How to make seed compost from supermarket potting compost

[Dom Tuk]

is it just a matter of adding equal part sand to equal part gardening/potting compost?

[heathfield]

Some info on the constituents of 'John Innes' composts here

[hammerman]

I think the difference is usually in the grading of the compost....seed compost is finer, and the nutrient content.....seed compost has less nutrients which could burn young seedlings.

Mix some silver sand and peat (equal quantities) to your compost and then mix well.....put through some sort of sieve such as a kitchen colander.

Hope this helps