Variabilidade espacial de indicadores físicos e visuais da qualidade estrutural dos solos da sub-bacia Santa Rita, Pelotas/RS

Abstract

Aiming at the sustainability of natural resources, information about the soil physical quality is necessary to subsidize farming and environmental conservation plans. In such perspective, and regarding water basins, the conservation of the natural environment is a theme of great relevance in the current economic, social and cultural contexts. This study aimed at evaluating the structural quality of the soils in the sub-basin Santa Rita based on physical and visual indicators, with the purpose of identifying and mapping their physical conditions and building up an initial data base of the soil physical quality for further studies on soil management and conservation. This research was carried out in the sub-basin Santa Rita, which is part of the Water Basin Arroio Moreira Fragata (BHAM), Pelotas, in the South of Rio Grande do Sul State, Brazil. BHAM is one of the main basins in the region, and is responsible for the water supply for Pelotas population. Initially, satellite images were classified to identify the classes of soils present in the sub-basin Santa Rita. Soil sampling was carried out at georeferenced and irregularly spaced points of a sampling grid in the sub-basin Santa Rita. The following soil properties were analyzed: soil structure visual evaluation, soil visual evaluation, weighted mean diameter, macroaggregates, microaggregates, aggregate tensile strength, friability, total organic carbon, soil penetration resistance, soil density, soil porosity, macroporosity, and microporosity in the 0.00-0.10m layer. To analyze the behaviour of these properties, the results were firstly submitted to descriptive statistics and the Kolmogorov-Smirnov test aiming at verifying the data distribution normality tendency. The spatial variability structure was evaluated using experimental and theoretical semivariograms and the respective fitting parameters (C0 nugget effect, C+C0 sill and range A). When the spatial dependence was identified, ordinary kriging was used to elaborate the map of variables; when it was not identified, the inverse distance weighted interpolator (idw) was employed. The spatial variability was identified and characterized and maps of the soil structure spatial distribution were created using these methodologies. The visual evaluation of the soil structure and the soil visual evaluation were practical ways of differentiating systems of use and management, and can be considered indicators of the soil physical quality. The soil physical parameters such as weighted mean diameter, macroaggregates, microaggregates, aggregate tensile strength, friability, total organic carbon, soil resistance to penetration, density, porosity, macroporosity and microporosity enabled the identification of alterations in the structural conditions regarding use and pedological units and are considered important indicators of soil quality.