Changes in soil profile characteristics through cultivation of an upland Vertisol in Zambia

Abstract. Crops grown on virgin upland Vertisols of Zambia, are reported to perform rather poorly. However, subsoiling followed by repeated cultivation over two years apparently improves crop growth. Highest yields were recorded under long-term cultivation (12 years). To evaluate the reasons for these differences in crop response to Vertisol management, physical and hydrodynamic characteristics of soil profiles were studied in three soil management systems. The management systems were: uncultivated or virgin land; land cultivated for two years; and land cultivated for 12 years. The mean soil aggregate size decreased with increased time of cultivation, mostly due to the decrease of the largest sized aggregates. The surface horizon dried more slowly on the long-term cultivation plot. A comparison of the hydraulic conductivities indicated that water intake in the deeper layers improved with increased period in cultivation. Oxygen diffusion measurements showed good aeration at field capacity, to a depth of 0.32m on the long-term cultivation plots, but only to 0.17 m and 0.25 m for 2 years cultivation and virgin plots respectively. Repeated cultivation was beneficial in improving surface soil tilth and in improving subsurface drainage, thus removing the problem of a perched water table which occurred close to the soil surface under natural conditions.     

Water requirements for salt control in rice schemes in the Senegal river delta and valley

The paper estimates the water requirements for salt control in rice schemes located on saline soils in the Senegal river delta. When the fields are not cultivated, salts are transported to the top soil by capillary rise from the very saline and shallow ground water table. During the irrigation season, the large quantity of irrigation water adds additional salt to the fields. If the percolation rate of the soil is small, salts will have to be removed from the schemes by flushing the standing water from the fields when the salinity of the water reaches the threshold value of 1.5 ds m^–1. The results indicate that if the schemes are located on the river banks (fondé), flushing at the beginning of the season and the percolation losses throughout the season may be sufficient to keep the salts out of the root zone. On the less permeable soils in the depressions (hollaldé), an extra flushing is required to evacuate enough salts from the fields during the irrigation season. In total about 2,300 m³ ha^–1 of water may be needed for flushing. If flushing is not practised, the schemes have to be abandoned after a few years of cultivation due to build-up of soil salinity.     

Soil erosion assessment at the Udi-Nsukka Cuesta (southeastern Nigeria)

Biophysical and participatory research methods were combined to examine factors contributing to soil erosion at field plot, village and regional scale on the sandstone dominated Udi-Nsukka Cuesta in southeastern Nigeria. At field plot scale, the properties of seven pedons were related to soil erodibility. Very high infiltration rates measured with a double ring infiltrometer and permeameter, were not in accordance to reported runoff and soil loss. The effect of groundcover and canopy height was incorporated into rainfall erosivity for plots under cashew, oil palm dominated forest and secondary natural vegetation. Cropping systems and field management practices were compared for different positions along a toposequence traversing the plateau and escarpment of the Udi-Nsukka Cuesta. Soil loss, calculated by a modified version of the universal soil loss equation, was 10 to 100 times higher on escarpment than on plateau plots. Farmers are adapting to the problems of interrill and rill erosion through careful crop selection and rotation, and contour ridging. At the village and regional scale, terrain observations were compared to archival research, historical accounts by villagers and geographic analysis of 1962 aerial photographs (1:40 000). Ravine and gully formations seemed influenced by a combination of infrastructure, geohydrology, topography, vegetation and land use. Both community efforts and state measures to combat erosion tend to be crisis managed, and are concentrated on repairing damage to economically important infrastructures. A conceptual diagram has been developed to show the complex interaction between various geophysical, agroecological, socio-economic and political components influencing soil erosion at farm, village and regional scale. Copyright © 1999 John Wiley & Sons, Ltd.