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-Al 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.