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VIII. Sodic Soils and the Threshold Concentration Concept

VIII. Sodic Soils and the Threshold Concentration Concept

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The Gapon equation, which has been widely used in research concerning

saline and sodic soils (Richards 1954), has been discussed by Sposito (1981) in

relation to the more conventional cation exchange equations. In the Gapon equation the ions are shown as reacting in equivalents:

NaX (s)

+ 0.5 CaC1,




+ NaCl(aq)


and the Gapon equilibrium constant is:

which may be rearranged to give

where E N , and Ec, are the equivalent fractions of sodium and calcium ions on the

exchange complex and the square brackets refer to concentrations in solution

(mmol liter-1) rather than activities. The ratio of ion concentrations is of a

similar magnitude to the corresponding ratio of ion activities over the concentration range common to salt-affected soils, even though the activities themselves

vary considerably. In considering K , values for individual soils it is realistic to

recognize that soil clays are frequently a mixture of clay mineral types and

contain four principal cationic species: sodium, potassium, magnesium, and

calcium. The ratio EN,/Ec, is referred to as the exchangeable sodium ratio

(ESR), and the ratio, if only sodium and calcium ions are present, E,,IEN, +

Eca, expressed as a percentage is the exchangeable sodium percentage (ESP);

more generally the ESP is the percentage of exchangeable sodium relative to the

exchange capacity. The ratio (Na+)/(Ca2+)0.5is referred to as the sodium adsorption ratio (SAR) of the equilibrium solution, and in practice Mg2+ is coupled

with Ca2+ in the denominator.

The Gapon equation has two interesting features. First, if a soil is equilibrated

with a solution having a given SAR value, and it is desired to percolate the soil

with a more dilute solution and at the same time maintain the ratio of sodium to

calcium on the soil colloid surfaces constant, then if the sodium ion concentration in the solution is reduced by a factor then the calcium ion concentration must

be reduced by the square of that factor. Second, if water being used to reclaim a

sodic soil is simply diluted, this dilution decreases the SAR value and thus favors

the adsorption of calcium on the soil surfaces, so the exchangeable sodium

percentage is reduced and reclamation is assisted.

Difficulties encountered in the irrigation of sodium-affected soils or in the

reclamation of saline-sodic soils can be circumvented by controlling the electrolyte level in the irrigation water; this prevents the deterioration of soil structure



and sustains permeability (Quirk and Schofield, 1955; Quirk, 1957, 197 1, 1986).

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. In practice a soil should be irrigated with

water which contains a level of electrolyte above the threshold concentration and

preferably has a SAR value which favors calcium adsorption onto the exchange


To determine the threshold concentration for various degrees of exchangeable

sodium saturation, Quirk and Schofield (1955) brought Sawyers 1 (an illitic

Rothamsted loam soil) to equilibrium with relatively concentrated mixed solutions of NaCl and CaCl, having different SAR values. When exchange equilibrium was established between a particular solution and the soil surfaces then the

permeability to a series of successively more dilute solutions was determined; the

dilutions were carried out in accordance with the Gapon equation so that each

solution in the series had the same SAR value. The threshold concentration is

taken as the concentration at which there is a 15% decrease in permeability; the

concentration is expressed in milliequivalents per liter (meq liter- I ) by adding

the concentrations of sodium and calcium in the threshold concentration solution.

Figure 9 shows a plot of the threshold concentration in relation to the SAR value

of the percolating water (Quirk, 1971). It may be noted that if the soil permeability is to be sustained, the electrolyte level in the water should be to the

right of the line, delineating decreasing permeability and unfavorable physical

behavior, from stable permeability; this situation is exacerbated the further the

concentration of the percolating water is removed from the threshold concentration. The relationship between the threshold concentration, cT, and SAR is

cT = 0.56 SAR

+ 0.6


The relationship covers the range of ESP values from 0 to 35. It may be noted

that Eq. (23) does not contain any parameters which are characteristics of the soil

solid phase. The SAR value of the saturated extract of a soil, through the Gapon

equation, provides the exchangeable sodium percentage. If a soil is irrigated with

water of such a quality that the ESP value of the soil increases over time, then

Eq. (23) can be used to assess the threshold concentration as the soil approaches

equilibrium with the SAR value of the water. The SAR value on the ordinate in

Fig. 9 can thus refer to the saturated extract of a soil or to the SAR value of an

irrigation water. A stylized version of Fig. 9 has appeared in work done by Oster

er al. (1984).

