Texture and mineralogy of soils with repeated additions of pyroclastics (in the Nguu Volcano Area—Machakos District, Kenya)

Abstract

From the Tertiary period onwards, Kenya has experienced repeated episodes of volcanicity, resulting in a large part of the country being covered by volcanic rocks. Soils developed on the volcanic rocks per se (for example, Nitosols and Andosols) have been widely studied. But the buried soils (paleosols), which had developed before the deposition of the volcanic rocks, have received very little emphasis. Little is also known of the soils developed on the overburden of volcanic rocks (surface soils), which in many cases have two or more parent materials. This study was conducted to determine the effect on soil texture and mineralogy of repeated additions of pyroclastics. The influence of the Nguu volcano was investigated in two directions: first the variation of the soils with distance from the volcano and secondly, variation with soil depth. A sample strip 2 km wide and running for about 14 km west of the Nguu volcano was selected. Eight soil profile pits at a distance of about 2 km apart were dug to the depth of 150 cm or to the rock, whichever was shallower. Sampling was done for every pedogenetic horizon in the pits and additionally, a composite sample was taken for each profile. Finally, all major rock types in the study area were sampled. The texture of the soils was determined using the pipette method. Clay and sand mineralogical analyses of the soil samples was also carried out. The rock samples were first crushed, then their mineralogy analysed. The mean particle size of surface soils decreases away from the volcano. The surface soils and paleosols are different in terms of their texture, sand mineralogy and clay mineralogy. The additions of the pyroclastics have enhanced the weatherable reserve of the surface soils.