Abstract <p>The influence of electrical properties of the soil on the results of electromagnetic soundings using TEM (Transient Electromagnetic) method with combined receiving and exciting antennas is studied. During three summer seasons, monitoring measurements of transient responses in the microsecond range were carried out at four sites within the same geological structure composed of moraine deposits. Simultaneously , air and soil temperatures as well as capacitance of a two-wire line placed in the soil to a depth of 10 cm were measured. The diurnal cycle of variations in the capacitance of “soil capacitor” and TEM responses were recorded. During the early afternoon, capacitance reaches its maximum, while TEM responses reach their minimum. At night and in the morning, the picture is mirrored: the capacitance is minimal while the transient responses are maximum. The diurnal cycle of these parameters with 5–15% day/night variations does not depend on the amount of precipitation, but correlates well with air temperature and solar radiation. It is assumed that the detected diurnal rhythm of variations is related to biological processes in the vegetation. This hypothesis is confirmed by field and laboratory studies of plant biorhythms under changes in temperature and illumination. In each field season, there were several periods when the intensity of polarization processes anomalously increased. These processes are induced in the near-surface layers by electric potentials arising in the antenna circuit with distributed conductivity and capacitance when current pulses are transmitted through the antenna (antenna polarization effect). During these periods, the transient responses inverted the polarity at late times in early afternoon and returned to positive values at night. Trains of variation fluctuations with a diurnal period were observed for several days to several weeks.</p>