Sodicity a dirty word in Australia
by Professor J.P. Quirk
Emeritus Professor of the University of Adelaide and the University of Western Australia; Honorary Senior Research Fellow, Faculty of Agriculture, University of Western Australia; Visiting Fellow, Research School of Physical Sciences and Engineering, Australian National University.
Sodicity (sodium rich) and salinity are soil characteristics responsible for soil degradation and affect agricultural production in several ways. Soils are classified as saline if the electrical conductivity of the saturated extract exceeds 4 decisiemens per metre (dS/metre). As the number of ions in the soil solution increases the electrical conductivity increases and above a conductivity of 4 dS/m the yield of sensitive crops may be reduced. There are a number of strategies available for managing saline soils one of these involves the use of crops with a moderate or high degree of salt tolerance. Significant amounts of salt in the soil solution can affect plant growth through a number of mechanisms which can be described as osmotic, ion imbalance or specific ion toxicities.
In general any irrigation system can only remain viable if drainage is installed so that excess salt accumulated as a result of irrigation can be removed from the root zone by the use of excess water(leaching requirement) at an appropriate stage in the irrigation cycle if insufficient rainfall is not available to effect this purpose.
The Diagnosis and Improvement of Saline and Alkali (Sodic) Soils (USDA Handbook 60) provides a wealth of detailed information concerning the management of sodic and saline soils under irrigation practice.
A soil is composed of a range of particle sizes varying from sand to clay; a texture description such as sand, sandy loam, clay loam and clay reflects the relative proportion of the different sized particles. The clay particles in a soil are described as the active fraction on account of their small size and large surface area. The clay fraction of a soil consists of a large number of tiny particles usually smaller than 1 micrometre (a thousandth of a millimetre) and the surface area of these particles exposed to the solution in a soil is large varying from 10,000 to 100,000 square metres per kilogram (10 hectares/Kg) of clay depending on the clay minerals present. The clay particles (individual crystals) are flat being relatively small in thickness compared with their lateral extent and are referred to as being plate shaped. These plate-shaped crystals exist in a soil as assemblages of many crystals in more or less parallel alignment. These assemblages are the operative particles in a soil and are described as clay domains. In normal circumstances when calcium is the dominant ion, the clay domains are stable however this stability is considerably reduced when significant quantities of sodium ions are present and the irrigation water has a small electrical conductivity.
The clay minerals found in soils belong to the layer lattice alumino-silicates. Each clay crystal has an over-all permanent negative charge as a result of ion replacement in the crystal lattice such as divalent magnesium replacing trivalent aluminium and trivalent aluminium replacing tetravalent silicon; this is referred to as isomorphous replacement. The permanent negative charge of the clay crystals is balanced by positive ions (counterions) which are usually predominantly calcium and magnesium however in certain circumstances these ions can be replaced (exchanged) to a significant degree by sodium ions.
A soil is a porous material, with the pores or interstices existing between the particles. Soil structure is defined as the arrangement of soil particles. When a soil is wet the pores are filled with a solution which contains varying amounts of calcium (Ca2+), magnesium (Mg2+) sodium (Na+) and potassium (K+) ions as well as the anions chloride (Cl-1) sulphate , bicarbonate and small quantities of various other cations and anions.
The cations in the soil solution are in equilibrium with the cations or counterions balancing the negative charge on the clay. This equilibrium is governed by the empirical Gapon equation
which states that the ratio of sodium ions balancing the charge of the clay surface (Nac) to the divalent ions on the clay surface (Cac+Mgc) is proportional to the ratio of the sodium ion concentration ([Na]) in the soil solution to the square root of the total divalent concentration . The right hand side of the equation is referred to as the sodium adsorption ratio (SAR) and the percentage of the exchange capacity occupied by sodium ions is called the exchangeable sodium percentage (ESP).
The Gapon equation is important because it shows in a simple manner how the salinity level influences the amount of sodium ions on the clay surface. An important consequence of the Gapon equation is that the ratio of Nac/Cac varies with the salinity of the soil solution. If for instance, the level of salts in the soil solution is halved then this favours the exchange of calcium and Nac/Cac is reduced because to retain the initial ratio would require, according to the Gapon equation a reduction of the calcium ion concentration in solution by a factor of four. The dilution of an irrigation water thus favours reclamation - 'the dilution effect'. Conversely - an increase in the total concentration of a water favours the adsorption of sodium ions on the clay surface.
