Agriculture Reference
In-Depth Information
4.5.4 w
ater
-h
oldInG
C
apaCItY
The knowledge of water dynamics in soil is essential for a better management of irrigation, fertiliza-
tion, and leaching of nutrients and heavy metals from a soil profile (Gerard et al., 2004). One of the
most important effects of OM addition to the soil is that it changes the soil's water retention char-
acteristics, which are generally positively related to crop production. A reduction in the available
water capacity is considered the foremost contributing factor in the loss of soil productivity caused
by erosion. This reduction in the available water capacity is attributed to changes induced in the
soil's water-holding characteristics of the root zone or by reduction in the depth (thickness) of the
rooting zone (Bauer and Black, 1992). Most of the OM is generally concentrated in the plow layer of
the soils (Fageria et al., 1991). Frye et al. (1982), in Kentucky, reported that the available water-hold-
ing capacity of an eroded Maury soil (fine-silty, mixed, misic typic paleudalf) was 4 × 6.1 lower in
the upper 15 cm on a volume basis than its noneroded counterpart. Biswas and Khosla (1971) also
observed a similar increase in the soil-water retention characteristics and hydraulic conductivity
from applying farmyard manure to soils over a 20-year period. Mays et al. (1973) found an increase
from 11.1% to 15.3% in the water content corresponding to the 0.33 bar (in modern terminology,
the pascal (Pa) is the unit of pressure, 1 bar = 10
5
Pa = 10
2
kPa = 0.1 MPa) suction of a silt loam soil
after an application of 327 metric tons ha
−1
of municipal compost for 2 years.
Gupta et al. (1977) also reported that the amount of water retained at 15 bars increased linearly
with the increase in sludge addition of OM in a coarse sandy soil. Scoot and Wood (1989) reported
a linear increase in the water retention of Crowley silt loam soil with increasing OM content. OM
content alone could account for 84.4% of the variability of water retained at 10 kPa. The slope of the
line indicates that each 10 g kg
−1
of OM could account for an increase of 5.6% by volume of water
retained (Scoot and Wood, 1989). Martens and Frankenberger Jr. (1992) studied the effects of differ-
ent organic amendments on the soil physical parameters and water infiltration rates on irrigated soil.
The incorporation of three loadings (75 Mg ha
−1
each) of poultry manure, sewage sludge, barley
straw, and alfalfa into an Arlington soil for 2 years increased the soil respiration rates (139-290%),
soil aggregate stability (22-59%), organic C content (13-84%), soil saccharide content (25-41%),
and soil moisture content (3-25%), and decreased soil bulk density (7-11%). The changes in soil
physical properties resulted in significantly increased cumulative water infiltration rates (18-25%)
in the organic-amended plots as compared with unamended plots. The increase in water retention of
soil due to the addition of OM may be related to the following factors: (i) decreased bulk density and
increased total porosity, (ii) change in the aggregate size distribution (which may change the pore
size distribution), and (iii) increased absorptive capacity of the soil (increase in the total surface
area) (Fageria and Gheyi, 1999).
Lado et al. (2004a) reported that the saturated hydrolic conductivity of soil with high OM content
(3.5%) was higher than that with low OM content (2.3%). Similarly, Lado et al. (2004b) reported that
in sandy loam soil, an increase of OM content from 2.3% to 3.5% reduced the aggregate breakdown,
soil dispersivity, and the seal formation at the soil surface under raindrop impact conditions. These
authors suggested that the final infiltration values were lower in the low than in the high OM soil
because (i) there was a more extensive breakdown and dispersion of the aggregate at the surface of
the low OM soil than at that of the high OM soil, so that a more continuous crust was formed on the
former soil, and (ii) the rearrangement of the detached and dispersed particles in the crust differed
between the two soils, so that a thicker, higher-density crust was formed on the low than on the high
OM soil.
4.6 OM VERSUS SOIL CHEMICAL PROPERTIES
OM brings many significant changes in soil chemical properties such as reducing Al toxicity and
decreasing allelopathy in crop plants. It improves the availability of macro- and micronutrients
to crop plants. OM in soils also controls fluctuations in the pH buffering capacity. The main soil
