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Inorganic Phosphorus in Calcareous Rockland Soils of the Bahamas¹

ABSTRACT

The phosphorus (P) status of cultivated and forest calcareous "rockland" soils of the Bahamas was investigated to acquire information on available P, forms of inorganic P, and P adsorption maxima. Available P was extracted by four methods: Bray P-1, 0.5M NaHCO₃, 0.01M CaCl₂, and distilled-deionized H₂O. Soil P was fractionated according to the Chang and Jackson procedure, and Langmuir P adsorption maximum was determined by the method of Olsen and Watanabe for soils.

For cultivated soils the order for amount of P removed by the extractants was generally NaHCO₃ > Bray > H₂O > CaCl₂. The pattern for forest soils, having less available P, was NaHCO₃ > H₂O > CaCl₂ > Bray. Phosphorus fractionation data of selected soils indicated that 55.7 to 99.9% of the inorganic P was in the Ca-P fraction. Correlation analysis between P fractions and available P revealed that all four extraction methods were positively correlated only with the Ca-P fraction. Most of the adsorption maxima (computed from region I of the isotherms) for cultivated and forest soils were in the upper medium range. The Langmuir P adsorption constant, K₁, related to the energy of adsorption, was generally smaller for cultivated soils, suggesting that P, being more loosely bound to these soil surfaces, would be more available to plants. Some fine-textured forest soils had very high K₁ values, ranging from 52 x 10⁴ to 222 x 10⁴ liters/mole. The implication is that these soils will require heavy fertilizer P applications to adequately supply crops with P when cultivated initially.

¹ Contribution from the Dep. of Plant Science, School of Agric. and Home Ec., Tennessee State Univ., Nashville, Tenn.

² Research Soil Scientist (formerly of the Bahamas Agricultural Research Center, San Andros, Andros Island, Bahamas) and technician, respectively.

Received for publication October 9, 1980. Accepted for publication February 4, 1981.