Some quick explanations of the jargon around soil moisture and technology for measuring it.
Soil water potential
Soil water potential measures the amount of force that plant roots need to exert to extract water out of the soil. Soil water potential is not dependant on soil type so a reading of 10 hPa in different types of soils reflects the same amount of effort for a plant to extract water. Different crops will require irrigating at different soil water potentials as some are more able to extract water from drier soils than others.
Volumetric water content
Volumetric water content measures the percent of volume in the soil, taken up by water molecules. The total volume of water that a soil can hold depends on the amount of pore space in the soil, that is the air space that exists between individual soil particles. Because soil particles vary immensely in shape and size, the total pore size will too. Organic soils will tend to have large, irregularly shaped particles which fit poorly together (leaving lots of space) while sandy soils have small particles that fit together very tightly. For this reason, volumetric soil moisture content readings will vary significantly depending on soil type.
Tensiometers are simple instruments designed to simulate the amount of "suction" that the surrounding soil has on water molecules (soil water potential). They consist of a ceramic cup at the end of a tube with a pressure sensing dial (or other recording device) on the end. Water flows back and forward between the ceramic cup as it equalises with the surrounding soil. This flow creates positive or negative pressure which can be read by the pressure sensor. Readings of soil suction are in hPa with a zero reading for saturated soils, to about 1000 hPa for very dry soils.
Like tensiometers, gypsum blocks when buried, equilise with the surrounding soil water, giving a measurement of soil water potential. Electrodes are encased in the block and changes in electrical resistance measured as soil water increases and decreases. Gypsum blocks have traditionally been short lived as the measurement process and burial conditions degrade the sensor, however newer model blocks contain circuitry to lengthen their useful life.
Time Domain Technology (TDR)
Unlike tensiometers or gypsum blocks, time domain methods measure the volumetric soil water content (%) of a soil. They are based on a specific property of water (it's dielectric permittivity) that is so distinct that, when measured in soil, provides a very accurate water content reading. The volumetric water content can be used as is, or converted to mm water content or deficit by accounting for local soil conditions. A range of portable, datalogging and wireless solutions use this technology.
Available Water Capacity (AWC)
The amount of water in mm that a soil can hold without runoff occurring. Due to differences in particle size, this is different for different soil types. The following lists values that can be used as an estimate: