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Variations in Soil Stability Within and Among Soil Types

“Variations in Soil Stability Within and Among Soil Types” Soil Science Society of America Journal 56 (1992): 1412-1421.
I. LeBron and D.L. Suarez

ABSTRACT

One of the most important considerations when irrigating with marginal water or managing sodic soils in maintaining or restoring good water quality parameters and assumed that soils behaved similarly. We examined the flocculation behavior of three different types of micaceous soils at variable electrolyte concentration, sodium adsorption ratio [SAR = Na/(Ca + Mg)0.5, where concentrations are expressed in mmol L-1], and pH. The variability in critical coagulation concentration (CCC) among soil types and among samples within the same soil type also was investigated. The CCC values were calculated at electrolyte concentrations of 2, 5, 10, 20, and 40 mmolc L-1 at SAR values of 5, 10, 20, 40, and 80 (mmol L-1)0.5, and at pH values of 6.0, 7.5, and 9.0. The variability in CCC was large among soil types of similar mineralogy, and even within the same soil type. This variability was greater at higher pH, suggesting that at least part of the variation results from differences in variable charge. Increasing pH had an adverse effect on dispersion, especially when the SAR was high enough to break apart the illite domains. There was no relationship among CCC, organic matter, and Fe and Al oxide contents for the soils studied. In the range investigated (1-4 g organic C kg-1 soil), only the Ebro Basin soils showed increases in CCC with increasing organic matter at high pH and SAR. Aggregate stability was measured on aggregates of the soils after saturation with solutions having salt concentrations of 2 and 40 mmol, L-1; SAR values of 5, 20, and 80 (mmol L-1)0.5; and pH values of 6.0, 7.5, and 9.0. No relation was found between the flocculation test data and the aggregate stability results. Apparently the measured parameters in these two tests are governed by different soil factors.Particle-size distribution and porosity, as well as the drying and rewetting processes, exert a major influence on aggregate stability, but are not considered in flocculation tests. Regardless of the test used, differences in results for sample of the same soil type preclude the use of generalized threshold lines for soil stability.

MAINTENANCE OF SOIL STABILITY is an important aspect of agricultural management. Poor aggregate stability, clay dispersion, and low infiltration are often aggravated in arid soils by chemical conditions such as high exchangeable Na. Eaarly recommendations for maintaining good soil structure advocated reducing the ESP of soils to < 15 by the application of chemical amendments(U.S. Salinity Laboratory Staff, 1954). Subsequently, stability lines including solutionsalt concentration as well as ESP were recommended (e.g., Quirk and Schofield, 1955; Rhoades, 1982; Ayers and Westcot, 1985), since it had been noted that increasing solute concentration increased soil stability. Additional variable such as Fe oxides (McNeal and Coleman, 1966), organic matter (Kemper and Kock, 1966; Gupta et al., 1984), clay content and mineralogy (Frenkel et al., 1978), soluble silica (Shanmuganathan and Oades, 1983), pH (Gupta et al., 1984; Suarez et al., 1984), and soil weatherability (Shainberg et al., 1981) have also been shown to relate to soil stability. More recently, Pratt and Suarez (1990) have shown that a unique relationship considering only salinity and exchangeable Na cannot provide a general prediction of hydraulic conductivity changes for arid soils, because similar soils differ greatly in their structural stability. The objectives of this work were to: (1) compare the stability to dispersive conditions for soils having the same mineralogy (micaceous); and (ii) examine the degree of variability within a specific soil type to determine whether a predictive relationship can be developed for soils of a specific soil series or within a given mapping unit. Soil stability was evaluated at variable salt concentration, SAR, and pH using both a dispersion test and an aggregate stability test. We examined the relationship of soil stability to differences in soil properties such as organic matter, CaCO3, texture, and content of Fe and Al oxides. We also compared the results of the aggregate stability and dispersion tests. MATERIALS AND METHODS Three micaceous soils were studied. Their classification and mineralogy are given in Table 1. Particle-size analysis was determined by x-rays, measuring the concentration of particles remaining at decreasing sedimentation depths as a function of time with a Sedigraph 5000 (ET Micrometrics, Norcross, GA1). Mineralogy was studied by XRD of the < 2-┬Ám fraction, and by FTIR. Di- and trioctahedral mica were determined by examining XRD d spacings in the 0.15- to 0.154-nm region (Fanning et al., 1989), along with the IR absorbance resulting fromoctahedral cations (e.g., Mg and Fe for trioctahedral mica, Stubican and Roy, 1961). Oxides were extracted using the method of Jackson et al. (1986). Cations were determined by ICP, and organic and inorganic C were analyzed by CO2 coulomtery (UIC Corp oration, Joliet, IL). A dispersion test was performed for each of the Ebro Basin, Ramona, and Clarence soils (34 soils in all). Two soils from each type were chosen for the aggregate stability test: Ebro Basin 1 and 18, Ramona 1B and 7B, and Clarence 1 and 9. Different solution compositions also were used in an attempt to test the response of the soils in these experiments to different irrigation waters. The solution were prepared as described below.