Although not commonly recommended, it is possible to reuse old potting soil for seed starting and transplanting. Most techniques to "clean" old potting soil are not environmentally friendly or cost effective. But recently, the California Department of Food and Agriculture has developed a soil solarization technique that is safe and effective. The double-tent system can heat potting soil to 140° F, killing weed seeds and some disease spores as well.
To build the solarization tent, lay black plastic on a solid foundation. Lay a wooden pallet on the foundation and place the old potting soil in pots, bags, or piles so it is no more than 1 foot deep. Water the soil well, and cover with a sheet of clear plastic sealed at the edges to prevent air leakage. Cover with a second sheet of clear plastic supported by wire hoops so there is a small air gap between the layers, and seal the edges of this sheet as well. Under sunny conditions with ambient air temperatures of 70%deg; F, one to two days of heating should be enough to properly treat the potting soil. With cooler temperatures and cloudy weather, treatment will take longer.