Total phosphorus content of U. P. soils ranged from 19.00 to 82.50 mg P/l00 g soil. Organic, available, saloid, aluminium, iron and calcium phosphates comprised 32.7,4.3, 2.9, 8.8, 8.2 and 40.4 per cent of total phosphorus. Total and organic phosphorus, in general, decreased with depth, whereas inorganic phosphorus increased. Soils under investigation were medium to high in available phosphorus content except Rakar soil of Jhansi district. Most available fractions were saloid and aluminium phosphates, as these showed positive correlations with available Phosphorus, i.e. r=0.924** and r = 0.807** respectively. Individual physico-chemical characteristics did not have any consistent effect on phosphorus availability. Organic phosphorus showed positive correlation with silt plus clay percentage (r = 0.640*), whereas it had negative correlation with pH (r=−0.570*). Organic phosphorus might be mineralized into aluminium and iron phosphates, as it had significant positive correlations, i. e. r=0.706* and r=0.789* respectively. Aluminium and iron phosphates decreased with rise in pH, whereas calcium phosphate showed direct relationship with pH. The significant correlations between iron phosphate and pH and calcium phosphate and pH were=−0.705* and r =0.746* respectively. Calcium phosphate was observed to have declined trend where soils were rich in organic matter content (r=−0.568*). Iron phosphate showed negative correlation with calcium phosphate, i.e. r=−0.661*, which might explain that calcium phosphate was converted to iron phosphate by chemical weathering.
Various P-forms, interrelationship with physico-chemical characters
