Alterações físicas em um Cambissolo tratado com biofertilizante: indicadores de qualidade e refinamento do método de avaliação pelo Srelativo

Abstract

The knowledge about soil physical changes and soil quality is important for the adequate targeting of management strategies to be adopted when soil is used for cropping. Considering the hypotheses that a) cultivation worsens soil quality, compared to soil under natural vegetation, for degrading its properties related to the porous geometry; b) biofertilizer application (organic matter) in soil under cultivation promotes an improvement in its physical attributes, compared to the soil under natural vegetation, for acting as a cementing agent between particles; c) soil physical changes can be assessed through indices and interpreted under the qulitative aspect; and d) the Srelative index obtained using the soil-water retention curve determined as close as possible to the structural porosity is more sensitive to physical changes than the Srelative obtained using a soil-water retention curve determined from air-dried soil, the objectives of this study were: 1) evaluate the effects of cultivation and biofertilizer application on the physical quality of a Cambisol cultivated with Ficus carica L., irrigated by drip system; 2) verify the efficiency of indicators at assessing changes in physical attributes; and 3) refine the method of obtention of the Srelative index, aiming to increase its sensitivity to the soil physical changes. In order to evaluate physical quality, five soil scenarios were analyzed: under fig cultivation without biofertilizer application (control), with application of 20%, 40% and 60% of the biofertilizer through irrigation, and secondary native forest (additional control), until the depth of 0.3 m, in the layers of 0.0-0.1 m, 0.1-0.2 m and 0.2-0.3 m, and four replicates. In these layers, disturbed and undisturbed soil samples were collected in order to perform physical analyses. The completely randomized design was adopted. For the refinement of Srelative, with the soil-water retention curve containing only textural porosity (reference curve), soil dispersion was performed in water and with the addition of 1 N sodium hydroxide (with and without removing sodium through washing). F tests were applied for the variance analysis and Dunnett test for mean comparison. Line parallelism and intercept tests were performed for the regressions between soil physical variables and Srelative obtained using air-dried soil, with dispersion in water and addition of 1 N sodium hydroxide (with and without washing). A multivariate analysis was also performed in the dataset. It was concluded that: 1) the porous network quality is improved, or kept, when soil is cultivated under the conditions described in this experiment; 2) when cultivated, biofertilizer application improves or, at least, maintains the quality of soil physical attributes in all considered layers, except for the soil air intrinsic permeability in the layer of 0.0-0.1 m; 3) regarding the soil under native forest, biofertilizer application improves or, at least, maintains the quality of soil physical attributes in all considered layers, except for the clay floculation degree in the layer of 0.0-0.1 m; 4) cases where the quality of soil physical attributes was worsened as a result of the applied treatments, although they were not considered as critical for plant development, are an indication that the adoption of specific management techniques is needed to avoid soil degradation; 5) most of the selected soil physical quality indicators are efficient at quantifying changes imposed to the soil structure; and 6) the Srelative index obtained from the method of soil dispersion in water is more sensitive to soil physical changes than the Srelative obtained using air-dried soil.