Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi-110 012, India
1This invitational lecture was delivered by Dr. V. Thulasi during the 14th Annual Convention of the Clay Minerals Society of India held at NRL, IARI, New Delhi during 11-2 August, 2005.
Minerals, organics and microbes profoundly affect the physical, chemical and biological properties of soils. Their interaction has multitude of consequences. From the fertility point of view, clay-organic-microbe interactions can be classified into mineral weathering and processes leading to fertility alterations. Mineral weathering is arguably an important geochemical phenomenon resulting in the formation of soil. Microbes mediate weathering through organic acids and humified materials and bring about dissolution and compartmentalization of elements. Alterations in soil fertility can be viewed from chemical, biological as well as physical aspects. The interactions affect sorption and desorption of elements in soil and thus alter nutrient availability. For example, humic and other organic acids found to compete with sulphate and phosphate ions for adsorption sites, whereas humin-clay interaction is the most important one for phosphorus adsorption in Andisols. Clay type and surface properties were reported to be the major factors determining the extent of carbon sequestration in soil. Siderophores, the microbial secretions, even when bound to clay, increase the availability of iron to plants and have a solubilizing action on heavy metals like cadmium thus impairing fertility. Montmorillonitic clays decrease the lag phase of nitrification and bind enzymes, peptides and amino acids thus affecting their stability, activity and bio-availability. Urease has been reported to retain its activity even when bound to HDTMA-Smectite. Organisms and humic substances showed a significant role in maintaining the structure and water retention properties of soil. Apart from these, clay-organic-microbe interactions enable soil to act as effective filters and sinks, thus protecting ground water and food chains from excessive exposure to many pollutants.
