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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.

This article has been cited by other articles:

				D. Curtin and J.K. Syers Lime-Induced Changes in Indices of Soil Phosphate Availability Soil Sci. Soc. Am. J., January 1, 2001; 65(1): 147 - 152. [Abstract] [Full Text]