Sodic soils exchangeable

Hydration swelling, 108-115 Layer charge and CEC, 113-115 Structure, 111 Sodic soils, 407 Dispersion, 414 Exchangeable Na+, 409, 412 Toxicity to plants, 407-408 Sodium, 411... 

Wetlands of humid climatic zones often emit H2S, as is evident from the rotten-egg odor of marshes and swamps. As long as the dominant exchangeable base cations are Ca " and Mg ", which is the case in most freshwater wetlands, H2S formation should not cause the soils to become strongly alkaline. In these nonsodic soils, alkalinity generated by reduction forms precipitates of Ca (and Mg) carbonates. The low solubility of these carbonates prevents the pH from rising much above 8. In sodic soils, however, reaction 7.64 causes alkalinity to build up in the form of soluble Na carbonates (see Chapter 8, Section 8.1). 

Figure 8.4. Relative cation composition of exchange sites for sodic soils compared with nonsodic ("normal" and acid) soils. (Adapted from G. H. Bolt et al. 1976. Adsorption of cations by soil. In G. H. Bolt and M.G.M. Bruggenwert (eds.). Soil Chemistry. New YorIcElsevier.)...

The typical compositional range of exchange sites in soils, shown in Figure 8.4, reveals that sodic soils have higher than normal levels of exchangeable Na" ". The important ionic concentrations in soil solution that affect the level of exchangeable Na are [Na" ], [Ca " ], and [Mg +], The sodium adsorption ratio, or SAR, is a solution property defined by the equation... 

There are basically three potential hazards of salt-degraded soils to plants—salinity, sodicity, and alkalinity. The first two hazards are used to classify soils as saline or sodic, or both, as described in Table 8.4. The alkalinity hazard (measured by RSC) is not usually applied directly to the classification of soils, but the pH values reported in Table 8.4 indicate that sodic soils are the most likely to have a problem of high alkaUnity. This arises partly from the hydrolytic exchange reaction, described in Chapter 3, which is enabled by the presence of appreciable exchangeable Na" and low salt concentrations, conditions that are diagnostic of sodic soils. Hydrolytic exchange can be viewed as a two-step reaction ... 

Frenkel, H., Gerstl, Z. Alperovitch, N. (1989). Exchange-induced dissolution of gypsum and the reclamation of sodic soils. J, Soil Set, 40, 599-611. 

Fig. 8.26 Exchangeable sodium percentage (ESP) along a Burleson soil column, as a function of the sodium adsorption ratio (SAR) of irrigation water. The values were obtained by percolating the soil columns with sodic water (total electrolyte concentration of 11 meq/L). Each curve corresponds to a given applied volume of solution (Thomas and Yaron 1968)...

The increase of soil volume from clay swelling in the field has important practical consequences, particularly in soils containing smectites. The extent of this problem depends very much on the nature of the exchange cation, as illustrated by the swelling volumes of smectites reported in Table 8.3. These results emphasize the management problems likely to be encountered with soils that are sodic. 

The solution to equation 8.36 is ESP = 16.3 percent, so that the soil is predicted to eventually exceed 15 percent exchangeable Na" and acquire sodic features. 

High exchangeable Mg is sometimes associated with low water permeability, soil crusting, and high pH, similar to the characteristic conditions of sodic (Na-rich) soils. This is sometimes the result of soil formation under marine conditions, where Na 1 and Mg2+ predominate. The Na+ may have produced the poor soil structure, leading to low water permeability, and then leached away, leaving a Mg soil with an inherited soil structure. Serpentine (an Mg silicate rock)-derived soils have high Mg2+ levels that repress Ca availability to plants. 

Early estimates of sodicity were based on sodium content. Because of the strong preference of most soil particles for divalent cations over monovalent cations, however, waters with high Na contents may still produce relatively low exchangeable Na levels in soils, if the Ca + Mg concentration is appreciable,... 

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