P.J.H., Waverley.

Reuse potting mix safely. I fill a 3 litre plastic ring basin with the old mix (moist) and give it 12 minutes on high in the microwave. Give it a stir at about 10 minutes. I test with my big thermometer and if its reading over 90 degrees so all the nasties have been zapped. If you don’t have a suitable thermometer give it 15 minutes in the microwave to be sure. This is much better than fresh potting mix for sowing seeds in just as it is . But for planting out plants some slow release fertiliser needs to be added.