Understanding the chemistry behavior in acid soils during the decomposition of plant residues is very important for the management of these soils. The effects of incubation (0, 15, 30, 60 and 90 days) of finely ground residues of oil seed radish (Raphanus sativus), soybean (Glycine max) and wheat (Triticum aestivum) (at 2 and 4%) with samples of Bw horizon of three Oxisols of the state of Paraná, Brazil, on the soil pH, dissolved organic carbon (DOC) and soluble and exchangeable Al, Ca, Mg and K were verified during 1995-1996. DOC and total and soluble Ca, Mg and K were determined in the plant residues. Immediately after application of plant residues (zero time), there were increases in pH, in exchangeable and soluble Ca, Mg, and K, in soluble Al and DOC and a decrease in exchangeable Al. The intensity of these changes was associated with the DOC and with the soluble Ca, Mg and K in the residues, in the following order: oil seed radish > soybean > wheat. During incubation, the DOC of the soil solution rapidly decreased. DOC decrease with the incubation time did not affect K, but drastically reduced Al, Ca and Mg in the soil solution, showing the importance of DOC in the maintenance of cations in solution through the formation mechanism of organic complexes. The chemical speciation estimated that more than 90% of the total Al in solution was in the organic form. The organic and inorganic compositions of the water soluble fraction of plant residues were the main factor responsible for the chemical changes observed in the acid soil samples.
green manure; dissolved organic carbon; organic complexes