Neutron probes are another way to measure soil moisture content. A probe inserted in the ground emits low-level radiation in the form of neutrons. These collide with the hydrogen atoms contained in water, which is detected by the probe. The more water content in the soil, the more neutrons are scattered back at the device.
Neutron probes are very accurate measurement devices when used properly but are expensive compared to most other measurement methods and generally have to be registered with the federal government due to radioactive elements used to emit the neutrons.