Plant residue compost, urban waste compost, farmyard manure, and peat can be used to ameliorate soil acidity. The diversity of these materials and their highly variable composition mean that their reliability in increasing the soil pH is uncertain because of lack of a method to test their acid ameliorating capacities. Incubation of a Spodosol from Sumatra, an Oxisol from Burundi and an Ultisol from Cameroon with 1.5% by weight of four composts, a farmyard manure and a sedge peat resulted in increased soil pH and decreased aluminum (Al) saturation measured at 14 days of incubation. The increased soil pH was directly proportional to the protons consumption capacity of the organic materials. This was measured by titrating the organic material from their natural pH values down to pH 4.0. This measure of acid neutralizing capacity provides a simple test method that was reliable across the variety of materials used. The final pH of the soil treated with organic material can be predicted with reasonable accuracy by determining the buffer characteristics of the soil and organic matter separately. The pH at the intersection of the two buffer curves predicts reasonably accurately the final pH of the treated soil. This suggests that a major mechanism of acid amelioration may be proton exchange between the soil and organic matter buffer systems. The increased soil pH was also directly proportional to the base cations [calcium (Ca), magnesium (Mg), and potassium (K)] contents of the added organic material. Such a relationship was presumably obtained because the base cations reflected the content of weak organic acid functional groups that are capable of binding protons and Al.