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Abstract

The present investigation used both soil and solution culture experiments to examine the effects of acid soil infertility factors, particularly aluminium and low pH on nodulation and growth of peanut, cowpea, soybean and green gram. Initially, a pot experiment was conducted to identify those factors limiting growth of the four legume species in the A horizon of three acid soils, viz. a Wallum sand, a yellow podzolic soil and a gilgaied acid clay, all from southeast Queensland. The legumes were grown at pH values ranging from pH 3.5 to 6.5 (1:5, soil:0.01 M CaC12) obtained by either acidification or liming. All species responded positively to lime application; however, the magnitude of the lime responses varied among species. Calcium deficiency appeared to restrict growth of all species in the acidified and unlimed gilgaied acid clay, whereas manganese toxicity was identified as a limiting factor for all species in the acidified yellow podzolic soil. Nevertheless, growth responses to lime application in all species were closely associated with decreasing activities of monomeric aluminium species in the soil solutions. Maximum growth and nodulation of most species occurred at the highest lime rates, in which little or no aluminium was detected in the soil solution. Acidification treatments markedly increased ionic strength and monomeric aluminium in soil solutions; low pH (3.53) and high activities of aluminium monomers (EaA1 mono @ 2000 µM) were lethal to cowpea, soybean and green gram, but not to peanut. An attempt was made to employ aluminium in soil solution to predict aluminium toxicity using the method of Blarney et al. (1983) to determine monomeric soil solution aluminium in the three acid soils. Although this method has been used successfully to discriminate between monomeric and polymeric aluminium in solution cultures, it has since been shown to fail to discriminate between inorganic monomeric aluminium and organically bound aluminium in soil solutions containing organic acids. Thus, soil solution analyses using this method yielded differing relationships between soil solution aluminium and plant growth across the three soils.