High phosphate adsorption in oxidic tropical soils changes the charge balance on the surface of particles and can increase dispersion. Consequently, it can affect pore size distribution, aggregate stability and susceptibility to compaction. In order to measure the effect of phosphate sorption on compression curves and consistence limits, a Dark-Red Latosol (LE) (very fine, allitic, isothermic Typic Hapludox) and a Cambisol (C) (very fine, isothermic Oxic Distropept) were sampled at 0-3 and 27-30 cm depth, at Campos da Mantiqueira, Minas Gerais, Brazil. Phosphate solution was added to half of the samples so that the P adsorption capacity could be reached. Preconsolidation pressures were obtained from the compression curves, which represent the bulk density of soil as a function of log of applied load. Liquid, plastic, and contraction limits were also determined for each sample under both P and no P conditions. The results showed a shift on compressibility curves towards lower preconsolidation pressures as moisture increased and P was adsorbed to samples. This shift was more evident on LE samples, mainly at 0-3 cm depth, which means a decrease on the load support capacity of soil. Phosphate adsorption also changed consistency limits, decreasing the range of friability and, consequently, affecting trafficability and workability of soil.
soil moisture; compaction; charge balance; tropical soils