Cation Retention Exchange in Soils

Probably the most important and distinctive property of soils is that they can retain ions and release them slowly to the soil solution and to plants. The retention prevents concentrations that are too high and too low. The evolution of plants has taken advantage of this buffered range of ion concentrations that soils make available in the soil solution. Over most of the earth s surface, the availability of these ions in the soil solution is adequate, but not necessarily ideal, for plants. Crop and horticultural plants and a desire for maximum yield place greater demands on the soil and may require adjusting the native soil solution. Adjustments by fertilization, liming, and salt removal are usually temporary. The soil and climate tend to return the soil to its native state. 

Ion retention is actually ion exchange. Soils give up other ions, H+ or OH- and HCOj, in equal amounts to those retained. When trace ions are removed from the soil solution, the ion exchange to the soil solution is often unnoticed. The retention of organic, nonionic substances usually results in their degradation by soil microbes and conversion to CO2 and water. This chapter is concerned with the exchange, the retention and release, of cations between soil particles and the soil solution. 

Cation retention by soils can be roughly divided into the weaker electrostatic interaction of soil particles with the alkali and alkaline earth cations and the soil s stronger 

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Kennedy VC, Brown TC (1965) Experiments with a sodium ion electrode as a mean to studying cation exchange rate. Clays Clay Minerals 13 351-352 Khachikian C, Harmon TC (2000) Nonaqueous phase liquid dissolution in porous media Current state of knowledge and research needs. Trans Porous Media 38 3-28 Kookana RS, Aylmore LAG (1993) Retention and release of diquat and paraquat herbicides in soils. Austral J Soil Res 31 97-109... 

Barium is also adsorbed onto soil and subsoil through electrostatic interactions (Bodek et al. 1988 Singer 1974). The cation exchange capacity of the sorbent largely controls the retention of barium in soils (Bodek et al. 1988). Barium is strongly adsorbed by clay minerals (Kabata-Pendias and Pendias 1984 Lagas et al. 1984). 

Large retardation factors can mean that the leaching of metal cations from the soil surface into the subsoil is slow, even assuming that metal adsorption is by exchange processes only. As will be explained in Chapter 4, many metals adsorb in addition by strong forces, and this form of metal retention in soils is likely to be practically irreversible, leading to long-term immobilization. 

It may be instructive to analyze in some detail the situation of a reversibly adsorbed ion, whose mobility in the soil is relatively high. A case in point is Cd, a toxic metal that adsorbs by cation exchange in acid and near-neutral soils, but may precipitate into carbonates and phosphates in calcareous and alkaline soils. The theory needed for the analysis of ion leaching through soils, in which the main retention mechanism is ion exchange, is developed in Chapter 3 (section 3.5) using Cs as the example ion. 

Exchanging one cation for another in the presence of a third (complementary) cation also becomes easier as the retention strength of the third cation increases. For example, replacing calcium by ammonium is easier from a Ca2+-Al3+ soil than from Ca2+—Na+ soil. The fraction of the CEC satisfied by the tightly bound A1 is in effect blocked off, and the Ca and ammonium ions compete for a smaller number of exchange sites. 

Soil pH measurements can be ambiguous. Two factors that affect soil pH measurements are the soil-solution ratio and the salt concentration. Increasing either factor normally decreases the measured soil pH because H and A1 cations on or near soil colloid surfaces can be displaced by exchange with soluble cations. Once displaced into solution, the A1 ions can hydrolyze (Eq. 10.2) and further lower the pH. Preferential retention of hydroxy aluminium polymers by soil colloids drives the hydrolysis reactions further toward completion and leads to lower pH. Increasing the neutral salt concentration to 0.1 or 1 M can lower the measured soil pH as much as 0.5 to 1.5 units, compared to soil pH measured in distilled water suspensions. 

Lim CH, Jackson ML, Koons RD, Helmke PA (1980) Kaolins Sources of differences in cation exchange capacity and cesium retention. Qays Clay Miner 28 223-229 Low PE (1981) The swelling of clay III Dissociation of exchangeable cation. Soil Sci Soc Amer J 45 1074-1078... 

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