Resíduos orgânicos no processo de compostagem e sua influência sobre a matéria orgânica do solo em cultivo de cana-de-açúcar

Abstract

Due to the growing demand for fertilizers, cycling nutrients in the soil-plant system is a fundamental management strategy. This is especially adequate to such crops as sugarcane that yields a high amount of residues. The work was carried out to evaluate the effect of using vinasse to water piles of residues of the sugarcane industry. The quantitative and qualitative aspects of the humic substances in both the compost and the soil were studied in order to assess the agronomic efficiency of these substances for sugarcane cropping. The compost piles consisted of a mixture of sugarcane bagasse, filter cake, cow manure, and Gafsa rock phosphate that were submitted to irrigation with increasing proportions of vinasse (0, 25, 50, 75, 100%). A treatment with the same solid phases (except vinasse) enriched with potassium sulfate and a treatment with filter cake only were included. The results showed that the quality of the composts treated with vinasse was similar to the compost irrigated with water. However adding vinasse to compost increased the cation exchange capacity (CEC). All the composts posed N, C and pH values as well as C/N ratio within the limits set by legislation hence they are suitable for agricultural use. The humic acids in compost, and mainly in the filter cake, were more aliphatic than aromatic. The filter cake presented the highest CEC probably due its fulvic acids concentration. Composting proved to be effective for vinasse recycling, yielding a product with high agronomic value and free of potential polluters. The effects of the composts and filter cake on soil fertility, humic substances, and crop productivity were evaluated for three cycles of sugarcane cropping. The treatments comprised four rates of composts or filter cake (0, 9, 18 and 27 Mg ha-1) plus a control with mineral fertilizers. There was an increase for pH and concentrations of P, Ca, Mg, K and organic matter, being the greatest effect owing to filter cake addition and the lowest to the compost enriched with potassium sulfate. The compost amended with vinasse and potassium sulfate increased water soluble carbon for the 0-10 cm soil depth whereas filter cake increased nitrate contents up to the 40 cm depth. The humic acid in soil treated with filter cake tended to incorporate alkyl carbon and carboxylic groups in its structure, which promote high recalcitrance and reactivity. On the other hand the humic acids in the compost watered with vinasse presented more phenolic compounds. The sugarcane yield depended on the soil fertility improvements due to organic residues added to soil. Such additions resulted in productivity (80 to 96 Mg ha-1) statistically similar to that obtained by mineral fertilizing (79 Mg ha-1).