The plant minimal exchangeable K (EPl,min) defines the lower accessible limit of the most available pool of soil K to plants. It is also an index of long-term K reserve in soils. However, its estimation by the classical method of exhaustion cropping is laborious. This study aimed at comparing EPl,min values obtained by the exhaustion cropping method with EPl,min values estimated by an alternative approach based on the cationic exchange capacity (CEC) of the infinitely high selective sites for K (i.e., always saturated with K) in the K-Ca exchange (EK-Ca,min). A set of 45 soil samples, corresponding to the various fertilization K treatments of 15 long-term K fertilization trials, was used in this study. The selected soil samples presented a wide range of texture, CEC, and exchangeable K. The plant minimal exchangeable K was found more or less independent of the K treatment, whereas EK-Ca,min increased when the soil exchangeable K content increased. The plant minimal exchangeable K was systematically lower than EK-Ca,min, showing that EK-Ca,min is at least partially available to the plant. Hence, EK-Ca,min is not a surrogate of EPl,min. Conversely, the plant minimal exchangeable K was strongly, positively correlated to soil CEC (measured at soil pH; r2 = 0.90***). This soil property can consequently be used as a proxy of EPl,min.