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RESEARCH ARTICLE
Contents Vol 43(3)

Organic compounds at different depths in a sandy soil and their role in water repellency

C. P. Morley A D , K. A. Mainwaring A , S. H. Doerr B , P. Douglas A , C. T. Llewellyn A and L. W. Dekker C
+ Author Affiliations
- Author Affiliations

A Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK.

B Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK.

C Alterra, Land Use and Soil Processes Team, PO Box 47, 6700 AA Wageningen, The Netherlands.

D Corresponding author. Email: C.P.Morley@Swansea.ac.uk

Australian Journal of Soil Research 43(3) 239-249 https://doi.org/10.1071/SR04094
Submitted: 25 June 2004  Accepted: 7 January 2005   Published: 25 May 2005

Abstract

The causes of soil water repellency are still only poorly understood. It is generally assumed that hydrophobic organic compounds are responsible, but those concerned have not previously been identified by comparison between samples taken from a water repellent topsoil and the wettable subsoil. In this study we separated, characterised, and compared the organic compounds present at 4 different depths in a sandy soil under permanent grass cover that is water repellent in the upper 0.30 m but wettable below this.

Soil samples were extracted using a mixture of isopropanol and aqueous ammonia (7 : 3 v : v). Samples were wettable after extraction and re-application of the extract from each sample onto wettable sand induced water repellency. The chloroform-soluble portions of the extracts were analysed by gas chromatography and gas chromatography-mass spectrometry. The compounds identified at all soil depths included long-chain carboxylic acids (C16–C24), amides (C14–C24), alkanes (C25–C31), aldehydes or ketones (C25–C29), and more complex ring-containing structures. 1H and 13C nuclear magnetic resonance spectroscopy, and the carbon/hydrogen ratio as determined by microanalysis, confirmed the predominantly aliphatic character of the extracts.

Both wettable and water repellent samples contained hydrophobic compounds. The 3 water repellent samples contained far more organic material, although the amount extracted was not related to the degree of water repellency. Perhaps more importantly, they contained polar compounds of high relative molecular mass, which were almost absent from the wettable subsoil. It may be speculated that these are the compounds in this soil whose presence in significant amounts is necessary for water repellency to be exhibited.

Additional keywords: hydrophobicity, gas chromatography.


Acknowledgments

The authors thank K. Oostindie for sample collection and shipment, I. Matthews for technical help with the GC (FID) instrument, the staff at the EPSRC National Mass Spectrometry Service Centre in the Department of Chemistry, University of Wales Swansea, for advice and the use of their GC-MS instrument, M. Nettle for running the NMR spectra, and S. Szajda for technical help. The financial assistance of the University of Wales Swansea in providing a postgraduate studentship (KAM) and a postgraduate bursary (CTL) is acknowledged. We also thank HEFCW for financial support. This study was supported by EU grant FAIR-CT98-4027 and NERC Advanced Fellowship NER/J/S/2002-00662(SHD). This work does not necessarily reflect the European Commission's views and in no way anticipates its future policy in this area.


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