Spectrum WaterScout SMEC 300, Руководство по эксплуатации продукта, Страница: 20 / 24

20
Volumetric Water
Content
The SMEC 300 measures volumetric water content. The
volumetric water content (VWC) is the ratio of the volume
of water in a given volume of soil to the total soil volume.
At saturation, the volumetric water content (expressed as a
percentage) will equal the percent pore space of the soil.
In-field soil moisture content will range from air-dry to
saturation. However, plants cannot extract all the water in
a saturated soil and can extract none of the water in an air-
dry soil. Instead, two other moisture content levels,
field
capacity and permanent wilting point are often used to
indicate the upper and lower limit of plant available wa-
ter . Field capacity is defined as the condition that exists
after a saturated soil is allowed to drain to the point where
the pull of gravity is no longer sufficient to remove any
additional water. Water draining from a soil profile can-
not, in general, be taken up by plant roots. On the oppo-
site end of the spectrum, permanent wilting point is the
highest moisture level at which an indicator plant cannot
recover turgor after being placed in a humid environment.
Irrigation should be scheduled somewhere between these
two extremes. One rule of thumb is to apply water when
half the plant available water has been depleted. How-
ever, individual circumstances may dictate a more conser-
vative or liberal approach. Figure 1 illustrates the plant
available water range for the 12 USDA-defined soil tex-
tures. Keep in mind that these numbers are merely guide-
lines and will vary for individual soils.