Soil acidity and liming in Natal.
1. Effects of Aluminium 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 A1 toxicity rather than to improved P availability. Growth response to amelia rants 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 A1 existed and since lime did not decrease P fixation despite its ability to eliminate soluble A1, it is concluded that P fixation is 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 the available P in the soil per se could thus be in serious error. 2. Lime Requirements of Natal Oxisols based on Exchangeable Aluminium The exchangeable Al status of eight Natal Oxisols is a suitable criterion for the measurement of lime requirement defined as the amount of lime necessary for maximum crop production. The principal function of lime in these soils is to eliminate A1 toxicity; it has little or no effect on P availability. A "critical value" for exchangeable A1 below which 'trudan' did not respond to lime application was found. On the average the amount of lime necessary for maximum growth and exchangeable Al control was approximately one sixth the amount required to raise the soil pH to 6.5. 3. Cation Exchange Capacity and Exchangeable Aluminium in Natal Oxisols Positive charges in acid soils reduce CEC at low electrolyte concentration probably by double layer interaction. The resultant net CEC (determined by washing soil free of salt with water) is the effective CEC under field conditions and the difference between net CEC and exchangeable bases is accordingly a convenient measure of exchangeable A1. A reaction scheme is proposed which relates the large pool of non-exchangeable Al (extractable with N NH0Ac-pH4) in these soils to the relatively small amounts of exchangeable AI; this reaction scheme is governed primarily by net CEC and exchangeable bases rather than by pH. 4. Amelioration of Subsoil Acidity in Natal Oxisols, The large pH dependent CEC in Natal Oxisols effectively limits the downward movement of lime. Although heavy fertilisation, particularly with acid forming nitrogenous fertilizers increases the rate of movement, relatively small amounts of Ca salts having little ability to neutralise subsoil exchangeable Al could be leached from limed topsoil. In contrast, bases equivalent to 80% of that applied leached rapidly from gypsum treated topsoil. Although gypsum did not eliminate subsoil exchangeable A1, it was considerably more efficient than lime in this respect. However, gypsum caused severe loss of exchangeable Mg which could have serious nutritional consequences if not corrected.