Agriculture Reference
In-Depth Information
RetentionofNutrientsbySoilOrganicMatter
Soil organic matter has a heterogeneous composition not only because of the vari-
ety of organic residues deposited on the soil but also because of the many bio-
chemical processes, driven by microorganisms, which occur in soil. The role of
polysaccharides and polyuronide gums (sticky materials) in stabilizing soil struc-
ture is discussed in “Soil Aggregation,” chapter 4. Here I describe the properties
of charged compounds, such as polymerized phenols and organic acids, which are
important constituents of humified organic matter.
As plant and animal residues decompose, a dark, humic material slowly forms,
most easily seen beneath the litter from broadleaf trees or grasses (figure 3.6), or in
well-madecompost(see“OrganicViticulture”).Humiicationleadstoanincrease
in the number of organic chemical groups that can dissociate H
+
ions and become
negatively charged. These groups are weak acids of varying strength so that they
gradually release H
+
as the pH increases from less than 3 to more than 8.
The negative charge on the organic matter increases as the pH increases and
this translates into an increase in
CEC
. Compared on a unit weight basis, humi-
fied organic matter has 3 to 10 times the
CEC
of illite or montmorillonite clay
Humified layer
Figure 3.6
Example of
well-humified organic matter
formed at the soil surface under
grass litter.
