Apparatus for measuring the dry unit weight of a soil, either directly or with reference to a known soil variable (e.g., the Proctor density) by means of a measuring probe containing two electrodes in the soil under investigation. The measuring probe is energized with a constant a.c., voltage, and is connected in series to a measuring resistance at which a voltage drop occurs proportional to probe current (I). The voltage tapped at measuring resistance overlaps a continuously adjustable part of the said constant a.c. voltage, and, in a phase-sensitive rectifying indicating device, a proportion, continuously adjustable from zero, of the sum voltage obtained by the superposition is superposed on a second component, continuously adjustable from zero, of the constant a.c. voltage.