An experiment on soil moisture remote sensing over bare, grass and alfalfa fields was conducted during July--September 1981 with 0.6 GHz, 1.4 GHz, 5.0 GHz, and 10.6 GHz microwave radiometers mounted on mobile towers. Ground truth acquisition of soil moisture content, ambient air and soil temperatures was made concurrently with the radiometric measurements. Biomass of the vegetation cover was sampled about once a week. Soil density for each of the three fields was measured several times during the course of the experiment. The results of the radiometric measurements confirm the frequency dependence of moisture sensing sensitivity reduction reported earlier by Kirdiashev et al. (1979). The present work extends the frequency range of earlier measurments down to 0.6 GHz. Observations frequency, a result contrary to the expectation based on the estimated over the bare, wet field show that the measured brightness temperature is lowest at 5.0 GHz and highest at 0.6 GHz dielectric permittivity of soil-water mixtures and current radiative transfer model in that frequency range.