Influence of lime and farm yard manure
par Dieudonne MUGOBOKA
ISAE Busogo - Bachelor's Degree 2008
Total Phosphorus concentration in surface soils varies between about 0.02 and 0.10 %. In most soils, 50 to 75% of the P is inorganic, although actual percentage can vary widely.
In neutral and calcareous soils, inorganic Phosphorus in the soil solution precipitates as Calcium Phosphate minerals.
The amount and particle size (i.e., surface area) of Calcium Carbonate (CaCO3, i.e., lime) minerals will increase the precipitation of Calcium Phosphate minerals on its surfaces. In acid soils, inorganic Phosphorus in soil solution precipitates as Iron (Fe) and aluminum (Al) phosphate minerals (Khasawneh. et al, 1986).
In neutral and calcareous soils, inorganic Phosphorus in soil solution becomes adsorbed to the surfaces of clay and lime minerals. In acid soil, inorganic Phosphorus in soil solution becomes adsorbed to surfaces of iron and aluminum oxide and clay minerals. Soils containing large quantities of clay (i.e., large surface area) will fix (i.e., adsorb) more Phosphorus than soils with low clay content.
A "portion" of the inorganic Phosphorus adsorbed on minerals such as lime, iron/aluminum oxides, and clay surfaces can desorb (i.e., go into solution) to buffer decreases in solution Phosphorus taken up by plants. The pH range of 6.0 to 6.5 is associated with minimum Phosphorus adsorption.
Labile Phosphorus is orthophosphate ions (H2PO4- and HPO42-) adsorbed to mineral surfaces. Labile Phosphorus is the readily available portion (fraction) of the total Phosphorus that exhibits a high dissociation rate and rapidly replenishes decreases in solution Phosphorus due to plant uptake. The remaining portion of adsorbed Phosphorus that does not readily desorb is called Non labile, and is not available to plants (Rayar, 2000).
Organic Phosphorus represents about 40 to 60% of the total Phosphorus in soils and typically varies between 20 and 80% in most soils. The Phosphorus content of soil organic matter ranges from about 1 to 3%. Mineralization (decomposition) of organic matter, primarily plant residues, can supply solution Phosphorus.
Organic Phosphorus mineralization on most conventional cropping systems probably contributes about 5 to 15 pounds/acre/year of plant-available Phosphorus.
In general, applications of manure can move organic Phosphorus compounds to a greater depth than can inorganic Phosphorus in solution.
Plants use perhaps one-tenth as much phosphorus as they do of nitrogen, yet adequate phosphorus to plants is a widespread problem. This is due to the insolubility of soil phosphate. Added soluble phosphates will readily combine with cations in soil solution to for low-solubility substance. Whatever the facts low solubility of soil phosphates are the major problem in getting and keeping soil phosphates available to plants (Raymond et al. 1990)