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DIVISION S-2 - SOIL CHEMISTRY

Gypsum Effect on the Aggregate Size and Geometry of Three Sodic Soils Under Reclamation

U.S. Salinity Laboratory, Riverside, CA

* Corresponding author (ilebron{at}ussl.ars.usda.gov)

Reclamation of sodic soils is imperative in many areas where deterioration of land and water resources is in progress. While the chemical mechanisms involved in the reclamation of sodic soils are well described and understood, the in situ physical processes undergoing during the salt leaching and cation substitution are normally not taken into consideration. Three sodic soils mixed with different amounts of gypsum were packed in columns and leached under saturated conditions for a period of time between 1 and 3 mo. Saturated hydraulic conductivity (K_sat) was measured and a thin section was prepared for each of the columns. We used scanning electron microscopy (SEM) and image analysis to measure the size and shape of the aggregates and the pores, and correlated these with chemical and physical parameters. We divided the area by the perimeter (A/P) to quantify the size and A/P² to express geometry for both aggregates and pores. The size of the aggregates had a good correlation with the exchangeable Na percentage (ESP), bulk density, pore size, and K_sat. There was no significant relationship between pore size and texture, indicating that transport models using particle-size distribution to infer porosity may not be successful in predicting water transport in soils under reclamation. The linear relationship between aggregate size and pore size indicates that the pore space is determined by the packing of the aggregates not the individual particles, this relationship may have implications not only for water transport but for modeling hydraulic properties in general.

Abbreviations: A, area • DLVO, Derjaguin, Landau, Verwey and Overbeek • ESP, exchangeable Na percentage • GR, gypsum requirement • K_sat, saturated hydraulic conductivity • P, perimeter • R_h, hydraulic radius SAR, Na adsorption ratio • SEM, scanning electron microscopy

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