Comparison of several ways of measuring soil phosphorus availability

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Abstract
Laboratory indices of P availability were compared for six soils. Phosphorus soluble in sodium bicarbonate, ammonium fluoride and citric acid solutions, and in water was determined. Maximum P adsorption capacity was calculated from data by a batch method obtained from equations using the Langmuir isotherm. Adsorbed P values were derived from the same equations. Measurements of P adsorption obtained by leaching soil samples with a phosphate solution yielded asymptotic curves which also allowed the maximum adsorption capacity to be calculated. Results obtained by the two methods were very similar. Leaching of soil samples by water was the basis for calculating maximum P desorption values; these agreed well with the adsorbed P values obtained from the Langmuir isotherm.Phosphorus uptake by plants, determined in a greenhouse experiment, was correlated with the various data and parameters obtained in the laboratory; there were no significant correlations with the P values from extraction methods, but there were good correlations with some of the parameters from the equilibration and leaching experiments.