Soil water status and temperature are critical factors that affect the activity of organisms in soils. Soil water content also has important effects on the quantity of water that eventually becomes available to surface water, both through its influence on infiltration/runoff relationships (wet soils have lower infiltration rates) and because of its potential contribution to the groundwater. Soil temperature influences soil water availability, the growth rates of plants and microorganisms, and rates of chemical/biochemical reactions.

A number of useful soil physics texts are available that cover the general theory and application of soil water and temperature (e.g., Baver et al. 1972; Hillel 1980a, 1980b; Marshall and Holmes 1988).  In addition, several books contain detailed summaries of current measurement techniques (Klute 1986; Smith and Mullins 1991; Topp et al. 1992). These references should be used to supplement material in this chapter.

Soil water content has been measured for many years. Earliest measurements were semi quantitative. To the experienced fieldworker, the appearance and feel of the soil can provide valuable information on the amount of water it contains, especially the total water content. Wet soils are generally darker than dry soils and feel "smoother" and heavier. However, appearance and feel can be deceiving. Furthermore, they do not provide quantitative measures of soil water status. Earliest measures of soil water content involved collecting a moist field sample, weighing it, drying it in air or in an oven, and determining the weight loss. Weight loss on drying at 100- 110 °C is usually attributable to water evaporation. The ratio of weight loss to dry weight has been termed gravimetric soil water content.