Abstract
Volcanic ash weathers quickly in the humid tropics and in a few years may develop an appreciable phosphorus adsorption capability. Apparently sulphate adsorption capacity develops more slowly. Phosphate and sulphate adsorption curves clearly reflect chemical and mineralogical changes in soils associated with weathering. For soils developed from ash (amorphous colloids) phosphate adsorption was inversely related to Si solubility. Phosphate adsorption by several oxisols of tropical America was 200 to 600 ?g P/g soil at standard supernatant concentration (0·2 p.p.m. P). Capacity for P adsorption was 20,425, 1900 and 2500 ?g/g for the weathering sequence: fresh volcanic ash, Umbric Vitrandept, Typic Dystrandept and Oxic Dystrandept, respectively. Phosphate adsorption by andepts was inversely related to silicon solubility. Sulphate accumulates in large quantities in subsoils of the most highly weathered Oxisols and Dystrandepts. Some oxisols, such as those of the Llanos Orientate of Colombia are generally low in adsorbed sulphate throughout their profiles. Sulphur deficiencies in these soils can be expected if other limiting nutritional problems are corrected. The A horizon of Nipe soil was only three per cent sulphate saturated and sulphate solubility was 0·4 p.p.m. Mean SO4-S saturation of Subsurface horizons was 68 per cent and mean solubility was 4·8 p.p.m. Soils of the Llanos Orientales usually contained <10 ?g/g of adsorbed S while mean adsorption capacity was about 150 ?g/g. Volcanic ash soils retain sulphate in proportion to weathering but apparently Sulphate adsorption capabilities develop more slowly than phosphate adsorption capability. Cation adsorption curves predict that Suitable intensities of K and a nutrition can be reached with small additions of these cations to oxisols but that capacity for supply is low especially for soils of the Llanos Orientales. Attempts to stock these soil with cations may result in elevated soil solution concentrations, luxury uptake by plants and leaching. Aluminium dominates the exchange complex of soils of the Llanos Orientates.
