CHANGES IN SOIL-PROFILE CHARACTERISTICS THROUGH CULTIVATION OF AN UPLAND VERTISOL IN ZAMBIA

Crops grown on virgin upland Vertisols of Zambia, are reported to perf orm rather poorly However, subsoiling followed by repeated cultivation over two years apparently improves crop growth. Highest yields were r ecorded under long-term cultivation (12 years). To evaluate the reason s for these differences in crop response to Vertisol management, physi cal and hydrodynamic characteristics of soil profiles were studied in three soil management systems. The management systems were: uncultivat ed or virgin land; land cultivated for two years; and land cultivated for 12 years. The mean soil aggregate size decreased with increased ti me of cultivation, mostly due to the decrease of the largest sized agg regates. The surface horizon dried more slowly on the long-term cultiv ation plot. A comparison of the hydraulic conductivities indicated tha t water intake in the deeper layers improved with increased period in cultivation. Oxygen diffusion measurements showed good aeration at fie ld capacity, to a depth of 0.32 m 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 surfac e soil tilth and in improving subsurface drainage, thus removing the p roblem of a perched water table which occurred close to the soil surfa ce under natural conditions.