2.1.6. Factors affecting soil phosphorus availability in
soil
Most crops recover only 10 to 30% of fertilizer Phosphorus
during the first year of application. Recovery percentage varies widely,
depending on Phosphorus source, soil type, crop grown, application method and
weather, but much of the residual will be available to succeeding crops.
Phosphorus availability varies with the following factors:
Amount of clay: Soils high in clay content
will fix more P than those containing less clay.
Type of clay: Soils high in certain types of
clay minerals like kaolinite, Al, Fe oxides and
hydroxides (common in the regions of high rainfall and
temperatures), and amorphous clay minerals like allophone, imogolite and
humus-Al complexes (common in soils formed in volcanic ash) retain or fix more
added P than other soils .Regardless of clay type, fertilizer P is converted to
less available forms.
Time of application: The longer the soil and
added P are in contact, the greater the chances for fixation. On high-fixing
soils, the crop must use fertilizer p before fixation sets in.
Aeration: Oxygen is necessary for plant
growth and nutrient absorption. It is also essential for microbiological
breakdown of soil organic matter, an important P source.
Compaction: Compaction reduces aeration and
pore space in the root zone. This reduces P uptake and plant growth.
Compaction also decreases the soil volume plants roots penetrate, limiting
their total access to soil P. the fact that P moves such short distances in
most soils adds to the problem of restricted root growth and nutrient uptake
brought on by on by compaction.
Moisture: Increasing soil moisture to optimum
levels makes P more available to plants. But excess moisture reduces
O2, limiting roots growth and slowing P uptake.
Phosphate status of soil: Soils that have received more P
fertilizer than crops have removed for several years may show an increased
level of available Phosphorus. Current fertilization may be reduced if the soil
level is high enough. It is important to maintain high soil P levels to
support optimum crop production.
Temperature: When temperatures are right for
good plant growth, they affect P availability very little. High temperatures
encourage organic matter decomposition. But when temperatures are too high or
too low, they can restrict P uptake by the plant.
Other nutrients: Applying other nutrients may
stimulate Phosphorus uptake. Calcium on acid soils and sulphur, on alkaline
soils seem to increase Phosphorus availability, as does ammonium-N. But Zinc
fertilization with borderline P deficiency tends to restrict P uptake
further.
Crop: Some crops have fibrous root and others
tap root systems. Therefore, crops differ greatly in their ability to extract
available P from the soil. Time and methods of P fertilization should be
matched to the cropping system to ensure most efficient use.
Soil pH: IN soil dominated by 2:1 type clays,
solubility of various P compounds are largely determined by pH. Phosphates of
Fe, Mn and Al have low solubility. They dominate in acid soil. Insoluble Ca
and Mg compounds exist above pH 7.0. The most soluble or available P forms
exist in the 6.0 to 7.0 range.
The mechanisms of P fixation in highly weathered soils of the
tropics (Ultisol and Oxisols dominated by Al and Fe oxides and koalinite) and
in soils derived from organic ash (Andisols) are different.
This process retain appreciable amount of applied P in the pH
range from 5 to 7.0 Lime application on tropical soils corrects Al toxicity and
Ca deficiency, and the correction of these factors leads to an increase in P
uptake even though liming has very little direct effect on P fixation (Gupta,
2004).
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