Imagine you have a sunflower growing in a large pot of soil. When that sunflower dies, how could you reuse the soil?

I read that you can mix it half-and-half with soil from a compost heap, but I don't have one, and my Mum doesn't want one.

Could you mix fertiliser into it? Or what about crop rotation? (We learnt about it at school, but I can't remember how it works).

It seems counter-productive to keep buying more compost, when gardening is supposed to be economical.