making John Innes No.3 ??

I have read that the correct 'medium' for planting my grape vine is "John Innes No.3"

Now I know I could and buy some, however I have plenty of VITAX Q4 compost ... which is a pretty good gp compost.
What could I add (or do I need to add) to convert it to JI.3 ?

 

This site gives the formulae John Innes Composts explained

Its a mixture of 7 parts loam (ie soil), 3 parts peat (ie compost), and 2 parts sharp sand. I think all the other items basically boil down to adding a high potassium feed (ie tomato feed). You can get sharp sand from a garden centre or a builders merchant - but it must be sharp (for the drainage) not builders sand.