I have had an 8' x 8' greenhouse for many years and the walls sit on a hardcore and cement base, leaving it possible to plant straight into the earth. The first year I planted into the original ground with loads of compost dug in. Thereafter, each winter I have dug out all the topsoil and distributed it about the garden, as a soil improver, and replaced it with bags of compost and the contents of about twenty growbags. This has worked out very well for me as I have never had any disease in the greenhouse - it's washed down each year before the soil is replaced. Most years I never water my tomatoes (I grow sungold and gardener's delight and both of these should never be overwatered ) as the roots grow to a tremendous depth. In 1995 I had to give in and water them a few times as it was such a very hot tummer.