The management of acid soils

In this paper our current understanding of soil acidification processes, and of the effects of soil acidity on yield are reviewed and expressed as a yield response model. The model is used to show that large effects on plant yields are caused by differences in plant tolerance to acidity, in subsoil acidity levels, and in the acid addition rate. Differences in the plant demand for subsoil resources are also significant, and the effectiveness of the ameliorant in neutralising subsoil acidity is an important variable. Plant tolerance to acidity is useful to increase yields on acid soils, to change the cost/price ratio to values more favourable for amelioration, to utilize the water and nitrogen resources of acid subsoils, to decrease acid addition by reducing nitrate leaching, and to diversify types of production on acid soils. Means of managing subsoil acidity include mechanical mixing of limestone with soil, mixing by earthworms and ants, utilizing root excretion of bicarbonate in response to manipulation of nitrate leaching and uptake, and using ameliorants with different leaching characteristics. Acid addition rates can be reduced by using less acidifying nitrogen fertilisers, by closely matching nitrogen supply with plant demand for nitrogen thus minimising nitrate leaching, and by managing the ecosystem to minimise unnecessary accumulation and export of organic anions. Lastly some acid soil infertility factors can be treated using fertilisers or other low cost techniques, such as the lime pelleting and inoculation of legume seed, to overcome limitations to plant growth. It is emphasised that the simple technology of liming to a soil pH at which maximum yields occur is an inadequate prescription for managing acid soils. A broad view of the many management options should be taken. The appropriate mix of management inputs for a given situation depends on their relative costs and returns.