Quirk and Schofield (1955) foreshadowed that materials such as oxides and

organic matter would protect a soil from the adverse effects of raised levels of

exchangeable sodium and envisaged that the threshold concentration concept

should be especially significant with respect to the irrigation of arid and semiarid

soils. McNeal and Coleman (1966) and Rhoades (1982) have presented results,














Electrolyte concentration (meq I-’)

Figure 9 The relationship between the SAR (NaKao.5) for an irrigation water or saturated soil

extract and the electrolyte level required to maintain a stable permeability-threshold concentration.

(After Quirk, 1971 ,) Equation (23) describes the relationship.

for soils from the western United States, in reasonable agreement with Eq. (23)

up to SAR values of about 30. Most of the soils investigated by McNeal and

Coleman (1966) contained a montmorillonitic component and thus the very

variable behavior for the permeability above a SAR value of 30 is probably due

to the crystalline swelling of the montmorillonitic materials in these soils.






Quirk and Schofield (1 955) noted that dispersed particles did not appear in the

percolate from permeameters until the electrolyte level was about one-quarter of

the threshold concentration. The concentration for dispersion, cD, is given approximately by the relationship,

cD = 0.14 SAR

+ 0.20


The amount of dispersed material in the percolate increased markedly with

increasing exchangeable sodium percentage at the dispersion concentration and

lower. These observations emphasize that factors other than dispersion are pri-



marily responsible for decreases in permeability which occur for sodium-affected

soils between the threshold and dispersion concentrations. Rowel1 et al. (1969),

working with a brown loam, corroborated the findings of Quirk and Schofield

( 1955) and established correspondence between concentrations at which increased swelling of oriented flakes of the clay extracted from the soil is observed

and the threshold concentration. They concluded that (a) the permeability begins

to decrease at the same concentration as additional clay swelling begins, (b) the

changes in permeability are directly controlled by the swelling of the clay until

clay dispersion and movement begins, (c) the concentration at which clay disperses depends on the mechanical stress applied, and (d) large mechanical

stresses may disperse more of the clay even at small exchangeable sodium

percentages. This behavior is reminiscent of the movement, described earlier, of

a soil from the overconsolidated condition toward the normal consolidation

curve. The failure that occurs within a soil at low velocities in a permeameter

could be described as sodic failure and furthermore the presence of exchangeable

sodium assists any mechanical work applied in altering the structure by removing

particles from primary potential minima and the disruption of the clay domains.

Frenkel et al. (1978) concluded that particle dispersion and pore blocking were

the cause of decreased permeability of sodium-affected soils. This conclusion has

lead to comprehensive studies of the dispersion-flocculation transition (Goldberg et al., 1991) which have considerably advanced our knowledge of soil

colloids. However, this review has stressed that interacting clay particles have to

first be removed from within potential minima before extensive swelling takes

place and this is followed by spontaneous dispersion if the solution is sufficiently

dilute. The evidence presented in this review is that, within a soil clay, particles

reside in potential minima when the soil is dry; even for Na-montmorillonite the

X-ray spacing is 19 A at 1.5 MPa suction. The crucial issue is that the structure

of a clay domain is very different from that of a floccule.

There is an intermediate stage between swelling and dispersion which precedes dispersion. A soil, as a result of the additional swelling which takes place

when some particles are removed from within potential minima, fails internally

and this leads to pore size reduction or even blocking. This failure also provides

surfaces to yield dispersed clay particles. These processes are probably concomitant as the electrolyte level is progressively reduced. Internal failure is considered to be a prerequisite to copious dispersion.

The introduction made reference to the fact that the balance between forces

when Ca clay particles are within a potential minimum must be delicate since

15% sodium or even less on the exchange sites can dramatically alter the physical

behavior of a soil. Particle in this sense could refer to the interaction between

quasicrystals or crystals in a clay domain or between the domains themselves. In

this latter case because of ion segregation, whereby Ca2+ is adsorbed within the

domain, there will be an accumulation of Na+ on the surface of intrinsic failure



pores. As a result, the interaction between surfaces will be strong where these

pores are relatively narrow or form a nexus between the walls of the pore.

