I used to have a 40ft by 8ft greenhouse when i was feeding the nine of us from the garden,veg. and eggs from chickens and ducks and I cultivated the 3ft wide borders either side of the path as i would outside i.e. by rotating the crops so as not to deplete the soil of essential minerals. That and plenty of cow or horse manure gave me magnificent crops for over 20 years and saved me lots of money and trips to the shops. It was a forcing greenhouse from s nursery which closed down and sold its land for housing and so the bottom was brick built. It cost my husband £40 and the dismantling and removing from the site, then he had to build it for me. It paid for itself three times over in its first year and I was winning prizes for dwarf beans etc. at horticultural shows as it was a forcing house! Your trial would be interesting but not wise as , when your crop takes goodness out of the soil you must replace it. Better to give the tomatoes a rest for a year(grow them outside) and grow beans whose nodules will put nitrogen back into the soil.
Gardeners' World Web User
I shuld think, JohnB, that Pippa's suggestion of the top 8inches being renewed every year(just like a fresh growbag really)should work for a few years. A build-up of soil pathogens would be bad news for tomatoes. As every greenhouse border varies in size it is difficult to specify when you should dig right down to the subsoil and replace. The old method of crop rotation seems more sensible to me.