Dinâmica de liberação de nutrientes de rochas silicáticas em solos de diferentes texturas

Abstract

Brazilian Agriculture utilizes a large volume of fertilizers to obtain the maximum food production. Most of fertilizers are imported generating a dependency in the external market. An alternative to fertilizers use is the silicate rocks, in particular those that are potassium carrier in their structure. This work aims in general to perform a review about potassium supply potential to agriculture from silicate rocks as well as evaluate four silicate rocks rich in potassium with relation to their nutrients release capability in leaching columns and in incubation experiment. The silica rich rocks used in the experiments were: Monzogranite, Dacite, Ultrapotassic rock and Granada biotite gnaisse. Both studies were conducted at the “Estação Experimental Terras Baixas” of Embrapa Clima Temperado, Capão do Leão, RS State by using two soil types: A Luvic Planosol collected in field with 5 years of fallow at Capão do Leão, RS State an a Rhodic Nitisol collected in the surroundings of an experimental field area in Sertão, RS State. The experimental design in Study I - incubation was randomized block design with four repetitions and factorial design 2 x 4, being the factor 1 (rock type) was qualitative – with five levels: Control (no rock), Monzogranite, Dacite, Ultrapotassic rock and Granada biotite gnaisse, and factor 2 (rock dose) was quantitative, with four levels: 0; 5,000; 10,000 e 20,000 kg ha-1. The incubation test was conducted in controlled environment at room temperature, where the experimental unit was composed of a 0.4 dm-3 soil + rock volume in a plastic pot. The incubation period was four months. In the study 2 – leaching columns experiment arrangement was composed by 5 treatments, 2 soil types and 3 repetitions, totaling 30 experimental units. To simulate rainwater regime, 300 mL (40 mm) of distilled water was added gradually to the leaching columns, once a week for 15 weeks. Among the evaluated lithologies, the Ultrapotassic rock (Bage, RS) presented the best performance to the others regarding the improvement of chemical attributes of the two types of soil. The Ultrapotassic rock provided Ca, Mg and altered the acidity and base saturation indexes of the studied soils. The soil P contents were positively altered by all rocks in at least one of the doses in the incubation experiment, being this effect likely related to the release of monosilicic acid by displacing PO4- ions to the available phase. Considering the K, rocks little or no altered the mean values of extractable K of soils and the accumulated quantities, probably because the silicate rocks do not have significant levels of water soluble K. Thus, the methodology of incubation rock:soil used in this study seems to be appropriate to evaluate the release dynamics of mostly nutrients from rocks to the soil, unless K. In future studies, it is recommended the use of experiments with plant species such as grasses, with high K extraction capacity.