17.1.2  Sodic soils

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Aside from salinisation, about 30% of agricultural land in Australia is already sodic. Such land is difficult to manage and susceptible to degradation (including erosion) due to poor soil structure. Soil physical properties are adversely affected by an accumulation of Na+ salts. Clay particles, by their very nature, carry a negative charge. When more than 6% of these charges are balanced by sodium ions, those soils are regarded as sodic. The extent of sodium adsorption in soils depends on the proportion of sodium to multivalent ions in the soil solution (especially Ca2+ and Mg2+) and is expressed as a sodium adsorption ratio (SAR) defined as:


where [Na], [Ca] and [Mg] are the concentrations of sodium, calcium and magnesium, respectively, in soil solutions or 1:5 soil:water extracts, and expressed as millimoles L–1.

In dry sodic soils, clay particles are linked by Na+ ions and remain as aggregates. On wetting by rain or fresh water of low salinity, Na+ ions become highly hydrated leading to separation of clay particles. Further wetting leads to extensive hydration. Clay particles then disperse as single individuals. Such soil is technically ‘dispersed’ (i.e. an absence of water-stable aggregates) and lacks structure. Notably, however, if a dry sodic soil is wetted with saline irrigation water, dispersion of clay particles does not occur to the same extent and some structure is retained. This property can be used to good effect during reclamation of sodic soils, but has to be managed carefully. Relying on NaCl to maintain clays in a stable state is a hazardous tactic and risks osmotic and ion toxicity effects on plant growth.

In contrast to a Na+-affected (sodic) soil where clay particles disperse on wetting, Ca2+ linked or Mg2+ linked clays swell to a more limited extent. As a consequence, those clay particles do not disperse as soils wet and structure is retained (Rengasamy and Olsson 1991).

Swelling and dispersion of sodic clays destroy soil structure, and by so doing reduce porosity and permeability which predisposes to waterlogging. Poorly structured sodic soils offer a high resistance to root growh that can be inferred from
penetrometer data (Table 17.1). Root growth is restricted, because sodic soils are either too wet (immediately after rain or irrigation) or after a few days of fine weather, too dry. In effect, there is only a narrow range of available water over which plant roots can grow and sustain plant function in a sodic soil (see net range of available water in Table 17.1).


Table 17.1

Sodic soils can be reclaimed via gypsum application, or in the case of acid sodic soils, gypsum + lime. Ca2+ ions in both gypsum and lime act by displacing Na+ ions which are sub-sequently leached out of the profile. As clay particles become charged with these divalent cations in place of monovalent ions, such clays cease swelling and dispersing so that soil structure is immediately improved by this change in their physico-chemical properties. Root growth ensues, and soil structure is further improved by accumulation of organic matter.