After an initial evaluation of several solvents, the efficiency of Soxhlet extractions with isopropanol/ammonia (s.g. 0.88) (70 : 30 v : v; 24 h) in extracting compounds associated with water repellency in sandy soils was examined using a range of repellent and wettable control soils (n = 15 and 4) from Australia, Greece, Portugal, The Netherlands, and the UK. Extraction efficiency and the role of the extracts in causing soil water repellency was examined by determining extract mass, sample organic carbon content and water repellency (after drying at 20°C and 105°C) pre- and post-extraction, and amounts of aliphatic C–H removed using DRIFT, and by assessing the ability of extracts to cause repellency in acid-washed sand (AWS). Key findings are: (i) none of organic carbon content, amount of aliphatic C–H, or amount of material extracted give any significant correlation with repellency for this diverse range of soils; (ii) sample drying at 105°C is not necessarily useful before extraction, but may provide additional information on extraction effectiveness when used after extraction; (iii) the extraction removed repellency completely from 13 of the 15 repellent samples; (iv) extracts from all repellent and wettable control soils were capable of inducing repellency in AWS. The findings suggest that compounds responsible for repellency represent only a fraction of the extract composition and that their presence does not necessarily always cause repellency.

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.

Summary In previous studies, extracts from water‐repellent soils have been applied to wettable substrates to examine the role of organic matter in causing water repellency, but the effects of individual compounds found in water‐repellent soils have not been examined. To address this research gap, acid‐washed sand (AWS) and wettable sandy soil were coated with a range of hydrophobic organic compounds that had been identified by gas chromatography‐mass spectrometry (GC‐MS) analysis of extracts from a range of water‐repellent soils in previous published work. Saturated and unsaturated long chain acids, alkanes, amides, esters, cholesterol and β‐sitosterol were applied at various loadings to investigate the effects of chain length, molecular shape, functional group, heating, particle size and packing efficiency on soil wettability. No single compound induced repellency on AWS at loadings equivalent to the level of detection of individual compounds in water‐repellent soils by GC‐MS. The extent of water repellency induced on AWS varied considerably with compound type, suggesting that it depends more on the presence of specific compounds rather than their quantity. A combination of long chain acid and alkane was found to be the most effective at inducing water repellency. Heating at 105°C for 24 hours caused a marked increase in repellency. Particle size had no effect on degree of induced repellency across the approximately 2.5 times variation in diameter examined.

Separation and chemical characterisation of specific compounds responsible for soil water repellency has not previously been achieved. Here we describe the extraction, separation and analysis by gas chromatography-mass spectrometry of organic compounds found in wettable and water repellent sandy soils from the Netherlands and United Kingdom. Fatty acids (C 16 -C 24 ), amides (C 14 -C 24 ), alkanes (C 25 -C 33 ), aldehydes/ketones (C 23 -C 31 ) and complex ring-containing structures were detected in all samples. We found a greater abundance of high molecular mass polar compounds in the water repellent samples.