A sodic soil is defined as one in which more than 10-15 per cent of the clay's negative charge is balanced by sodium ions. This may occur for soils formed in arid and semi-arid environments or may be man made by repeated use of an irrigation water in which the level of sodium is relatively large compared to the divalent ion concentration. The Gapon equation is used to assess the quality of an irrigation water with respect to the potential increase in sodicity of a soil. Sodicity may also be caused by water tables approaching the surface in the lower parts of a landscape as a result of over-irrigation at a higher level or by the changes in hydrology in the upper part of a landscape by the replacement of deep rooted trees by crops.
Electrical double layer
The electrical double layer at the interface between the clay surface and the soil solution is made up of the permanent negative charge of the clay and the counterions in the soil solution balancing the negative charge. The counterions or cations balancing the negative charge are influenced by two countervailing forces - the electrical force attracting the positive ion to the negative surface and the diffusive or thermal forces (Brownian motion) which facilitate the movement of the cations away from the surface. The balance of these two forces gives rise to a distribution of cations in water adjacent to the clay surface; this distribution, described as a diffuse electrical double layer or simply diffuse double layer, is made up of a negative clay surface and the spread out (diffuse) distribution of the counterions.
The electrical double layer, in effect, occupies the space between the clay surface and the soil solution and has a thickness less than one-millionth of a centimetre (10-6 cm). The thickness of the double layers decreases with an increase in the electrolyte concentration when the double layer is said to be compressed. The double layer is thinner or less extensive when Ca-ions balance the charge rather than monovalent ions such as sodium.
When two clay crystals, in parallel alignment, sit close to one another the double layers of the two particles over-lap or interact and as a consequence the total concentration of the ions at the plane mid-way between the two particles is greater than that in the soil solution in which the particles are immersed so that there is a difference in osmotic pressure which will draw water between the particles causing them to move further apart and this is described as swelling. This is the essential basis for the large (extensive) swelling of Na-clays in dilute solutions. However this extensive swelling can be decreased to a considerable extent in the presence of concentrated solutions.
The situation with respect to the interaction of Ca-clay crystals is considerably different for two reasons. First because the doubly charged Ca-ions are more strongly attracted to the clay surface the thickness of the double layer is less and the tendency to swell is correspondingly less but secondly and much more importantly the organization of clay crystals within a domain is profoundly influential in restricting or limiting the swelling which takes place between Ca-clay crystals.
The arrangement of clay crystals within a domain may be thought of as something similar to a brick wall with many of the bricks missing but in the case of a clay domain the pores are small being determined by the thickness of the clay crystals which has a magnitude of 5x10-6 cm. Where the clay crystals over-lap the separation is very much smaller than this being about 10-7cm and when calcium or other polyvalent ions balance the charge the electrostatic attraction forces are very strong indeed and are responsible for the stability or coherence of the clay domain. These very strong electrostatic forces have only recently been theoretically explained as a major extension of the classical van der Waals forces between macroscopic objects first identified in Holland in the early decades of this century.
The amount of electrolyte required to prevent the deterioration in soil structure is called the threshold concentration. The threshold concentration, simply expresses the minimum level of electrolyte required to maintain the soil in a permeable condition for a given degree of sodium saturation of the soil colloids; it is described by the following equation which relates the electrical conductivity expressed in decisiemens per metre to the sodium adsorption ratio (SAR) of the Gapon equation:
This relationship can be used to describe an irrigation water or the saturated extract of the soil solution. At about one-quarter of the threshold concentration dispersed clay particles appear in the percolate from a soil or in the irrigation water ponded at the soil surface. The equation (2) has been found to be widely applicable but may be affected by a number of factors such as the presence of oxides, acting as cements stabilizing the aggregates or by levels of organic matter above those found in arid or semi-arid soils. Sodic effects are more pronounced at alkaline pH values.
As a result of the presence of the strong electrostatic forces the clay crystals where they over-lap are said to reside in a potential well or minimum. The nature of this potential minimum can be well illustrated by reference to the mineral montmorillonite which is a layer lattice silicate in which water can enter between the individual crystal layers; the change in the separation of the crystal layers can be followed by x-ray diffraction. When the lattice charge is balanced by Ca ions the amount of water entering between the silicate layers or lamellae is limited to a thickness of three water molecules and furthermore and most importantly this crystalline swelling is not affected by the concentration of calcium chloride with which the clay is in contact. Sodium montmorillonite however develops diffuse double layers on each of the montmorillonite lamellae surfaces and in dilute sodium chloride solutions may swell to ten to twenty times its initial dry volume. This swelling is suppressed in the presence of increasing concentrations of sodium chloride. Since the crystalline swelling montmorillonite parallels the behaviour of soils in relation to the marked different responses to electrolyte levels the behaviour of montmorillonite serves as a model for the interaction of clay crystals at their areas of over-lap. However the clay domain structures would not have the same degree of order and hence it would not be expected that the crystals constituting a clay domain would have a uniform relationship with neighbouring crystals. As a result some expansion would occur between some particles at relatively small percentages of exchangeable sodium. As the exchangeable sodium increases an increasing number of interactions of this type occur so that at an exchangeable sodium percentage of 10 to 15 there would be significant disruption of the clay domains and this gives rise to pore blocking and decreased permeability. It has been shown experimentally that whatever the level of exchangeable sodium the permeability of a soil can be maintained by adjusting the level of electrolyte in an irrigation water. The greater percentage of exchangeable sodium the larger the quantity or concentration of electrolyte in the soil solution required to maintain the permeability as indicated in equation (2). The greater amount of electrolyte can be said to compress the diffuse double layers and thereby the stability of the clay domains is retained.
