Amelioration of Al toxicity and P deficiency in acid soils by additions of organic residues: a critical review of the phenomenon and the mechanisms involved

High rates of lime and fertilizer-P are characteristically required to obtain high crop yields on highly weathered acid soils. Much of the agriculture in the southern tropical belt, where acid soils predominate, is carried out by resource-poor, semi-subsistence farmers who are unable to purchase large quantities of lime and fertilizer. There are, however, a number of reports that additions of organic residues to acid soils can reduce Al toxicity (thus lowering the lime requirement) and improve P availability. The literature regarding these effects is sparse and disjointed and an integrated overview of the probable mechanisms responsible and their implications is presented and discussed. During decomposition of organic residues, a wide range of organic compounds are released from the residues and/or are synthesized by the decomposer microflora. The two most important groups in relation to Al toxicity and P availability are soluble humic molecules and low molecular weight aliphatic organic acids. Both these groups of substances can complex with phytotoxic monomeric Al in soil solution thus detoxifying it and they can also be adsorbed to Al and Fe oxide surfaces consequently blocking P adsorption sites. During residue decomposition, there is often a transitory increase in soil pH and this induces a decrease in exchangeable and soil solution Al through their precipitation as insoluble hydroxy-ill compounds. It also confers a greater negative charge on oxide surfaces and thus tends to decrease P adsorption. The increase in pH has been attributed to a number of causes including oxidation of organic acid anions present in decomposing residues, ammonification of residue N, specific adsorption of organic molecules produced during decomposition and reduction reactions induced by anaerobiosis. There are also mechanisms specific to either Al detoxification or improved soil P status. For example, regular applications of organic residues will induce a long-term increase in soil organic matter content. Complexation of Al by the newly-formed organic matter will tend to reduce the concentrations of exchangeable and soluble Al present. As organic residues decompose, P is released and this can become adsorbed to oxide surfaces. This will, in turn, reduce the extent of adsorption of subsequently added P thus increasing P availability. The practical implication of the processes discussed is that organic residues could be used as a strategic tool to reduce the rates of lime and fertilizer P required for optimum crop production on acidic, P-fixing soils. Further research is, therefore, warranted to investigate the use of organic residues in the management of acid soils

Comparative liming effect of four organic residues applied to an acid soil

The effects of additions of four organic residues to an acid Oxisol on soil pH, exchangeable and soil solution Al and maize growth were investigated in a glasshouse experiment. The organic residues used were: grass residues, household compost, filter cake (a waste product of sugar mills) and layer poultry manure and they were added at a rate equivalent to 20 Mg ha(-1) and incubated for 6 weeks. Additions of organic residues generally raised concentrations of exchangeable cations and extractable P and increased the electrical conductivity in the soil solution. Residue additions also increased the pH and decreased exchangeable Al in the order: poultry manure>filter cake>household compostgreater than or equal tograss residues. The major mechanisms responsible for the elevated pH were suggested to be the high CaCO3 content of poultry manure and filter cake, the proton consumption capacity of humic material present in the household compost and decarboxylation of organic acid anions during decomposition of the grass residues. Concentrations of total (Al-T) and monomeric (Al-mono) Al in soil solution, and the proportion of Al-T present as Al-mono, were depressed to a similar extent by additions of all the residues regardless of the magnitude of the increase in pH or decrease in exchangeable Al that was induced. Additions of residues increased the concentrations of soluble C in soil solution and the decrease in the proportion of AI(T) present as was attributed to complexation of Al by this soluble organic matter. Soil solution Al-mono concentrations were decreased t

Increases in pH and soluble salts influence the effect that additions of organic residues have on concentrations of exchangeable and soil solution aluminium

It has been suggested that additions of organic residues to acid soils can ameliorate Al toxicity. For this reason the effects of additions of four organic residues to an acid soil on pH and exchangeable and soil solution Al were investigated. The residues were grass, household compost, filter cake (a waste product from sugar mills) and poultry manure, and they were added at rates equivalent to 10 and 20 t ha(-1). Additions of residues increased soil pH measured in KCl (pH((KCl))) and decreased exchangeable Al3+ in the order poultry manure>filter cake>household compost>grass. The mechanism responsible for the increase in pH differed for the different residues. Poultry manure treatment resulted in lower soil pH measured in water (pH((water))) and larger concentrations of total (Al-T) and monomeric (Al-mono) Al in soil solution than did filter cake. This was attributed to a soluble salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al3+ and H+ back into soil solution. The considerably larger concentrations of soluble C in soil solution originating from the poultry manure may also have maintained greater concentrations of Al in soluble complexed form. There was a significant negative correlation (r = -0.94) between pH((KCl)) and exchangeable Al. Concentrations of Al-T and Al-mono in soil solution were not closely related with pH or exchangeable Al. The results suggest that although additions of organic residues can increase soil pH and decrease Al solubility, increases in soluble salt and soluble C concentrations in soil solution can substantially modify these effects