As in plants the basic difference in measurements is between soil water content and water potential. Again, they are generally correlated but the relationship is unique with every soil. Unfortunately, the relationship is not linear and has some rather sharp changes when the water potential the plant senses can change rapidly with only small changes in water content.
Example of the relationship of soil water content to water potential for 3 different soils. Source: NRCCA Soil and Water Management Study Guide.
Soil Water Potential Measurements
For water potential in soils, the classic tensiometer in use for decades, is still the standard principle of measurement as it basically mimics the plant. However, the very limited range of soil moisture it can read and the excessive maintenance required has greatly limited it use by growers. The microtensiometer has a range of up to 100X and overcomes all these major issues.
There are many indirect methods based on measuring a physical principle of the soil and correlating it to water potential. However, all of these have limitations of range or interference due to factors such as soil salinity, soil texture, temperature, etc.
Soil Water Content Measurements
Water content is useful to know amount of water in the soil, how much was used, so how much irrigation is needed to replace the loss. But that does not tell you what stress the plant is sensing and how the plant will react.
The variability of soils across most farms and with depth means that there will be a myriad of unique content-potential relationships. For deep-rooted crops, where the water is available is not easily known. So measuring water content will never be a good measure of crop stress across a farm. For that a measurement of soil water potential is needed.
There are many soil water content estimating methods of many different principles listed in the table below. The “gold standard” is the neutron probe that is based on the reflection of neutrons from a radioactive source by water. It is quite insensitive to interferences. However, it is an expensive manual instrument that is heavily regulated.
Many other instruments are based on varying principles but none are as good as the neutron probe.
Table 1: Comparison of commonly used techniques to measure soil water status