Clearing out your greenhouse for the winter gives you a chance to clean it and let in more light, but also to look after the soil. If you grow tomatoes and other crops in grow-bags or containers, don’t put the peat from them in the bin. It is a scarce and dwindling resource that should be used to the full; use it to improve the soil elsewhere in your garden, or mix it with soil for planting trees and shrubs. Alternatively, give it to a neighbour who can use it.
The organic matter in greenhouse soil decomposes quickly in the warm conditions. You need to replace it or the soil becomes hard and cloddy, and plants grow poorly. Sowing green manure, digging it in and waiting for it to rot down in the soil delays the next crop; the quicker option is adding well-rotted garden compost or other organic matter. Level the soil and spread the compost on top. You could leave it there and let nature take its course, but read up on no-dig gardening first. If digging it in, turn it under with a spade. Work slowly to avoid damaging the sides or cladding of the greenhouse. A small greenhouse is one of the few places where a short-handled back-breaker spade might be better than a proper long-handled one. Leave it to settle for a few weeks before sowing or planting in another crop.