Comprehensive assessment of the clay mineralogical composition of lowland soils in West Africa

The clay mineralogical composition of 87 topsoil (0-15 cm) samples from inland valleys (IVs) and flood plains (FPs) in seven West African countries, namely Côte d'Ivoire, Ghana, Guinea, Mali, Niger, Nigeria and Sierra Leone, was examined using X-ray diffraction analysis to gain basic information for the development of sustainable lowland agriculture in the region. The results of the present study revealed that the clay fraction (< 2 m) of these samples consisted of 68.4% of 7 Å minerals (low-activity clays such as kaolin minerals), 26.6% of 14 Å minerals (relatively high-activity clays such as smectite and vermiculite) and 5.1% of 10 Å minerals (illite or clay micas) and showed a high variability in the composition. With respect to the soil material classification based on the relative abundance of these three types of minerals, Type 7 (rich in 7 Å minerals) accounted for 42.5% of the total samples, while 39.7% of all the samples were Type 7-10 and 7-14 with a predominance of 7 Å minerals and with negligible amounts of 10 and/or 14 Å minerals, respectively. Type 14-7 accounted for 17.2% of the total samples and was only recorded in Nigeria. The other clay mineral types (i.e. Types 7-10-14, 14, 10, 10-7, 10-14 and 14-10) that were composed mainly of 10 and/or 14 Å minerals were hardly found in the West African lowland soils, whereas Types 14 and 7-10-14 were observed in a vertic soil of Southeast Ghana and in northern Ghana, respectively. In contrast, no significant differences in the clay mineralogical composition were found between the IV and FP soils. Geographical distribution of the soil types showed that the soils in the eastern part of West Africa contained more 14 Å and 10 Å minerals than those in the western part. Although the effect of agro-climatological differences was not conspicuous, soils in the Sahel and Sudan savanna zones showed a higher percentage of 14 Å clay minerals than those in the Guinea savanna and equatorial forest zones. The findings were as follows: (1) the low fertility status of the lowland soils in the region was closely associated with their poor mineralogical characteristics (i.e. predominance of 1:1 type clay minerals and a lower amount of 2:1 type clay minerals), (2) no significant differences in the mean clay mineralogical composition were observed between the IV and FP soils, indicating that the lower fertility of the IV soils mainly resulted from the lower clay content, (3) the clay mineralogy of the West African lowland soils was more strongly influenced by the nature of the parent materials than by the climatic conditions and relief.