NOTES INFLUENCE OF RELATIVE HUMIDITY ON THE REACTION PRODUCTS OF PHOSPHATES AND NONCRYSTALLINE HYDROXIDES OF ALUMINUM AND IRON

Aluminum and iron in noncrystalline constituents in soils play a major role in phosphate fixation. Non-crystalline aluminum and iron hydroxides have often been used as model materials of these soil constituents. Previously, phosphate sorption on noncrystalline alu-minum and iron hydroxides were carried out in sus-pension (Nanzyo, 1984, 1986); however, phosphate fertilizers and soils ordinarily react in much less aqueous environments. Lehr et al. (1959) reported that 27-34% of monocalcium phosphate monohydrate (Ca(H2PO4)2' H20) was converted to d ica lc ium phosphates at 0.5-1.0 moisture equivalent in soils. Reaction of Ca(H2PO4)2"HzO to give dicalcium phosphate dihy-drate (CaHPO4.2H20) can be formulated as follow