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Water storage capacity

The water that is 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 different sizes that are filled with air and/or water. The pores can make up 30 to 60% of the total volume. The porosity - the ratio of the void volume to the total volume - depends on the pore volume. In dry soil, all pores are filled with air.

Wasserspeicherungs-Vermögen - Graphik mit Bodenstruktur und Begriffen

Adhesive water

The infiltrating water first displaces the air in the fine pores until finally, in moist soil, air only remains in the coarse pores. Adhesive or capillary water is water held against gravity that sticks to pores smaller than 10 µm (10 thousandths of a millimetre) due to the surface tension of the water (meniscus = concave water surface). The amount of adhesive water that the soil can hold with its pores against the force of gravity is referred to as water capacity (WC) or field capacity (FC). It is highly dependent on the type of soil and is given in % vol (volume) or % mas (weight). For example, 30% vol FC means that one m³ of soil can hold 300 litres (a barrique barrel holds 225 litres) of water, which is an average value for clay soils.

Water holding capacity

Water is bound to varying degrees in the pore types of the soil, which results in the water retention capacity. The coarse pores ensure good aeration. The medium pores contain the largest proportion of the water available for the plants, while the fine-pored part is characterised by very strong adsorption. This binds part of the water so strongly that it is not available to the plants. This is why this proportion is referred to as dead water, which can be up to 150 litres/m³ in clay soils. In heavy clay soils with a field capacity of up to 400 litres/m³, the dead water can even be up to 300 litres/m³.

Clay soils have a balanced ratio between available water and dead water and are also among the highest-yielding soils. In sand or gravel soils, the water seeps away very quickly. Here, only 100 litres/m³ of field capacity can be retained, of which 50 to 70 litres are available. The monoculture often practised in viticulture for centuries can lead to soil compaction and thus to waterlogging and poor water storage capacity.

Evaporation

The water retained in the soil rises to a lesser extent to the soil surface and evaporates, which is known as evaporation. The usable field capacity (minus dead water) varies greatly between plants. This proportion of water is absorbed by the roots, whereby the suction tension in vines is extremely high compared to other plants at around 16 bar (only 10 bar in wheat). However, only a small proportion of this water remains in the plant and most of it evaporates via the stomata of the leaves(transpiration). If the maximum storage capacity of the soil against gravity is exceeded, percolation of the soil occurs.

Water drainage

The excess water seeps deeper and deeper and eventually reaches the groundwater. However, this requires good drainage. An optimal water balance depends on the type of soil, the amount of precipitation, the depth of rooting and the water absorption of green plants. The storage capacity can be improved by adding organic material such as humus or mulch. These materials promote the formation of coarse and medium pores. In clay-rich soils, stones incorporated into the soil can be effective. Sufficient rainfall and a balanced water balance are important criteria for suitability for viticulture. See also under vineyard care.

Graphic: presumably by FMoeckel, own work, public domain, link
edited by Norbert F. J. Tischelmayer 3/2019

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