[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

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J.D. DUTCHER ET AL.
Soil organic matter is composed of partially decayed and partially synthesized
plant and animal residues. Although, the organic matter content of a mineral soil is
generally only about 3–5%, its influence on soil properties and plant growth are
great.
Due to the work of soil micro-organisms, organic matter should be constantly
renewed by the addition of plant residues. Legumes such as crimson clover break
down quickly; however their root systems remain tough and fibrous, contributing to
the accumulation of organic matter. The addition of organic matter to soils improves
soil structure, increases water holding capacity, increases cation exchange capacity
(the ability of the soil to act as a short term storage bank for positively charged plant
nutrients), and provides more efficient storage of nutrients.
Organic matter functions as a “granulator” of soil mineral particles. In most
cases, the higher the soil organic matter, the more loose, easily managed, and
productive the soil. Organic matter can also serve as a partial source of N, P and S.
Through its effect on the physical condition of the soil, organic matter can increase
the ability of the soil to hold moisture and make soil water more available for plant
growth.
Cation exchange is one of the most common and important of soil reactions.
The cation exchange capacity (CEC) of a soil represents the capacity of the soil to
hold cation, or positively-charged nutrients such as Ca +2 , Mg +2 , K + , and NH 4 + . The
CEC is determined by the amount of clay or organic matter present in the soil. Soils
with a higher clay and organic matter content have a higher cation exchange
capacity than sandy, low organic matter soils. Hydrogen ions from the root hairs and
soil microorganisms replace nutrient cations from the exchange complex. These
nutrient cations are then forced into the soil solution, where they can be more readily
assimilated by the root surface (Brady, 1974). Due to their effects on improving soil
organic matter, legumes can aid in this process.
Legumes help to increase the total number and diversity of soil organisms,
which is the key to a healthy, well functioning soil. As organic matter increases,
especially if succulent and subject to relatively rapid decay, it encourages microbial
action of the heterotrophic organisms responsible for basic decomposition, as well as
“free-living” bacteria, such as Azobacter, which can also fix N from the atmosphere.
Legumes are closely associated with beneficial fungi, the mycorrhizae, which
produce a water-insoluble protein known as glomalin, which binds and glues
together particles of organic matter, plant cells, bacteria, and other fungi.
Well aggregated soils are less prone to compaction. Heavy farm implements
such as tractors, sprayers, mowers, shakers, and harvesters often make numerous
passes over the orchard floor in a given season. Mycorrhizal fungi also have an
efficient method of absorbing phosphorous (P) from the soil, which they pass on to
their host. Without this relationship, P builds up in the soil. Although it is not
leached, it can runoff into streams and rivers through soil erosion. The filaments of
the mycorrhizal fungi effectively extend the root system and help the plants tap
more P from the soil. Keeping P in an organic form is the most efficient way to keep
it cycling in the soil.
The culture of cool-season legume crops has both soil and nutrient conserving
properties that are highly advantageous and readily applicable under most humid