Characteristics and genesis of pisolitic soil layers in a tropical moist semi-deciduous forest of Ghana

In the tropical moist semi-deciduous forests of West Africa, gravelly soil horizons common on the summits and upper slopes in the landscape, are often covered by a thin layer of gravel-free soil. Previous investigations have shown that these top layers are formed by termites transporting gravel-free soil material from the subsoil to fallen logs and branches on the soil surface. The present study located at the upper part of a catena at Kade in Ghana shows that the gravel consists of iron nodules or pisolites of varying degree of cementation, quartz and a few remnants of unweathered metamorphic stones. According to chemical and mineralogical studies of pisolites from E, Bt and Bt/Cg horizons, pisolites in the three horizons consist of well crystallized goethite and hematite, and the pisolite content and hardness decrease with increasing soil depth. Furthermore, the very hard pisolites from the E horizon are richer in total iron oxides, especially hematite, than the pisolites in the horizons below but iron oxide crystallinity is similar in the three horizons indicating that pisolite hardening depends on the total content of iron oxides, in particular hematite. Based on the location of the gravel-rich soil horizon in the landscape, a general route for development of pisolite-rich soil horizons in West Africa is proposed including the following processes: (i) An upward transport of gravel-free material to the soil surface due to termite activity, (ii) ongoing removal of this material by water erosion, (iii) weathering of the rather soft metamorphic bedrock present at shallow depth and (iv) gley formation. These processes will over time lead to a lowering of the land surface and concentration of pisolites at the soil surface or in a layer near the soil surface.