English

Abstract

Ethiopian soils, formed from old weathered rocks, are naturally low in fertility. Moreover, crop production is constrained by non-sustainable cropping practices, particularly repeated plowing and hoeing, which enhances loss of soil organic matter. Field trials were therefore conducted to determine the integrated effects of tillage system and nitrogen fertilization on organic matter content of Nitisols at five sites using maize as test crop during 2000 to 2004 in Western Ethiopia. Three tillage systems [minimum tillage with residue retention (MTRR), minimum tillage with residue removal (MTRV) and conventional tillage (CT)] and three N levels (the recommended rate and 25% less and 25% more than this rate) were combined in factorial arrangement with three replications. After 5 years, the two measured indices of organic matter, viz. the organic carbon (C) and total nitrogen (N) content were both significantly higher in the MTRR soil compared to the CT and MTRV soils. However, the influence of tillage systems on organic C and total N was confined to the upper 0 to 7.5 cm soil layer. On average, organic C and total N in this layer were 17 and 25%, and 20 and 29% higher with MTRR than with CT and MTRV, respectively. Application of N fertilization for 5 consecutive years showed profound effects on organic C and total N. Increasing levels of N application led to higher organic C and total N irrespective of tillage system. Both organic C and total N showed however a steady (115 kg ha-1 N level) to declining (69 and 92 kg ha-1 N levels) trend over time. Based on these results, replacement of CT with MTRR should be beneficial and sustainable to soil quality in the study area. However, MTRV is not an option at all to replace CT from a soil quality point of view.  This study’s findings may be applicable to other highland regions in Africa where cropping on Nitisols is common.   Key words: Conventional tillage (CT), crop residues, minimum tillage, organic carbon, total nitrogen.