Clay soil changes throughout the year
Clay soil changes throughout the year

The structure and physical properties of buffer zone clay soil change throughout the year. Kimmo Rasa, a Senior Researcher from MTT Agrifood Research Finland, discovered in his research for his doctoral thesis that the structure of pores and conductivity of water in clay soil change considerably as conditions relating to moisture in the soil change.

These results are significant for water protection as conductivity of water is one of the factors that affect the functioning of buffer zones. If the soil structure is conducive to water infiltration, soil particles and nutrients will not flow directly into the water system.

The changes that take place in the physical properties of soil throughout the year are well known at a practical level but there has been little research in Finland into the phenomena related to soil moisture changes. The results from Rasa's research help to develop more accurate models with which to describe the movement of water and nutrients in the soil.

Aggregates, water channels and micropores

The material of the research consisted of soil samples collected in spring 2005 from the perennially vegetated buffer zones located at clay fields in Jokioinen. The samples were analysed for soil structure and hydraulic properties as well as soil shrinkage and water repellency. Measurements were carried out both in laboratory and field conditions. Thin sections (0.03 mm) were produced from soil samples and then analysed for the shape, size and number of pores.

"The surface of clay soil consists of sub-angular blocky, angular blocky or platy aggregates. Inside these are microscopic pores that store water in the soil. There are elongated and irregular pores between the aggregates and water moves through these macropores," describes Rasa.

Drying changes soil structure

The research discovered that drying has a significant effect on the structure of clay soil. When soil dries, it shrinks by up to 10%, and this shrinkage changes the structure of pores. Structural changes are particularly distinct in the spring.

Rasa explains that water saturation as well as freezing and thawing outside the growing season shape the structural units of the soil, and they will become rearranged more favourably as the growing season progresses. Strong shrinkage is indicated by cracks on the soil surface. However, these cracks close up as moisture swells up the soil again.

Water repellency of soil is another phenomenon that has attracted little research in Finland but is related to dry soil. If soil is very water repellent, it does not immediately absorb water into its aggregates.

"In dry conditions, cracking and water repellency occur simultaneously which allows for fast flow of water, and substances in the water, into a subsurface drain. Water repellency is presumably a fairly common feature in Finnish agricultural soil in dry conditions but this no longer has any effect as soil gets wet," says Rasa.

Wet conditions create new aggregates

When the soil thin sections were analysed under a microscope, Rasa observed that they had dense concentrations of clay substance which may have been formed when water dispersed fine clay particles from the soil.

"In the spring, clay-rich water does not infiltrate the soil as lower soil layers are still frozen but instead clay suspension becomes dried in the surface horizon. As clay particles dry, they form clusters, or aggregates, which we presume have different physical properties from earlier aggregates," says Rasa.

The formation of aggregates shows that water saturation outside the growing season weakens soil structure, but on the other hand produces new structural units.

Grazing decreases infiltration of water

The research also showed that the management of buffer zones has a clear effect on soil structure. One of the buffer zones in Rasa's study was in its natural state, one was harvested once a year and the third was grazed by cattle. The most unbeneficial conditions for water infiltration were created by grazing.

"The surface horizon of the grazed site was compacted and it had only a few pores. The bigger pores were also more horizontal which prevents water from easily infiltrating the soil. Grazing pressure should be kept light so that it does not weaken soil structure," concludes Rasa.

The public examination of Kimmo Rasa's (M.Sc. Agriculture and Forestry) Doctoral Thesis "Physical properties of a boreal clay soil under differently managed perennial vegetation" will be held on 25 November 2011 at the University of Helsinki Faculty of Agriculture and Forestry. Rasa's Opponent will be Professor Nicholas Jarvis from Swedish University of Agricultural Sciences and the Custos will be Professor Markku Yli-Halla from University of Helsinki.

Physical properties of a boreal clay soil under differently managed perennial vegetation:

For more information, please contact: Senior Scientist Kimmo Rasa, MTT Agrifood Research Finland, tel. +358 40 179 2794,

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