Polytunnel soil question

Hi, I was wondering what happens to the soil inside polytunnels (and greenhouses) after repeated use without being exposed to the elements and 'washed' by the rain.
Does it need rotating say with soil from elsewhere on the allotment to keep it fresh, or does the digging in of manure suffice?
sorry if these are obvious questions, but i am ignorant of the principles of ploytunnel growing.
Are there any good on-line resources for polytunnel growing anyone can recommend?

regards

Paul