Soil amendment with organic materials (crop residues animal manure, and green manure) reportedly has positive effects on soil properties, from acidity to plant-nutrient availability. To examine that hypothesis, an incubation study was conducted to assess the changes in some chemical properties of three different tropical soils (Andisol. Ultisol, and Oxisol) amended with chicken manure and green manure (Leucaena leucocephala) at the rate of 10 t ha(-1). The results showed that organic amendments raised soil pH and EC, regardless of the type of manure used. Manuring lowered the concentrations of Mehlich-3 extractable Ca. P, Mn and Si in all soils and decreased the concentration of Mg in the Ultisol and Oxisol. However, manure amendment led to increases in the concentrations of Mg and K in the Andisol. Organic amendments caused a decrease in KCI extractable Al. Initial soluble C levels were highest in the Oxisol (60 mu mol g(-1)) and lowest in the Andisol (20 mu mol g(-1)). The concentration of soluble C decreased exponentially with duration of incubation. Three low molecular weight organic molecules (acetic acid, catechol and oxalic acid) out of the eight tested were found in all manure-amended soils. This study quantified the release of some Al chelating organic acids, the reduction of exchangeable Al, and the changes in major plant-nutrients when organic materials were added to nutrient poor, tropical acid soils. (c) 2008 Elsevier Ltd. All rights reserved.