High rice husk ash doses applied to the soil can lead to phosphorus water contamination

The goal of this work was to evaluate the rice husk ash (RHA) effect on soil phosphorus (P) adsorption and to analyze the probable consequences of these in the parameters that affect P availability to the plants and the risk of soil P loss by superficial run-off. For this, a factorial experiment was conducted in laboratory with samples of two soils (factor A: Hapludult and Hapludox) that were submitted to five doses of RHA (factor B: 0; 25; 50; 75 and 100 g RHA kg^-1 of soil). The experimental units were plastic pots with 200 g of air-dry fine earth from horizon A of each soil. The experimental design was completely random with three replications. After 30 days of soils incubation where moisture was maintained in field capacity, the phosphorus adsorption isotherms were made in subsamples of each experimental unit. The RHA decreases sharply the phosphorus adsorption power but decreases very little the maximum soil phosphate adsorption. The RHA increases the soluble silicium and phosphorus contents, but these are not responsible for the decrease in phosphorus adsorption later applied in the soil. The major cause of the negative effect of RHA in the soil phosphate adsorption is the increase in soil pH. As a consequence of these effects, the RHA decreases the buffering ability and increases the phosphorus effective diffusion coefficient in the soil. Moreover, as a consequence of the adsorption power decrease, the RHA increases the risk of environmental contamination by phosphorus by superficial runoff, especially in soils with originally low phosphorus adsorption ability.