The importance of
soil organic matter:
"The nation that destroys its soil destroys itself" - Franklin D Roosevelt
S
oil organic matter consists
of living parts of plants
(principally roots), dead
forms of organic material
(principally dead plant parts), and
soil organisms (micro-organisms
and soil animals) in various stages
of decomposition. It has great impact upon the chemical, physical
and biological properties of the soil.
Organic matter in the soil gives the
soil good structure, and enables the
soil to absorb water and retain nutrients. It also facilitates the growth
and life of the soil biota by provid-
ing energy from carbon compounds, N for protein formation,
and other nutrients. Some of the
nutrients in the soil are held in the
organic matter, comprising almost
all the N, a large amount of P and
some S. When organic matter decomposes, the nutrients are released into the soil for plant use.
Therefore, the amount and type of
organic matter in the soil determines the quantity and availability
of these nutrients in the soil. It also
affects the colour of the soil.
Dead matter constitutes about 85
percent of all organic matter in
soils. Living roots make up about
another 10 percent, and microbes
and soil animals make up the remainder.
Organic matter that has fully undergone decomposition is called humus. The origins of the materials
after formation of humus cannot be
recognized. Humus is dark in colour
and very rich in plant nutrients. It is
usually found in the top layers of a
soil profile. The dark colour of hu-
Dead plants and animals add organic matter to the soil, spawning an entirely different web of soil
microorganisms. Bacteria are decomposers of fresh, green plant matter, while fungi prefer the more difficult to
decompose lignin and cellulose found in many tree leaves and in wood. Protozoa and nematodes help too,
although they also enjoy munching on the microorganisms smaller than themselves (and on each other.)