Phosphorus sorption and desorption in oxide-rich Ferralsols of New Caledonia

Phosphorus (P) availability in soils is one of the main limiting factors fo r crop production in Ferralsols that contain high amounts of aluminium and iron oxides. Highly weathered Geric Ferralsols cover large surface areas of south Pacific islands, some of which are required for agricultural develop ment. Sorption and desorption of P in Geric Ferralsols of the island of Mar e (New Caledonia) were investigated. The soils contain large quantities of oxides, especially of aluminium. The clay fraction consisted exclusively of fine, discrete oxide particles. Both Freundlich and Langmuir equations pro vided a good fit to the data for equilibrium P concentrations ranging from 0.05 to 850 mg P/L. The extremely high P sorption capacities of New Caledon ian Geric Ferralsols were exemplified by the large Langmuir sorption maxima which ranged from 6400 to 9250 mg P/kg. A strong negative correlation betw een P sorption and organic matter was noted, resulting in a larger P availa bility in topsoils. This relationship can be used to provide a first assess ment of P sorption capacities of Geric Ferralsols across the island. Manage ment of organic matter is likely to be of major importance in the control o f P availability in Geric Ferralsols of south Pacific islands.