Agriculture Reference
In-Depth Information
most soils and so are not generally added as fertilizer. Both calcium and magnesium are
instrumental in controlling soil pH, for which purpose they are added to soil, and so
they have a dual role in plant production.
9.7
SOIL ORGANIC COMPONENTS
Soil organic components come from living and dead plants, animals and microorgan-
isms, and humus, which is synthesized when organic matter is decomposed. Plants
add organic matter from both tops and roots. Typically, soil that has grass, which in
some areas dies in the fall each year, will have a deeper darker surface horizon than
other soils. Animals add organic mater in the form of excreta, hair, and skin while
they are alive. In death they add the organic matter of their body to the soil. Micro-
organisms add organic matter that they cannot digest and waste products from
decomposition of organic matter (OM). In addition they add their bodies when they die.
Aerobic microbial
degradation
OM þ O
2
!
CO
2
þ H
2
O þ Energy þ Released plant nutrient þ Humus
(9:1)
Anaerobic microbial
degradation
OM !
CO
2
þ H
2
O þ CH
4
þ Energy þ Released plant nutrient þ Humus
(9:2)
Organic matter added to soil undergoes rapid decomposition, especially if the soil
is moist and at a temperature above 158C. Decomposition is generally faster under
oxidizing conditions, illustrated in reaction (9.1), as opposed to reducing (anaerobic)
conditions, illustrated in reaction (9.2). Under either set of conditions small pieces of
organic matter undergo the fastest decomposition.
A great number of changes occur during the organic matter decomposition process.
First, the complex organic and biomolecules are broken down. This releases plant nutri-
ents, commonly abbreviated by capital letters, that is, nitrogen compounds (N), phos-
phorus (P), potassium (K), sulfur (S), and potentially all the other plant nutrients that
happen to be present in the organic matter. In all cases these are intended to be forms
available to plants. Humus, which has high absorptive capacity for water, plant nutrients,
organic molecules, and other organic material, is produced, or synthesized, during this
decomposition process. Humus also readily releases sorbed nutrients for plant use.
Soil organic matter is highly variable from soil to soil and region to region. Organic
matter can be less than 1 percent in many African soils, around 1 percent in many soils
in the United States, and 2 percent in some soils in the central Philippines. However, it
is not uncommon to find soils containing 10 percent organic matter. There is even a soil
order, the Histosols, that are primarily organic matter; see Table 9.3.
Generally speaking, organic matter is not taken directly into plants. However,
organic matter has dramatic beneficial effects on soil, especially in relationship to
