It's not my garden, it's Peter's and it's not really a garden but a number of planters on a balcony, hence he will be buying pricey potting soil by the bag rather than a few cubic yards of top soil plus amendments. If, as I suspect, it's a matter of bad fungi, heating the soil to 180 degrees will kill the fungi and adding some rich compost and nutrients will do the trick. It's up to Peter whether he would rather try to rehab the soil or toss it out.