Decomposição e liberação do nitrogênio, fósforo e potássio da palhada de duas variedades de cana-de-açúcar

Abstract

Sugarcane is one of the main crops in the Brazilian economy, making Brazil one of the largest producers. Recently, the sugar cane sector has undergone several changes, including the use of a new harvesting system. Sugarcane harvesting with previous fire clearance was, for many years, the most widely used harvesting method. Mechanization has gradually replaced the old harvesting methods, due to environmental and labor laws and shortages of labor in rural areas. In this new system, there is the deposition of organic matter under the soil surface, resulting from dry leaves, pointers, green leaves and varying amounts of stalk pieces, forming a mulch called straw. This coverage increases the infiltration of water in the soil, improves the soil structure, increases the CTC, decreases erosion and evaporation and will later be a source of nutrients for the macro and microflora of the soil and for sugar cane. In this context, the objective of the present study was to evaluate the decomposition and release of nutrients from the straw of the varieties: RB867515 and RB92579. The mineralization study was conducted in Boca da Mata, with chemical evaluations carried out in private laboratories. Straw samples from RB867515 and RB92579 were collected in the field, immediately after mechanized harvesting, dried in the sun, manually chopped with fraction, at an average size of 15 cm in length, and stored in a shed until the beginning of the study. In this plant material, the content of nitrogen (N), phosphorus (P) and potassium (K) was quantified. In order to conduct the study, nylon canvas bags (25 mm diameter mesh) were made, with the dimensions of 1.0 x 0.6 m. A straw mass equivalent to 15 t of dry matter per hectare was transferred to these bags. These bags containing the straw were taken to the field in mid-March. The study consisted of three sampling times: 45; 105 and 165 days after the transfer to the field, of the bags containing the straw. The experimental design was randomized blocks, with five replications. At each sampling time, the bags' straw was collected, weighed and homogenized. A subsample was kiln dried and analyzed for N, P and K contents. The mass balance calculated the release rate of N, P and K from the straw of the varieties. The N release from the straw was 15%, compared to the initial mass. For phosphorus there was no significant release. On the other hand, potassium release was high, at 45 days the concentration of this element in the straw was 33% for RB867515 and 36% for RB92579, at 165 days, more than 95% of potassium had been released into the soil compared to the initial concentration. Thus, the potassium contained in sugarcane straw contributes significantly to the nutrition of sugarcane in the next cycle, but for N and P there is no significant contribution.