II.5. NUTRIENT BALANCE PERCEPTION AND IMPORTANCE.
II.5.1. Nutrient element balance as perceived generally.
The soils upon which plants depend for their food materials
developed from minerals. Plants with different nutrient requirements developed
on soil having different powers to supply nutrients. In SSA, many soils, in
their virgin state, do not furnish a balanced nutrient supply for agricultural
crops. Man, however, has not restricted his cultivation to the soils best
suited to the crops he wants to grow. Furthermore, he has continued to crop the
soil for years and has failed to return to the soil all of the nutrients
removed in the crops produced. The organic matter of the soil, one function of
which is to act as reservoir of slowly available nutrients, has also been
allowed to be depleted.
II.5.2. Plant nutrient balance in Sub Sahara Africa
Agricultural production in SSA is constrained by low soil
fertility, climatic conditions, lack
of infrastructure, and low availability and use of
agricultural inputs (particularly mineral fertilizers). In sub-humid and humid
regions, savanna and forest areas have a high variability of nutrients
losses/outputs and nutrients inputs.
For smallholder farmers cultivating small acreage in SSA,
utilization of straight fertilizers may be more economical but will also depend
on farmers' knowledge of nutrients required. Other practices such as crop
rotations including legume crops, recycling of residues and INM
(Intensification Nutrient Management) are needed to improve plant nutrition.
(Vanlauwe and Giller, 2006). Even in resource-limited smallholder agriculture
not all fields are continuously mined; some fields have very positive nutrient
balances, usually through concentration of nutrients from other parts of the
farm (Scones, 2001; Tittonell et al., 2005). This arises from the diversity of
plot management, as most organic resources and mineral fertilizers are used on
the home gardens and infields, and rarely on the outfields further away from
the homestead. The development of gradient of declining soil fertility with
distance from the homestead may not be a deliberate form of management, but
probably an inevitable consequence of the limited availability of cattle manure
and other nutrient resources. Preferential application of nutrients to the
infields and homestead gardens ensures good crop yields in these limited areas,
and save labour in terms of the distance the nutrients are transported
(Vanlauwe and Giller, 2006)
II.5.3. Availability of major elements in soil and their
importance
Small farmers use animal manure as a source of crop nutrients
which are good amendment and they contain N, P, K, Ca, Mg and micronutrients.
Nitrogen is the key nutrient for crops production. This element is the most
mobile and also most easily exhausted nutrient in the soil (Ghicuru et al.,
2004).
The effect of nitrogen to crop may be summarized as
follows:
1. it increases leaf size and therefore the potential for
greater photosynthesis, which will increase root growth, total dry matter and
yield of useful product;
2. it increases the protein content of storage organs, that is
grain, tubers, and roots:
3. it increases the proportion of water in the plant fresh
weight because of increased plant protoplasm:
4. it increases the site of plant cells and reduces the
thickness of their wall (Nyombaire,2001).
Phosphorus is the major element limiting crop production in
the tropics (Ghicuru et al., 2004). It plays a role in photosynthesis,
respiration, energy storage and several other processes in the living plant. It
promotes early root development and growth, it improves the quality of many
fruit, vegetable and grain crops. Phosphorus is also vital in seed formation
and its concentration is higher in the seed than any other part of the mature
plant (Nyombaire, 2001). Potassium is essential for plant growth unlike
nitrogen and phosphorus, potassium does not form organic compounds in the
plant. It is also essential in protein synthesis and helps the plant use water
more efficiently by promoting turgidity to maintain internal pressure in the
plant (Nyombaire, 2001).
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