ISSN: 0041-3216 (Online), 0041-3216 (Print)
Volume 62 Number 1
Tropical legumes for N production: growth and N content in relation to soil pH*
Growth rates, N accumulation, and soil cover potential of six green manure legumes were compared on a manqaniferous Oxisol. Responsiveness of Crotalaria juncea L., Mucuna sp., Lablab purpureus (L.) sweet. Psophocarpus tetragonolobus (L.) DC, Sesbania grandiflora (L.) Pers. and Sesbania cannabma Poir. to increasing soil pH (4.7-6.9) was examined in a field experiment. N content of the legumes was compared with that of Zea mays L. to estimate N-fixation by the legumes. Dry matter and N content increased with increasing pH to > 6.0. When N yield at pH 4.7 was expressed as a percentage of maximum N yield, b. cannabina was most sensitive to low pH (3%), followed by lablab (16%), crotalaria (20%). mucuna (36%), and maize (70%). N accumulation in the tops of S. grandiflora and Crotalaria ranged from 10 to 72 kg N ha-1, respectively, during the 10-week growth at soil pH of 5.8. Maize growth and N percentage indicated low levels of available soil N, suggesting that legume N content resulted primarily from the plant-rhizobia symbiosis rather than from N recovery from soil. High levels of soil Mn were probably the principal limitation to legume growth at pH < 5.3. Mucuna and lablab provided the greatest and most rapid initial soil cover and thus should be most effective in controlling soil erosion by rainfall.
Keywords: Nitrogen; Soil; pH; Legumes