Organic waste materials used to tackle soil acidity include undecomposed plant materials, composts, manures, peats, and coal products. Organic amendments are suitable for resource-poor farmers, as these farmers are unable to buy large quantities of lime and fertilizer phosphorous needed for their lands because of economic reasons. Some authors have reported an increase in soil pH after addition of organic materials to soil, followed by a decrease of aluminum saturation and an improvement of plant growth, depending on the type of residue, its rate of application, and the buffering capacity of the soil (Hue 1992; Noble etal. 1996). The rise of soil pH is due to the flow of protons from the soil (lower pH) to the organic matter sites (higher pH), decom­position of less stable materials in the soil resulting in mineralization and nitrification of organic nitrogen, and microbial decarboxylation (Haynes and Mokolobate 2001; Wong and Swift 2003). A long-term increase of soil pH is dependent on the balance between proton production and consumption in the system (Helyar and Porter 1989). The role of humic substances in increasing phosphorous availability is unclear. Some authors have reported the role of humic substances contained in organic matter in competing for adsorption soil sites and subsequent decrease in phosphorous adsorp­tion (Bolan et al. 1994; Perrott 1978), whereas other authors have stated the unim­portance of soil organic matter in increasing phosphorous availability (Borggaard et al. 1990). Humic substances concomitantly with organic acids, organic residues, and release of inorganic phosphorous have been found to be the main factors involved in increase of available phosphorous. A decrease in aluminum phytotoxicity is directly linked to phosphorous availability.

A conceptual model of the major processes that lead to the detoxification of soil aluminum and an increase of phosphorous availability when wood ash compost is applied to acid soils is summarized in Fig. 7.1.