ABSTRACT
Phosphorus (P) is one of the essential macronutrients needed for plant growth but its use efficiency is reduced due to the sorption of a substantial amount added to the soil. The study was conducted on twelve (12) soil samples obtained from four (4) soil depths namely (0-30, 30-60, 60-90 and 90-120 cm) in Ahiara soil series of NIFOR, Benin City, Edo state, Nigeria. The P sorption capacity was determined by equilibrating the samples for six (6) days in 30 ml of 0.01 M calcium chloride (CaCl2) containing various concentrations of P as KH2PO4. The physical and chemical properties of the soils were determined using standard laboratory methods while the Freundlich model was used to express the P sorption capacity at different depths. The data obtained were subjected to statistical analysis using Genstat statistical package. The means were separated at 5% level of probability using Duncan multiple range test. The results from the physical and chemical properties of the soils at the various depths revealed that there were no significant differences for sand and clay between the soil depths, silt showed significant differences between the soil depths and the soil textural class was Sandy loam for all the soil depths. The soils were generally acidic with a mean pH value of 4.68, organic carbon and total nitrogen decreased with increased soil depths. The available P and K showed no significant differences among soil depths and Ca did not increase with increased soil depths, however, Mg decreased while Na increased with increased soil depths. The mean values of OC, N, P, K, Ca, Ng and Na were 7.01gkg-1 , 0.35, 0.42 mgkg-1 , 0.26, 0.73, 0.28 and 0.09 cmolkg-1 respectively and were all low except Ca and Na which were adequate when compared to critical limits. EA and ECEC increased while CEC and % BS did not increase with increased soil depths with mean values of 0.50, 1.69, 1.12 cmolkg-1 and 69.74 % respectively. The rate of P sorption increased with increase in concentration of P added to the soils. At 20, 40, 60, 80 and 100 mg P L-1 , the mean values of the amount of P adsorbed were 19.70, 37.81, 58.31, 72.27 and 92.94 mgkg-1 respectively. The P sorption capacities of the various soil depths considering the Freunlich model decreased in the order of 60-90 cm > 30-60 cm > 90-120 cm > 0-30 cm with values of 45.290, 43.152, 36.296 and 28.184 mgg-1 respectively. The Freundlich model showed mean R2 value of 0.840. P sorption determination and proper P management such as application of organic and inorganic fertilizers are recommended to reduce P sorption for the benefit of plant growth.
