Selective sorption of potassium in a weathering sequence of volcanic ash soils from Guadeloupe, French West Indies

Volcanic ash soils are usually associated within the weathering sequence glass→allophane→halloysite. On the eastern slopes of the volcano La Soufrière, Guadeloupe, this sequence occurs in a toposequence Molli–silicic Andosol–Humic Andosol–Haplic Andosol–Haplic Nitisol, where the average annual rainfall decreases from 4000 mm at 420 m a.s.l. to 2500 mm at 30 m a.s.l. Of this sequence, we selected four profiles and examined the relationships between weathering stage, clay mineralogy and exchangeable K+–Ca2+ selectivity. With increasing weathering, the total content of alkaline and alkaline-earth cations of the B horizon steadily decreased from 185 cmolc kg−1 soil in the Molli–silicic Andosol to 24 cmolc kg−1 soil in the Nitisol. K+ selectivity was small in the Molli–silicic Andosol, but significantly increased with increasing weathering stage and increasing content of halloysite. In the Haplic Andosol and Nitisol, the very strong selectivity for K+ ions may be linked with the presence of a 2:1 clay mineral, but could not be attributed unequivocally to this mineral or to halloysite. The 2:1 mineral occurred as a discrete phase mixed with halloysite and exhibited X-ray diffraction features, suggesting its vermiculitic nature.