CHAPTER 2. LITERATURE REVIEW
2.1 Phosphorus in soil
2.1.1 Role of phosphorus in plant growth
Phosphorus is one of major essential plant nutrients .It is
essential for plant growth, cell division ,root growth and lengthening seed and
fruit development and early ripening .It is absorbed as
H2PO4-- and HPO4--
forms.
According to Miller (1990), it is the one key plant nutrients
.It is an essential part of nucleoproteins in the cell nuclei, which control
cell division, and deoxyribonucleic acid (DNA) molecules, which carry
inheritance characteristics of living organism. In its many compounds,
phosphorus has roles in hastening plant maturity, energy transformation within
the cell, and in fruiting and seed production.
According to Russell (1995), root crops suffering from severe
phosphorus shortage are also stunted and the effect of added phosphorus can be
spectacular .As matter of history, the early workers were so impressed with the
great increase in the yield of roots obtained by phosphate fertilizers, which
they assumed the phosphate had a specific action in encouraging root
development.
2.1.2
Deficiency symptoms of P
Under the P deficiency, the plant has restricted growth of
both tops and roots characteristics with thin, erect and spindly stems,
restricted foliage consisting of narrow leaves, suppressing lateral buds. The
leaves turn into bluish color.
Older leaves become bronzed, reddish, brown or purple in color
having dead tissues all the tips. The symptom first appears on the tips or on
the margins.
In case of cereals and millets, the plants have bluish -green
leaves, reddish-purple tints at the internodes, leaves or even heads, while
Potatoes tubers develop rusty lesions. The ear heads are poorly formed,
maturity is delayed and lack of poor seed and fruit development is observed
(Moughalib, 2005)
2.1.3 Availability of
phosphorus in soil
Chemical elements (Iron and Aluminum), both from compounds of
low solubility with po4 ions in acid soils .From pH 5.0 up to neutral point,
soil phosphates have an appreciable solubility on account of basic ions present
that maintain the high pH and at the same time form some Ca and Mg phosphates
in soil .In soil from about pH 5.0 down ward, however, complex phosphates of
ion and Aluminum may be formed, which have a very low solubility and do not
supply sufficient phosphorus to plants. The Ca and Mg phosphates are more
soluble in the presence of CO2, which comes from decaying organic
matter, but this solubility effect decreases up to about pH (Millar, 2004)
According to Mohsin (1995), there are three important problems
that are generally associated with the management for crops sequences:
Ø Small amount of phosphorus in soil
Ø Low availability of native phosphorus soil
Ø High fixation of added phosphorus in soil
Behavior of P in acid soils depends to some extent the
concentration of soluble and active Aluminum and Iron besides P carriers used.
Soil characteristics like soil reaction, sesquioxides content etc...influence
the response of crops to added fertilizers in acid soils. Addition of phosphate
fertilizers to soil may not solve the problem as 71 to 80% of it remains in
soil and only 10to15 %reaches the plant.
2.1.4. Forms of soil
phosphorus
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