Agriculture Reference
In-Depth Information
The role of SOM in B absorption and desorption processes in soil is not yet clearly understood.
Elrashidi and O'Connor (1982) showed a positive relationship between SOM content and B absorp-
tion, but they were not able to find any precise correlation between the content of OM and the
occurrence of significant hysteresis during B desorption from soil. Gupta (1968) found a positive
correlation between OM and hot-water-soluble B contents in soils. Since solubility in hot water is
considered an availability index, this result supports the hypothesis that OM is the main reserve of
B easily available to plants. Marzadori et al. (1991) reported that OM present in the soil has a char-
acteristic influence on the behavior of B in the soil. SOM appears to be responsible for occluding
important adsorption sites and hinders the possible hysteric behavior, that is, it plays a positive role
in B release from soil surfaces by conferring reversibility characteristics on adsorption processes.
Usually, lower Cu application rates are required with the organic than the inorganic carrier to
correct Cu deficiency by either a hard or broadcast application (Fageria and Gheyi, 1999). Similarly,
lower rates of broadcast Zn as an organic and as an inorganic carrier can generally be used to cor-
rect Zn deficiency (Moraghan, 1983). Humic substances are ubiquitous in soils, sediments, and
natural waters. They play a vital role in enhancing the availability of Cu (II) and other micronutri-
ent cations to plants and other living organisms and in reducing toxicity effects due to free Cu (II)
(Stevenson and Chen, 1991). Manganese plays a very important role in enzyme synthesis and in the
growth and development of crop plants. OM is the storehouse for Mn availability. Shuman (1985a,b)
studied the effect of texture and other soil properties on fractionation of Mn in soils and found that
Mn was primarily in the organic and manganese oxide fractions in fine-textured soils. Xiang and
Banin (1996) reported that Mn may be present in many forms in the soils such as OM, Fe oxides,
and primary and secondary minerals, and that these may also be partly available to plants.
4.6.3 C
atIon
e
xChanGe
C
apaCItY
OM, depending on its level in the soil, can make a significant contribution to the soil's CEC.
Increasing the OM level in the soil increased the soil CEC (Kapland and Estes, 1985; Fageria and
Gheyi, 1999). The marked effect of OM on soil CEC can be explained by the high CEC of OM
(Helling et al., 1964). Kapland and Estes (1985) reported that an incremental 1% increase in SOM
on a dry weight basis (starting near zero) resulted in a corresponding increase of 1.7 cmol CEC kg
−1
of soil.
Martel et al. (1978) also reported that CEC was highly correlated with OM in the surface horizon
of clay-rich soils in lowland Quebec, Canada. Results of the above-cited studies have shown that
OM makes a significant contribution to the CEC of the soil, but the actual contribution depends on
the soil pH. The results of Helling et al. (1964) show that, for each unit change in pH, the change
in CEC of OM was several times greater than for clay. At pH 2.5, only 19% of the CEC of several
grassland and forest soils was caused by OM, but this value rose to 45% at pH 8.0.
4.6.4 a
lumInum
t
oxICItY
OM plays an important role in controlling the level of aluminum in the soil solution (Bloom et al.
1979a,b). When grown at the same pH, plants from soils high in OM do not exhibit the symptoms
of Al toxicity common to plants grown in soils low in OM (Thomas, 1975; Coleman and Thomas,
1964). Hargrove and Thomas (1981) established that OM can, in part, alleviate Al toxicity. In the
24 soils of this study that contained no amendments, the variation in top dry weight and root dry
weight was attributed to the interaction of OM and Al (significant at P < 0.01). Foy (1964) suggested
that the reason alfalfa could grow in a Bayboro soil with high Al was because the Al was chelated by
OM, thus reducing the amount of Al in solution. Kapland and Estes (l985) reported that the critical
Al level of alfalfa was correlated with SOM levels (r = 0.88). An increase of 1% in SOM on a dry
weight basis (starting from about zero) increased the critical Al level by 0.3 cmol kg
−1
. Hargrove and
Thomas (1981) found in a study using artificial media that, by increasing SOM with peat, the critical
