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Characteristics of Silica Sorption and Solubility as Parameters to Evaluate the Surface Properties of Tropical Soils: I. The Index of Silica Reactivity¹

ABSTRACT

Silica sorption by some Alfisols and Ultisols from southern Nigeria was studied. The percentage of silica lost from a solution brought into contact with a soil is called the index of silica reactivity (ISR).

For reference samples, the scale of ISR follows the order kaolinite < < goethite < gibbsite. For soils where Iron oxides are the only sorbing phases, ISR is ruled by the part of the total specific surface area due to these oxides. More generally, however, ISR is sensitive to both the nature and the magnitude of the surface area so that soils rich in gibbsite exhibit higher ISR than the soils devoid of this mineral. To become useful as a comparative index, ISR should be amended so as to also take into account the specific reactivity of the different minerals with respect to soluble silica.

¹ Contribution of Int. Inst. of Tropical Agric. Ibadan, Nigeria, and Section de Physico-Chimie Minérale du Musée Royal de l'Afrique Centrale, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium. IITA Journal paper no. 88.

² Research Fellow, IITA: Scientist, M.R.A.C. Tervuren and Professor, Catholic University of Louvain; and Soil Scientist, IITA respectively.

Received for publication May 16, 1977. Accepted for publication July 20, 1977.