Gardening Methods: Sterilization
Sterilized seed and potting composts have revolutionized greenhouse techniques and made first-class results available to everyone. Soil sterilization can also be invaluable in the garden and in particular for the vegetable plot. Nowadays, a wide range of sterilized composts is sold in garden shops. However, you should realize that once the containers are opened they may soon lose their sterile nature, unless certain precautions are taken. Bags should be tightly closed immediately after anyhas been removed. Alternatively, the compost may be transferred to clean plastic drums with lids. Small dustbins are useful for this purpose, and convenient to keep in a greenhouse or potting shed. If you make up your own seed and potting composts, make sure that the ingredients are clean and sterile.
It is not a difficult matter to sterilizeas well as compost ingredients. Outdoors, a solution of the chemical formalin is generally the most effective and least troublesome method. Formalin can be bought at most good garden shops; the label on the container will give full instructions about diluting it with water (generally to give a 2% solution). It can be used on soil both outdoors and in the greenhouse.
When using formalin to sterilize open ground, you must first clear the soil of any wanted plants. The best time to sterilize is in late winter or early spring, but sterilization can be carried out before your sowing and planting programmes, provided that you leave enough time between treating the soil and growing crops in it for the fumes to escape from the soil: outdoors three weeks should be adequate, but under cover allow at least six weeks.
After you have diluted the formalin, apply the solution generously to the soil so as to wet it thoroughly. The soil should be moist—neither wet nor dry— before applying the formalin. Afterwards, lay some sheets of polythene or some other impermeable material over the soil surface so that the fumes are retained overnight. This is obviously more important when open ground is being treated. After about 48 hours, remove the covering, and dig over the soil lightly to help the fumes to escape, now that they have done their work.
Formalin fumes have a characteristic pungent, irritating odour, and you should make every effort to avoid inhaling them. When used as instructed, however, formalin is not dangerous.
Diluted formalin can also be used to sterilize pots, boxes and other garden containers, also tools, by simply immersing them for 48 hours and washing them thoroughly afterwards. If you treat clay pots in this way, you must not use them for a week to allow all fumes to clear.
Sterilization with sulphur dioxide
Provided it is empty and all wanted plants are removed, a greenhouse can be safely sterilized by fumigation with sulphur dioxide gas. This will not penetrate soil very far unless it is very dry, but it is useful for killingthat may be concealed in the structure and framework. Sulphur dioxide fumigation is particularly useful when a greenhouse has been out of use for some time or before you start a new crop of a pest-and-disease-prone plants, such as tomatoes. Its great advantage is that planting can begin almost immediately after fumigation.
The gas is produced by burning flowers of sulphur or special sulphur candles, obtainable from good garden shops and horticultural specialists. To aid ignition, you can mix the flowers of sulphur with some wood shavings. About 0.45 kg (1 lb) of sulphur will be needed to fumigate 28 cu m (1,000 cu ft) of greenhouse. The candles should be used according to the manufacturer’s instructions. You must take special care not to inhale any sulphur dioxide fumes, as these are poisonous and extremely irritating, even in small amounts. As soon as the sulphur is alight, make a hasty retreat and see that no-one enters the greenhouse until the next morning when the fumes will have cleared. Ensure that all vents are closed tightly or sealed before you start fumigation.