This part of ISO 23753 specifies a method for determining the dehydrogenase activity in soil using 2,3,5-triphenyltetrazolium chloride (TTC). It is not applicable for determining the dehydrogenase activity in the upper layers (L, F, H horizons) of forest humus forms with low microbial activity (e.g. mor), or in soils showing reducing properties (e.g. waterlogged soils).