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The Slow Reaction which Continues After Phosphate Adsorption: Kinetics and Equilibrium in Some Tropical Soils¹

ABSTRACT

Dissolved phosphate was mixed with topsoil samples, and the decline in solution phosphate concentration (P) was followed for 200–300 days by periodically shaking and extracting subsamples with 1 or 10 mM CaCl₂ (1:10).

During the first 20–40 days, (P) declined faster in soil suspensions that were being shaken than it did in undisturbed soil at 0.1 bar moisture. After 40 days of reaction, shaking time had little effect.

The slow fixation had first-order kinetics with respect to (P). The relative rate was faster in an Andept than in three Oxisols. It was unaffected by lime, though lime increased the strength of adsorption.

Equilibrium was achieved at 50 days in an Andept and 100–200 days in three Oxisols. At equilibrium, the amount of adsorbed phosphate remaining labile was estimated from values of (P), using 6-day adsorption isotherms. Labile phosphate so estimated amounted to 30–50% of the added phosphate, implying that the residual value of phosphate added to these soils should be substantial and permanent except for removal by crops and erosion.

Desorption isotherms diverged from adsorption isotherms less markedly with increasing time after phosphate addition, as if the slow reaction caused much of the apparent hysteresis.

¹ Joint contribution from Dep. of Soils & Plant Nutrition, Univ. of California, Davis, CA95616, and Dep. of Agronomy & Soil Science, Univ. of Hawaii. Journal series no. 1954 of Hawaii Agric. Exp. Stn. Supported by a grant from U.S. Agency for International Development to Univ. of Hawaii under section 211-D of Foreign Assistance Act, 1961.

² Associate Prof., Univ. of California, and Professor, Univ. of Hawaii, respectively.

Received for publication June 16, 1975. Accepted for publication September 3, 1975.

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