Soil Is Never Without Water - Soil: The Skin of the Planet Earth - page 112

already in Chap. 7 that natural-occurring soil pores are variously shaped and curved,
change in size, occupy irregular spaces between solid surfaces of particles and those
within and between aggregates, and manifest a diversity of simple to complex spa-
tial interconnectivity. Because capillarity is linked to changes of water surface cur-
vatures within soil pores on a microscale and consequently to changes of surface
tension or pressure, it is important to detect and understand the relationship between
soil water content and macroscopically measured capillary pressure. The smaller is
the soil water content, the lower should be the pressure at equilibrium. Because this
meniscus pressure is negative compared to the pressure on a horizontal, plane water
surface, its absolute value is higher. Let us explain with a simple example. We mea-
sured the soil water content of a loam to be 32 % when the meniscus pressure was
−15 kPa, and when the pressure was −100 kPa, we measured a smaller water content
of 26 %. The negative sign indicates that we have to use less energy for extracting a
small drop of water from soil having a water content of 32 % than the energy neces-
sary for extracting the same small amount of water from the loam having a smaller
water content of 26 %. Or, vice versa, water is bound in this loam by a higher energy
if its water content at 26 % is less than that at 32 % or higher soil water contents.
Although these values of pressure are often carelessly referred to as the potential of
soil water, it is more precise to identify them as pressure potential.
Values of soil water pressure potential should be measured on “undisturbed” soil.
Such samples are obtained by pressing a metallic cylinder (volume of at least
100 cm 3 ) into the soil without disturbing the soil's natural arrangement inside of the
ring. The cylinders are extracted together with the undisturbed soil and placed on
the porous ceramic plate in the chamber of a pressure plate apparatus; see Fig. 8.10 .
Fig. 8.10 Pressure plate apparatus on the left side is provided with a liquid manometer just for
simplicity. Routinely it is connected to a gauge. The applied air pressure pushes out of the soil
sample all water kept by capillary pressure (in absolute value) lower than is the applied overpres-
sure, indicated here by water manometer. Its value is the same as in the equipment on the right-
hand side which practically imitates the decreased water content due to the depth of groundwater
level. The apparatus name is tension plate apparatus