Abstract. Since K fixation in softs is largely a function of clay mineralogy, an effort was made to elucidate the influence of mineralogical variations in soil clays on K fixation, under the moisture-temperature regimes usually prevalent in tropical upland rice softs. Beidellitic clay turned out to be the most severe fixer of added K (80%), followed by vermiculitic (69%) clays. Fixation is not appreciable (< 15%) in clays consisting of montmorillonite, x-ray amorphous material, chlorite, hydrous mica, kaolinite and halloysite. Fixation by beidellite and vermiculite clays is reduced by the simultaneous occurrence of other mineral species.Potassium is usually fixed in the clay fraction; and the phenomenon is largely a function of clay mineralogy. Some soil processes such as alternate wetting and drying operative in upland tropical rice soils have been shown to influence K fixation appreciably [11,14,17]. Most documented studies have, however, been conducted either on relatively pure clay mineral aggregates isolated from geological deposits [2,3,6,8,10,16,18,20] with the assumption of identity between them and their counterparts in soils, or on soils with httle or vague mineralogical information or simple clay mineralogies; under often extreme moisture conditions, temperature and nutrient concentrations not prevalent in upland rice soils [1,4,5,7,9,11,12,13,14,15,17,19]. Extrapolation of results from such studies to fertility management of soils in natural environments thus could prove hazardous.The present study was made to gain information on K fixation of some tropical upland rice soils with range of clay mineral compositions under the moisture, temperature and nutrient situations usual to these soils. Hopefully, the results can be utilized to improve fertilizer efficiency through the development of management practices suited to soils of specific clay mineralogies.
