Effects of low-molecular-weight organic acids on phosphorus release from phosphate rocks

In order to study the effect of low-molecular-weight organic acids (LMO) on phosphate rocks solubility for direct use in agricultural activities, a factorial experiment was conducted at completely randomized design in three replication. LMO acids were acetic, salicylic, malic, oxalic and citric at five concentration levels (0.5, 1, 2, 5 and 10 mM) and a control (no acid), on two phosphate rocks (RP) including igneous of Yazd (RPi) and sedimentary of Yasooj (RPs). Results showed that use of low-molecular-weight organic acids caused to release large amounts of phosphorus from both phosphate rocks. Among organic acids, citric and oxalic were more effective than others. Citric acid increased soluble P from 21 to 487 μg/L in RPi and oxalic acid increased soluble P from 36 to 400 μg/L in RPs. With increasing concentration of organic acids from 0 to 10 mM, soluble P increased significantly in both phosphate rock. Soluble P increased from 21 to 888 μg/L in RPi and from 36 to 546 μg/L RPs. The sufficiency level of soluble P for most plants (200 μg/L) was found in the 0.5 and 5 mM of LMO acids for RPi and RPs, respectively. It is concluded that the release of phosphate from RP was affected by types and concentrations of organic acids and composition of RP.