SUMMARYIn a field experiment on a dark‐red latosol (haplustox) in the Cerrado of Brazil, green manure, including maize residues, increased yield of maize grain. Samples of soil from the experiment were investigated in the laboratory. Green manure caused no effect on amounts of phosphate extracted by Bray's solutions, nor by anion exchange resin. Adsorption measurements showed that green manure did not affect the total amount of phosphate adsorbed by the soil, in relation to the amount added nor to the concentration of phosphate in solution. However, green manure increased the proportion of added phosphate that became isotopically exchangeable so that after desorbing some phosphate in two stages, 99% of the adsorbed added phosphate was exchangeable in soil with green manure, but 89% was exchangeable in soil without it. Green manure increased the concentration of phosphate in solution that was maintained by a specific amount of exchangeable phosphate held by the soil. Buffer power increased with time, without and with green manure; but it was less in its presence indicating that phosphate was less strongly held, and would be more mobile in soil in the field.The results indicate that easily decomposable organic material, such as green manure and crop residues, makes fertilizer phosphate more effective for crops growing in an oxiso