Alteração da textura pela dispersão mecânica e retenção de água na fração areia em neossolos derivados de rocha vulcânica

Abstract

The definition of the time for mechanical dispersion that ensure total dispersion without altering the amount of sand, silt and clay in soils with fragments of saprolite is still a research gap. Moreover, the effect of saprolite fragments on soil water retention is still not well understood. Therefore, this work had as main objectives: (i) to evaluate how different mechanical dispersion strategies alter the amount of sand, silt and clay in Entisol derived from volcanic rock, in which fragments of saprolite still remain in the soil mass, (ii) to investigate if fragments of saprolite in the sand fraction of Entisol have a high water retention capacity. In relation to the first objective, samples of the Entisol were dispersed using a electric mixer at 12,000 rpm for 5, 15, 30 and 60 min, a horizontal shaker at 150 cycles min-1 for 1, 2, 4, 8 and 16 h, with and without the use of nylon balls, and a sonication agitation with energy of 70 and 110 J mL-1. Regarding the second objective, the contribution of the coarse and fine sand fractions, obtained from the texture analysis, in the water retention was analyzed using their water retention curves, which were compared to the retention curve of sand only by quartz. It was found that the mechanical dispersion strategies affected the particle size fractions. The most sensitive fractions were the coarse sand and clay fraction, with an average variation of 14% and 18% respectively. Some strategies caused a significant change in the quantity of granulometric fractions, mainly due to the fragility of the saprolite fragments. The results indicated that the most appropriate dispersion method is the horizontal shaking at 150 cycles min-1 for one or two hours without the use of nylon balls. The available water was 0.081, 0.318 and 0.015 cm3 cm-3, respectively for Entisol coarse sand, Entisol fine sand and quartz sand. Regarding the water availability in the quartz sand, two fold available water was observed in the Entisol coarse sand and 12 times in the Entisol fine sand. The results showed that the presence of fragments of saprolite and rock in different degree of alteration positively contribute to water retention and available water, in contrast to what occurs in sands with a predominance of quartz and other low-porous minerals.