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Leaching Effect on the Dissolution of Two Phosphate Rocks in Acid Soils

Dep. of Agricultural and Environmental Science, The University, Newcastle upon Tyne, NE1 7RU England, UK

N. S. Bolan

Fertilizer and Lime Research Centre, Massey Univ., Palmerston North, New Zeland

*Corresponding author.

ABSTRACT

Dissolution of phosphate rock (PR) in soil is a necessary prerequisite for the P in the PR to become plant available. The dissolution of two PR materials, Gafsa (GPR) and Christmas Island A (CIPR), in six acid, Malaysian soils (pH in water 3.0–5.4) was examined in closed-incubation and open-leaching systems. Dissolution was determined from the increase in either 0.5 M NaOH-extractable P (P) or 1 M BaCl₂-triethanolamine (TEA)-extractable Ca (Ca) in soil amended with PR, compared with the control soil. In the closed-incubation system, dissolution of the reactive GPR was higher (22–71%) than that of the less reactive CIPR (26–42%). Despite an adequate supply of protons, the extent of dissolution of both PR materials was very small at 50 d, after which there was a negligible increase in the amount of P released. Calcium released from the PR materials occupied most of the negatively charged exchange sites and there was an insufficiently large Ca sink for dissolution to continue. Although the extent of PR dissolution increased with increasing P-retention capacity of the soil, in the closed-incubation system the size of the sink for Ca determines the extent of PR dissolution in these high P-sorbing soils. With one exception, the dissolution of both PR materials was larger (56–94%) in the open-leaching system and there was less difference in the extent of dissolution between the two PR materials and among the various soils. Continuous leaching resulted in the removal of cations (between 4.2 and 29.9 mmol_c kg^–1 soil), thus providing new sites for the Ca released from PR. The renewal of the Ca sink during leaching was the main reason for the continued dissolution of PR under leaching conditions. Studies of PR dissolution in soils using a closed-incubation system provide information that is less relevant to the field situation than do related studies involving an open-leaching system.

Contribution from the Dep. of Agricultural and Environmental Science, The University, Newcastle upon Tyne.

Received for publication March 7, 1991.