Explanation Of Terms - UMS HYPROP Operation Manual

Explanation of terms

The terms tension, matrix potential, water tension and pF value refer to the
same physical value: they describe the energy that attracts water mole-
cules – to pores capillary or to soil particles adhesively. Plants e.g. must
overcome this binding energy (or attraction force), to suck water from the
soil matrix.
The water is under tension in the soil. This tension or attraction force can be
directly measured as a vacuum of the water compared to the atmospheric
pressure. So for the measurement water in the tensio shaft is "offered" to
the soil. The soil sucks it with the same force as it retains the water. As the soil has
a capillary contact to the water in the tensio shaft through the pores of the
ceramic the pressures of the water in the soil and in the tensio shaft finally match.
As the tensio shaft water is locked air tight, it cannot flow into the soil before
the soil is so dry and accordingly the vacuum is so high that the first pore
layer gets empty. This vacuum (tension) can be measured with a pressure
sensor. If the vacuum is strong enough to suck the small pores empty the
water entry point has been reached: air enters the tensio shaft, the pressure
rises up to the atmospheric pressure and the pressure read out drops to zero.
The matrix potential is the negative value of the tension. It is often expressed
in hPa or kPa. In the soil science also the pressure head is used, e.g. in the
unit "cm water column". The pF value is the decimal logarithm of a wa-
ter column in cm. A soil water tension of -100 hPa is equivalent to a water
column of approximately* 100 cm. So its pF value is 2.0
The terms retention function, pF curve and pF/WC characteristics mean the
same. They describe a soil characteristic dependent on the binding energy
(pF value, water tension) and on the water content (WC). As an example:
Sand can retain only little water, clay a lot. At or near saturation sand con-
ducts water very well, clay however poorly. Dry sand conducts water very
poorly, compared to this clay is a bit better (water conducting horizons).
Please see the following retention curves for comparison.
* as g = 9.81 m/s²
78 | Theory