Disc harrowing intensity and its impact on soil properties and plant growth of agropastoral systems in the Llanos of Colombia

Oxisols such as those of the Colombian Eastern Plains (Llanos) are suscepti ble to physical, chemical and biological degradation once brought into cult ivation, especially under intensive use of machinery. The main objective of this study was to determine the impact of intensive disc harrowing (2, 4 o r 8 disc harrow passes per year over 3 years) on soil physical and chemical properties, soil phosphorus dynamics, plant growth and nutrient acquisitio n of contrasting agropastoral systems on an Oxisol. The three: main systems tested after 2 years of upland rice cultivation were grass-alone pasture ( Brachiaria dictyoneura), green manure (Crotalaria juncea), and maize (Zea m ays). Native savanna treatment was used as a control. Intensive disc harrow ing improved macroporosity values of 0-5 cm soil layer up to 59% for grass- alone pasture system compared to native savanna. Disc harrowing significant ly reduced bulk densities for pasture and green manure systems compared to the native savanna in the 0-5 cm soil layer. Intensive disc harrowing signi ficantly improved volumetric moisture content of green manure and maize sys tems at 5-10 cm soil depth. The distribution of biologically, moderately an d sparingly available P, organic P and total P varied under green manure, m aize and grass-alone pasture systems. Two passes of disc harrow per year we re sufficient for grass-alone pasture, while maize showed greater abovegrou nd production and nutrient acquisition at 8 passes of disc harrow per year. The maize and green manure cropping systems were better than the grass-alo ne pasture system at separating the effect of increased number of disc harr ow passes on soil physical and chemical characteristics. (C) 2001 Elsevier Science B.V. All rights reserved.