The Assessment of Soil Sodicity
S.C. Boucher
School of Geography and Environmental Science
Monash University, Victoria 3800
Introduction
Briefly, the surfaces of clay particles in sodic soils typically exhibit a net negative charge which is neutralized by a distribution of ions characterized by an increase in the cation concentration with increasing proximity to the surface and an opposing trend for anions (e.g. Rengasamy and Sumner, 1998). The resulting environment surrounding the particle is known as a diffuse electric double layer (e.g. Leeper and Uren, 1993) and detailed discussions as to the nature of the forces involved have been presented elsewhere (e.g. van Olphen, 1977; Quirk, 1994; 2001; Goldberg et al., 2000). The relative concentrations of the four major exchangeable cations, Ca2+, Mg2+, Na+ and K+ are important in that they strongly affect both soil pH and the wetting reaction of clay particles (Leeper and Uren, 1993).
Soil Sodicity Measurement
Rengasamy and Churchman (1999) reported that two parameters, ESP and/or SAR, are normally used to evaluate soil sodicity. These are defined as follows:
| ESP = | (100 . Exchangeable Na) Cation Exchange Capacity | |
| or if the soil pH is not acidic: | ||
| ESP = | (100 . Exchangeable Na) Exchangeable (Ca+Mg+Na+K) | |
SAR = | [Na] ([Ca] + [Mg])0.5 2 | |
The sodicity of soil solutions and irrigation water is determined using the SAR, and concentrations are in mmol/L. The assessment of ' Exchangeable (Ca+Mg+Na+K)' as a surrogate for Cation Exchange Capacity (i.e. CEC) was discussed by Sumner (1993) and Sumner et al. (1998), with the link to acidic soils with respect to Exchangeable Al having been examined by Rengasamy and Churchman (1999). Various relations between ESP and SAR were summarized by Sumner et al. (1998).
In both the ESP and SAR, Ca, Mg and Na are the most effective in affecting the structural stability of the clay fraction of a soil in water. Although Ca is beneficial to the maintenance of soil structure, Mg and Na tend to exert the opposite effect, with the latter proving by far to be the most harmful. Either the ESP or SAR is used together with other chemical and/or physical data to characterize the dispersibility, and hence erodibility, of soil.
Whilst an ESP of six was proposed by Northcote and Skene (1972) to be the lower limit of soil sodicity, values of five (van Beekom et al., 1953) and two (Mitchell, 1976) have been suggested to cause a deleterious effect on soil structure. Spontaneous clay dispersion occurred in Ca-Na aggregates at an ESP of five, but was observed in Mg-Na samples when the ESP was only 3 (Emerson and Bakker, 1973). However, as mentioned in Soil Sodicity in Victoria: an Overview, the dispersion of clay particles was found to be strongly dependent on the electrolyte concentration in the soil solution and this topic will be addressed in a future article entitled Aggregate Slaking and Clay Dispersion.
References
Emerson, W.W. and Bakker, A.C., 1973: The comparative effects of exchangeable calcium, magnesium, and sodium on some physical properties of red-brown earth subsoils. II. The spontaneous dispersion of aggregates in water. Australian Journal of Soil Research 11, 151-157.
Goldberg, S., Lebron, I. and Suarez, D.L., 2000: Soil colloidal behaviour'. In Sumner, M.E. (ed. in chief). `Handbook of Soil Science'. CRC Press, Boca Raton, pp. B195-240.
Leeper, G.W. and Uren, N.C., 1993: `Soil Science: an introduction'. (Fifth Edition). Melbourne University Press, Carlton.
Mitchell, J.K., 1976: `Fundamentals of soil behaviour'. John Wiley and Sons Inc., New York.
Northcote, K.H. and Skene, J.K.M., 1972: `Australian soils with saline and sodic properties'. CSIRO Australia, Division of Soils Soil Publication No. 27.
Quirk, J.P., 1994: Interparticle forces: a basis for the interpretation of soil physical behaviour. Advances in Agronomy 53, 121-177.
Quirk, J.P., 2001: The significance of the threshold and turbidity concentrations in relation to sodicity and microstructure. Australian Journal of Soil Research 39, 1185-1217.
Rengasamy, P. and Churchman, G.J., 1999: Cation exchange capacity, exchangeable cations and sodicity. In Peverill, K.I., Sparrow, L.A. and Reuter, D.J., (eds) `Soil Analysis: an Interpretation Manual'. CSIRO Publishing, Collingwood, 147-157.
Rengasamy, P. and Sumner, M.E., 1998: Processes involved in sodic behaviour. In Sumner, M.E. and Naidu, R., (eds) `Sodic soils - distribution, properties, management and environmental consequences'. Oxford University Press, New York, 35-50.
Sumner, M.E., 1993: Sodic soils: new perspectives. Australian Journal of Soil Research 31, 683-750.
Sumner, M.E., Rengasamy, P. and Naidu, R. 1998: Sodic soils: a reappraisal. In Sumner, M.E. and Naidu, R. (eds) `Sodic Soils - Distribution, Properties, Management and Environmental Consequences'. Oxford University Press, New York, 3-17.
van Beekom, C.W.C., van den Berg, C., de Boer, Th.A., van der Molen, W.H., Verhoeven, B., Westerhof, J.J., Zuur, A.J., 1953: Reclaiming land flooded with salt water. Netherlands Journal of Agricultural Science 1, 153-163 and 225-244.
van Olphen, H., 1977: `An introduction to clay colloid chemistry'. (Second Edition). John Wiley & Sons Inc., New York.
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