ELECTRO-CHEMICAL PROPERTIES OFSELECTED OXISOLS AND ULTISOLS OFKERALA WITH SPECIALREFERENCE TO

CHARGE CHARACTERISTICS ANDSURFACE MINERALOGY

By

P.RAJENDRAN

. THESIS

Submitted in partial fulfilment ofthe requirement for the degree

DOCTOR OF PHILOSOPHY

Faculty of AgricultureKerala Agricultural University

DEPARTMENT OF SOIL SCIENCE AND AGRICULTURAL CHEMISTRY.COLLEGE OF AGRICULTURE .

Vellayani -Thiruvananthapuram

1992

CERTIFICATE

Certified that this thesis entitled "Electro-chemical properties of selected Oxisols

and Ultisols of Kerala with special reference to charge characteristics and surface

Mineralogy" is a record of research work done independently by Mr. P. Rajendran under

my guidance and supervision and that it has not previously formed the basisfor the award

of any degree,fellowship or associateship to him.

Vellayani27-1-1992.

Dr. R. SUBRAHMONIA IYERChairman '

Advisory CommitteeProfessor and Head (Rtd.).Department of Soil Science andAgricultural Chemistry.

ABSTRACT

A study has been conducted in seven selected profiles of Oxisols and Ultisolsrepresenting the important pedological units with a wide geographical distribution in thestate to have a deeper insight into the electro-chemical behaviour of these soils. Amultipronged approach to the studies made are highlighted to enable a clearunderstanding of the achievements as against the major objectives and approaches made.

A laboratory study with thirty six samples from seven profiles representing.six Ultisols and one Oxisol has been carried out. Path coefficient analysis of importantthirteen charge contributing factors against two parameters for measurement of chargeand the inter-relationships of 15 soil characters show that organic matter, clay % , Rz 03%, Alo% and Feo% are the major factors that control the sudace charge behaviour ofthe soils. The factors studied explained only 55% and 48% of the variability of cationexchange and anion exchange respectively.

Study of the distributionof the electric charges in the surface and sub-surfacehorizons of the soilswas made by means of potentiometric titrations and by measurementof adsorption of ionsin thepresence ofvarying concentrationof electrolytes. The titrationcurves at different ionic strengths crossed at the common point of intersection, thezero-point of charge (zpc). Thus the electro-chemical behaviour af these soils was foundto be similar to that exhibited by many metallic oxides in which the surface potential ofa reversible double layer is determined solely by the activity of potential determiningions, H+and OH-in the bulk of the solution. The zpc for the surface horizons was foundto be lower than the sub- surface horizons in all the soils studied. Soil to soil variationin zpc between surface and sub-surface horizons were more or less the same for allsamples. Thus zpc cannot be recommended as a taxonomic tool in soil classification todistinguish,Oxisols from Ultisols.

From known values of surface area and zpc of these soils, the values for netelectric charge was calculated by the application of the Gouy-Chapman model of doublelayer and was found to obey the theory only at a critical electrolyte concentration. Asthe soils were found to be similar to that of constant potential systems, the chargedistribution varied substantially with pH and electrolyte concentration. Directmeasurement of adsorption of ions from solutions of KCl, NaCI and CaCh showed that

. the,nature and valence of index cations also influenced the magnitude of the negativecharges on the soil particles. On the basis of the influence of pH, electrolyte concentration

. andt the valence of the counter-ions on the electric charges of the soils, most of the

conventional meth~ds of ion- exchange determinations using buffered electrolytes at highconcentrations appears to be inappropriate for tropical soils.

In a study for the evaluation of ion-exchange measurements it was found thatthe compulsive exchange method is most suitable for the combined estimation of CECand AEC. The traditional ammonium acetate method was found to give over estimationsof CEC and thus very high values. Calcium chloride 0.002 M was found to be equallyeffective but owing to the simplicity of the procedure, the former appeared better. Thecompulsive exchange method is free from the inherent defects of neutral normalammonium acetate and hence would seem well suited for the Oxisols and Ultisols ofKerala.

Although sesquioxidic components in combination with organic matterapparently dominate the charge properties of these soils, there was evidence from thetitration curves to show the presence of small amounts of clay minerals with permanentnegative charge. This was confirmed by different methods and was found in goodagreement with the estimated theoretical results. However under field conditions theexpression of this constituentwas minimum and the variable charge components such asFe, Al oxides certainly control the electro-chemical characteristics.

In a separate experiment to find out the contribution of organic matter andsesquioxides towards exchange properties of soils, it was observed that about 64% ofthe negative sites and 8% of the positive sites was contributed from organic matter.Sesquioxides explained only 11%of the variability in CEC and 22% of the variability inAEC. The combined effect of organic matter and RzO3removal was highly significantwith respect to CEC and AEC in all the soils studied.

Mineralogical investigation revealed thepresence of kaolinite as the dominantclay mineral. Appreciable amounts of smectites, quartz and gibbsite was also identified.The presence of smectites was not reflected in the CEC of any of the samples. Scanningelectron micrographs did not give much information oHlerthan the presence of a thickcoating of iron oxide and amorphousmaterials over clay aggregates. The specific surfacemeasured by ethylene glycol retention method invariably showed higher values than theestimated values obtained from negative adsorption data. The specific surfacedetermined closely followed the organic matter content of the samples inspite of thevertical increase in clay content within profiles.

. I