SOIL SOLID AND SOLUTION PHASE-CHANGES AND MUNG BEAN RESPONSE DURING AMELIORATION OF ALUMINUM TOXICITY WITH ORGANIC-MATTER

A soil incubation and short-term root growth experiment was conducted to investigate the effects of organic matter application on Al toxicity alleviation in a highly weathered acid soil. Ground leaves of a tree legume (Calliandra calothyrsus Meissn.), ground barley (Hordeum vulgare L.) straw, or CaCO3 were mixed at various rates with A-horizon soil of a red podzolic soil (Epiaquic Haplustult) and incubated at 90% of field capacity for 4 or 10 weeks. After the incubation, a short term (48 h) root growth test was conducted using mung bean (Vigna radiata (L.) Wilczek), followed by the analysis of the solution and solid phases of the post-harvest soil. Adding either CaCO3 or organic matter increased root length in mung bean largely by decreasing the activity of monomeric Al in the soil solution. With organic matter, the major mechanisms of this decrease were presumed to be precipitation of soluble Al and the formation of Al-organic matter complexes. The former effect was predicted from the pH increase accompanying the organic matter addition, the increase being larger with legume leaves which had the higher exchangeable and soluble Ca and Mg contents. The concentration of Al complexed with soluble organic matter also was shown to increase with increasing rate of organic matter addition, the effect again being larger with legume leaves. The sum of monomeric Al species activity and Al3+ activity was negatively correlated with relative root length for the organic matter and CaCO3 treatments. However, indices which took into account the possible alleviation effects of basic cations in soil solution on Al toxicity provided an improvement in correlation with relative root length. The efficiency of the two organic amendments relative to CaCO3 in decreasing Al toxicity was assessed by comparing the rates required to reduce Al3+ activity below 10-mu-M, the value found to be associated with 90% relative root length for mung bean. The rates of CaCO3, legume leaf and barley straw required to reach this critical value were 0.75, 14, and 42 t ha-1 respectively.