Water capacity

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The ability of the soil to absorb water and hold it against gravity is referred to as water capacity or water holding capacity .

Potential concept

The binding force with which the water is held in the soil matrix is ​​described as the matrix potential ψ m . In relation to the groundwater surface, the term soil water tension or suction tension is also used for the forces.

Field capacity

The maximum amount of water that a soil can hold is called field capacity . The binding forces are in the range of 60 to 300  hPa .

Water tension curve

Water tension curve (Ss = pure sand, Uu = pure silt, Tt = pure clay, Lu = silty loam; see soil type )

If a water-saturated soil is drained, the water seeps away from the coarse pores first. The finer the pores, the stronger the capillary and adsorption forces that hold the water against gravity. The remaining water forms thin water films around the soil particles; Swelling processes of clay colloids and humus particles keep the water in the soil. Finally, soil salts bind water of crystallization with potentials of up to 600 MPa with the highest energy .

The course of the water holding capacity of a soil in relation to the matrix potential ψ m is shown as a water tension curve. This curve correlates with the grain size distribution and the humus content of the soil and reflects the distribution of the pore volume , the pore size distribution, of the soil.

The water tension curves are characteristic for each type of soil ( texture ). The figure shows the drainage curves of sand, loam, clay and peat. When saturated with water, the mineral soils sand, loam and clay store around 40% water volume, the organic peat soil even 90% of its volume.

At a potential of 100 kPa (corresponding to about −9.81 m groundwater level), the peat has already given off 55% water volume, sand 25%, clay 10% and clay even less water.

For the vegetation, this has the consequence that sandy and humus soils provide significantly more plant-available water in more humid conditions, while loamy soils or clay soils have a greater water holding capacity in dry periods.

Dynamic processes: hysteresis

The water capacity of a soil is not the same for drainage and slow irrigation for a given matrix potential ψ m , rather a hysteresis curve results .

To a certain extent, the current capacity also depends on the previous history - the reasons for this are complex and not yet fully understood. In fact, however, various physical mechanisms are responsible for this. While the drainage runs from the coarse to the fine pores, the finer pores are filled first with slow irrigation due to the capillaries. The air displacement and compression depends in turn on the flow speed of the water during filling. Swelling processes change the pore size and thus the capillary effect. Soils containing peat can be downright water-repellent when dry, even due to electrical charges.

See also

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