TDR is used for soil moisture measurement and is based on the change in the soils permittivity or dielectric constant with varying soil moisture. It uses a transmission line surrounded by the soil along which an electromagnetic pulse is propagated. The propagation velocity and therefore propagation time is determined by the bulk dielectric constant of the surrounding soil.
Used for measuring soil water content and electrical conductivity [1].
Commercial.
A good overview of the TDR measurement technique for measuring soil water content and electrical conductivity [1].
1. Noborio, K. (2001) Measurement of soil water content and electrical conductivity by time domain reflectometry: a review. Computers and Electronics in Agriculture, 31(3), 213-237.