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Influence of Exchangeable Calcium on Phosphate Retention by Weakly Acid Soils¹

ABSTRACT

The extractability of P from two weakly acid soils that had been amended with three forms of P [KH₂PO₄, single superphosphate (SSP), and Chatham Rise phosphorite (CRP)] was investigated in an incubation study under field conditions. In a sequential extraction sequence with water, P release from soils incubated for 9 and 27 weeks with KH₂PO₄ and SSP declined with increasing extraction number. In contrast, P extracted from CRP-treated soils increased, indicating a slow dissolution of phosphate rock. Washing with 0.1 M NaCl to remove exchangeable Ca resulted in a large increase in P extracted subsequently with water, regardless of whether sorbed P (as in KH₂PO₄- and SSP-treated soils) or particulate Ca phosphate (as when CRP was added) was the source of the extracted P. A high proportion (up to 80%) of the added P could be recovered by water extraction when exchangeable Ca was replaced by Na. Significantly, P recovery did not decrease with time of contact between soil and P as it did before exchangeable Ca was replaced with Na. The removal of exchangeable Ca accelerates the dissolution of Ca phosphates by creating a sink for Ca. The results obtained suggest that sorbed P may enter into chemical association with Ca, possibly at metal oxide surfaces, and that this interaction with Ca constitutes an important P retention mechanism in acid soils that contain adequate exchangeable Ca.

¹ Contribution from the Fertilizer and Lime Research Centre, Massey Univ., Palmerston North, New Zealand.

² Former Visiting Lecturer, Postdoctoral Fellow, and Professor, respectively. Present address of the senior author is Dep. of Agricultural Chemistry and Soil Science, University College, Dublin, Ireland; that of the other two authors is Dep. of Soil Science, The University, Newcastle upon Tyne, NE1 7RU, England.

Received for publication July 7, 1986.