Field capacity

Below field capacity refers to the amount of water that an initially water-saturated soil against the gravity can hold after 2 to 3 days yet. The field capacity is of great importance in practice-oriented issues in arable and horticultural fields , e.g. B. for questions about the water supply of the plants, the plant availability of water-soluble nutrients , the leaching of water-soluble substances as well as the irrigation .

determination

The water content is determined by drying a soil sample in the laboratory for 24 hours at 105 ° C and determining the weight of the soil sample before and after drying. The difference in the measured weights gives the water content. The value is specified (for example values, see pore volume ):

• as a percentage ( vol .-% )
• based on the volume [cm ³ / cm ³ ] or
• based on a certain floor height [mm / dm].

Because the drainage behavior of a soil u. a. strongly depends on the type of soil (or its stratification ), a convention has been agreed for determining the field capacity in the laboratory for soils not influenced by groundwater : the water content at a soil water tension of pF  1.8.

In central European latitudes, full field capacity is usually reached towards the end of winter (March / April). Even then, the soil is not completely saturated with water ; The coarse and macro pores still contain air. The soil can therefore have a higher water content during precipitation than determined by the field capacity.

Relationship with the pore size

The field capacity is u. a. depends on the pore size distribution (see pore volume ). It is usually assumed that pores with a diameter of more than 10 or 50 µm can not hold water in place by capillary forces ; the water seeps through the ground.

The retained water stored in the soil is also not completely available for plants. The part of the field capacity  (FC) that can be absorbed by the plants through the roots is the usable field capacity (nFK), often also called soil water reserve :

${\displaystyle nFK=FK-TOT}$.

Dead water  (TOT) is so strongly held in pores under 0.2 µm by the adhesive forces that plants can no longer remove it from it. Here the water tension is over 15,000 hPa or 15 bar, this value is also called permanent wilting point  (PWP).

swell

• Ad hoc working group Soil: Soil-Scientific Mapping Instructions. Published by the Federal Institute for Geosciences and Raw Materials in cooperation with the State Geological Services. 5th improved and enlarged edition. Schweizerbart, Stuttgart 2005, ISBN 3-510-95920-5 ( http://www.schweizerbart.de/pubs/isbn/bgr/bodenkundl-3510959205-desc.html ).
• DIN 4220: 2008-11 - Soil science location assessment - Marking, classification and derivation of soil parameters (normative and nominal scaling).
• Hartge, KH & R. Horn (1989): The physical investigation of soils. Enke publishing house, Stuttgart.
• Scheffer , Paul Schachtschabel: Textbook of soil science. 15th edition, reprint. Spectrum Akademischer Verlag, Heidelberg 2008, ISBN 978-3-8274-1324-6 ( spectrum textbook ).