As a result of the presence of strong electrostatic forces, as described above, the swelling of Ca-clay soils is hardly affected by the concentration of calcium chloride in the equilibrium solution and thus the behaviour of Ca clays in a soil is not determined by the interaction of diffuse double layers. Nevertheless a Ca-clay soil containing 60 per cent of clay particles may increase in volume by 30 per cent from the dry to wet state in nature. This type of swelling results from the entry of three layers between the clay plates at regions of particle over-lap in a domain. This swelling is said to be limited as distinct from the extensive crystalline swelling of Na-clays which is well illustrated by the behaviour of Na-montmorillonite in dilute solutions. Increasing sodicity of a soil represents the various stages or partial transition between these two states.
Two features of clay domains are important as a basis for understanding the physical behaviour of a soil. First the clay domains in a soil are in random array and as a result the macroscopic swelling of a soil is isotropic and is expressed along three orthogonal axes. The closure of cracking which occurs at the surface of clay soils in the field on wetting is the horizontal expression of the three-dimensional swelling. The vertical expansion is a rise of the soil surface for clay soils or soils with a relatively thick clay horizon in the profile; this rise from the dry condition at the end of summer to the fully wet profile during the winter is 8 centimetres for a black earth profile and 5 centimetres for a red-brown earth profile. Such swelling in the absence of appropriate foundations can lead to cracking of sealed roads, domestic housing and other structures. The swelling of clay domains is thus fundamental to the water storage by soils. Secondly, as noted earlier, the texture of a soil relates to the proportion of clay particles in a soil for instance a sandy loam may contain 10 per cent of clay particles and a heavy clay more than 60 per cent of clay particles. In each case the clay particles are associated with one another to form clay domains. For the sandy loam the clay domains do not form a continuous phase but are contained in the pores between the large sand particles. In either case the behaviour of the clay domains is basic and there is a continuum in behaviour from limited domain swelling and various stages of extensive swelling within the domain depending on a multiplicity of physico-chemical factors. In the presence of free water at a soil surface the interaction of the diffuse double layers of a sodic sol may be such as to cause clay dispersion. Swelling is a precursor of particle dispersion.
Reclamation and management of saline and sodic soils
Modern irrigation practice derives principally from California and the USDA Handbook 60 has been regarded as a definitive statement on the management of saline and sodic soils. The recommendation for the amelioration of sodic soils is the application of various amendments to replace the exchangeable sodium in the top 5 to 10 centimetres of soil by Ca ions. The main amendment used for this purpose is gypsum - calcium sulphate (CaSO4 H2O). The amount of gypsum applied varies considerably depending on the amount of exchangeable sodium, expressed in centimoles per kilogram of soil, to be replaced. It can be shown that, for a soil containing 1 centimole of exchangeable sodium per kilogram, the amount of gypsum required to replace the exchangeable sodium in one hectare of soil to a depth of 10 centimetres is 1.3 tonnes. For a soil having 20 per cent exchangeable sodium and an exchange capacity of 20 centimole per kilogram the gypsum requirement would be 5.2 tonnes per hectare. The powdered gypsum is broadcast and worked into the soil with appropriate machinery.
For sodic soils with a pH value less than 6 calcium carbonate (CaCO3) has been successfully used as an amendment. For calcareous soils sulphur can be used as an amendment because the sulphur is oxidized to sulphuric acid which is effective in dissolving the lime to product gypsum.
