Debatable problems of modern soil chemistry

This paper is the edited text of the author's lecture presented at the Semi nar on Theoretical Problems of Soil Science. It is a brief review on the ev olution of some basic terms and concepts of soil chemistry in the 20th cent ury. The advantages and limitations of some common soil chemical indices ar e shown in the light of modern concepts. The basic terms of soil chemistry are analyzed, such as cation exchange, soil acidity, soil chemical composit ion, etc. The terms "fulvic acid," "humic acid," and "humin" are considered , with particular attention paid to the differences between native and isol ated humic acids. Some urgent tasks of soil chemistry are outlined.