Ion exchange and soil pH

Exchangeable ions are those that are held temporarily on materials by weak, electrostatic forces. If particles with one type of adsorbed ion are added to an electrolyte solution containing different ions, some of the particle-surface adsorbed ions are released into solution and replaced by those from the solution (Fig. 4.20).

We have seen that the interlayer sites of clay minerals, particularly smectites, hold ions weakly, giving these minerals a capacity for ion exchange. Clay mineral ion exchange can also be a surface phenomenon. Edge damage to minerals can break bonds to expose either uncoordinated oxygens (sites of net negative charge) or uncoordinated silicon or other metal ions (sites of net positive charge). These surface charges are balanced by electrostatic adsorption of cations and anions respectively.

In soils with neutral or alkaline pH (see Box 3.5), ion-exchange sites are typically occupied by exchangeable base cations, for example Ca2+, Mg2+, Na+ and K+ (Box. 4.11). These exchangeable base cations are in equilibrium with the soil pore water and H+ ions. However, changes in soil pH, for example to more acidic conditions (higher solution H+ concentration), encourage exchange of base cations for H+ ions, causing H+ pore water concentrations to fall (pH increases) along with an increase in pore water base cation concentration. The presence of the base cations effectively buffers the soil water pH (see Section 5.3.1), minimizing change. Clearly, a lack of exchangeable base cations, for example in oxisols (Section 4.7), will often lead to low soilwater pH as there is little or no buffer capacity. Understanding the buffer capacity of a soil to pH fluctuations is important, as some fundamental chemical processes (e.g. dissociation, Box 4.5), chemical species, for example phosphate (see Section 5.2) and elements, for example aluminium (see Section 5.4), are very sensitive to pH change.

12K+ Cl-(solution)

Fig. 4.20 Schematic diagram to show ion-exchange equilibria on the surface of a clay particle. Potassium ions in solution are exchanged for other cations, causing the exchange equilibrium to move from left to right.

Table 4.10 Representative cation exchange capacity, CEC (meq* 100g-1drywt), for various soil materials. Table adapted from Pedology, Weathering and Geomorphological Research by P. W Birkeland, copyright 1974 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.

Non-clay materials CEC Clay minerals CEC Cation exchange site

Quartz, feldspars 1-2

Hydrous oxides 4 Kaolinite of Al and Fe

Illite Chlorite

Organic matter 150-500 Smectite

3-15 Edge effects

10-40 Mainly edge effects, plus some interlayer

10-10

80-150 Mainly interlayer plus some edge effects

*A milliequivalent (meq) is the charge carried by 1.008 milligrams (mg) of H+ (i.e. the atomic mass of hydrogen), or the charge carried by any ion (measured in mg l-1), divided by its relative atomic mass and multiplied by the numerical value of the charge. If, for example, divalent Ca2+ displaces H+, it occupies the charged sites of 2H+ ions. Thus the amount of Ca2+ required to displace 1 meq of H+ is 40 (atomic mass of Ca) divided by 2 (charge) = 20 mg, i.e. the mass of 1 meq of Ca2+.

Under high soil pH conditions, damaged soil clays with exposed OH groups in octahedral layers may dissociate, forming a negative charge, which is neutralized by other cations in the soil water, for example:

Clay-mineral-O--H(s) + K +aq) ^ clay-mineral-O--K(s) + H+aq) eqn. 4.18

Micrometre-sized clay minerals have a large surface area: volume ratio and constitute a significant sink for some anions and cations in soil environments. Despite this, in smectite clays, surface ion exchange is much less important than inter-layer site exchange (Table 4.10). The cation exchange capacity (CEC) of a soil provides a useful indication of the availability of essential trace metals (which are fundamental for plant health) and is often considered the best index of soil fertility. In soils it is usually impossible to calculate the amount of ion exchange caused by clay minerals alone, since other components, especially organic matter, have significant CECs (Table 4.10). Organic matter provides a significant proportion of exchange sites in many soils, created by the dissociation of acidic or alcoholic/phenolic functional groups at pH above 5. The high CEC of soil organic matter (150-500 meq/100gsoil; Table 4.10) means that organic matter content, rather than clay mineral content, often provides the most significant contribution to a soil's CEC. Addition of organic matter to soil is thus an easy and effective means of increasing the CEC, and thereby the soil fertility.

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