutrients is another form of chemical degradation. Conversely,
soils can become toxic through the buildup of elements (e.g., high Al, Fe, and Mn
concentrations can be toxic to plants). Continual increases in soluble salts in the root
zone that increase the electrical conductivity above 4 dS m -1 create salinization of
the soil. Addition of Na ions through sodic salts can lead to alkalinization of the soil.
Erosion can cause nutrient depletion in soils, and the degradation of soil structure
can increase denitrification of N because of reduced oxygen content. Vanderpol and
Traore (1993) observed these processes to be evident in the soils of Mali with the
magnitude of the nutrient losses equivalent to 40% of the annual farm income. In the
sandy loam soils of Georgia, Sainju et al. (2002) observed that SOC and nitrogen (N)
concentrations in the soil could be maintained by reducing the mineralization and
erosion losses through the use of no-tillage coupled with cover crops and N fertiliza-
tion. In northern China, Su et al. (2004) compared the differences in soil properties
in a cultivated and ungrazed area in a degraded grassland ecosystem. They found
that cultivation had a significant negative impact on N and phosphorus (P) avail-
ability, soil biological activity, and soil enzyme activity. These effects were reversed
by maintaining the soil with a permanent grassland cover, and they concluded that
tillage has a detrimental impact on soil properties associated with high soil quality.
Lal (1997) proposed that soil degradation could be described as a function of soil
properties ( S ), climate ( C ), terrain ( T ), vegetation ( V ), management ( M ), and time ( t ),
as shown in Equation 2.1:
S d = f ( S , C , T , V , M ) t
(2.1)
He posited that these factors would interact and affect soil degradation. Soil prop-
erties, texture, and the structure of the soil are particularly affected, of which the
latter can be greatly affected by SOM and biological activity. Climate factors include
precipitation, temperature, evapotranspiration (ET), and seasonal patterns of these
parameters. The terrain parameters of slope length and gradient along with slope
aspect can determine how factors like soil temperature, vegetative growth, and soil
water affect degradation. Slope shape is also likely to affect erosion and, conversely,
deposition of eroded soil. The vegetative factors associated with soil degradation
include the amount of ground cover and canopy height, species composition, and suc-
cession in native systems, which are related to SOC dynamics and nutrient cycling
(Lal 1997). The cumulative effect of these different factors is thought to contribute to
marginality in soils and the rate of change from a high-quality to a low-quality soil.
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