There are several
factors which are known to be the influencing factors of the availability of
nutrients to activated plants. In some instances, these nutrients maybe present
in the soil and may not be absorbed by the roots of the plant. This phenomenon
is caused by some factors among which are; leaching, effect of oxidation,
effect of pH, excess of other nutrients and effect of burning.
Leaching
Leaching is
the washing down of the nutrients in the top soil into the subsoil. It is
favored by the movement of water and takes place most readily in permeable
soils which are in organic matter. Thus, excessive water can drain away some
dissolved cations while some will accompany the percolating water and then
accumulation in the deeper layers where the plants roots cannot reach. Carbondixide
which is dissolved in rain water forms carbonic acid. This carbonic acid
ionizes to give hydrogen ions which can then replace the alkaline ions which
are present in the soil. This process leads to an accumulation of hydrogen ions
in the soil, thus making it acidic.
Effect of oxidation
The effect
of oxidation is the breaking down of the soil organic matter in the presence of
oxygen. Ammonium compounds which are in the soil maybe oxidized to ammonia, and
nitrates may as well be reduced to nitrogen or oxides of nitrogen by
denitrifying bacteria. The gases produced from these reactions escape into the
atmosphere leaving the soil with low nitrogen content.
Effect of soil pH
The pH of
the soil is an index of the hydrogen ion concentration in soil solutions. pH=-log10H+.
Majority of the soil solutions have more hydrogen ions than hydroxyl ions and
are therefore acidic. Some have more hydroxyl ions than hydrogen ions and this
are alkaline, while others have equal concentrations of hydrogen and hydroxyl
ions and are termed neutral. The soil pH may influence absorption of nutrients
and plant growth. If the soil is too acidic, that is below 5.0, and then
aluminum iron and manganese dissolved in sufficient quantities than they become
toxic to the plants. However, if the soil is too in the nature of alkalinity,
the elements will not dissolve and will not be available to plant.
Excess of other nutrients
Micronutrients
in plants are those nutrients required by plants in very small amount. They
become toxic when their concentration exceeds the amount required for optimum
growth. For instance, chlorosis, which is cause by the lack of iron, is common
in soils containing large qualities of calcium which can also depress the
intake of magnesium and potassium. The application of large qualities of
phosphate fertilizer can affect the supply of some of the micronutrients such
as iron and zinc very adversely.
Effect of burning
The practice
of burning vegetation and crop residues has some advantages as well as
disadvantages. The advantage of burning lies mainly on the reduction of labor
required to clear the piece of land. However, the disadvantages outweigh the
advantages. The disadvantages include loss of carbon, nitrogen, sulphur and
phosphorus in gaseous form. Fire in its own nature also destroys organic matter
which is very rich in major plant nutrients. It also destroys the micro –
organism which helps in the improving of soil structure. The lost nutrients can
be replenished through rotational cropping, organic matter manuring, inorganic
fertilization, fallowing, liming, cover-cropping as well as mulching.
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