Steam sterilization for garden soil
Using steam to sterilizeis the best and most effective method, but it is essential to control the temperature and other conditions carefully so that beneficial soil organisms are not killed as well as the harmful ones. If these useful organisms are destroyed, the soil becomes infertile and plant growth will be poor and slow.
In large commercial greenhouses, steam pipes are installed as permanent fixtures, if crops are grown in the ground soil. If you need to sterilize your greenhouse soil regularly, and you make up your own seed and potting composts, it is worth buying or making a steam sterilizer. On a very small scale, it is possible to use a saucepan and cooker.
In all cases the soil must be reasonably fine and dry and should not have had fertilizers added before sterilization. Also, made-up potting composts should not be sterilized—only the separate ingredients. Peat and sand do not normally need to be sterilized.
The basic principle of most steam sterilizers is that water is boiled in a suitable container and the steam is allowed to pass through or around the soil or other material to be sterilized, into which a thermometer is inserted. Ideally the soil temperature should be raised to about 82°C (180°F) as quickly as possible and held at this temperature for about 10 minutes. You should then turn out the soil onto a flat, clean surface to allow it to cool.
A home-made sterilizer can be constructed from a thoroughly cleaned oil drum or similar watertight container provide with a lid. Place this on an oil stove or gas ring, and pour a few inches of water into the bottom, which will boil quickly to produce the steam. Put the soil or other material to be sterilized into a hessian bag and suspend over the boiling water by tying it securely to a rod wedged in the drum. Insert a thermometer through the bag near the top so that it can be easily read by quickly removing the lid.
For sterilizing small quantities of soil, boil 1.5 cm (¾”) of water in a saucepan and add the dry soil to the brim. Simmer for about 15 minutes and turn out to cool.
Sterilizers of different capacities can be purchased. These are usually electrically heated and consist of a water boiler on which is placed a closely fitting metal box with wire grid at the bottom. The steam passes through this grid and a thermometer is inserted at the top, through the lid.
A few sterilizers are designed to work on the principle that moist soil heats up when a current of electricity is passed through it. In this case the soil must obviously be put into the sterilizer in a moist condition.