From what I have read, heating the soil in the microwave or an oven can sterilize the soil by killing all the bacteria/fungi in it with heat. It can also kill weed seeds that might be in the soil. However, some thermophilic (thermo means heat, philic means loving, so, heat-loving) bacteria can survive this. Some farmers use steam to sterilize soil, which can be more powerful. From what I've seen, there are lots of methods of doing this including putting clear plastic over the soil to heat it in the sun. Apparently even ancient Egyptians and Indians did this, but not with plastic which was only invented fairly recently.