7. Green manuring with vetch on acid soil in the highland region of Rwanda.
Biol. Agric. and Hort., 7, 1991, pp. 303-310
The purpose of this study is to assess the value of vetch used as a green manure crop on acid soil prevalent in the highland region of Rwanda. Bean (Phaseolus vulgaris) and wheat (Triticum aestivum) yields and soil chemical analysis following incorporation of vetch were used to evaluate success in the improvement of soil fertility.
The study was carried out during four cropping seasons.
The soils are classified in the USDA system as Oxisols.
The advantages ascribed to legume green manuring are numerous and include: improvement in soil fertility, increased cation exchange capacity, increased water retention, enhanced microbial activity and improved soil structure.
Vetch (Vicia sativa L.) is a potential green manure crop in the East-Central African highland region where soils are generally acidic.
In this area, vetch grows relatively fast, assumes a quick soil cover to check ground erosion and produces high biomass.
Incorporation of 5-month old vetch lowered soil pH as well as raising exchangeable Al and H and reducing exchangeable Ca, Mg, and P.
Consequently, bean and wheat yields for the subsequent season were reduced by 71 and 33%, respectively. There was no response to fertilizer N on either main treatment. There were no significant yield differences for either crop with respect to vetch treatment during the second season. Third season crop yields were superior on the control plot, with that of beans being significantly different. Application of lime raised exchangeable Ca, K and reduced exchangeable Al. Bean was more affected by exchangeable Al than wheat. Half-life for decomposition of the vetch was 3.5 weeks and nutrient release pattern followed the order: K>N>P>Ca indicating that liming may be required to supply Ca and to neutralize soil acidity at the initial stages of decomposition.
Concluding, it can be stated that in the Rwandan highlands vetch grows rapidly and provides a quick cover to control erosion. Its use for green manuring on the acid soils in this region must be treated with caution.
Decomposing vetch material initially made the soil acidic and crops which immediately followed vetch incorporation suffered severe yield losses. It may be advisable to allow three to four weeks after incorporation before seeding food crops. Similarly, lime application may be necessary to counteract adverse effects of soil acidity during early stages of decomposition and also to supply Ca.
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Asia, Philippines, humid tropics, lowland, field experiment, cropping systems, preceding crops, organic manure, nitrogen fertilizer, mungbean, sesbania, green manure, rice, maize, yield, residual effects
MEELU, O.P. et al.
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