Solution Phosphate Concentration as an Estimate of The Fertilizer P Requirement of Soybean (Glycine max (L) Merr.) in Some Inceptisols in Benue State, Nigeria

Abstract

Some Inceptisols in Benue state were examined to determine the solution phosphate concentration (SPC) as a parameter for the estimation of the P fertilizer required to obtain optimum yield of soybean. Soil inorganic P was fractionated using standard procedures. Sorption characteristics were determined in 0.01 M CaCl₂ solutions of various P concentrations. For each soil, the amounts of P that gave 0.025, 0.05, 0.075, 0.100, 0.125, 0.150, 0.175, 0.200, 0.225, 0.250 mg kg^-1 solution concentration were estimated from adsorption curves. In the greenhouse, 4 kg of soil from each location was placed in plastic pots. Amount of P estimated from sorption study was added as KH₂PO_4. The treatments were laid out in Randomized Complete Block Design (RCBD) and soybean (TGx 1448-2E) was planted and observed to maturity. At harvest,  the shoot was dried, weighed, milled and digested in a 4:1 HNO₃:HClO₄ mixture and analyzed for P. SPC was determined for each soil in relation to yield. Field trials were conducted at Otobi-Akpa and phosphate fertilizer quantity that resulted in 0.0, 0.5 SPC, 1.0 SPC, and 2.0 SPC (as estimated from pot experiment) was added per plot and replicated thrice in RCBD. Soybean seeds were planted by drilling. At harvest, SPC that gave highest grain yield was evaluated for each soil and the quantity of P required to achieve this concentration was calculated. High grain yield (t ha^-1) was obtained at 0.025 mg P kg^-1 in Otobi-Akpa (12.5) and Abeda-Mbadyul (8.5) while Tor-Donga and Odoba produced 5.4 and 4.1 t ha^-1 respectively at 0.10 mg kg^-1 in the green house experiment. The estimated SPC value also gave high grain yield (5.0 t ha^-1) in the field trial at Otobi-Akpa. It was concluded that Odoba soil would require highest fertilization of 604.93 Kg P ha^-1, Tor-donga (112.31 Kg P ha^-1), Abeda-Mbadyul, 105.93 Kg P ha^-1 while the Otobi-Akpa soil would require the least fertilization of 33.18 Kg P ha^-1.

Keywords: Solution phosphate concentration (SPC), Inceptisols, P-Requirement, Soybean