Variable grain legume yields,responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms |
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Authors: | Shamie Zingore Hebert K Murwira Robert J Delve Ken E Giller |
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Affiliation: | (1) Tropical Soil Biology and Fertility Institute of CIAT, Mt Pleasant, P.O. Box MP228, Harare, Zimbabwe;(2) Plant Production Systems, Department of Plant Sciences, Wageningen University, P.O. Box 430, Wageningen, 6700 AK, The Netherlands |
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Abstract: | Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets
on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources
to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile
fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super
phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy
soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on
productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha−1) and maize yields (<0.7 t ha−1) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients.
Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also
significantly increased the proportion of N2 fixed by soyabean (measured using 15N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP
and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger
on clay soil with moderate fertility (0.4–0.7 t ha−1 and 2.0–2.3 t ha−1 respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher
when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of
manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In
the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha−1 respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields
were poor (< 0.5 t ha−1) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with
available P (R
2 = 0.5–0.7) and soil organic C (SOC) contents (R
2 = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more
fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced
by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income
by targeting manure to soyabean on the more fertile soils. |
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Keywords: | Soyabean Soil fertility gradients P use efficiency Manure Rotations |
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