A distinctive podzol B horizon, characterized by accumulated humus, low-iron content, lateral uniformity in appearance and composition and frequently hard cementation, develops at the top of the water table in quartz sands on coastal plains in the subtropics and tropics. Soils with such B horizons are separated from freely-drained podzols in many classifications, such as groundwater podzols1, humus podzols2, hydromorphic humic podzols3, aquods and humods4, because of a distinctive morphology and possible differences in genesis. However, while an association between the humus-rich B horizon (Bh) and the underlying water table has long been recognized, most explanations offered for the mode of formation of the Bh have been essentially similar to those for Bhir (iron-rich Bh) horizons in freely-drained podzols. These do not provide a satisfactory explanation of the mechanism whereby organic matter is precipitated in an environment that is virtually free of sesquidoxes. We suggest here that such Bh horizons may form as the result of coprecipitation in the B horizon between organic-rich surface waters and aluminium from the groundwater. Further, such a mechanism can lead to the development of organic-cemented hardpans, commonly called coffee rock3,5, that are so frequently associated with hydromorphic podzols in both temperate and tropical regions3.