ISO 23753-2:2005 specifies a method for determining soil dehydrogenase activity using 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT). As the INT reduction is less sensitive to O2, the method is more reproducible than the TTC-method described in ISO 23753-1. It is not applicable for determining the dehydrogenase activity in 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).