Leanne,
Humus is the organic part of any soil and providing that it hasn't any unnatural fertilizer added,it should be ok.
This fertilizer is usually seen as small white or yellowish nodules in the soil.Perlite is also added to some composts,this is also a small, round and white polystyrene type material and also should be avoided incase of any newts etc swallowing them
Damian