The heavy metal Chromium has several oxidation states but it occurs predominantly in two forms: trivalent (Cr3]) and hexavalent (Cr6+) states. Unlike trivalent chromium, the hexavalent form rarely exists naturally being originated by anthropogenic sources. Hexavalent chromium (Cr (VI)) is very often an issue in ecotoxicity studies for two main reasons: (i) due to its importance as a pollutant that threats especially aquatic ecosystems and (ii) due to its value as a reference toxicant in standard toxicity tests. Cr (VI) is used in industry (wood preservation, leather tanning, metal finishing, pigments, refractory, etc., Very often wastes from such industries are used as a fill material at numerous locations where Cr (VI) easily reaches groundwater by leaching and seepage from the soils. The tanning industry especially contributes to chromium aquatic pollution, being estimated that in India alone about 2000 to 3200 tone of elemental chromium escape into the environment annually. Generally these works show that Cr (VI) acts as a strong oxidizing agent.

A book on agriculture microbiology should include a study of the occurrence of microorganisms in the soil, their activities and their role in soil processes. It is this last phase which has been studied least and where the information available is far from satisfactory in explaining what is taking place in the soil. This is due largely to the limitations of the subject which depends for its advance on botany, zoology, bacteriology, chemistry, including biological and physical, and especially upon the advance of our understanding of the physical and chemical conditions of the soil. There are various kinds of audiences to which a book on soil micro-biology may appeal. There is the scientific farmer who may search for a better understanding of the processes taking place in the soil, those processes which control the growth of his crops and indirectly influence the growth of his animals. There is the agronomist, who is interested in the fundamental reactions controlling soil fertility.This book will be useful not only for students, teachers, and researchers but also for those interested in agriculture microbiology, plant pathology, ecology, environmental science, and agronomy.