The major sources of phosphorus in soil are parent material (the apatite group of primary minerals), plant and animal residues and inorganic fertilizers.
Total soil P contents vary considerably over a wide range and depends on the organic matter content, parent material and degree of weathering. A value of 120 to 1880g g -1 (or 268.8 to 4211.2kg per hectare) had been quoted for soils in some areas.
Inorganic Phosphorus in Soils
Inorganic phosphorus constitutes 20 – 80% of total phosphorus in most soils and is usually associated with calcium and magnesium or with the oxides and hydroxides of aluminum, iron, and manganese. The various forms of inorganic phosphorus in soil are explained as follows:
1. Solution-Phosphorus
This form is also called available phosphorus because it is the form taken up by plants. These P-forms are H 2 PO – 4 and HPO 4 2- , the combined concentration of which is very low in soils with a range of about 1-3 mg P per ml. A value of 0.01 to 0.02mg P per liter were found in saturated extracts of three Northern Nigerian ferruginous soils by Bache and Rogers (1970).
Phosphate in solution represents the intensity factor while the other phosphorus forms constitute the quantity or capacity factor. The ratio of H 2 PO – 4 and HPO 2- 4 in soil solution is pH dependent. That is, high H + concentrations shift the equilibrium to the more protonated form.
At pH5, HPO 4 is almost absent whereas at pH 7.0 both phosphate species are present in almost equal proportions.
Usually, the amount of phosphorus in solution is much less than plant need. The soil must therefore contribute to P-nutrition of plants from other sources such as the labile-P form.
2. Labile-P or Surface-P
Labile – P is the phosphorus adsorbed onto the surfaces of soil mineral particles and it is usually exchangeable with solution –P. Surface-P replenishes the P being depleted from the solution of rapid equilibrium reaction.
How is surface P formed? Surface P is formed by the attraction of phosphate anion, H 2 PO 4 – , onto the surfaces of aluminum, iron and manganese oxides and hydroxides in acid soils or adsorption to calcium in alkaline calcareous soils, thus making phosphorus unavailable. Thus in acid soils, phosphate is locked up as A1 and Fe – hydroxyl phosphate.
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3. Non-Labile or Sub-Surface P
As phosphate is added to the soil solution, some OH – is replaced on the mineral surfaces, this process continues until some of the H 2 PO – 4 ions begin to migrate into the soil mineral particles to form part of the sub- surface P.
The migration is detected by a slow equilibrium reaction unlike the surface-P reaction which is in rapid equilibrium with solution P.
That is, the sub-surface P is slow equilibrium with surface P while the surface P is in rapid equilibrium with solution P. The sub-surface P is available with difficulty unlike surface P which is exchangeable and moderately available.
3. Occluded Phosphorus
The migration of surface P to sub-surface may be at such magnitude that some of the migrating P may lose contact and be completely blocked out of reach of some of the surface P and form occluded P.
The occluded P is in very slow equilibrium with the sub-surface P because the occluded P has migrated further into the A1 (OH) 3 or Fe (OH) 3 crystals. The P becomes unavailable. This process could happen when time is too long after P fertilization before cropping.
4. Clay Lattice Locked-Up Phosphorus
At the edge of silicate clay minerals, there are hydroxyl ions, OH – which could be replaced by H 2 PO – 4 ions.
This is called exchange reaction between OH – and H 2 PO 4 – whereby P is made less available than the amorphous form.
The highly weathered tropical soils (laterites) usually contain high contents of aluminum, iron and kaolinite clays which usually mop up phosphate ions either applied as fertilizer or released from mineralized process.
Reactions of Phosphorus with calcium
In calcareous soils, pH range is usually around 7-8.5. In these alkaline soils, calcium precipitates phosphorus; that is, low soluble calcium phosphate is formed, soluble phosphate ions (H 2 PO – 4 , HPO 2 – ) can also be adsorbed to surfaces of solid calcium carbonate.
This mono hydrogen orthophosphate has solubility of 0.29g per 100ml. Calcite (CaCO 3 ), Ca(H 2 PO 4 ) 2 and CaHP 4 are the three forms of phosphorus present in calcareous soils and are in equilibrium which is mainly disturbed by the concentration of CO 2 .
As CO 2 increases solubility of P increases, for example in the rhizosphere soil, CO 2 is given off by the microbial and root population and P becomes more soluble.
However, in alkaline soils but non-calcareous there is no problem of P- fixation because KH 2 PO 4 , K 3 PO 4 , NaHPO 4 or Na 3 PO 4 which are formed in alkaline soils are usually soluble.
In the presence of CaCO 3 , rock phosphate (apatite) is formed. Powdered rock phosphate can be used directly as fertilizers but with low solubility.
From the forgoing information on the behavior of phosphate in both acid and alkaline soils, it could be concluded that maximum phosphorus availability is at pH of 6.5 for mineral soils and about 5.5 for organic soils and ox sols.
Liming practices which increase soil pH from 5.5 to 6.5 improve phosphorus availability to crops.
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Organic Phosphorus in Soils
This fraction of phosphorus in soils varies considerable ranging between 20 and 80% of the total P in the surface layer of soils. In Nigeria, organic P constitutes about 17 to 72% of the total P and a range of 46 – 70% for Ghana soils.
There are three major groups of organic phosphorus compounds identified in soils; namely, inositol phosphates, phospholipids and nucleic acids out of which inositol phosphate predominates.
The mineralization of soil organic P is brought about by the activities of soil microorganisms.
Phosphorus Deficiency and Toxicity
Since P is a mobile element, deficiency symptoms first show up in older leaves of plants.
The deficiency symptoms may appear as: Stunted overall growth of whole plant compared to normal plants. Dark green color, dark red to purple discoloration of stems, and at times dull green.
Protein synthesis is impaired, vegetative growth is depressed. P-deficient plants have limited root system and thin stems. In cereals, tailoring is affected.
There is a deposition of starch in roots. Stems of annual plants have reddish green color because of formation of anthocyanins. Leaves are tinged with brownish color and fall off prematurely. Phosphorus contents of P-deficient plants are usually low (0.1% P).
When cereal and herbage are supplied with P, their P-content may go up to about 0.3 to 0.4%. Phosphorus toxicity is not common, but when it occurs it leads to reduced growth due to retardation of up-take and translocation of micro-nutrients including zinc, iron and copper.