Enhancement of Phosphorus Solubility by Humic Substances in Ferrosols1

An investigation was conducted to study the effect of humic substance (HS) on the phosphorus (P) solubility in acidic soil. The soil (2.5 g), HS (0, 0.5, and 2.5 g), and P as monocalcium phosphate (0.31 and 1.25 g P kg−1 soil) were mixed with 50 mL distilled water and two different sequences of adding HS and P were used. The results indicated that the P concentration in water and 0.01 mol L−1 CaCl2 solution increased with increasing amounts of humic substance. The concentrations of Fe and Al were also increased. However, Olsen P decreased with increasing amount of humic substance. Water-soluble P concentrations from P rates at 0.31 and 1.25 g P kg−1 soil in the treatment with 0.5 g (2.5 g) humic substance addition were 360% and 70% (500% and 90%) higher, respectively, than those in the treatment with no humic substance addition. P extracted by 0.01 mol L−1 CaCl2 in the treatments with 0.5 and 2.5 g humic substance addition was increased by 400% and 540%, respectively, compared with that in the treatment without humic substance at the rate of 0.31 g P kg−1 soil, while the corresponding P concentrations were increased by 80% and 90% at the rate of 1.25 g P kg−1 soil. The order of mixing humic substance and phosphate did not significantly affect desorbed P and labile P extracted with CaCl2.