The development and testing of a transient adsorption/desorption model that describes the response of biochar particles to nutrient pulses simulating the application of fertilizer is presented in this study. Intraparticle nutrient transfer occurs both by diffusion through liquid-filled pores and by surface diffusion, and nutrient adsorption is described by Langmuir-Freundlich (Sips) isotherms. Simulation results show that the ability of a biochar to adsorb and then slowly release the nutrient is modulated by a complex interplay of external mass transfer, intraparticle diffusion (both pore and surface diffusion), and adsorption dynamics. The nutrient retention potential of biochar-amended soils is quantified and is shown to depend on multiple factors that include chemical and physical biochar properties, soil permeability, water flow, and the method of fertilizer application. These findings may explain why biochars with similar properties can potentially have widely different impacts on crop yields, as has been repeatedly reported in the literature. (C) 2017 American Institute of Chemical Engineers