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 (PO₄), 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 (NaNO₃ or CaCl₂) and two conditions of added PO₄ (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 NaNO₃ and CaCl₂ 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 PO₄ 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.