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

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.