Agriculture Reference
In-Depth Information
4.5.1 t
exture
Soil texture is the relative proportion of the various soil separates in a soil. The three soil separates
that make soil texture are sand (2-0.02 mm in diameter), silt (0.02- 0.002 mm), and clay (0.002 mm
or less in diameter). Soil texture is a fundamental parameter in soil science and a major component
of the soil natural capital (Robinson et al., 2009). Texture is widely used in agriculture and engineer-
ing as well as in basic research to estimate, for example, the water release curves in flow and trans-
port modeling (Schaap et al., 2001; Wuddivira et al., 2012). Soil texture, especially the clay content,
controls the magnitude and rates of many physical, chemical, and hydrological processes in soils.
The important soil phenomena such as nutrient storage, nutrient availability, water retention, and
stability of aggregates may vary across the field in response to the spatial variability of clay percent-
age. Soil moisture, which is the major control for the rainfall-runoff response in a watershed, has
been directly linked to clay variability (Crave and Gascuel-Odoux, 1997; Wuddivira et al., 2012).
Particle size fractionation is a useful indicator of SOM dynamics (Borchers and Perry, 1992;
Parker et al., 2002; Gartzia-Bengoetxea et al., 2009; Norris et al., 2011; Grand and Lavkulich, 2012).
SOM associated with the clay fraction is considered to be the most stable fraction, with physical
occlusion and the formation of complexes with mineral elements contributing to its stabilization
(Paul, 1984; Sollins et al., 1996; Eusterhues et al., 2003). In contrast, silt- and sand-sized SOM
fractions are considered to be more reactive due to weaker interactions with minerals (Tiessen and
Stewart, 1983; Six et al., 2002).
Soil texture is unchanged by cultural and management practices. The OM content of the soil is
highly related to its clay content. Kadebra (1978) found a correlation coefficient of 0.84 for organic
C and percent of clay in soils developed from basaltic rocks. No such relationship was found for soils
from acidic rocks. Broersma and Lavkulich (1980) reported that 24-48% of OM in selected soils of
Canada was associated with the clay fraction and 40-60% was associated with the fine silt fraction.
The close correlation between clay mineral concentration and OM in soils may be accounted for by
the binding of organic C to clay minerals (Loll and Bollag, 1983; Mengel et al., 2001).
Nichols (1984) studied the relationship of organic carbon to soil properties of Mollisols of the
southern Great Plains of the United States. Stepwise multiple regression analyses were made on
the percent of organic carbon versus the percent of clay, percent of silt, base saturation, mean
annual precipitation, and mean annual temperature. Results indicate a significant relationship
between organic carbon and clay with a lesser relationship with precipitation. The other variable
did not improve the predictive equation. Six et al. (2000a) confirmed the conclusions of Hassink
(1997) showing that the quantity of clay- and silt-size particles is positively correlated with SOM
in the same size fractions across a broad range of climates and soil types. Texture has an effect on
aggregation (Six et al., 2000b), and there is evidence that the extent of physical protection of SOM
increases with increasing clay content (Six et al. 2000a; Vandenbygaart and Kay, 2004). Similarly,
many studies have reported that a major part of OM in predominantly inorganic soils is usually
found in the clay- and silt-size fractions (Anderson et al., 1981; Tiessen and Stewart, 1983; Catroux
and Schnitzer, 1987).
Soil compaction affects the physical, chemical, and biological properties of soils, and has been
considered one of the main causes of agricultural soil degradation (Imhoff et al., 2004). The resis-
tance to compaction depends on the intrinsic soil attributes, with texture being one of the most
relevant (Larson and Gupta, 1980; McBride, 1989). OM plays an important role in soil compaction
(Larson and Gupta, 1980; McBride, 1989; McBridge and Watson, 1990). The susceptibility to com-
paction decreases as the SOM content increases (Zhang et al., 1997).
4.5.2 s
truCture
The soil is a porous mixture of inorganic particles, OM, air, and water. This mixture also contains
a large variety of living microorganisms. The inorganic particles and OM make up the soil solids,
