Banca de DEFESA: RICARDO DE CASTRO DIAS

Feasibility of using silicate rocks as a source of potassium in agriculture

Fertilization. Soil fertility. Remineralizer

Brazil is a major importer of fertilizers, especially potassium fertilizers. Domestic production is small and insufficient to meet national demand. Given this scenario, the search for alternative sources to conventional soluble fertilizers has grown a lot in the last two decades. Thus, the present work aimed to evaluate the potential of different silicate rock powders as a source of potassium in Brazilian agriculture. Therefore, the work was divided into four chapters. In chapter I, a systematic review of the literature was carried out followed by a meta-analysis in order to present the state of the art on the efficiency of silicate rock powders as a source of potassium for cultivated plants. In general, the effect of the application of rock powder was superior in relation to the non-application of K and inferior in relation to the application of soluble fertilizer. Doses of up to 5 Mg of rock dust ha^-1 provided a positive effect, however significantly lower in relation to the application of doses higher than this value. No benefit was observed as a result of the application of rock dust in granulometry greater than 2 mm. Long-term studies showed results similar to those of short-term studies, showing that there is no residual effect in the application of silicate rock powder. In Chapter II, assays were performed on the kinetics of potassium release from silicate rock powders using different extractors. The results showed that there was a higher proportion of soluble K in the Serpentinite residue, rich in phyllosilicates, after 48 h of extraction. In general, the release of K from rock powders was relatively fast in the first 4 h, becoming practically stable after this period. Chapter III aimed to evaluate and correct the chemical properties of the soil used in the research. For this purpose, a bench test was carried out to determine the amount of CaCO₃ necessary to correct the pH to approximately 6.0, and a test in a greenhouse to define corrective fertilization, as well as mathematically model the dry mass production and accumulation of K in corn plants as a function of potassium fertilization. The best fit for dry mass production was obtained using the quadratic root model, which indicated the need for 163 mg kg^-1 of K to reach 90% of the maximum dry mass productivity. The accumulation of K showed a quadratic behavior as a function of increasing the dose. In Chapter IV, an experiment was carried out in a greenhouse to define the efficiency of rock dust, compared to KCl, in the production of dry mass of the aerial part of maize plants, adopting the treatment without application of K as a negative control. The rock powders were applied in three different granulometries (Ø < 103 µm, Ø < 300 µm and Ø < 4,800 µm), incorporated or on the surface. After two successive cultivations, the dry mass production obtained by the application of serpentinite powder (Ø < 300 µm) provided a dry mass production equivalent to that obtained by the application of KCl. The accumulation of K provided by the incorporated application of serpentinite powder in a granulometry of less than 103 µm was equivalent to that provided by the application of KCl. The other rock powders provided values of dry mass and K accumulation significantly lower than those provided by the application of KCl. The application of rock powders in granulometry classified as mash (Ø < 4,800 µm) resulted in a significant decrease in the performance of rock powders.