Effect of tillage system and nitrogen fertilization on the pH, extractable phosphorus and exchangeable potassium of Nitisols in Western Ethiopia

Abstract

Maize production in Western Ethiopia is constrained by non-sustainable cropping practices, particularly repeated conventional tillage and insufficient N fertilization.  A change in these cropping practices have usually a profound influence on soil chemical properties of which some may affect crop performance.  Field trials were therefore conducted to determine the integrated effects of tillage system and nitrogen fertilization on the pH, extractable phosphorus and exchangeable K of Nitisols at five sites using maize as test crop from 2000 to 2004 in Western Ethiopia. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) 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 five years the influence of the tillage systems on pH, extractable P and exchangeable K was confined to the upper 0-7.5 cm. The soil was acidified much more with MTRR than with either MTRV or CT. However, MTRR resulted in higher contents of extractable P and exchangeable K than MTRV and CT.  Application of N fertilization for five consecutive years significantly decreased pH irrespective of tillage system. Neither extractable P nor exchangeable K were affected by N fertilization. The enhanced soil acidification that coincides with MTRR may impact negatively on maize production in the long run without a proper liming program. Liming is currently not a common practice in Western Ethiopia because of resource poor farmers. This aspect should be taken into account when the replacement of CT with MTRR is recommended. The findings of this study could be useful to other highland regions in Africa where cropping on Nitisols is common.   Key words: Conventional tillage, crop residues, minimum tillage, plant nutrients, soil acidity.