pF value

In soil science , the dimensionless pF value (p of power , F of free energy of the water) characterizes the energy with which the soil water is held against gravity in the soil matrix. The higher the pF value, the drier the soil or the lower its water content .

definition

Conversion table

pF value	Soil water tension in hPa	Soil water tension in mWS	Soil moisture
0	−1 hPa = −10 0 hPa	−0.01 mWS
1	−10 hPa = −10 1 hPa	−0.1 mWS	wet
2	−100 hPa = −10 2 hPa	−1 mWS	wet
3	−1,000 hPa = −10 3 hPa	−10 mWS	fresh
4th	−10,000 hPa = −10 4 hPa	−100 mWS	dry
5	−100,000 hPa = −10 5 hPa	−1000 mWS	dry

The pF value is defined as the decadic logarithm of the amount of soil water tension in hectopascals (this is also called suction tension or matrix potential  ): ${\displaystyle \psi _{\mathrm {m} }}$

${\displaystyle \operatorname {p} F=\operatorname {log} _{10}\left|{\frac {\psi _{\mathrm {m} }}{\mathrm {hPa} }}\right|\;.}$

If the soil water tension is not given as pressure in hectopascals, but

• in meters of water column  (mWS)
• in Pascal  (Pa) or
• in geotechnical engineering in kPa,

it must first be converted into hectopascals to determine the pF value.

Under hydrostatic conditions, the pF value corresponds to the decadic logarithm of the vertical distance to the water table in centimeters.

Application examples

• In the hydrostatic state, a groundwater level of 60 cm below the soil surface corresponds to a soil water tension at the soil surface of −60 hPa and thus a pF value of 1.8.

• The field capacity of soils is between 1.8 and 2.5 pF

• The permanent wilting point PWP is defined at a pF value of 4.2 (corresponding to approx. 160 m distance to the groundwater ) ; Sunflowers, Helianthus annuus , and pines, Pinus sylvestris , can no longer withdraw enough water from the soil, even if they are well rooted, and wither irreversibly. 

• A soil sample in equilibrium at 90% relative humidity , i.e. H. Dried out until the water content of the soil has adapted to that of the air, corresponds to a pF value of 5.

See also

• Water capacity
• Water potential
• Tensiometer

literature

• Karl Heinrich Hartge: Introduction to Soil Physics. E. Schweizerbart'sche Verlagsbuchhandlung oHG, Stuttgart 1978, ISBN 3-432-89681-6 , pp. 132-140.
• Diedrich Schroeder: Soil science in brief. Hirt Verlag, Unterägeri 1984, ISBN 3-266-00192-3 .

Web links

• Christine Alewell: Soil Water: Capillarity, Potentials, Darcy, nFK , Chapter 8 in Fundamentals of Soil Science . Institute for Environmental Geosciences, University of Basel (PDF file; 1.69 MB).
• M. Schöniger, J. Dietrich: Matrix potential and water content . In: HydroSkript - lectures on hydrology and water management.