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Nutrient availability after fertilising agricultural soils is affected by many factors, including soil moisture conditions and physicochemical properties. Herein, the availability of potassium in soil enriched with biochar is studied, considering either saturated or unsaturated moisture conditions and questioning key ion exchange approaches, such as equilibrium exchange (E.E.) and kinetic exchange (K.E.). Potassium release is simulated from a soil–biochar mixture of 0, 0.5, 1 and 2% by coupling HYDRUS-1D and PHREEQC models. The water flow, mass transport and geochemical processes are simulated for a cultivation period that imitates agronomic and environmental conditions of a common agricultural field in Northern Greece. Potassium is released gradually during the irrigation period in the case of unsaturated flow conditions as opposed to its complete release over a few days in the case of saturated flow conditions in the soil. Regarding ion-exchange processes, the soluble amount of potassium is more readily available for transport in soil solution when using the E.E. approach compared to the K.E. approach that assumes a kinetically controlled release due to interactions occurring at the solid–solution interface. The increased proportion of biochar in soil results in a doubling of available potassium. Among the four modelling schemes, although the total mass of potassium released into soil solution is similar, there is a significant variation in release time, indicating that simplified saturated conditions may lead to unrealistic estimates of nutrient availability. Further experimental work will be valuable to decrease the uncertainty of model parameter estimation in the K.E. approach. [ABSTRACT FROM AUTHOR]