Agriculture Reference
In-Depth Information
while the soil pore space is occupied by air and water. Soil structure is the combination or arrange-
ment of primary soil particles into secondary units or peds. The secondary units are characterized
on the basis of size, shape, and grade. Each ped is in turn made up of small clusters or aggregates of
soil particles. Exceptions are sandy soils that exhibit single-grain characteristic.
Soil structure is an important property that mediates many physical and biological processes
and controls SOM decomposition (Van Veen and Kuikman, 1990; Mikha and Rice, 2004). From
the agronomic standpoint, soil structure affects plant growth through its influence on infiltration,
percolation and retention of water, soil aeration, and mechanical impedance to root growth. The
major binding agents responsible for aggregate formation are the silicate clays, oxides of iron and
aluminum, and OM and its biological decomposition products (Duiker et al., 2003; Denef et al.,
2004). The stability of aggregates is fundamental for the favorable physical conditions in the soil
(Tisdall and Oades, 1982; Six et al., 1999; Carter, 2002) and for sustaining crop productivity
(Eynard et al., 2004). Iron and aluminum oxides and OM increase the stability of soil aggregates.
SOM compounds bind the primary particles in the aggregate, physically and chemically, and this,
in turn, increases the stability of the aggregates and limits their breakdown during the wetting pro-
cess (Emerson, 1977; Lado et al., 2004a,b). Chaney and Swift (1984) used wet sieving to measure
the aggregate stability of 26 agricultural soils with differing properties, and they found a high posi-
tive correlation between aggregate stability and OM content, suggesting that OM is an important
controlling factor. Benito and Diaz-Fierros (1992) studied the effects of various cropping systems
on the structural stability of soils containing various OM contents. They found that a decrease of
OM content in the soil led to a decrease in soil structural stability. Emerson (1977) suggested that
OM stabilized the aggregates mainly by forming and strengthening bonds between the particles
within them.
In addition, activities of plant roots and soil fauna and microorganisms also influence soil aggre-
gate stability (Tisdall and Oades, 1982; Marquez et al., 2004). Several studies have elucidated the
relationship between aggregates and associated SOM dynamics (Elliott, 1986; Jastrow, 1996; Six
et al., 1998, 2000a,b; Denef et al., 2004). Surface soil with high OM content has a well granulated
and stable structure. Carter (2002) reported that soil mineralogy and particle size distribution regu-
late the capacity of a soil to preserve OM and control soil aggregation (Carter, 2002). A marked
reduction in the quantity and/or quality of OM often leads to a deterioration of the soil structure
(Nemati et al., 2000; Zaher et al., 2005).
4.5.3 B
ulk
d
ensItY
The bulk density of soil is defined as the mass of dry soil per unit bulk volume. The unit of soil bulk
density is Mg m
−3
. Bulk density is a physical property of the soil that can be used as a simple index
to the general structural condition of the soil. Although it cannot be interpreted in a specific manner
as with the degree of aggregation, aggregate stability, or pore size distribution, bulk density does
provide a general index to air-water relations and impedance to root growth. The bulk density of
most surface soils usually ranges from 1.0 to 1.6 Mg m
−3
(Fageria and Gheyi, 1999). SOM signifi-
cantly influences soil bulk density. The higher the OM, the lower is the bulk density and vice versa.
Bockheim et al. (2003) reported that bulk density significantly decreased in a quadratic manner
with increasing organic soil carbon in the tundra soils of arctic Alaska. Prevost (2004) reported that
the SOM content was found to be closely related to bulk density and porosity after clear cutting and
mechanical site preparation.
Soil bulk density significantly influences the physical, chemical, and biological properties of
soil-plant systems and consequently the nutrient uptake. Bulk density is often used as an index of
assessing soil compaction and productivity. Heuscher et al. (2005) reported that organic carbon
content was the strongest contributor to bulk density prediction. These authors reported that organic
carbon shows a negative relationship with bulk density, indicating that bulk density decreases as
organic carbon increases.
