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ORGANIC MATTER
In the last column we looked at the role of soil micro-organisms (MO) in
building soil fertility, with the soil food web. This column will look more
closely at the role of Organic Matter (OM) in soil fertility.
Organic matter feeds plants through nutrient exchange and through nutrient
release upon its decomposition. As organic matter decomposes, it is a continual
slow-release source of nutrient for plant uptake and for the soil microbial
population.
Organic acids, produced by organic matter decomposition to humus, help dissolve
minerals in the soil, making the mineral nutrients available to plants and for
the soil microbial populations. The organic acids also increase the permeability
of plant root membranes and therefore promote the plant roots’ uptake of water
and nutrients.
Organic matter is the energy source for the soil’s microbial life-forms, which
are an integral part of soil health. In 1 gram of humus-rich soil there are
several billion bacteria, 1 million fungi, 10 to 20 million actinomycetes, and
800,000 algae. Organic matter is necessary to create healthy conditions for
organisms that live in soil, including earthworms and beneficial fungi that
fight nematodes and other soil pests.
The microbes that feed on organic matter in the soil temporarily bind the soil
particles together. The fungi, with their threadlike mycelia, are especially
important They literally sew the soil together. The microbes secrete compounds
into the soil as they live, metabolize, and ultimately decompose. Their
secretions are a bacterial glue (polysaccharides) that holds soil particles,
thus improving the soil’s structure. Organic matter helps to break up clay and
clods and binds together sandy soil. It helps make proper aeration in clayey and
sandy soil possible.
Plants can obtain 96% of the nutrients they need from the air, sun, and water. A
loose, healthy soil assists in diffusing air and moisture into the soil and in
exchanging nutrients. Carbon dioxide released by organic matter
decomposition diffuses out of the soil and is absorbed by the canopy of leaves
above in a bed of closely spaced plants.
Good structure is vital to soil productivity because it ensures good aeration,
good drainage, good water retention and erosion resistance. A soil with good
organic matter content soaks up rain like a sponge and regulates water supply to
plants. A soil stripped of organic matter resists water penetration, thus
leading to crusting, erosion and flooding.
In order to maintain good soil fertility, about 4% to 6% (by weight) organic
matter is needed in the top 6 inches of soil.
Our next three columns will explore techniques organic gardeners can use to
improve soil fertility by increasing micro-organism (MO) and organic matter
(OM).
Most of the material for this column was obtained from the seventh edition of
John
Jeavons classic book “How to Grow more Vegetables”. We highly recommend this
book.
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Sustainable
Living Through Organic Gardening