Introduction to Soil Chemistry

88 soil basics iv
Figure 4.10. Pressure apparatus for measuring the amount of water held in soil at different pressure;
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 -10 to -30kPa. Other
similar pressure plate apparatus can be used to determine water at pressures
to -1500kPa [10].
4.13.2. Field Methods
Field methods of measuring soil water are designed primarily to measure
water, in the range of -10 to -1500kPa of pressure. However, different instruments
have different ranges as shown in Table 4.1. Tensiometers, porous