Evaluation of the effects of conservation practices carried out in the 1970s on soil properties in the Eastern Region of Paraguay

Soils in subtropical and tropical regions are extremely fragile to perturbations due to their sandy texture, acid nature, low soil organic carbon (SOC) and low cation exchange capacity. In Paraguay, deforestation and subsequent cultivation affect 60% of the land. Certain conservation practices were carried out in the 1970s in response to the important degree of degradation caused by the soil degradative processes. Although the management of crop soils is still very intensive, to date there is no data on how the practices are affecting the sustainability of the system. The objective of this study was to evaluate the chemical and physical properties of three subtropical soils (Ultisol, Alfisol and Oxisol) under different agricultural systems and types of management, aimed at conservation, in the Eastern Region of Paraguay. The agricultural systems differ in the main crop (soybean or maize) and rotation crop (oat, turnip, crotalaria, lupins, maize). The soil properties were compared with those in disturbed natural forests and eucalypt plantations established on former agricultural land. The physical and chemical properties of the soil were determined in the 0 to 40 cm layer. The soils in the disturbed natural forest systems were less acidic and contained more Ca, Mg and K than the cultivated soils. Relative to the seminatural forests, the cultivated soils contained 20 to 40% less SOC and had lower macronutrient reserves and higher bulk density (up to 1.45 g cm(-3)). The SOC was higher in the agricultural systems with oats sown as a rotation crop in winter. By contrast, the N content was higher in systems with lupins or turnips grown in winter. Even when managed using conservation practices (direct sowing, retention of harvest residues, green manure), the soils in the agricultural systems under study displayed signs of physical and chemical degradation. The findings show that these conservation practices are not sufficient to correct the impact of intensive agricultural management.