Agriculture Reference
In-Depth Information
Types of organic matter in soil
Most gardeners in Britain and Ireland work with mineral
soils that contain between 2 and 5% organic matter.
The main types of organic matter in the soil are:
X
living organisms
X
dead organic matter
X
humus.
Soil organic matter is derived from living organisms
such as plants, earthworms, insects, fungi and
bacteria. On death these plant and animal remains
decompose and are recycled along with any other
organic matter that is added. The remains of cultivated
plants are incorporated directly by digging them in
or indirectly having been used as a mulch or been
composted. More bulky organic matter can be added
in the form of material imported from elsewhere
such as farmyard manure (FYM), garden compost,
mushroom compost, composted municipal waste,
leaf mould, chipped bark, composted straw and green
manure.
decompose very slowly, so its physical effect persists,
but nutrient contributions are low. The distinction
between the 'green' and 'brown' organic matter is
crude, but a useful one when selecting materials for
composting (see p. 161 and Table 13.1).
Benefits of dead organic matter in the soil:
X
microbial activity is increased because it is food for
soil organisms
X
soil is physically opened up and aeration improved
by the dead but recognizable organic matter
X
water-holding capacity of the soil is improved by
fi ne (unrecognizable) organic matter
X
it is a dilute source of slow-release nutrients as
the organic matter decomposes and releases
minerals.
Humus
is one of the end products of decomposition
in aerobic soils. Large quantities of organic matter
yield tiny amounts of this black jelly that coats soil
particles, darkening the soil where it is present. It is
also sticky when wet so it helps in the creation of
crumb structure in soils where there is little clay to
bind particles together, but it also forms clay-humus
complexes which helps heavy (i.e. high clay) soils
to crumble more readily. Humus also has 'buffering
capacity' so, along with clay particles, plays an
important part in holding on to the nutrients that
would otherwise be leached from the soil profile while
continuing to release them to plants.
Benefits of humus in the soil:
The role of organic matter in the soil
The organic matter in its many forms has important
effects on the soil colour (see p. 144), soil structure
(see p. 147), water availability (see p. 154) and the
cultivation window (see timeliness p. 148).
Most of the
living organisms
contribute to the
decomposition of organic matter and the formation
of humus, but many also affect the soil structure by
moving soil and creating a network of interconnected
tunnels thereby improving aeration and water
movement.
Benefits of living organisms include:
X
plant and animal debris are converted to minerals
and humus
X
Rhizobia
and
Azotobacter
spp. fi x gaseous nitrogen
X
soil structure is improved by the activity of plant
roots, earthworms and other burrowing organisms
X
detoxifi cation of harmful organic materials such as
pesticides is undertaken by many bacteria species.
Dead organic matter
is a source of food for the
living organisms, but has two other important roles
in the soil. In its undecomposed state it 'opens up'
the soil, that is, creates bigger gaps between the soil
aggregates, increasing air and water movement. As it
decomposes, it can yield plant nutrients. In general,
'green' (succulent, leafy) organic matter decomposes
very rapidly, if conditions are right. Consequently, it
tends to have just a short-term physical effect, but
yields nutrients, especially nitrogen compounds.
The 'brown' (fibrous/woody) plant material tends to
X
in sandy and silty soils it helps to form stable
crumbs
X
in clays the surface charges on humus are capable
of combining with the clay particles, thereby
making them less sticky and more friable
X
'buffering capacity' is increased, which reduces the
leaching of some nutrients from the profi le
X
soils warm up more quickly in the spring because
darker soils absorb more of the sun's radiation
X
improves water-holding capacity.
Buffering capacity
is the ability of the soil to
retain nutrients including lime against loss by
leaching.
Decomposition of organic matter
As in any other plant and animal community, the
organisms that live in the soil form part of the
food
web
(see p. 38). The organic matter derived from
dead plants and animals of all kinds is digested by a
succession of species: large animals by crows, large
