Tomatos in Compost Bags rather than Grow Bags

Multipurpose compost should be fine. You won't need to add any fertilizer - just feed as usual when the flower trusses set.

I grow mine in MPC tipped into pots (flower buckets).

i might be wrong but the compost in the either bag wont be too different,the compost bag nourishment will easily last til the first truss sets, and then i would use tomato fertiliser as normal,with that size bag i would bury the young plant up to,or even take off the lowest branch of leaves and bury it as low as possible giving the maximum amount of roots to fuel a great crop,i did something similar some years ago with bags from the previous year,given to me by someone who was moving,on their sides, edges,just 2 toms per 60ltr bag,they grew over 8ft tall and had way over the recommended amount of trusses,but old raymond next door kept saying that you will see when they run out of steam,so i left them and to this day we have never had such a big crop,30 years on,i would probably have difficulties just lifting the bags,and with the black loam in the garden/greenhouse,nearly 3ft deep now,the toms grow well enough to keep the two of us in toms/pasta sauces etc for the year,so give it a go,it worked for us....you will need a bigger freezer though.