This past growing season was the first attempt I've made at gardening. I've been thinking about how I'm going to do it next year. I want to create raised beds with cement blocks. One problem I'm having is that the soil where I live is heavily clay.

Here's my question: Can I fill my raised beds completely with purchased topsoil or do I still need to mix it in with the native soil?

Thank you

Depends how many blocks high you are going. Most vegies won't put down roots which are over say 10" deep. If it's only a single block then some prep' of the clay would be necessary but for two blocks high i'd just cover the clay with gypsum and chicken manure, then a layer of newspaper or cardboard to kill off any persistent weeds like couch grasses. Maybe a layer of sand above that, then your imported compost/soil. Worms and other critters will tranfer the layers and do your deep digging for you over time.
Bear in mind that you will need to ensure there is drainage out of the beds or your vegies may end up swimming during very heavy rain and the soil will become sour. So weep holes between every second block and/or perforated drainpipe below the beds would be a good idea..

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The secret is the soil.