The protective role that organic materials can have is best illustrated by Emerson (1954) who showed that soil from a pasture maintained a high permeability

to low electrolyte levels even when Na saturated. It seems that the organic matter

has prevented the failure but would not have been expected to decrease the

particle interaction since large polymers would not be expected to enter pores

much less than 100 A across, bearing in mind the free form of polymers in

aqueous solution.

Results presented by Quirk and Schofield (1955) show that organic matter or

moieties of the organic matter enhance the repulsive forces which prevent flocculation of soil suspensions and that it is necessary to add higher concentrations

of electrolyte to achieve flocculation in the presence of adsorbed organic molecules. This is illustrated by reference to surface and subsurface soils at Rothamsted. Sawyers I, a surface soil, required 300 meq/liter to flocculate the suspension of the Na-saturated soil whereas only 20 meq/liter was required to flocculate

the Na-saturated subsoil with essentially the same mineral composition; this latter

value is characteristic of the minerals Na-montmorillonite and illite. Thus organic molecules have a negative effect on soil structure when the particles dispersed

by the action of raindrops lead to surface seals by the slow sedimentation of fine

particles and in-washing into pores (McIntyre, 1958). There is thus a paradox.

The presence of organic matter stabilizes aggregates against slaking and the

presence of organic molecules adsorbed on the clay serves to prevent flocculation

(peptization) and thus assists in the formation of surface seals.

Rengasamy et al. (1984) examined the dispersion behavior of 138 samples

taken from the surface and subsurface of Australian red-brown earths. They

reported that 30 of the samples did not disperse after 1 hr of end-over-end

mechanical shaking and that 28 samples dispersed spontaneously. For this last

group they obtained the following relationship between the total cation concentration (meq/liter-1) and the sodium adsorption ratio, TCC = 0.16 SAR + 0.14,

which is remarkably similar to Eq. (24) although the methods used are quite

different. They also found that for surface soils which were mechanically shaken

the boundary between dispersion and flocculation was defined by TCC = 1.21

SAR + 3.3. For subsoils the corresponding relationship was TCC = 3.19 SAR

- 1.7 indicating the possible presence of inorganic peptizing agents because the

organic matter content of the subsoils is very small. Rengasamy et al. (1 984)

considered that the shaking procedure represented the mechanical effect of raindrop impacts. The large difference in the coefficients for SAR for the spontaneously dispersed soils, the surface soils, and the subsoils is noteworthy and

reveals that factors other than the mineral species involved can have a major

influence on flocculation concentrations.







An example of the importance of the threshold concentration concept is gained

by considering the behavior of Cajon sandy loam in Arizona. This soil was

usually irrigated with Colorado River water, but little river water was available

during the years 1946 to 1948, and as a result it was necessary to rely on

underground water. This water contained 50 meq/liter of Na and 8 meq/liter of

Ca (plus Mg). After irrigating with this water for 3 years the ESP value was

reported as varying between 19 and 34, with 25 as the mean. Such a soil would

require a concentration of 1 1 meq/liter of irrigation water to maintain a stable

permeability (Fig. 9). The underground water, although it would have provided a

very saline environment, exceeded this concentration. However, the river water

contained 3.9 meq/liter, which is considerably less than the threshold concentration. McGeorge and Fuller (1950) reported that when river water was again

available in 1949 its use caused the soil to become impermeable and to develop

unsatisfactory physical conditions: the soil “froze up.” If the river water had been

modified by the addition of 8 meq/liter of Ca2+ ions or, alternatively, if as little

as one-seventh of the underground water had been mixed with the river water,

this problem could have been avoided. This would have given an electrolyte

concentration of about 11.6 meq/liter, which could have been progressively

reduced by monitoring the ESP value until it fell to a level that would have

allowed the use of river water alone.

Because it would not have been economical to add large quantities of gypsum

to soils used for pasture development in western New South Wales, Davidson and

Quirk (1961) added gypsum to the irrigation water to assist pasture-plant establishment on Riverina clay. This soil had an ESP value of 20 and the irrigation

water had an electrolyte concentration of about 1 meq/liter, and as a result

dispersion occurred when the soil was irrigated; water entry was poor and very

few pasture plants emerged because of the hard surface crusts that developed

between imgation events [see photographs in Davidson and Quirk (1960)l.

