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DIVISION S-2—SOIL CHEMISTRY

Critical Coagulation Concentration of Paddy Soil Clays in Sodium–Ferrous Iron Electrolyte

^a Institut de Recherche pour le Développement-UR067–BP 5045 Montpellier Cedex 1, France
^b Land Development Dep., Paholyothin Rd., Chatuchak, Bangkok 10900, Thailand
^c Institut National de la Recherche Agronomique, Route de St. Cyr, 78026 Versailles Cedex, France
^d Lab. of Soil Science, Swiss Federal Inst. for Technology, EPFL-ENAC-ISTE, 1015 Lausanne, Switzerland

^* Corresponding author (pascal.boivin{at}epfl.ch).

Sodium affected rice (Oryza sativa L.)-cropped fields are very common. Due to their high exchangeable Na percentage, clay dispersion is one of the major risks for these soils when they are flooded. During flooding, Fe²⁺ may become a major cation due to reductive dissolution of Fe oxides, but the flocculation effect of Fe²⁺ is not known. In this paper, the effect of Fe²⁺ on clay flocculation is studied by establishing critical coagulation concentration (CCC) curves of clays extracted from a paddy soil of Northeast Thailand. The effect of Na-Fe²⁺ and Na-Ca electrolytes on the CCC values is compared for sodium adsorption ratio (SAR) values ranging from 0 to 40 and total electrolyte concentration (TEC) ranging from 0.5 to 10 mmol L^–1. The extracted clay is a mixture of kaolinite and smectite but only the smectite and some poorly ordered kaolinite could be dispersed. The CCC values largely reflected the behavior of smectite, in agreement with previous studies. The CCC values were equal for electrolytes with Fe²⁺ or Ca²⁺ cations, suggesting that Fe²⁺ strongly adsorbs on smectite exchange sites and behaves similarly to Ca²⁺. The Fe²⁺ concentration needed to flocculate the clays was at maximum 0.6 mmol L^–1 for the most dispersive electrolytes, which is a common concentration in flooded rice fields. Reductive dissolution of Fe can protect sodic soils from clay dispersion upon flooded rice cropping.

Abbreviations: CCC, critical coagulation concentration • CEC, cation-exchange capacity • EC, electrical conductivity • EGME, ethylene glycol monoethyl ether • SAR, sodium adsorption ratio • TEC, total electrolyte concentration • XRD, X-ray diffraction