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Changes in Water-extractability of Soil Inorganic Phosphate Induced by Sodium Saturation

Dep. of Agronomy, Oklahoma State Univ./USDA-ARS, Water Quality and Watershed Res. Lab., P.O. Box 1430, Durant, OK 74702-1430

D. Curtin and J. K. Syers

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

*Corresponding author.

ABSTRACT

The effect of replacing indigenous exchangeable cations with Na on the subsequent release of inorganic phosphate (P) from 34 USDA-SCS benchmark soils representing nine soil orders was investigated by sequential extraction with distilled water. Sodium saturation substantially increased the water extractability of P regardless of whether Ca-P or sorbed P was the predominant form of inorganic P in the soil. On average, Na-saturated soils released about 2.5 times more P than the untreated soils. Sequential chemical extractions were used to identify the sources of P extracted. In soils that had not been Na saturated, water-extractable P appeared to originate largely from the NaHCO₃-extractable fraction. The additional P released following Na saturation was accounted for by a disappearance of Ca-P compounds (HCl-extractable) in soils where such entities were present or a decrease in the sorbed-P fraction extracted by NaOH. The results indicate that the distribution of P between solid and solution phases is strongly dependent on the nature of the exchangeable cation population. These results have important implications to soil fertility and P availability in irrigated soils influenced by Na.

Received for publication July 20, 1987.