Propriedades, erodibilidade e produtividade de camadas de podzólico vermelho-amarelo eutrófico e cambissolo eutrófico, submetidas à erosão simulada

Abstract

Soil conservation research have dealt, predominantely, with quantification of soil and water losses assuming that crop yields would be inversely proportional to the increase of erosion. Consequently, information on the erosion's impact on productivity and erodibility as related to exposure of layers of different soil depths to cultivation is scarce. However this information is increasingly important as the concern for preserving the productive potential of soil arise and the need for restoring productivity on degraded lands and its associated costs tend to increase. A greenhouse experiment was conducted in order to detect the effects of simulated erosion through exposure of surface soil and subsoil layers to maize and cowpea growth at varying depths of 0 - 5, 5 - 10, 10 - 20, 20 - 40 and 40 - 60 cm on an eutrophic Red Yellow Podzolic and on an eutrophic Cambisol of Ceará State, Brazil. Chemical and physical properties of these layers as well as their erodibilities were determined and 3 kg soil samples correspondent to each depth were used in plastic bags where maize and cowpea were grown during 60 and 77 days, respectively, with three replications for each layer on both soils. As soil depth increased, progressive reduction in soil quality and unfavorable conditions mainly related to compaction due to clay increase as well as decreases in organic matter and less availability of N, P, K, Ca and Mg, were detected in those layers below 5 cm. Consequently, emergence was delayied, and plant height, dry matter yield and nutrient absorption decreased gradually from maximum values measured on plants grown on the surface layer (0 - 5 cm) to minimum values of these parameters determined on the deeper layers until 40 - 60 cm. Removal of surface layers through erosion would exacerbate their susceptibility to soil losses since while the Cambisol erodibility increased only slightly from the surface (0,037) to intermediate layers (0,041) it attained the highest value of this factor. In this same direction the Podzolic virtyalIy doubled the value of this parameter at intermediate and lower layers (0,040) as compared to topsoil layers (0,018 - 0,021). Regression analyses relating alI the layers' soil properties with crops reIative yields, detected organic matter as the soil property which showed the highest correlation coefficent to maize (r = 0,97) and cowpea (r = 0,89) relative yields on the Podzolic. On the Cambisol, P content (r = 0,98) and K content (r = 0,84) were attributes more correlated to cowpea and maize relative yields, respectively. The data obtained through simulated erosion of uniform layers of the soil profile although of relative importance due to their artificial characteristics of exposure of varying depths, clearly indicated that maintenance of topsoil thichness and integrity is associated with high soil productivity and less vulnerability of these soils to erosion due to better soil conditions and smalIer erodibility values. Effectively natural erosion tends to decrease topsoil depth. However, unfavorable conditions of the subsoil environment related to nutrient depletion, organic matter decreases, higher soil density and drastic reduction in productivity detected on both soils, even with combination of attributes due to layers mixture during soil tilIage, tend to increase the magnitude of soil-related constraints to sustainability of higher production levels and plant growth.