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Influence of lime and farm yard manure

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par Dieudonne MUGOBOKA
ISAE Busogo - Bachelor's Degree 2008

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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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