SWT4 User Manual v3.1
Introduction
7
1.6 SWT4 and SWT4R
1.6.1 Soils and soil water
All water movement in soil is directly dependant on the soil water
tension, because water, both in the soil and on the surface, will always
move from a point of higher potential to a point of lower potential.
The majority of soil water flow take place in response to small water
tensions. Only Tensiometers allow the direct and precise measurement
of these small tensions.
Natural soils in the ground are heterogeneous. It is not just
precipitation and evaporation that matter, but also the soil texture,
particle size distribution, cracks, compaction, roots and cavities. All
these heterogeneities cause the soil water tension to vary. It is prudent
therefore to have multiple measuring points, particularly in soil
horizons close to the surface.
1.6.2 Intended use
Tensiometers measure soil water tension
– a measure of the soil
matrix potential
– which is the work the plant needs to do in order to
extract water from a unit volume of the soil. These Tensiometers work
from +100 kPa (water pressure) to -85 kPa (suction or soil water
tension).
If the soil gets drier than -85 kPa, the Tensiometer runs dry and must
be refilled as soon as the soil is sufficiently moist again (see Fig 6.1).
Soil water and Tensiometer water have contact through the ceramic
which is porous and permeable to water. A wetted porous ceramic
creates an ideal pore/water interface. The soil water tension is directly
conducted to the pressure transducer which offers a continuous signal.
The atmospheric reference pressure is provided through a membrane
on the cable, a unique patented method.
1.6.3 Types
The SWT4 is available in 2 versions: the standard
SWT4
without
refilling tubes, and the
SWT4R
with refilling tubes for refilling the
Tensiometer in the field with a syringe.
Separate refilling instructions for the SWT4R are provided in section 4.
