Effects of Aluminum Toxicity and Phosphorus Fixation on Crop Growth on Oxisols in Natal

Abstract

Abstract Simple, routine methods for estimating exchangeable Al and P‐fixing capacity of acid soils are described. A glass‐house study on eight Oxisols revealed marked growth response of ‘trudan’ ( Sorghum sudanense ) to amelioration by lime, gypsum, and Ca silicate which is ascribed to elimination of Al toxicity rather than to improved P availability. Growth response to ameliorants took place up to the point of elimination of exchangeable Al after which a significant reduction in yield occurred. P fixation is shown to be a major fertility limitation in the soils studied. Since no apparent relationship between P fixing capacity and exchangeable Al existed and since lime did not decrease P fixation despite its ability to eliminate soluble Al, it is concluded that P fixation is the result of an adsorption reaction rather than a precipitation reaction. Although the soils studied are all capable of fixing large quantities of P considerable variation exists between them. Fertilizer recommendations based only on an estimate of available P in the soil per se could thus be in serious error.