There are several ways to measure soil moisture levels, but these four methods are the most common ones:
- Electrical resistance blocks
- Dielectric constant methods
- Neutron probe units
1. Electric resistance blocks
These are simple, cheap and easy to use. The electrical resistance is measured by an electronic circuit connected to a block containing Gypsum material, which is a very soft mineral. The block needs good soil contact usually secured by digging a hole and making a slurry from the water and the dug up soil and pouring it back into the hole, where the block has been placed.
Although the block will last for years it will deteriorate over time compared to other soil moisture meters.
Equivalent to “mechanical roots”. They measure how hard it is for the plants roots to extract water from the soil. A moisture release curve is needed to make a correct measurement.
3. Dielectric constant methods
Time Domain Reflectometry, TDR:
Two metal rods are inserted into the soil and electronics measure the dielectric constant of the soil by looking at wave propagation. Because of the relationship between the dielectric constant and soil water content the soil moisture can now be calculated and shown on the meter.
Frequency Domain reflectrometry, FDR:
The soil is used as dielectric in an oscillator circuit. Changes in moisture levels causes changes in dielectric values and this will change the operating frequency of the oscillator circuit, resulting in a change on the soil moisture meter display.
4. Neutron probe units
These contain a radioactive neutron source, and when the neutrons are reflected by hydrogen in the soil, you will be able to measure water content in the soil. This type of meter will probably require a licence due to the radioactive source in the meter.