The amount of water in the soil depends on its composition: sandy soils with large soil particles can contain up to 0.12 g of water per g of soil, loamy soil (0.25 g g–1) and clay (0.40 g g–1) with smaller soil particles can contain substantially more water and organic material such as potting compost up to 1.5 g g–1.
Soil water content can be measured as with different sensors such as:
- frequency domain sensor such as a capacitance sensor. FD sensors are relatively cheap and have a fast response time. However because of the complex electrical field around the probe, the sensor needs to be calibrated for different soil types.
- neutron moisture gauge, which utilizes the moderator properties of water for neutrons, slowing high-energy neutrons
- time domain transmission (TDT) and time domain reflectometry (TDR) measure the dielectrtric constant which is high for water, the methods determine an apparent TDR permittivity from the travel time of an electromagnetic wave that propagates along a transmission line, usually two or more parallel metal rods embedded in a soil. Water content affects the dielectric permittivity.
It is essential to characterize soil water status as soil water potential rather than soil water content, since the water potential determines the water availability for plants. The relation between water content and water potential is a permanent characteristic of a specific soil, named water release curve (>>watering).
Tardieu F, INRA. The protocol is also available at http://www.dropsproject.eu