Boron Adsorption and Interaction With Phosphate in a Volcanic Soil From the Humid Tropic

Background Boron (B) deficiency is a major constraint for crop production in volcanic soils from humid tropical regions. Its availability can be influenced by adsorption to short‐range order (SRO) minerals and soil organic matter (SOM) and by interactions with other nutrients. Aims To analyze the adsorption behavior and surface speciation of B in a volcanic soil from the humid tropics, including its interaction with phosphate (PO 4 ), and to test the applicability of a mechanistic multi‐surface geochemical model for describing these interactions. Methods Boron adsorption isotherms were determined on a volcanic soil using two electrolyte solutions (NaNO 3 or CaCl 2 ) and two conditions of added PO 4 (0 and 1.6 mM). The results were interpreted using an advanced multi‐surface ion adsorption model that combines the charge distribution multisite ion complexation (CD‐MUSIC) model for describing ion adsorption to SRO minerals and the nonideal competitive adsorption (NICA)–Donnan model for adsorption to SOM. Results The model simulations, conducted in a fully predictive scheme, showed an excellent agreement with the experimental B adsorption data in both NaNO 3 and CaCl 2 solutions, even at micromolar B concentrations. Under natural soil pH conditions, SRO minerals were the dominant surfaces controlling B adsorption, whereas SOM played a minor role. The addition of PO 4 had a limited effect on B adsorption at low B concentrations; however, a moderate competitive effect was observed at higher B concentrations. Conclusions This study provides mechanistic insights into B adsorption and speciation in volcanic soils, emphasizing the importance of SRO minerals as binding surfaces and the limited role of phosphate competition. These findings can contribute to optimize B management in volcanic soils systems from the humid tropics.