CASE STUDY 15.1 Water-repellent sands

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Margaret Roper

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Figure 1 A droplet of water on water-repellant sand. Beading of water is characteristic of such sands, greatly reducing infiltration and hence moisture availability to plant roots. Droplet diameter c. 5mm (Macrophotograph courtesy Margaret Roper)

 Drought stress can be exacerbated or prolonged if rain that does fall is not absorbed and is therefore not available to roots. Water repellence is mainly associated with areas of siliceous sands, generally occurs within 100–150 mm from the soil surface and is easily recognised by beading of water on the soil surface (Figure 1). Water repellency has been observed in about five million hectares of sandy soils in agricultural land in southwestern Australia, from Geraldton to east of Esperance and in southeastern Australia on Eyre Peninsula and in a region south of Adelaide extending to western Victoria. Water-repellent sands are not confined to coastal regions, having been observed in the mallee region of western New South Wales. Because of the reduced and uneven infiltration of water in repellent soils, germination of plants is generally uneven and delayed, stand establishment is poor and there is an increased risk from wind and water erosion (Tate et al. 1989).

Water repellence is caused by the formation of a skin of hydrophobic substances around sand grains. Plant waxes and their biodegradation products appear to be major contributors to hydrophobicity but repellence has also been linked with populations of basidiomycete fungi and actinomycetes. Water repellence occurs in soils under native vegetation and in agricultural soils. McGhie and Posner (1981) tested a range of plant species for their effect on water repellence by mixing the plant material with wettable sand. They found that species of subterranean clover (Trifolium subterraneum) caused strong water repellence while cereals gave wettable mixtures. Litter from native trees such as mallee (Eucalyptus sp.) and sheoke (Casuarina heugeliana) caused severe repellence. A more detailed list of the vegetation associated with water-repellence is given by Wallis and Horne (1992). Within a water-repellent soil, the particulate organic matter fraction contributes significantly to the repellence of the soil as a whole, first, by its strongly hydrophobic nature and, second, by acting as a reservoir of hydrophobic waxes (Franco et al. 1995). These authors showed that hydrophobic materials from the particulate organic matter diffuse onto sand grains during heating, and particularly during wetting/heating/drying cycles. More extreme heating resulting from burning of the litter layer vaporises hydrophobic substances which move down to cooler layers of soil where they condense, resulting in an increase in the thickness of the water-repellent layer.

Confirmation that hydrophobic material in soils is primarily of plant origin has been provided by analyses of plant waxes. For example, free alcohols, fatty acids and alkanes with carbon chain lengths similar to those found in soils have been found in leaf waxes of grasses and clover. There are also strong similarities between plant and animal fats and indeed water repellence is often exacerbated in areas where animal fats are concentrated, for example sheep camps.

Amelioration

Strategies for amelioration of water repellence include physical treatments, chemical additives and biological processes.

Considerable success in improving soil wettability has been achieved by adding montmorillonite clays to promote oxidation of the hydrophobic fraction in soils. Kaolinite may be more efficient than montmorillonite at reducing water repellence because this type of clay is able to cover the hydro-phobic surface of repellent sand. The amounts of clay required to eliminate water repellence are high, up to 100 t ha–1 (Blackwell 1993). Introduction of clays to water-repellent sandy soils is therefore only economic if clays occur below ground where they can be brought to the surface, thereby eliminating transport cost.

Non-ionic wetting agents (surfactants) have been used to increase wettability. Addition of a wetting agent decreases surface tension so that in water-repellent soil infiltration rate of the solution containing the surfactant is increased. How-ever, due to their cost wetting agents are of limited use, and there are also indications that they have negative effects on plants (Wallis and Horne 1992).

Biodegradation of waxes by microorganisms can remove or reduce the cause of water repellence via either of two approaches: by developing treatments which promote the wax-degrading activity of naturally occurring microorganisms, or by introducing microorganisms which have superior wax-degrading capabilities. The first approach has been used successfully to break down hydrocarbons in environments affected by oil pollution. A wide range of wax-degrading bacteria (including actinomycetes) and fungi has been isolated from soils and other sources including a soil bacterium (Micrococcus cerificans) which is capable of using cabbage paraffin (n-nonacosane) for energy. By introducing efficient wax-degrading microorganisms and applying treatments/manage-ments which promote microbial activity in soils, it may be possible to improve soil wettability or at least achieve a sustainable level of crop/pasture production.

References

Blackwell, P. (1993). ‘Improving sustainable production from water repellent sands’, Western Australian Journal of Agriculture, 34, 160–167.

Franco, C.M.M., Tate, M.E. and Oades, J.M. (1995). ‘Studies on non-wetting sands. I. The role of intrinsic particulate organic matter in the development of water-repellency in non-wetting sands’, Australian Journal of Soil Research, 33, 253–263.

McGhie, D.A. and Posner, A.M. (1981). ‘The effect of plant top material on the water repellence of fired sands and water repellent soils’, Australian Journal of Agricultural Research, 32, 609–620.

Tate, M.E., Oades, J.M. and Ma shum, M. (1989). ‘Non-wetting soils, natural and induced: an overview and future developments’, in The Theory and Practice of Soil Management for Sustainable Agriculture, A workshop for the Wheat Research Council, 2–3 February 1989, 70–77, AGPS: Canberra.

Wallis, M.G. and Horne, D.J. (1992). ‘Soil water repellency’, Advances in Soil Science, 20, 91–146.

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