Swt5 And Swt5X; Soils And Soil Water; Intended Use - Delta-T Devices SWT5 User Manual

1.6 SWT5 and SWT5x

1.6.1 Soils and soil water

All water movement in soil is directly dependant on the soil water
tension, because water, both in soils and on the surface, will will move
from a point of higher potential to a point of lower potential.
The majority of soil water flows 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/level) to -85 kPa (suction / soil water
tension). The SWT5x operates to even lower tensions.
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.
The SWT5 Miniature Tensiometer is specially designed for point
measurements, e. g. in soil columns, pots or laboratory lysimeters, or
when the measurement of a minimal span is desired.
2
With an active surface of only 0.5 cm and a diameter of 5 mm the
ceramic tip has all advantages of small dimensions: little soil
disturbance, point measurement and fast response.
8  Introduction
SWT5 User Manual v2