ABSTRACT
Mohr, a Dutch soil scientist surveying the soils of Indonesia during colonial times, was struck by iie diversity of soils he saw there. Mohr was attempting to group soils into distinct behavioral and performance classes. This chapter summarizes what scientists now know about Mohr's senile soils of humid tropics and offers glimpses into what the future holds for them. Unlike the clay minerals whose surface charges originate from internal defects and are permanent and almost always net negative, the charge on oxides arises from adsorption of potential determining ions. The most important potential determining ions in soils are the hydrogen and hydroxyl ions. In some ways organic matter is chemically very much like quartz. Both quartz and organic matter have isoelectric points in the pH range of two to three. This means that like quartz, organic matter is negatively charged in the pH range usually found in soils. The chapter explores management options for isoelectric soils of the humid tropics.
