Till the existing soil as deeply as possible first. Then add about 4" of compost, and a thin layer of perlite, and till as deeply as possible again. I generally have to re-till the edges a third time.
Any organic material you add to soil is a good thing. It will reduce dependency of fertilizers as it encourages micro-organisms in the soil and of course worms. Just don't use raw materials. Let it age for 6 months first. Otherwise it will consume the nitrogen in the soil as it decomposes and starve your plants, making them severely anemic.
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