These difficultieswere overcome when 10 meq/liter of Ca2+ as gypsum (approx.

0.6 t ha-1) was added to the irrigation water. It was only necessary to add

gypsum to the first irrigation to achieve satisfactory pasture establishment and


Reeve and Bower (1960) remarked that “the use of waters of poor quality, that

is those having a high salt content and a high proportion of sodium, for reclamation has been unthinkable in the past, and at first thought such use would seem to

be a questionable practice.” These authors used water from the Salton Sea mixed

with water from the Colorado River to reclaim soil (ESP = 37) in the Coachella

Valley, California, to sustain the permeability by ensuring that the electrolyte



level in the manufactured irrigation water was always above the threshold concentration. As a result the reclamation time was shortened and was assisted by

the “valence-dilution effect” which, as discussed in relation to the Gapon equation, favors the exchange of Ca for Na as progressively more dilute waters are

used as reclamation proceeds.

Oster et af. (1984) have discussed the threshold concentration in relation to the

reuse of drainage waters for irrigation in the San Joaquin Valley in California as a

means of obtaining more water for irrigation.


Dexter (1988) discusses the hierarchical order within soil aggregates as

The lowest hierarchical order is the combination of single mineral particles,

such as clay plates, into a basic type of compound particle such as a domain.

The next higher order is larger compound particles such as a cluster of domains. The next higher order is when a number of clusters are combined to

form microaggregates. Compound particles of a lower hierarchical order are

more dense than those of higher hierarchical order. This is because the order

excludes the pore spaces between the particles of the next higher order“porosity exclusion principle.” Compound particles of a lower hierarchical

order have a higher internal strength than particles of higher hierarchical order.

Dexter also observes that not all soils possess all hierarchical orders.

Quasicrystals, crystals, clay domains, and intrinsic failure are all features of

the hierarchical order as are the pores which readily drain in attaining field

capacity. To extend our knowledge of the hierarchical order which underlies

physical behavior, more attention should be given to the soil as a porous system,

particularly for soils of varying textures. We should be concerned not only with

pore sizes, their interdependence, and their probability of continuity but also the

disposition within the pore space of organic matter and other materials which

promote aggregation.

Over the past decade or so there has been substantial progress in the theoretical

and experimental approach to interparticle forces, and although much remains to

be achieved, it seemed timely to provide a bridge between mainstream colloid

and surface chemistry and soil science in which field we have, through the

activity of clay mineralogists and others, an unusually detailed knowledge of the

surface chemistry of clays and oxides.

The simple fact remains that soils and the clay materials in them are condensed



systems and it is the particle interaction at short range which holds the key for an

understanding of clay-water interactions and their effect on the various aspects

of the hierarchical organization within a soil. Two simple facts are relevant.

First, even unstable aggregates do not slake when immersed in nonpolar liquids

and second, a clay material such as Willalooka illite which increases in volume

by 50% from the dry to wet state must, even in the wet state, have the predominant part and its surface area in fine pores (
The close range interaction of clay particles influences other aspects of the

hierarchical order. For much too long there has been an overemphasis of the

flocculation-dispersion transition and as a result the very real issue of shortrange interaction between particles has not received attention commensurate with

its importance.

The mutual influence of the physical and biological worlds within a soil is

exceedingly complex. In an admirable review Foster (1985) has discussed “the

localisation of organic matter in soils.” The nature of the interface between these

two worlds constitutes a serious challenge if progress is to be made in unraveling

the basis for the profound effect that organic materials can have on soil physical

behavior and structural stability. The basis of soil structural degradation must be

at an hierarchical level above domains since it is improbable that domains in a

soil would be disrupted by cultivation.

Clearly bacteria, extracellular enzymes, and large macromolecules do not have

access to the reactive (high surface area) pore space in soils and low molecular

weight materials are rapidly degraded by soil microorganisms. The nature of the

interface between the biological and physical worlds within a soil can only be

appropriately evaluated if much greater knowledge of the soil as a porous system

is attained.


It is a pleasure to acknowledge the role that Dr. R. S. Murray of the Waite Agricultural Research

Institute has played in the refinement and development of some of the ideas presented here; I am also

grateful for his helpful advice on this review.


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