I don’t think this is a good idea.
The problem with potting composts appears to be that several are not doing what they claim, and until using them we do not know which ones are good, which bad. Chempak fertilisers are water soluble, if you add that to an already good compost there are then too many nutrients, so some leach away and growth may be stunted. Bad for the environment, bad for plants.
For me the strange thing is how compost companies appear unable to measure the nutrient flows, or estimate them, when using woody substrates.
