Agriculture Reference
In-Depth Information
Figure 4.10.
Pressure apparatus for measuring the amount of water held in soil at different pres-
sure; pressure gauge is to the left, and a pressure plate is on top of the vessel.
Table 4.1. Common Instruments for Measuring Soil Water Content in the Field
Instrument
Useful kPa Range
Characteristics
Tensiometers
0 to -86
Limited range and depth
Thermocouple
50 to -10,000
Wide range but limited accuracy
psychrometers
Porous blocks
-100 to -1500
Accuracy and range are limited
Neutron probe
0 to -1500
Cannot be used in highly organic soils
Time-domain
0 to -10,000
Accurate and can be installed at various
reflectrometry
depths
(TDR)
The other common laboratory method uses pressure plates and membranes
to measure the amount of water held by soil at various pressures (Figure 4.10).
One advantage of this method is that it gives a pressure value that can be used
in many calculations relevant to movement of water in soil. The plates in the
apparatus shown are used in the pressure range of
30 kPa. Other
similar pressure plate apparatus can be used to determine water at pressures
to
-
10 to
-
-
1500 kPa [10].
4.13.2.
Field Methods
Field methods of measuring soil water are designed primarily to measure
water, in the range of
1500 kPa of pressure. However, different instru-
ments have different ranges as shown in Table 4.1. Tensiometers, porous
-
10 to
-
