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Soil Organic Matter
The beneficial effects of soil organic matter have been mentioned under
various sections, relating to nutrient supply, soil texture and water
management. The importance of maintaining soil organic matter cannot be
overstressed, particularly in relation to low-input, rainfed cropping systems.
Soil fertility decline under intensive cropping is often
associated with decline of soil organic matter. Mechanised agriculture tends to
ignore this resource, substituting its services with more fertilisers and
irrigation. However, changing the farming system to promote soil organic matter
can make it more efficient and more sustainable.
Increasing the organic matter content of the soil has a
number of beneficial effects. The gradual decomposition of this material
provides a steady supply of plant-available nutrients. The organic particles may
also provide a suitable substrate on which soil nutrients can be held in an
available form. Organic matter increases the soil’s ability to resist
acidification. It also increases water retention so that the soil takes longer
to dry out, and gives the soil an open texture so that more air can get to the
roots.
Organic matter is increased by leaving crop or fallow residue
on the field, without burning, or by bringing plant material from another site.
If the need for field sanitation prevents leaving the crop residue, then options
may be to compost it and return it later, or to use it to mulch another crop
which is not at risk from the pest. Often sweet potato vines are removed and
used to feed stock. Returning the animal manure to the field replaces some of
the nutrients, but it will not maintain the organic matter content of the soil
unless supplemented by plant material from fallow or green manure crops.
Improved fallows
The long fallows of traditional swidden farming systems allow organic matter
to accumulate in the soil, and this forms the basis for restoring soil fertility
for cropping. However, in tropical conditions, organic matter is quickly
degraded during cropping, and this source of fertility is lost within two or
three years. In many areas where swidden farming was practiced, land pressure
has meant that fallows have become shorter, and often the tree-dominant fallows
have been replaced by grasses or woody weeds, which are less effective in
restoring fertility (although grass fallows can be quite effective in restoring
soil texture).
In these situations, consideration needs to be given to improving the quality
of fallow vegetation, and reducing the losses of organic matter. Burning of
fallows is one of the biggest sources of loss, which can be avoided. Communities
which have traditionally burnt fallows will find it difficult to move away from
this practice. To overcome these prejudices, non-burning should be introduced
with planted fallows, selecting fallow species which are easy to clear, are
non-weedy and suppress weeds during the fallow. The concept that gardens should
be “clean”, with no debris lying on the ground, should be challenged, and
the benefits of surface mulches demonstrated.
Improved fallow species are most commonly legumes, which have the ability to
replenish soil nitrogen by “fixing” nitrogen from the atmosphere. Legumes
may be either trees or herbaceous species, and a large body of literature is
available on the merits of various species in particular situations. However,
non-legume species may also be included in an improved fallow, particularly
species which provide some other utility, such as production of firewood or pole
timber, fruits, fibres or medicinal substances, or which suppress pests or
diseases.
Typically an improved fallow may consist of an herbaceous legume “cover
crop”, which will establish quickly and suppress weeds, together with one or
more tree species, which may later take over from the cover crop. Trees which
can be propagated from pole cuttings are easy to establish among grasses and
weeds, but may not root as deeply as trees grown from seed, as they do not
produce a true tap root. Deep rooting may be an advantage in accessing nutrient
reserves and water from the subsoil, but this depends on local soil conditions.
Tree seedlings may be established during the cropping cycle, to give them a
head-start in the fallow. However, this may be difficult where mechanical
tillage is used.
Very short fallows (one year or less) may be restricted to herbaceous species
only. This becomes in effect a continuous cropping system, where one crop in the
rotation is dedicated to soil improvement.
It should be understood that the nitrogen fixed by a leguminous plant is
contained in the plant tissues. It does not replenish soil nitrogen until the
plant is decomposed in the soil (or recycled through the droppings of animals or
insects eating the plant). Growing a leguminous crop, such as peanut, does
not appreciably increase soil nitrogen as the nitrogen is removed with the
harvested crop.
Contributed
by:
Jane O'Sullivan |
Further
topics on Soil Management:
Soil management
Soil
structure
Plant
nutrients
Fertilisation
Causes
of nutritional disorders
Diagnosing
nutritional
disorders
Correcting
nutritional
disorders
Other topics on Crop Management: Land
preparation
Production
of planting materials
Planting
Soil
management
Vine
lifting
Integrated
pest management
Harvesting
Postharvest
practices
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