The thermalised neutron method involves the emission of high energy neutrons into the soil and the detection of lower energy (thermalised) back scattered neutrons.
Since hydrogen is the most effective at scattering and slowing the neutrons this method correlates well to soil moisture content. This technique is employed by the commonly used neutron probes.
Requires an active neutron source.
Commercial.
A good overview of field measurement of soil moisture using neutron probes [1].
1. Chanasyk, D. S. & Naeth, M. A. (1996) Field measurement of soil moisture using neutron probes. Canadian Journal of Soil Science, 76, 317-323.