Compost is a rich soil amendment that often makes up at least a third of many seed starting mixes and homemade potting soils. While the high temperatures in the compost pile kills many weed seeds and disease organisms, some may still survive. Sterilising the compost before use in soil mixes will neutralise any remaining harmful elements. Sterilising also allows you to reuse compost mixes with no worries of spreading plant diseases to your new plantings.
Check the compost for any large, uncomposted elements, such as twigs or chunks of wood. Remove these and return them to the compost pile to finish breaking down.
Preheat the oven to 93.3 degrees C. Move the oven rack to the lowest position inside the oven.
Fill a roasting pan with compost to a depth of 4 inches. Cover the pan with foil, crimping it around the edges for a tight seal.
Stick an oven thermometer into the soil. Push it through the foil covering into the centre of the pan.
Cook the compost until the thermometer reads 180 degrees. Continue cooking for 30 minutes at this temperature.
Remove from the oven and allow to cool before using in your soil mixes. Keep covered with the foil until you are ready to use it so it remains sterilised.
Use the compost in a seed starting mix. Mix one part compost, one part peat moss and one part perlite. Sterilise small amounts of compost in the microwave. Moisten 0.907 Kilogram of soil and microwave on high for 90 seconds. Use this method to sterilise mixed potting mediums or garden soil that have been used before. Line your pan in foil first or use a disposable pan to sterilise soil.
Do not allow the temperature of the soil to go above 200 degrees while it is cooking. Watch the thermometer closely during the entire heating process. The compost may smell during and after sterilising. The odour may be unpleasant depending on how mature the compost is. More mature compost smells less.