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poultry manure (POM), and cow manure (COM) were respectively added at rates equivalent to 10, 20, 30, and 40 t ha-1 to an Ultisol having soil-water pH of 4.6 and KCl extractable acidity of 3.00 cmol kg-1. The
amended soils were incubated at 70% water holding capacity for 3 weeks. Animal manures significantly increased the soil pH from 4.6 to values above 5.6 and also reduced exchangeable acidity from 3.00
cmol kg-1 to values below 0.35 cmol kg-1. The mechanism that best explained the neutralization reaction was found to be microbial decarboxylation of calcium-organic matter complex leading to the release and subsequent hydrolysis of calcium ions. The hydroxyl ions released in the hydrolytic reaction then reacts with both the exchangeable hydrogen and aluminum ions to form water and insoluble aluminum hydroxide (Al(OH)3) respectively.