The contents of free radicals in preparations of humic and fulvic acids extracted from virgin and plowed podzolic, surface gleyic podzolic, and peaty podzolic-gleyic soils were determined. The concentration of paramagnetic centers in the humic acids was 1.5-2 times higher than that in the fulvic acids. The agricultural use decreases the paramagnetic activity of the humus compounds and promotes the accumulation of biothermodynamically stable organic compounds in the plow horizons.