This paper reports the effects of applying solid urban waste compost to 21 acid soils and five alkaline soils on the electrical conductivity (EC), pH (both in CaCl2 0.01 mol L-1 and in water), and potential acidity (H + Al). The compost was collected at São Matheus Treatment Plant, São Paulo. In acid soils, the compost was applied with and without mineral fertilizers and dolomitic lime (to reach 70% of base saturation). Lime was substituted by gypsum in alkaline soils. The experiment was carried out under greenhouse conditions on a split-plot completely randomized block design with three replicates. It was observed that the EC and the pH were increased, and potential acidity was decreased by compost application. However, the EC values due to the compost were lower than those due to the fertilizer. Consequently, the greatest EC values (> 2 dS m-1) were observed when acid soils were treated with compost + lime + fertilizer, and alkaline soils with compost + gypsum + fertilizer. In addition, the compost application was, on average, more effective than lime for soil pH and potential acidity amelioration. Thus the application of compost + lime caused both a greater increase in pH and a greater decrease in potential acidity. The treatments containing mineral fertilizers showed lower values of pH than those without fertilizer, mainly due to the nitrification reaction following urea hydrolysis. This effect was much clearer for alkaline soils. For acid soils, the higher the values of pH and EC, the lower was the difference between the pH measured in CaCl2 and in water. The application of compost to ameliorate soil acidity may be feasible in the field if fertilizers and lime (for acid soils) or gypsum (for alkaline soils) are also properly managed.
organic fertilizer; solid urban waste compost; physical-chemical soil properties