Abstract:
Nitrogen deficiency is widespread in southern Africa, but inorganic fertilizers are often unaffordable for
smallholder farmers. Short-duration leguminous fallows are one possible means of soil fertility restora-
tion. We monitored preseason topsoil (0 to 20 cm) ammonium and nitrate, fallow biomass production
and grain yields for three years in a relay cropping trial with sesbania [Sesbania sesban (L.) Merr.] and
maize (Zea mays L.). Sesbania seedlings were interplanted with maize during maize sowing at 0, 7400
or 14,800 trees ha –1 , in factorial combination with inorganic N fertilizer at 0 or 48 kg N ha –1 (half the
recommended rate). After maize harvest, fallows were allowed to grow during the seven-month dry season,
and were cleared before sowing the next maize crop. Both sesbania fallows and inorganic N fertilizer
resulted in significantly greater (P < 0.01 to 0.05) preseason topsoil nitrate-N than following unfertilized
sole maize. In plots receiving no fertilizer N, preseason topsoil inorganic N correlated with maize yield
over all three seasons (r 2 = 0.62, P < 0.001). Sesbania fallows gave significantly higher maize yields
than unfertilized sole maize in two of three years (P < 0.01 to 0.05). Sesbania biomass yields were
extremely variable, were not significantly related to sesbania planting density, and were inconsistently
related to soil N fractions and maize yields. Short-duration fallows may offer modest yield increases under
conditions where longer duration fallows are not possible. This gain must be considered against the loss
of pigeonpea (Cajanus cajan L. Millsp) harvest in the similarly structured maize-pigeonpea intercrop
common in the region.
