Adsorption water
So-called adsorption water describes the adsorption , i.e. accumulation, of water on the surface of soil particles , a film of water molecules forms in the process . The amount of water that forms this film is not constant, but depends on the soil water tension . The accumulations of water at the boundary surfaces of the soil particles are related or equilibrium with the menisci of the capillary bound water. Capillary water and adsorption water together form what is known as adhesive water , which is held in the ground against gravity .
Adsorption water and soil moisture
With a high soil moisture in the area of the field capacity of a soil, the water films around the soil particles are more pronounced. If, on the other hand, the capillaries in the pore volume of a soil are largely dried out, the remaining thin layers of water molecules are very strongly adsorbed. The first molecular layer around the soil particles is adsorbed with a binding pressure of 6 × 10 4 N · cm −2 (corresponding to a pF value of about 6.8).
Effect on the soil structure and stability
In sandy soils, the specific surface areas per volume of soil are much smaller than in soils with a finer grain size . The structural structure of the clay minerals also leads to larger surfaces in clay soils. Therefore, the amount of adsorption water and the effect on the soil structure is only small in sandy soils, higher in loamy soils and very high in clay soils. The adsorption water causes an increase in the plasticity of soils - with negative consequences for stability. This property is not only dependent on moisture but also on pressure. At high pressure, even an air-dry clay floor can be plastically deformed.
Cause and mechanisms
The water molecules are bound both osmotically and electrostatically as dipoles to the surfaces of the minerals and humus colloids in the soil. Hydrogen bridges , van der Waals forces and the water of hydration of existing cation exchangers provide additional binding forces for the water. The valency of the bonds of these ions plays a role. As a rule, monovalent ions bind more water than divalent or trivalent ions. The loss of potassium ions has a particularly strong effect on clay minerals. A structural change ( closing ) of the silicate layers reduces the surface area for water adsorption .
Properties of adsorbed water
The adsorbed water is a part of the kinetic energy as heat of wetting to the surroundings. The heat capacity is less than that of free water; the freezing temperature is lowered in a fine-grained bottom. The density , viscosity and vapor pressure are also influenced by adsorption.
Effects on the ground
The adsorption of water molecules changes the physical properties of the soil particles. In the clay fraction of the soil, the water dipoles cause peptization (under conditions of repulsion) or flocculation or coagulation of the clay minerals under predominant conditions of attraction of the colloidal clay . Swelling and shrinking processes change the water-holding capacity of the soil and ultimately lead to a hysteresis in the water tension curve .
See also
literature
- Karl Heinrich Hartge: Introduction to Soil Physics. Stuttgart 1978, ISBN 3-432-89681-6 , pp. 94-96.