Clay-humus and oxide-humus complexes :

Extensive studies have been done on complexes of montmorillonite, illite, kaolinite, hematite, goethite, gibbsite and boehmite with humic and fulvic acids. New concepts have been proposed to understand the mechanism of bonding, the influence of crystal edges, layer charge, cations, etc. in clay-humus bonding. These concepts have been developed based on an in-depth study of crystal structures, chemical reactions and high precision analytical data. Details of sites and modes of interaction and the reasons for behavioural differences between the minerals have been deduced. It was shown how (a) charges are distributed at various surfaces of crystals, (b) coordinate bonding with a crystal surface is possible and (c) strengths of cation-bridged bonds vary with the crystal structure.

Weathering of silicates by organics :

Reaction of olivine, epidote, hornblende, tourmaline, biotite and microcline by organic acids were studied. It was observed that the position of an ion within the crystal, its polymeric nature, steric factors and ligand strength, together influence dissolution of ions from the minerals; surface etching, fragmentation, layer opening and new crystal growth was demonstrated.

Dissolution of oxide coatings from clay minerals :

The short-comings of the Mehra-Jackson (DCB) method have been investigated. An improved method has been proposed.

Clay-humus Complexes

Oxide Humus Complexes
Silicate Weathering
Oxide dissolution



References :

1. Varadachari C and Ghosh K (2006) Development of computer generated structural models : Teaching aids in soil mineralogy. Clay Research 25 45-53.
2. Varadachari C, Goswami, G and Ghosh K (2006) Dissolution of iron oxides. Clay Research 25 1-22
3. Soil clay-humus complexes. I. Alkali dissolution, TEM and XRD studies : N. Ahmed, C Varadachari & K. Ghosh : Australian Journal of Soil Research (Australia) 40 : 691-703 (2002).
4.     Soil clay-humus complexes. II. Bridging cations and DTA studies : N. Ahmed, C Varadachari & K. Ghosh : Australian Journal of Soil Research (Australia) 40 : 705-713 (2002).
5.     The crystallo-chemistry of oxide–humus complexes : C. Varadachari, T. Chattopadhyay & K. Ghosh : Australian Journal of Soil Research (Australia) 38 : 789-806  (2000).
6.     Organic  solvent  separation of  iron  oxides  from  layer  lattice  silicates  :  G.Goswami, C. Varadachari & K. Ghosh : Journal of the Indian Society of Soil Science  46 : 135-137  (1998).
7.     Complexation  of  humic  substances  with  oxides of   iron and aluminum : C.Varadachari, T. Chattopadhyay & K. Ghosh : Soil Science (USA) 162 : 28-34 (1997).
8.     Dissolution  of   iron  oxides   by   a   dithionite-oxalate-carbonate method :  G.Goswami, C. Varadachari & K. Ghosh : “Clays Controlling the Environment” (Eds. G.J. Churchman, R. Fitzpatrick & T. Eggleton) CSIRO Publishing, Collingwood, Victoria, Australia (1995).
9.     The influence, of crystal edges on clay-humus complexation : C. Varadachari, A.H. Mondal & K. Ghosh : Soil Science (USA) 159 : 185-190  (1995).
10.     Clay-humus   complexation   :   Effect  of   pH  and  the  nature of   bonding :  C.Varadachari, A. H. Mondal, D.C. Nayak & K. Ghosh : Soil Biology & Biochemistry (UK) 26 : 1145-1149 (1994).
11.     Weathering  of   silicate  minerals  by  organic  acids.   II.  Nature  of  residual products : C. Varadachari, A.K. Barman & K. Ghosh : Geoderma (Netherlands)  61 : 251-268 (1994).
12. A novel method for  the determination of  total acidity of  humic substances :  C.Varadachari, D.C. Nayak & K. Ghosh : Journal of the Indian Society of Soil Science  40 : 682-687 (1992).
13. Weathering  of   silicate  minerals  by   organic  acids   :   I.  Nature  of  cation solubilisation : A.K. Barman, C. Varadachari & K. Ghosh : Geoderma (Netherlands) 53 : 45-63 (1992).
14. Some  aspects of  clay-humus  complexation :  Effect of exchangeable cations and lattice charge : C. Varadachari, A.H. Mondal & K. Ghosh : Soil Science (USA) 151 : 220-227 (1991).
15. Influence  of   organic  acidic   functional    groups  of   humic substances in complexation with clay minerals : D.C. Nayak, C. Varadachari & K. Ghosh : Soil Science (USA) 149 : 268-271 (1990).
16. Synthesis  and   characterisation  of   crystalline  aluminum  compounds :  M. Datta &  C. Varadachari : Clay Science (Japan) 7 : 337-342 (1990).
17. Studies on decomposition of humus in clay-humus complexes : C. Varadachari, N.K. Biswas & K. Ghosh : Plant & Soil (Netherlands)  78 : 295-300 (1984).
18. On  humus  formation  :  C. Varadachari  &  K. Ghosh  :  Plant  &  Soil (Netherlands) 77 : 305-313 (1984).