Introduction to Soil Chemistry

86 soil basics iv
A
Figure 4.9. Shown from left to right are a tensiometer, soil drying can (A) and porous block (B)
and meter for determination of soil water.
4.13. MEASURING SOIL WATER
Soil water can be measured using either laboratory or field methods. Common
laboratory measurements include the percentage of water on a dry-weight
basis and water content as a function of pressure. For the analyst the determination
of percentage water on a dry-weight basis is the most important.
Field methods include tensiometers, porous blocks (shown in Figure 4.9), psychrometers,
time-domain relfectrometry (TDR), neutron probe, and others
designed primarily to determine plant available water.
Soil water content may be reported in a number of different ways. Most
commonly it is reported as the amount of water in grams per gram of oven
dry soil, that is, the mass water content (see equation 4.3a below). It may
also be reported as the volume of water per volume of soil, that is, the
volumetric water content. For analytical purposes it is most often simply
presented as a percent of water on a mass:mass ratio, that is, grams of water
per gram of oven-dry soil. In field measurements, the water content of
B