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Hedged agroforestry (AF) demonstration plots with maize/bean intercrops were studied at Matanya in
Laikipia district, Kenya, between 1991 and 1995 inclusive, to understand crop yield behaviour due to
selected soil moisture conservation methods applicable in semi-arid areas. The treatments were:
Grevillea robusta trees root pruned, compared to unpruned, both in combination with (1) minimum
tillage and mulching with 3t/ha maize stalks harvested from the plots with additional stalks collected
from the nearby farms, and (2) the locally applied method of deep tillage practiced by the immigrants
from wetter regions, acting as the control. Results showed that: (i) plots with root pruned Grevillea
robusta trees that were mulched and minimum tilled had most soil moisture available in the shallower
layers, during the wettest and the driest season on which this paper is based; (ii) the variation of soil
moisture with distance from the Grevillea robusta trees showed patterns that were quite similar for plots
with root pruned trees in the dry and the wet season; (iii) beans had greater seed yields and maize had
more (stover) biomass and (only in the wettest season) grain in plots with pruned trees, minimum tilled
and mulched, than in other AF plots. In the wettest season this resulted in identical maize yields but
lower bean seed yields compared to those in the mulched and sometimes also the local control plots
without trees. In the driest season bean yields remained the same but maize biomass yields improved
above the control yields for the most successful agroforestry intervention applied; (iv) competition
between the six year old Grevillea robusta trees and the crops was indirectly confirmed to be stronger
than in earlier experiments in the same plots. This way the agroforestry demonstration plots were very
successful in showing the consequences of the ageing agroforestry system, where the soil moisture
conservation measures of pruning and mulching kept their effects. Statistical analysis only weakly
confirmed the positive effect of root pruning on reducing competition for soil moisture between crops
and trees that were very clearly shown to exist by the physical error analysis
