Delmhorst Instrument KS-D1 - Manual (Page 3)

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INTRODUCTION
The Delmhorst soil moisture measuring system consists of two
parts: the gypsum soil blocks (sensors) and the measuring
instrument, Model KS-D1 moisture tester. The blocks (GB-1)
are made of gypsum cast around two concentric electrodes.
The gypsum acts as a buffer against the effect that salts or
other chemicals might have on the electrical conductivity. The
concentric electrodes confine the flow of current to the interior
of the block, eliminating the effects of soil conductivity.
When a block is buried in the soil it absorbs moisture from
the soil or releases moisture into the soil, until its moisture
content approaches equilibrium with the moisture content of
the soil. When the block is connected to the meter, current
flows between the electrodes and the electrical resistance of
the gypsum is measured. Such readings are an indication of
the moisture available to the plants.
For irrigation purposes, the water of importance is that
amount which can be extracted from the soil by the roots
of the plants, and not the percent moisture content (which
is related to the weight of the soil). We refer to this reservoir
of moisture as “available moisture” and it is expressed within
a given range of soil moisture tension.
Each soil has a different capacity to hold water, depending
on its structure and texture. The maximum amount of water
available to the plants (called Field Capacity) is the amount
held by the soil against drainage by gravity. When virtually
all available water has been used (that is when no further
moisture can be extracted by the plant) soil moisture has
reached the level known as the “Permanent Wilting Point.”
With soil moisture at this level, plants permanently wilt
and die.
Gypsum block systems work best in finely textured soils such
as fine sandy loams and clay loams. These soils hold a greater
amount of water at field capacity then coarsely textured soils
such as coarse sandy loams. Blocks are not recommended
for use in sandy soils because the coarse texture of the sand
does not interface well with the fine gypsum texture. As a
guideline, blocks work best in soils with a water-holding
capacity greater than 1.2in/ft.
KS-D1 Op ManualQ7.qxp:Layout 1 4/14/11 9:25 AM Page 2