CHARACTERIZATION AND AMELIORATION OF DEGRADED SOILS IN THE ETHIOPIAN HIGHLANDS

Abstract

While millions of dollars and billions of hours of food-for-work farm labor have been invested in soil and water conservation practices in the (sub) humid Ethiopian highlands, sediment concentrations in rivers are increasing because land degradation and associated soil erosion remain a persistent problem in the Ethiopian highlands despite the conservation efforts. The objective of this dissertation research was, therefore, to better understand soil degradation and measures that can be taken to ameliorate the hard pans that have formed in degraded soils. The research was carried out in the humid Ethiopian highlands where land degradation is severe. The detailed study areas were the 113-ha Anjeni and 95-ha Debre Mewi watersheds in the Ethiopian highlands. In both watersheds, 0-45 cm soil penetration resistance (SPR, n=180) and soil physical properties (particle size, organic matter, pH, base ions, cation exchange capacity, silica content, bulk density and moisture content) were determined at 15 cm depth increments for three land uses: cultivated, pasture, and forest. In addition, 32 experimental plots were constructed in the Anjeni watershed to investigate the effects of ripping and liming of soil hardpans on runoff and erosion. The results show that the mean SPR of agricultural fields was significantly greater (at p<0.05) than that of forest lands. Dense layers with above SPR a critical threshold of ≥ 2000 kPa were observed in the cultivated and pasture lands starting at a depth of 15-30 cm but did not occur in the undisturbed forest land. Compared with the original forest soils, agricultural fields were: lower in organic matter, CEC, and exchangeable base cations. They were also more acidic, had a higher bulk density and more fine particles (clay and silt), and contained less soluble silica. Measurements in the ripped and limed soil in the field experiments in the Anjeni watershed showed that ripping to depths up to 60 cm increased infiltration and decreased runoff. Liming alone, on the contrary, increased runoff likely due to surface sealing. Deep ripping was not effective in reducing sediment yields and there was a tendency for deeper ripping to increase sediment concentration especially in the beginning of the rain phase. Liming decreased sediment concentrations compared with the unlimed plots. Overall, the findings suggest that land degradation is a process where soil physical and chemical properties in agricultural lands deteriorate after deforestation, causing disintegration of soil aggregates resulting in greater sediment concentration in infiltration water that clog macro-pores, thereby disconnecting deep flow paths found in original forest soils. This in turn decreased baseflow and increased direct runoff. This process is common in the Ethiopian highlands.