It is again emphasised that it is not possible to think solely in terms of the exchangeable sodium percentage without considering also the quality of the water with respect to the level of electrolyte in an irrigation water or in the soil pores (equation 2). It has been shown experimentally that the physical status of a soil, as measured by the ability to transmit water (permeability) can be preserved whatever the level of exchangeable sodium by ensuring that the level of electrolyte in the percolating solution is above the critical level (the threshold concentration). As the exchangeable sodium percentage of a soil increases the threshold concentration increases. In practice the threshold concentration concept is applicable in a range of circumstances as illustrated by the following:
- Cajon sandy loam in Arizona was successfully irrigated with Colorado River water which over time established an equilibrium exchangeable sodium percentage of and the electrolyte level in the water was sufficient to sustain the soil permeability. As a result of a shortage of river water, groundwater was used for 3 years and the exchangeable sodium percentage increased to 25 however when river water was again available it was used and the soil 'froze up' (became impermeable). The threshold concentration for this amount of exchangeable sodium would have an electrical conductivity of 1.1 dS m-1. This experience pre-dated the statement of the threshold concentration concept. If this information had been available it would have been appropriate to mix the well water (5 dS m-1) with the river water to obtain the threshold concentration and over time to increase the proportion of river water as the exchangeable sodium was reduced until the original condition of the soil in equilibrium with the river water was attained.
- In the Riverina District of New South Wales extensive areas of land were being developed by the sowing of improved pastures under irrigation. Immense difficulties were encountered as the soil was almost impermeable, the clay dispersed at the surface of the soil and when dried formed hard crusts which prevented the emergence of the pasture plants. The Riverina clay contained 23 per cent exchangeable sodium for which the threshold concentration solution would need to have an electrical conductivity of 1 dS m-1 The basis for the difficulties was that the irrigation water had a conductivity of 0.1 dS m-1. The problem was resolved by dissolving gypsum in the irrigation water to raise the conductivity to 1 dS m-1. To achieve this the gypsum is dissolved to the extent of one-third of its saturation value. For an irrigation of 7.5cm the amount of gypsum required for each hectare is about 0.6 tonne. One irrigation was sufficient for the successful establishment of the pasture plants. A similar result was obtained by broadcasting several tonnes of gypsum however such large applications were precluded on economic grounds. The same principles and technology for the dissolution of gypsum is currently being used to irrigate sodium affected soils for the production of sugar cane in the Burdekin Valley in Northern Queensland.
- Salton Sea water was mixed with Colarado River water to reclaim a saline soil with 37 per cent exchangeable sodium in the Coachelle Valley, California. By ensuring that the manufactured water had an electrolyte level always above the threshold concentration the soil permeability was sustained and thus the period required for reclamation was considerably shortened. During the reclamation process the sea water was, in stages, increasingly diluted with river water and this favoured the exchange of Ca ions as expected from the Gapon equation.
- In California as a water conservation measure drainage water is re-used. The threshold concentration concept is used as a reference together with the use of salt tolerant crops.
The significance of organic matter or some moeity of it in moderating the physical behaviour of sodium affected soils was dramatically demonstrated by experience in The Netherlands. During World War II areas of land were flooded by the sea. Subsequently it was found that pasture land, which had accumulated organic mater over time, was much more readily reclaimed than land which was arable at the time of flooding.
In the laboratory the use of the additional swelling pressure in the presence of increased sodicity is used as a 'chemical hammer' to assess to assess the effect of increasing periods under pasture on the stability of soil aggregates. As the organic matter increases with increasing periods under pasture the aggregate stability increases until the effect of exchangeable sodium is modified to a significant degree. The organic matter prevents the aggregates from failing internally and hence soil permeability is maintained.
Sodicity in Australia
Using an exchangeable sodium percentage of greater than 6 in the top metre of the soil profile as a definition of sodic it has been estimated that over 28 per cent of the total land area of Australia is sodic and that 50 per cent of arable land has sodicity related problems. These figures tend to over emphasise the sodicity problem in Australia for two reasons. First sodicity, where free water is available at the soil surface as a result of rainfall or irrigation, affect the physical behaviour of the top 5 to 10cm of soil and secondly sodicity cannot be defined without reference to the nature of the waters being used for irrigation. The exchangeable sodium value of 6 for sodic soils is a reflection of the generally small electrolyte content in Australian irrigation waters. In any case there is no sharp boundary between sodic and non-sodic so the hazard increases steadily with an increase in the exchangeable sodium percentage.
There is virtually no information available on the effect of exchangeable sodium levels on the behaviour of soils or sub-soils below a depth of 10 centimetres.
Australian soil with saline and sodic properties. K.H. Northcote and J.K.M. Skene, CSIRO Australian Soil Publication 60, 1972
Diagnosis and improvement of saline and alkali soils. United States Salinity Staff. L.A. Richards (ed.) USDA Handbook No. 60, 1954
Interparticle forces: A basis for the interpretation of soil physical behaviour. J.P. Quirk, Advances in Agronomy 53, 121-183, 1994
California Agriculture. Special Issue: Salinity in California 38, 1984
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