Tropical Legumes Assessed for Soil Improvement in Southern Africa.

Tropical legumes, grown as green manure/cover crops, are an important option that smallholder farmers in sub-Saharan Africa have for maintaining the productive capacity of their soils. Success, however, depends on proper legume selection. Farmers and development practitioners need to know the potential biomass, mineral contribution, and soil impact of legume species. This research was undertaken in South Africa, over four growing seasons/years, to provide such knowledge for dry, sandy soils low in organic matter (0.6-0.8%) and nitrate-nitrogen (3-4%). Treatments, arranged in a randomized complete block design with four replications, included two controls (bare ground and weedy fallow) and eight legumes (cowpea [Vigna unguiculata], horse gram [Macrotylyma uniflorum], jack bean [Canivalia ensiformis], lablab [Lablab purpureus], sunn hemp [Crotalaria juncea], tephrosia [Tephrosia vogelii], and two types [bush and vining] of velvet bean [Mucuna pruriens]). Each season, with no fertility inputs and semi-arid conditions, the amount of dry matter produced by weeds (3-8 t·ha-1) was matched or exceeded by lablab (5-13 t·ha-1) and vining velvet bean (4-10 t·ha-1). Nitrogen accumulated by above-ground weedy biomass ranged from 67 to 166 kg·ha-1; these levels were matched or exceeded by 3 to 8 legumes each season. Lablab accumulated as much as 345 kg·ha-1 nitrogen. Legume residues left on the soil surface favorably impacted soil health. At the end of seasons 1 and 2, for example, soil nitrate concentration with bare ground (6-7 mg·kg-1) and fallow (8 mg·kg-1) were exceeded (P<0.0001) with cowpea (13-15 mg·kg-1), lablab (14-22 mg·kg-1), and vining velvet bean (12-15 mg·kg-1). At the end of season 4, soil potassium was higher with lablab (52 mg·kg-1) than bare ground (30 mg·kg-1). Results are helpful for assessing the potential of tropical legume species to accumulate nutrients and maintain soil quality.