Responses of Bambara groundnut [Vigna subterranea (L.) Verdc.] to Phosphate Fertilizer Rates and Plant Spacing and Effects on Soil Nutrient Statues in a Degraded Tropical Ultisol Agbani Enugu South East Nigeria

The agronomic practices such as plant spacing and phosphate fertilizer requirements for the cultivation of Bambara groundnut [Vigna subterranea (L.) Verdc.] have not yet been fully determined in Agbani agro ecology South east Nigeria. Field trials were conducted in 2015 and 2016 planting season at the Teaching and Research Farm of the Faculty of Agriculture and Natural Resources Management, Enugu State University of Science and Technology, Agbani as a 4 x 3 factorial experiment in randomized complete block design with three replications. The treatments were four rates of single super phosphate fertilizer (0 kg P ha^-1, 25 kg P ha^-1, 50 kg P ha^-1and 75 kg P ha^-1) and three plant spacing regimes (intra and inter row spacing) 10 cm x 45 cm (222,222 plants hectare^-1) 5 cm x 45 cm (148,148 plants hectare^-1) and 20 cm x 45 cm (111,111 plants hectare^-1). Soil samples were collected from the top soil at a depth of 0 to 15 cm before planting, four and eight weeks after planting. The obtained results showed that Bambara groundnut spaced at 20 cm × 45 cm per plant grown on plots fertilized with 75 kg P ha^-1 single super phosphate fertilizer (SSP) had significantly (P = .05) the tallest plants [24.53 cm at 4 weeks after planting (WAP), 35.23 cm at 8WAP in 2015 and 25.00 cm at 4WAP and 36.00 cm at 8WAP in 2016 respectively] compared with the other treatments. The effects of single super phosphate fertilizer treatment on leaf area index (6.04 in 2015 at 4WAP,  45.86 in 2015 at 8WAP, 6.31 at  4WAP in 2016 and 46.51 at  8WAP in 2016) were significant (P = .05) at the highest at application rate of 75 kg P ha^-1 of SSP. Plots which had the lowest population density (111,111 plants hectare^-1) and received the highest rate of  SSP (75 kg P ha^-1) gave significantly (P = .05)  the highest number of fresh pods per plant at harvest (17.67 in 2015 and 19.00 in 2016) and weight of fresh pods per plant (248.03 g plant^-1 in 2015 and 290.76g plant^-1 in 2016 respectively) than the other treatments. Total nitrogen, phosphorus and potassium concentration 7 of soil were significantly (P = .05) higher in plots which received 75 kg P ha^-1 SSP than the other treatments. Plant population density of 111,111 plants hectare^-1 and SSP rate of 75 kg P ha^-1 is recommended for optimum agronomic performance of Bambara groundnut in a degraded tropical ultisol.