Phosphorus Adsorption as Affected by Concretionary Nodules of Oxic Rhodustalf in Southern Guinea Savannah Agroecological Zone of Nigeria

Hard, rounded masses of mineral matter, known as concretionary nodules, can be found in soil or sedimentary rock. These nodules are typically made up of minerals like iron oxides, hydroxides, and carbonates that have been deposited in groundwater. Their sizes can range from small pebbles to large boulders, and they often differ in composition or hardness compared to the surrounding rock or soil. Nodules act as a highly effective storage space for extra P, leading to a significant increase in overall P requirements. Phosphorus, although an essential element for all living organisms, including plants and animals, is scarce. Despite its importance, only a small fraction of the total phosphorus available can be readily absorbed by plants. Given the worldwide demand for phosphorus in food production, it is crucial to devise techniques for extracting it from different sources. However, there has been limited research on the understanding of phosphorus availability and adsorption mechanisms in these areas. Therefore, the study focused on exploring the impact of concretionary nodules on phosphorus sorption and the characteristics of low-activity clay soil in the Guinea savannah of Nigeria. Soil samples collected from the study area were used to investigate the soil’s ability to absorb phosphorus at depths ranging from 0 to 30-60-90-120-150 cm in different soil and concretion locations. Various soil and concretion types demonstrated distinct capacities for phosphorus adsorption, as indicated by the adsorption isotherm. The maximum monolayer adsorption capacities (Qmax values) were 161.0, 154.5, 149.6, 141.7, 139.8, and 139.3 mg/g for OBC, OBS, OC, OIS, OS, and OIC, respectively. At equilibrium with a 50-ppm solution, the pseudo-second-order rate constants for P sorption were 1.180 x 10<sup>–4</sup>, 9.740 x 10<sup>–5</sup>, 1.120 x 10<sup>–4</sup>, 1.140 x 10<sup>–4</sup>, 1.000 x 10<sup>–4</sup>, and 8.010 x 10<sup>–5</sup> g mg<sup>–1</sup> min<sup>–1</sup> for OIS, OIC, OBS, OBC, OS, and OC, in that order. In the 300-ppm equilibrium solution, the OIS, OIC, OBS, OBC, OS, and OC pseudo-second-order rate constants were 1.250 x 10<sup>–4</sup>, 1.130 x 10<sup>–4</sup>, 9.550 x 10<sup>–5</sup>, 1.040 x 10<sup>–4</sup>, 2.750 x 10<sup>–4</sup>, and 1.420 x 10<sup>–4</sup> g mg<sup>–1</sup> min<sup>–1</sup>, respectively. At the 500-ppm equilibrium, the pseudo-second-order rate constants for OIS, OIC, OBS, OBC, OS, and OC were 1.240 x 10<sup>–4</sup>, 1.090 x 10<sup>–4</sup>, 1.020 x 10<sup>–5</sup>, 1.100 x 10<sup>–4</sup>, 2.730 x 10<sup>–4</sup>, and 1.180 x 10<sup>–4</sup> g mg<sup>–1</sup> min<sup>–1</sup>, respectively. Consequently, the soil adsorption capacity increased with higher pseudo-second-order rate constants.