Water repellency in soils: a historical overview

Soil water repellency is much more wide-spread than formerly thought. During the last decades, it has been a topic of study for soil scientists and hydrologists in at least 21 States of the USA, in Canada, Australia, New Zealand, Mexico, Colombia, Chile, Congo, Nepal, India, Hong Kong, Taiwan, China, Ecuador, Venezuela, Brazil, Mali, Japan, Israel, Turkey, Egypt, South Africa, Germany, The Netherlands, Spain, Portugal, United Kingdom, Denmark, Sweden, Finland, Poland, Slovakia, Russia, France, Italy, and Greece. Although, water repellent soils already have been indicated at the end of the nineteenth century, they have been discovered and studied in most countries in the last decades. Water repellency is most common in sandy soils with grass cover and in nature reserves, but has also been observed in loam, heavy clay, peat, and volcanic ash soils. From 1940 to 1970 research was focussed on identifying vegetation types responsible for inducing water repellency and on developing techniques to quantify the degree of water repellency. Of special interest has been the effects of wildfire on the development of soil water repellency and its consequences for soil erosion. Due to increasing concern over the threat to surface and groundwater posed by the use of agrichemicals and organic fertilisers, studies on water repellent soils have also been focused on its typical flow behavior with runoff and the existence of preferential flow paths. Since the end of the 1950s, wetting agents and clay amendments have been studied to ameliorate water repellent soils. Since 1883, more than 1200 articles related to soil water repellency have been published in journals, reports, and theses. An exponential increase in number of publications started in 1960, resulting in an average of 200 publications per 5 years.

Exponential increase of publications related to soil water repellency

The author has researched the chemistry of soil organic matter for almost 50 years. In this chapter, he presents a personal account of how soil organic matter chemistry has evolved during the second half of this century from wet to computational chemistry. The chapter begins with a definition of soil organic matter and how it relates to humus and humic substances. Problems associated with the extraction of organic matter from soils, separation of the extract into humic substances, and purification of the resulting fractions are then discussed. New experimental approaches to the in situ analysis of organic matter in whole soils to overcome these problems are described. Investigations on the chemistry of soil organic matter are outlined in terms of (a) an analytical and (b) a structural approach. The analytical approach involves determinations of the characteristics of humic substances by chemical methods, infrared, 13C nuclear magnetic resonance, electron spin resonance spectroscopy, and electron microscopy, whereas the structural approach consists of oxidative and reductive degradations, pyrolysis-field ionization mass spectrometry, and Curie-point pyrolysis-gas chromatography/mass spectrometry. The author recounts how the results of the analytical and structural studies led to the formulation of a two-dimensional humic acid model structure and how the latter was converted with the aid of computational chemistry to a three-dimensional humic acid model structure and later to three-dimensional model structures of soil organic matter and whole soils. The next topics discussed by the author are advances in the chemistry of N-, P-, and S-containing components of soil organic matter. Especially noteworthy is progress in the chemistry of N in soil organic matter, which points to a prominent role of heterocyclic N. As far as colloid-chemical characteristics of humic substances are concerned, the three parameters that control the molecular characteristics (molecular weight, size, and shape) of humic and fulvic acids are (a) the concentration of the humic substance, (b) the pH of the system, and (c) the electrolyte concentration of the medium. In the last part of the chapter, the author discusses how humic substances interact with water, metals, minerals, pesticides, and herbicides; lists functions and uses of humic substances; and describes personal encounters with outstanding scientists who influenced his research.

A Lifetime Perspective on the Chemistry of Soil Organic Matter

Functional relationships between soil water content and water suction were examined and related to textural and organic carbon content data. Soil water retention curves between 5 and 10 000 kPa were determined on disturbed samples of 18 soils representing various soil Great Groups in the Canadian prairies. The best fit was obtained with a two-straight-line regression model. Correlation and regression analysis showed that texture was the main soil property influencing the shape and position of the water retention curve. Organic matter influenced primarily the water content at which a break in the curve occurred. Soil zone and cultivation history had little effect on water retention. Key words: Water retention, texture, organic matter, two-straight-line regression

Water retention equations and their relationship to soil organic matter and particle size distribution for disturbed samples

Soil water repellency is known to be a dynamic property, which varies in the short term or between seasons. In the short term, its temporal nature is often studied based on soil water content, assuming that it is reestablished after the soil dries out. In this study, we examined the reestablishment of soil water repellency after wetting and subsequent drying. The reestablishment process was studied on: (i) natural water-repellent soils (WRS) subjected to different leaching rates; and (ii) wettable sand subjected to wetting-drying cycles with dissolved organic matter (DOM) solution. After air drying, the soils were packed in columns, rewetted, and from the maximum height of capillary rise for the target soil (H eq1 ) and that of a "reference soil" (H eq0 ) the equilibrium (static) contact angle (ω eq1 ) was calculated, assuming that for the reference soil the contact angle ω eq0 = 0. Increasing the leaching fraction of an initially WRS resulted in a decrease in ω eq1 values. For an initially wettable soil, increasing wetting-drying cycles with DOM solutions resulted in an increase in ω eq1 . The variations in cosω eq1 were reflected in a similar ratio for the capillary-pressure-saturation relations (CSR). The prediction from scaling the "reference soil" CSR curve by cosω eq1 was found to be satisfactory, but more accurate where the effective saturation S 0.5, organic matter is more likely to detach and dissolve, changing the properties of the soil solution by altering its surface tension and the soil particles' surface, i.e., ω eq1 .

M. Schnitzer

Papers

Water adsorption on soil humic substances