The water vital for plants seeps into the soil after precipitation (rain, hail, sleet, snow) (as far as the soil conditions allow). A small proportion also comes from surface precipitation formed by condensation (dew, frost), which is absorbed directly by the dew roots located just below the surface. The non-solid soil substance consists of pores of varying sizes that are filled with air and/or water. The pores can make up 30 to 60% of the total volume. Depending on the pore volume results the porosity - the ratio of the void volume to the total volume. In dry soil, all pores are filled with air.
The infiltrating water displaces the air first in the fine pores until finally, in moist soil, air remains only in the coarse pores. Adhesive or capillary water is the water held against gravity, which sticks in pores smaller than 10 µm (10 thousandths of a millimetre) due to the surface tension of the water (meniscus = concave curved water surface). The amount of adhesive water that the soil can hold with its pores against gravity is called water capacity (WK) or field capacity (FK). It is strongly dependent on the soil type and is expressed in % vol (volume) or % mas (weight). For example, 30% vol FK means that one m³ of soil can hold 300 litres (a barrique barrel contains 225 l) of water, which is an average value for clay soils.
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