Abstract:
Yield levels in smallholder farming systems in semi-arid sub-Saharan Africa are generally low. Water
shortage in the root zone during critical crop development stages is a fundamental constraining factor.
While there is ample evidence to show that conservation tillage can promote soil health, it has recently
been suggested that the main benefit in semi-arid farming systems may in fact be an in situ water harvest-
ing effect. In this paper we present the result from an on-farm conservation tillage experiment (combining
ripping with mulch and manure application) that was carried out in North Eastern Tanzania from 2005
to 2008. Special attention was given to the effects of the tested treatment on the capacity of the soil to
retain moisture. The tested conservation treatment only had a clear yield increasing effect during one of
the six experimental seasons (maize grain yields increased by 41%, and biomass by 65%), and this was a
season that received exceptional amounts of rainfall (549 mm). While the other seasons provided mixed
results, there seemed to be an increasing yield gap between the conservation tillage treatment and the
control towards the end of the experiment, and cumulatively the yield increased with 17%. Regarding
soil system changes, small but significant effects on chemical and microbiological properties, but not on
physical properties, were observed. This raises questions about the suggested water harvesting effect
and its potential to contribute to stabilized yield levels under semi-arid conditions. We conclude that,
at least in a shorter time perspective, the tested type of conservation tillage seems to boost productivity
during already good seasons, rather than stabilize harvests during poor rainfall seasons. Highlighting the
challenges involved in upgrading these farming systems, we discuss the potential contribution of con-
servation tillage towards improved water availability in the crop root zone in a longer term perspective.
