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The many faces of soil

A new research project at Aarhus University will delve into the secrets behind soil properties in relation to the soil’s water content. This knowledge is relevant for agriculture and engineering.

[Translate to English:] Nogle jordtyper skrumper, når der er tørre. Foto: Colourbox

Here is a puzzle for you. Imagine you have a dry soil sample and you want it to soak up some water until it reaches a particular relative humidity. At the required humidity level, you carefully measure the amount of water the soil sample soaks up in order to characterize the soil’s so-called water vapor sorption. So far so good, right? You then decide to dry this soil sample so that it returns to its previous dry state. You work with great precision in the laboratory so that the amount of water the now wet soil sample gives up should equal the amount of water the dry soil previously absorbed, right? Wrong!


Scientists have found that there can be a difference in some soil’s water sorption ability depending on whether the soil is absorbing water or releasing it. The mechanism behind this is a puzzle that a new project at Aarhus University intends to shed more light on. Researcher Emmanuel Arthur from the Department of Agroecology is the leader of the project and has received a grant of DKK 4.5 mill. from Villum Fonden to carry out the four-year investigation.     


Soil with a past

The ability of the soil to hold water depends on its previous history, which is whether it is coming from a dry condition and absorbing water or coming from a wet condition and releasing water. This state of affairs is called hysteresis. If scientists can get a handle on the mechanisms determining sorption hysteresis they can develop a novel framework that can estimate soil properties that are otherwise difficult to measure, such as surface and engineering properties.


- Soil water vapor sorption and reflectance spectroscopy are two rapid and cost-effective methods for determining soil properties which are otherwise tedious and costly to measure. Currently, the mechanism behind the hysteretic nature of water vapor sorption is poorly understood despite its critical importance in modeling unsaturated water flows and other biological processes, Emmanuel Arthur explains.


Besides gaining a better mechanistic understanding of hysteresis, the scientists will also focus on soil surface properties like water repellency, which are essential for agronomic processes such as plant water utilization, preferential leaching of pesticides and nutrients. The researchers will also investigate other properties that are crucial for characterizing soil for engineering applications. These other properties include the soil’s Atterberg limits and its shrink-swell potential.


Soil takes many forms

And what are these the concepts “shrink-swell potential” and “Atterberg’s limits”, you may ask? Some soils, such as clay, change in size depending on their water content. When clay is wet it swells and when it dries it shrinks. The shrink-swell potential describes this property. The shrink-swell potential is important to identify when erecting buildings or other forms of construction on soil. Soils that shrink markedly when dry can also affect crops by damaging their roots.


The Atterberg limits help describe what happens when soil changes its form depending on its water content. Once again, think of clay. Very wet clay can be almost liquid, when partially wet it becomes plastic, and when completely dry it is hard enough to use as bricks for building but very difficult to plough. The limits between these stages are described as the Atterberg limits.


Based on a wide range of samples that will include pure clay, clayey soil, sandy soil, young Greenland soil, expansive and low activity soil, and organic soil, the project will combine measured soil water vapor sorption isotherms and reflectance spectroscopy with the following:


  • Clay and organic characteristics to understand hysteresis
  • Vapor sorption kinetic theory and organic matter fractions to predict water repellency
  • Clay type and reference data to develop and validate models to predict several engineering properties


- We expect that the results will facilitate rapid estimation of the selected properties from easy-to-measure data and assist in policy-making and research that requires knowledge of soil surface properties on regional or national scales, says researcher Emmanuel Arthur.


For more information please contact: Researcher Emmanuel Arthur, Department of Agroecology, e-mail: emmanuel.arthur@agro.au.dk, telephone: +45 8715 7635