Evaluation of Seven Forage Legumes for Biological Nitrogen Fixation (BNF) and Their Effects on Amaranthus cruentus in a Fluvisol (River Sand)

A six month screen house experiment was conducted at the Department of Crop Science, University of Benin, Benin City to assess seven forage legumes for N2 fixation and their effects on Amaranthus cruentusin a fluvisol (river sand). Cajanus cajan, Centrosema pascuorum, Leucanea leucocephala, Peuraria phaseoloides, Stylosanthes guianensis, Stylosanthes hamata and Lablab purpureus were fitted into a completely randomized design with three replications. All seeds except Lablab purpureus were scarified and treated with benlate (50% benomyl) before sowing in river sand. Measurements taken at 4- week intervals were root length (cm), root fresh weight (g) and root dry weight (g). At 8 weeks after sowing (WAS), number of nodules, number of effective nodules, nodules fresh weight and nodules dry weight were measured. Shoot and soil nitrogen (g kg-1), leaf chlorophyll index and carbon: nitrogen ratio was assessed at 12 WAS. Amaranthus cruentus followed legumes in sequence and number of days to emergence, plant height, number of leaves, root length, fresh weight of leaves, stems and roots (g) including dry weight of leaves, stems and roots (g) were assessed at 4 WAS. The seven forage legumes accumulated substantial quantities of nitrogen in their shoot (30.5–40.9 g kg-1) and also fixed considerable quantities of nitrogen in the soil (3.2–6.3 g kg-1). Centrosema pascuorum recorded the highest shoot nitrogen (40.9 g kg-1) whereas Stylosanthes hamata fixed the highest quantity of soil nitrogen (6.3 g kg-1). Leucanea leucocephala furnished the best (p = 0.05) root variables while Stylosanthes hamata exhibited the best root nodule characteristics. Carbon: nitrogen ratio ranged from 2.6 to 13.3. Amaranth seeds emerged within 2–7 days after sowing. Growth and yield of amaranth was significantly better in the Lablab-amaranth than other sequences. These positive responses indicate their usefulness for biological nitrogen fixation, forage production and soil fertility improvement. Lablab-amaranth sequence should be developed further for increased vegetable consumption.