Approximately 43 % of the arable soils in the tropics are highly weathered acid soils, that are inherently infertile or rapidly become infertile when they are cleared for agriculture. The application of high-analysis ameliorants and fertilisers, use of highly adapted crops and the application of organic residues from green manure, hedgerow and agroforestry crops and organic by-products from agricultural production have all been shown to overcome the infertility of these soils. The high cost of adopting intensive technologies such as inorganic fertilisers and highly selected crops generally precludes their use in traditional farming systems. Consequently, organic manuring and long-fallow practices, coupled with technologies and strategies that can identify appropriate crops to use as organic manures, need to be developed to enhance the sustainability of agriculture in the tropics.
The objectives of this thesis were (1) to investigate mechanisms by which organic materials ameliorate Al toxicity and Ca deficiency, and (2) chemically characterise organic materials that could be used to ameliorate acid soil infertility. It was also necessary to clarify which Al species cause Al toxicity in highly weathered acid soils. Thus, an additional objective was to determine if the toxic Al13 polycation would form in soil solutions of highly weathered soils.
The A113 polycation has generally been found to form in relatively simple synthetic solutions, containing Al, Ca, Mg, K, chloride and nitrate but not sulfate or silicate, undergoing partial neutralisation. To ascertain if the Al13 polycation could form in highly weathered acid soils, synthetic solutions containing cation and anion concentrations typical of highly weathered acid soils were partially neutralised and then analysed by 27Al nuclear magnetic resonance and the ferron analytical technique. The inclusion of sulfate m these solutions, at concentrations as low as 100 µM during partial neutralisation, reduced the formation of the A113 polycation by 80 %, while sulfate concentrations >250 µM prevented the formation of A113 altogether. Similarly, silicate concentrations >441 µM completely prevented the formation of A113. The sulfate ion was thought to bridge partially neutralised octahedral A1 ions as they condensed to form A113, destabilising its structure and resulting in the formation of A1(0H)3 micro-particulates and precipitates, whereas silicate was thought to combine with A1 to form soluble aluminosilicates that passed through a 0.025 µm filter. It was concluded that the ubiquitous nature and combined presence of sulfate and silicate in soil solutions of highly weathered acid soils would prevent the formation of the toxic A113 polycation. This finding negates the need to analyse soil solutions from highly weathered acid soils for A113, when these soils are being assessed for A1 toxicity.
To investigate the role organic materials play in ameliorating acid soil infertility, laboratory incubation trials were undertaken to compare the effects of organic materials, and/or their soluble, insoluble and ashed components, as well as lime, on an Al-toxic surface soil and a Ca-deficient subsoil and on the root length of mungbean cv. Berken grown for 2 days in those amended soils.
The application of organic materials to the A1-toxic surface soil and the Ca-deficient subsoil had variable effects on the chemical characteristics of these two soils. The applied organic decomposed to release low and high molecular weight organic acids into their soil solutions. Low molecular weight aliphatic and aromatic acids accounted for <10 % of the total soluble C in the soil solution, indicating that high molecular weight organic molecules were the major component of the soluble C in the soil solutions. It was calculated that low molecular weight organic acids complexed <10% of the complexed A1 in the soil solution of the A1-toxic surface soil and between 10 and 95% of the complexed Al in the soil solution of the Ca-deficient subsoil. No relationship was found between the activities of the low molecular weight organic acids and the A13+ activity in the soil solution of these organic material-amended soils.
Relative root growth of mungbean cv. Berken was strongly correlated with the activity of A13+ in soil solutions of the amended soils. The activity of A13+ in the soil solution of the two soils was strongly correlated with soil solution pH, indicating that organic materials ameliorate A1 toxicity mainly by increasing soil solution pH. Increases in soil solution pH by organic amendments were primarily related to the effect the organic materials had on the soil solution ionic strength and the relationship of the soil solution pH to the pH causing zero net surface charge. Organic materials, such as maize, that increased soil solution ionic strength resulted in a rapid decrease in soil solution pH and a substantial increase in A1 toxicity. The application of organic materials to the Ca-deficient subsoil generally induced A1 toxicity via this mechanism.
Organic materials with a ratio of Ca to all other cations in the organic material of <0.3 induced or maintained Ca deficiency, when applied to the A1-toxic surface soil or the Ca deficient subsoil, respectively. The exception was where the organic material was observed to have a high protein-bound condensed tannin and low fibre-bound tannin content, resulting in high NH4+ activity in the soil solution. The latter reduced the activity of Ca2+ in the soil solution and resulted in induced Ca deficiency characteristics. The application of organic materials to the Ca-deficient subsoil generally ameliorated the Ca deficiency, but may induce Al toxicity by increasing the activity of Al3+ in the soil solution of the treated soil.
The soluble component of organic materials was generally less effective in ameliorating acid soil infertility than the insoluble or ashed components, indicating that organic materials that have undergone some weathering prior to application to A1-toxic or Ca-deficient soils will have less of a detrimental effect on crop establishment than fresh organic materials.