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Relationships between Sesquioxides, Kaolinite, and Phosphate Sorption in a Catena of Oxisols in Malawi¹

ABSTRACT

A catenary sequence of Oxisols was examined to determine the phosphate sorption capacity of the oxic horizon and to identify the minerals responsible for this property. The phosphate sorption capacity of the six soil samples taken along the sequence was found to increase downslope as far as the midslope, then to decrease towards the valley bottom. All samples contained quartz, kaolinite, goethite, and hematite; gibbsite was identified in one sample. Aluminium-substituted goethite was found to be the mineral mainly responsible for phosphate sorption capacity and the one that caused variations in this property along the slope sequence. Significant correlations between phosphate sorption capacity and both Fe and Al extracted by citrate-dithionite are explained by their occurrence in this oxide. Amounts of noncrystalline oxides were low and there was no correlation between phosphate sorption capacity and amounts of either Fe or Al extracted by oxalate. Kaolinite was calculated to account for 12 to 29% of the phosphate sorption capacities of the soils but there was no significant correlation between the two. Hematite, which showed a progressive decrease in content downslope, had no measurable influence on phosphate sorption capacity. The distribution of Fe oxides along the sequence suggests rather slow pedogenetic change in the horizons sampled with goethite, the main oxide product of contemporary soil-forming processes.

¹ A contribution from the Soil Science Unit, University College of Wales, Aberystwyth, SY23 3DE U.K.

² Research Student and Senior Lecturer, respectively.

Received for publication March 15, 1983. Accepted for publication November 22, 1983.

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