ABSTRACT

 Due to paucity of research data on the sorption characteristics of agricultural soils in Akwa Ibom State, this research aimed at determining the sorption capacities of nitrate, sulphate and phosphate onto different soils; and to evaluate the relationship between the different sorption parameters and soil physicochemical properties. Twenty-seven (27) soil samples were collected from nine soil profile pits at three different soil depths (0-30, 31- 60, 61-90 cm). Soil physicochemical properties were determined using standard methods while sorption studies were carried out to determine NO3 − , SO4 2- and PO4 3- sorption onto homogenized soils (< 1.0 mm) using batch equilibrium procedure based on Organisation for Economic Co-operation and Development (OECD) guidelines. Results obtained showed that the soils were generally acidic with pH ranged between 2.96 and 5.02. The soils varied widely in their NO3 - , SO4 2- and PO4 3- sorption behaviours due to different physical and chemical properties. Results showed that sorption of NO3 − , SO4 2- and PO4 3- increased with increasing concentration with maximal sorption values varied from 2609.1 to 2778.8, 1952.9 to 2244.7 and 1159.2 to 2023.9 mg g-1 soil respectively. The Langmuir, Freundlich, Van Huay and Temkin models were applied to study the sorption characteristics and the relationship between sorption parameters and soil physicochemical properties were examined using Pearson correlation analysis. The Temkin and Freundlich model provided the best choice for nitrate (R2 = 0.9390) and phosphate (R2 = 0.9872) sorption respectively; while Langmuir isotherm provided best choice for sulphate sorption for IB-USR (R2 = 0.9846) and IB-USW (R2 = 0.9205). The equilibrium concentration for nitrates (EC-N), sulphates (EC-S) and phosphates (EC-P) were used as desorption index to determine whether the soils could serve as potential sources for their bioavailability to plants. Results of EC-N and EC-P ranged from 7.10 to 7.40 and 5.0 to 9.90 mgL–1 respectively, showing high vulnerability to nitrate and phosphate loss. Samples IN-ITM and IB-USR indicated low tendency for sulphate loss with EC-S values of 2.00 mgL–1 and 5.35 mgL–1 respectively, while sample EK-ONW (EC-S = 5.00 mgL– 1 ) indicated lowest tendency for phosphate loss. Relating phosphate sorption parameters with soil physicochemical properties, Langmuir sorption maxima (Qmax) correlated with organic carbon (r = 0.611), organic matter (r = 0.594), amorphous Fe2O3 (r = 0.660) and Al2O3 (r = 0.606)