Testing the ability of organic ligands and plant materials to reduce the toxic effects of aluminium in soils

Abstract

An acid soil from the Sedgwick region of central Victoria was modified to provide a range of aluminium (Al) concentrations in order to test whether incorporation of organic ligands, or plant material, could reduce plant-available (or 'reactive') Al in soils. Al concentrations in the soil were increased by addition of varying amounts of a solution of AlK(SO₄)₂, chosen after measurement of the adsorption of Al onto the soil. A similar study of citrate adsorption allowed estimation of the amount of citrate required to achieve a 1 : 1 Al : citrate ratio in the soil in order to test the effectiveness of organic ligands in alleviating Al toxicity. Citrate was found to decrease the level of reactive Al in the soil. Pot trials also showed that addition of citrate to Al amended soil caused some improvement in root length and dry weight of soybean plants (Glycine max) compared with the Al amended soil. Addition of oxalate also reduced the level of reactive Al but did not improve root growth. Incorporation of dried leaves from lucerne (Medicago sativa), rhubarb (Rheum rhubarbarum), oxalis (Oxalis pes-caprae), and soybean at application rates equivalent to 5 and 10 t/ha to a soil containing added Al tended to increase root length. The most surprising result was the relatively small effect of high Al concentrations on plant growth, with 2000 μm reactive Al reducing root length by only 50% after 15 days of growth. This is a much smaller reduction in root length than has previously been obtained in hydroponic systems with 500 μm reactive Al after 15 days growth (Ginting et al. 1998) and points to a major difference between plant growth trials in solution culture and soil systems. One possible explanation for this difference is that the roots are in contact with only a small volume of soil solution, and this can be modified by root exudates. Further research is required to test this hypothesis, which will require the development of a method of analysis for Al in soil systems that more accurately reflects reactive Al levels in the root-zone.