Soil moisture tension is measured by a Tensiometer or Electrical Resistance block and is usually expressed in bars, centibars or kilopascals. One kilopascal is equal to 1 centibar; 100 centibars equals one bar. The graph depicts available soil moisture on the y axis and a logarithmic scale of centibars on the x axis. The curves for Loamy sand, Fine sand loam and Sandy loam appear similar to logarithmic shapes. The curve for Clay appears similar to an exponential shape and the curve for Loam begins as exponential but then changes to logarithmic shape. With these instruments field capacity is always 0 to 10 centibars. The trigger point (50% available moisture) in centibars for each soil texture is: Loamy sand 25 cbar, Fine Sandy loam 30 cbar, Sandy loam 40 cbar, Loam 81 cbar, Clay 205 cbar.

The Davis Soil Moisture Sensor uses the gypsum block method and consists of two electrodes embedded in a block of gypsum. They can be either cylindrical or rectangular in shape with concentric or parallel electrodes (see image to the right). Wires are joined onto each electrode and extruded from the gypsum block to measure the resistance between the electrodes. The resistance between the two electrodes varies with the water content in the gypsum block, which will depend directly on the soil water tension. As the soil dries out water is extracted from the gypsum block and the resistance between the electrodes increases. Conversely as the soil wets, water is drawn back into the gypsum block and the resistance decreases.

Davis soil moisture probe

The Davis soil moisture sensor uses electrical resistance to measure the moisture level of the soil and does not require periodic maintenance. The sensor is buried directly into the soil at a depth of 30 cm. The soils here at the Otter Valley Weather Station are sandy-loam with the local geology being Triassic Sandstone. They are prone to drying-out rapidly during dry spells in the spring and summer. The graph to the left shows the most recent data regarding soil moisture at this station. Rainfall has been included and a trend should emerge during precipitation after lengthy dry spells where the soil moisture responds to recent rainfall. During the winter the soil moisture generally reads less than 10 (saturated) but from mid-February onwards the soil starts to dry out and the reading will begin to climb.

Soil water potential, or soil tension, is the amount of energy available in the soil for water movement. That is, soil water potential is a measure of how much energy is required by the plant to absorbed or extract water from the soil matrix. A soil water potential value of 0 (zero) means the soil is saturated, or flooded, and there is free movement of water. As soil water potential values become more negative, then the greater amount of work is required by plants to extract water from the soil. A value of -1,500 kPa (or 1,500 centibars) is called the permanent wilting point and is the point where most plants can no longer extract moisture from the soil.

Soil water potential should not be confused with soil water content. Soil water potential is an absolute measurement, such as temperature, as it is a measure of a state of energy. Soil water content, on the other hand, is a dependent measurement. Soil water content depends on the physical properties of the soil. For example, a soil water content value of 30% in a sandy soil means it is saturated, whereas 30% in a heavy clay soil may indicate that it is dry and there is little plant available water. A soil water potential value of -33 kPa means the soil is at field capacity whether it is measured in a sand or a heavy clay.