Atributos físicos do solo cultivado com cana-de-açúcar sob sistemas de manejo

Abstract

The knowledge of the spatial distribution of soil physical attributes, as well as practices of soil management and gypsum application is important to evaluate effects of agricultural operations on sugarcane productivity. In this context, the objective of this work was to study the best sample density, to determine the spatial variability, the effects of soil preparation with harrowing + subsoiling and the physical attributes of a dystrocohesive Yellow Argisol correlating with the productivity of sugarcane. In order to evaluate the spatial variability and sample density, a sample mesh was demarcated in a first cycle of sugarcane cultivation, where 32 equidistant samples were collected, spaced every 7 m. In order to evaluate the effects of the preparation and the gypsum on the physical attributes of the soil, an experiment was carried out in a randomized block design in a subdivided plots scheme with four replications, totalizing 32 experimental units. The treatments in the main plots were with mechanized preparation (harrowing + subsoiling) and the absence of soil preparation. In the subplots the doses of gypsum applied were 1; 2 and 4 t ha-1 plus the control (absence of gypsum), applied on surface to the haul before tilling the soil. The samples were collected at depths of 0-0.30 m and 0.30-0.60 m between the planting furrows. Tensile strength of the aggregates, weighted mean diameter, penetration resistance of soil, gravimetric moisture, bulk density, total porosity, macroporosity, microporosity, soil calcium content, soil water retention characteristic curve, adjustment parameters of the van Genuchten model, least limiting water range and sugarcane productivity. The minimum sample density to estimate sugarcane productivity was 14 samples per hectare and depended on the penetration resistance and total porosity of the soil. The natural densification of cohesive soils can be reduced by irrigation, favoring significant increases in productivity, independent of other soil physical attributes. The gypsum rates, in the management with harrowing and subsoiling, reduced the tensile strength of the soil aggregates. Among the soil physical attributes penetration resistance of soil was influenced by soil preparation with lower values in the mechanized plots. Soil preparation with subsoiling was efficient in increasing gypsum contents in depth. Subsoiling was more effective in increasing the critical soil bulk density range when used alone. The dose of gypsum with 2 Mg ha-1 improved the physical-water attributes of the soil, with greater availability of water to the plants, by increasing the critical bulk density of the soil, when prepared with subsoiling. The improvement in soil physical-water attributes due to the application of 2 Mg ha-1 increased cane-plant productivity.