What we have is fill over swamp and on this did start building my soil, with tons of woodchips, compost, leftover potting soil and so on. The soil test I did two years ago came out quite reasonable but my garden could grow better. I reckon that (apart from my new compost brew machine) more minerals, in general, would help. I know that there are basalt mines which are 'paramagnetic', because boral did some research in the past on that but they dropped it (they were able to do enough business with the building boom). That particular mine is a smaller company.