A gibbsitic Red-Yellow Oxisol with electropositive carater was used to study the effect of organic matter on electrochemical properties and dispersion mechanisms. Two organic sources were used, one characterized by non-humified compounds (cattle manure) and another by humified compounds (peat humic acids). The soil was incubated with 0; 7,5; 15,0; 22,5 and 30,0 g kg-1 of organic manure. Water dispersible clay, electrical conductivity, water and KCl pH, and the zero point of saline effect (ZPSE) and superficial electric potential (Ψo) were measured. Clay dispersion behavior was distinct when the soil was treated with cattle manure and humic acids. Soil dispersion increased with the addition of cattle manure, which may be related to the increase of electrical conductivity due to the amount of salts in the manure. On the other hand, the addition of humic acid induced clay flocculation, which was probably due to the formation of clay-humic complexes. The ZPSE linearly decreased with the addition of organic manure, especially with humic acids. The flocculation was not necessarily associated with the low modal condition of Ψo. Negative charge was observed in the treatments with high levels of humic acids, but no clay dispersion was observed, probably due to the electrochemical properties of the humic acids, as a result of its polyeletrolytic condition and the possibility of changing the macromolecule configuration.
organic manure; ZPSE; water dispersible clay; acric Oxisol