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2. Soil Fundamentals
Soil organic matter is composed of the
elements that make up the bodies of plant,
animal, and soil organisms—largely carbon,
oxygen, hydrogen, and nitrogen. You may
recognize that these four elements can be
found in the atmosphere as gases (carbon
dioxide, oxygen, and nitrogen) or vapor
(water). Photosynthesis, precipitation, and
nitrogen fixation bring these elements from
the atmosphere into the tissues of plants,
animals, and soil organisms. These elements
become soil organic matter through the
process of decomposition and reassimilation
by soil organisms, both large and small (see
“Biological characteristics” on page 51).
Because the largest quantity of soil
organic matter initially comes from
plants, in arid and semi-arid regions with
water limited plant growth, there is limited soil
organic matter in soils, often less than 2%.
The amount of soil organic matter can change
over human-relevant time scales, based on
management practices, but increases in soil
organic matter levels are constrained by
environmental factors such as temperature
and water availability. Remember that if you
are drastically altering the amount of organic
matter in your soils, you are changing the soil
ecosystem. Addition of soil organic matter
does not benefit all soil organisms and plants
equally — it is important to remember that
when you add soil organic matter you are
changing the physical characteristics of a
soil as well as the chemical characteristics —
specifically the nutrients, favoring growth of
some species over others. For example, some
noxious weeds benefit from increased organic
material or available nutrients, as they may be
better adapted to a nutrient rich environment.
Soil Organic Matter Component
Soil Organic Matter
(SOM, sometimes called just organic matter or OM) is biological material from plants and
animals at various stages of decomposition, the cells and tissues of soil organisms, and
substances produced by soil organisms.
Glossary