Hi Elaine, don't be confused as this isn't a simple question and needs a some what in depth understanding. What is top soil? Some are prefabricated from screened soil taken during house construction when the surface soil is taken away and sold separately in order to make more money and this can then be padded out with all sorts of other by products to again increase profits. Some of this padding includes shredded bark from the forestry industry etc. I use leaf mold which is the top inch or so from the beech woodland floor. You have to be sensitive here and cause minimal damage to the ecosystem. Pine based soils are acidic and may harm amphibian skin. You also have to be aware of possible contagions. It works for me very well. I'd not use top soil no matter. I will use a product (UK) called Irish moss peat which contains no additives as is a natural product from 'sustainable' sites though I am conscious about this coming from managed wetlands. I do feel guilty there but justify if based on my animals' needs. I mix it with semi decomposed beech, oak, sweet chestnut leaves. I'll see if I can find some info. Try putting your search into Google Scholar.
