Although Ferralsols result from a deep weathering, small amounts of potassium-bearing 2:1 phyllosilicates were identified their B horizons in many studies but the consequences on total potassium (K) content and K availability remain under discussion. Our objective was to discuss this issue by measuring the total amount of K, as well as the cation exchange capacity (CEC) and the amount of exchangeable K using chemical analyses of bulk samples, combined with backscattered electron scanning images (BESI) and chemical analyses using energy dispersive X-ray spectroscopy (EDS) on cross-sections of undisturbed samples. Our results showed that the total and exchangeable K were related to the presence of elongated particles corresponding to 2:1 phyllosilicates at varying degrees of weathering. These 2:1 phyllosilicates ranged from weathered muscovite to hydroxy-Al interlayered vermiculites (HIV). The quantities of K reserve potentially available over time for plant nutrition ranged from 259 to 5 044 g ha−1 m−1. The stock of exchangeable K+ ranged from 86 to 207 kg ha−1 m−1 when it was higher than the detection limit (18 kg ha−1 m−1). It was related to the density of the elongated particles < 2 μm and 2–20 μm in length because their K was more exchangeable than in larger particles. The CEC varied according to the clay content and the kaolinite proportion in the clay fraction. Finally, if the presence of these K-bearing 2:1 phyllosilicates results, as discussed in recent studies, from the upward material transport activity of termites from the saprolite, this raises the question of the consequences of soil cultivation on soil fauna and hence on the reserve of K and its availability for plant nutrition.