Soil properties we divide in moisture, temperature, soil heat flux, conductivity, and thermal properties: soil thermal conductivity (also soil thermal resistivity).
From a meteorological point of view moisture and temperature are the most important parameters. The WMO recommends to measure soil temperature at various depths: 5, 10, 20, 50 and 100 cm.
The measurement is usually done using specially sealed and quite robust sensors. For special sensors to measure high accuracy thermal gradients in the soil see Hukseflux.
The measurement of soil moisture is a heavily controversial subject. Fist of all there are different groups advocating the use soil moisture content measurement and others preferring water potential measurements (the latter is the work needed to extract water from the soil).
For soil moisture content the reference measurement consists of weighing soil samples before and after drying. This technique obviously involves too much work for routine use. Special equipment see Soil Moisture.
A second possibility is by radiological techniques, neutron scattering or gamma attenuation. Usually these techniques involve radioactive samples and require careful use. Increasingly popular are techniques based on the measurement of dielectric properties of the soil; Time Domain Reflectometry (TDR) and High Frequency Impedance techniques. Suppliers are Campbell Scientific, Soil Moisture, IMKO and Delta T.
For systems for non-scientific purposes, irrigation in agriculture, mining and turf, look at Sentec.
For measurement of water potential, the usual technique is called a tensiometer. Manufacturers are UMS and Soil Moisture. Tensiometers usually have low purchasing costs, but should not be exposed to below 0 temperatures.
Upcoming technology: soil psychrometers. No manufacturers known yet.
Soil heat flux is often measured in Evapotranspiration and Bowen Ratio systems to "close the energy balance". For heat flux sensors or heat flux plates see Hukseflux.
Soil conductivity (electrical) can offer a clue to salinity or dissolved nutrients in the soil. See Delta T.
Soil thermal properties like soil thermal conductivity, soil heat capacity are useful for surface energy balance measurements. In fact most soil energy balance models require the input of soil volumetric heat capacity. Also there is some redundancy when soil moisture is measured. See Hukseflux.
The inverse value of the soil thermal conductivity is also called soil thermal resistivity.