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

the soil particles and aggregates. Dispersion or flocculation of clays occurs because of the repulsion of similar Reclamation of sodic soils is imperative in many areas where detericharged clay platelets and the ability of the soil solution oration of land and water resources is in progress. While the chemical mechanisms involved in the reclamation of sodic soils are well deto mitigate this repulsion. Irreversible changes in soil scribed and understood, the in situ physical processes undergoing structure may occur when clay particles become disduring the salt leaching and cation substitution are normally not taken lodged if, for example, the electrolyte level is decreased into consideration. Three sodic soils mixed with different amounts of or the Na fraction increases. gypsum were packed in columns and leached under saturated condiThere are several studies in which the decrease of Ksat tions for a period of time between 1 and 3 mo. Saturated hydraulic has been related with increases in Na content (McNeal conductivity (Ksat ) was measured and a thin section was prepared for and Coleman, 1966; Frenkel et al., 1978; Shainberg and each of the columns. We used scanning electron microscopy (SEM) Letey, 1984; Suarez et al., 1984). However, there is no and image analysis to measure the size and shape of the aggregates quantification, to our knowledge, of the repercussion and the pores, and correlated these with chemical and physical paramthat increases (or decreases) of Na have on the soil eters. We divided the area by the perimeter (A/P ) to quantify the size and A/P 2 to express geometry for both aggregates and pores. The structure while water flow is occurring. Flocculation, size of the aggregates had a good correlation with the exchangeable Na slacking, and aggregate stability studies show that inpercentage (ESP), bulk density, pore size, and Ksat. There was no creases in ESP cause dispersion and a decrease in aggresignificant relationship between pore size and texture, indicating that gate stability and aggregate size, but these tests are transport models using particle-size distribution to infer porosity may performed in fractions of the soil placed in sieves or not be successful in predicting water transport in soils under reclamatest tubes. Despite all the information collected in the tion. The linear relationship between aggregate size and pore size laboratory there are some doubts about our capability indicates that the pore space is determined by the packing of the to predict the extent of aggregation and dislodging in aggregates not the individual particles, this relationship may have real soils. implications not only for water transport but for modeling hydraulic Explanation for the clay behavior has been typically properties in general. based in the Derjaguin, Landau, Verwey and Overbeek (DLVO) model, this theory has been used successfully to explain a great number of laboratory experiments. T need for reuse of secondary waters in agriculUnfortunately, there is evidence that the electrical douture generates a demand for tools to predict the ble-layer interactions between charged particles in conlong term impact that those waters will have on the fined geometries are different than in isolated environsoil structure. Models exist to simulate sodification and ments (Larsen and Grier, 1997; Grier, 1998; Bowen and reclamation scenarios, one of these models, UNSATSharif, 1998; Sader and Chan, 1999). Grier (1998) and CHEM (Simunek and Suarez, 1996), also simulates, Bowen and Sharif (1998) found that isolated pairs of based on a regression of the variables against saturated like charged spheres behave as predicted by DLVO hydraulic conductivity, the effect of the water compositheory but spheres confined by a concentration of other tion on hydraulic properties. With this program, we can spheres develop long-range attractions inconsistent with estimate the time as well as the amount of water needed DLVO. to obtain a given ESP in soils using different amendDilute systems may not properly represent the soil ments. In these calculations UNSATCHEM considers scenario, as geometrical confinement has a dramatic the cation affinities of the exchangeable complex of speeffect in the pairwise double-layer interaction between cific clays, kinetics of the dissolution and precipitation two clay particles. Sader and Chan (1999) found that the of different minerals, fluctuations of CO2, and changes interaction between two confined spheres with uniform in pH. constant surface charge is primarily affected by the elecWhile the chemical reactions involved in soil sodificatrical nature of confining plates. They also found that tion and reclamation are relatively well known, there when the interaction is between two spheres with uniis no direct information, to our knowledge, about the form constant surface potential, the interaction between physical mechanisms taking place in the soil while these the spheres is not only strongly affected by the potential processes are occurring. Changes in soil structure are and charge but is also affected by the electrical properquantified with reduction functions or simple correlaties of the confining plates. tions, but they typically do not explicitly account for Considerations of previous findings summarized the effect of solution chemistry on the arrangement of above, indicates that there is a need to reevaluate our Abbreviations: A, area; DLVO, Derjaguin, Landau, Verwey and OvU.S. Salinity Laboratory, Riverside, CA. Received 30 Mar. 2001. *Corerbeek; ESP, exchangeable Na percentage; GR, gypsum requirement; responding author ([email protected]). Ksat, saturated hydraulic conductivity; P, perimeter; Rh, hydraulic radius SAR, Na adsorption ratio; SEM, scanning electron microscopy. Published in Soil Sci. Soc. Am. J. 66:92–98 (2002).