The effects of nitrogen nutrition of plants on the development of acidity in Western Australian soils. I. Effects with subterranean clover grown under leaching conditions

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Abstract
Subterranean clover was grown, under leaching conditions, in pots on cultivated and virgin acidic sandy soils from two sites in Western Australia. Nitrogen (N) was supplied to the plants either as NH+4, NO-3 or through symbiotic fixation. There were marked changes in the acidity of the soil as the result of the differences in catiot/anion balance induced by the different forms of N supplied. Thus plants supplied with NH+4 depressed the pH by 0.9 pH units, and those dependent upon fixation by 0.5 pH units. There was little change in pH when NO-3 was supplied. Changes in soil pH were strongly related to cation/ anion balance, with increasing acidity resulting from the decreasing ash alkalinity of shoots of plants supplied with NH+4. There were marked differences both between the two soils from the same site, as well as between soils from the two different sites, in the relationships between pH and soluble aluminium and pH and exchange acidity. It is suggested that differences in the form, as well as differences in the total amount, of organic matter could play a major role in controlling the development of acidity in such soils of low, permanent cation exchange capacity.