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Water storage capacity

The essential for plants water oozes rainfall (Rain, hail, sleet, snow) in the ground. A small proportion also comes from the surface precipitation (dew, ripening) formed by condensation, which directly through the surface just below the surface Tauwurzeln is recorded. The non-solid soil substance consists of pores of different sizes, which are filled with air and / or water. The pores can make up 30 to 60% of the total volume. Depending on the pore volume, the porosity - the ratio of the void - to the total volume results. All pores in dry soil are filled with air.

Water storage assets - graphic with soil structure and terms

The infiltrating water first displaces the air in the fine pores, until finally, when the soil is moist, air only remains in the coarse pores. Adhesive or capillary water is the water held against gravity, which sticks in pores smaller than 10 µm due to the surface tension of the water (meniscus = concave water surface). The amount of holding water that the soil can hold with its pores against gravity is called water capacity (WK) or field capacity (FK). It is strong from soil type dependent and is given in% vol (volume) or% mas (weight). For example, 30% vol FK means that one m³ of soil can hold 300 liters of water, which is an average value for clay soils.

The water is bound with different strengths in the individual pore types of the soil, which results in the so-called water retention capacity. The coarse pores above all ensure good ventilation. Most of the water available for the plants is contained in the middle pores. And the fine-pored portion looks very strong adsorption, This force binds part of the water so strongly that it is not available to the plants. That is why this proportion is aptly referred to as dead water, which makes up for clay soils up to 150 l / m³. In the case of heavy clay soils with a field capacity of up to 400 l / m³, the dead water can even be up to 300 l / m³ in extreme cases. Clay soils have a balanced relationship between available water and dead water, which are also among the most productive. The water seeps away very quickly in sand or gravel soils. Only 100 l / m³ field capacity can be kept here, of which 50 to 70 l are available.

The water trapped in the soil rises to a lesser extent back to the surface of the soil and evaporates what you call evaporation (Evaporation). The usable field capacity (minus dead water) is very different for the plants. This proportion of water is absorbed by the roots, with the suction tension at vines is extremely high compared to other plants and corresponds to a pressure of about 16 bar (only 10 bar for wheat). However, this water only remains to a small extent in the plant and largely evaporates through the stomata of the leaves (Transpiration). If the maximum storage capacity of the soil in relation to gravity is exceeded, the soil seeps out (percolation).

The excess amount of water seeps in deeper and eventually ends up in the groundwater. But that's a good thing water discharge required. An optimal water balance depends on the soil type, the amount of precipitation, the depth of rooting and the water absorption of greening plants dependent. An improvement in storage assets is achieved by introducing organic materials such as humus or mulch reached. These substances promote the formation of coarse and medium pores. In clay-rich soils, stones placed in the ground can be an effective measure. Adequate rainfall and a balanced water balance are important criteria for the so-called Weinbauwürdigkeit, See also under Weingarten Care,

Graphics: probably. by FMoeckel , own work, public domain , link
edited by Norbert FJ Tischelmayer 3/2019

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