Soil Sterilisation For The Home Gardener

Press, Volume CII, Issue 30127, 10 May 1963, Page 6

Soil Sterilisation For The Home Gardener

A note has been received from a grower in connexion with a recent Gardener’s Query concerning the sterilisation of a glasshouse soil in the vicinity of a grape vine. The grape in this particular case had been planted outside the house so that the roots are outside while the vine itself is trained inside the glasshouse. The house is also used for growing tomatoes. For the last three years the house has received an annual washdown internally with two per cent, formalin at about this time of year. This is followed by sterilising the glasshouse soil with chloropicrin at the rate of five c.c.s. per square foot injected into the soil. This treatment caused the grape to drop its leaves prematurely, but there have not been any subsequent detrimental effects noticed in the cropping or growth of the vine.

What exactly is sterilisation, and why is it necessary to go to all the trouble that it involves?

Soils in which crops are grown for a number of years become subject to a build-up of pests, diseases and weeds. It has been explained several times in the past on this page that many disease organisms which attack plants can remain alive in the soil for considerable periods until such times as conditions again become suitable when fresh outbreaks of the disease will occur.

A wide range of pests live in the soil or spend part of their life history in it. These pests feed on the roots and injure the plant. Over the years weeds will build up in the soil so that at the end of each growing season there is a greater amount of weed seed present than at the beginning of the season.

When the same crop continues to be grown in the same soil, the build-up of these troubles can become so great that it becomes impossible to grow the crop or its returns are so poor that it is not worth while. If the crop is to be grown again, it becomes necessary to either replace the soil or ter sterilise to correct the troubles. Sterilisation will kill only the harmful organisms in the soil and leave the beneficial ones, such as those that break down or game matter, largely unharmed. Sterilisation -of soil that is to be used in potting composts is also desirable as it removes harmful organisms so that plants grown in it will have no adverse conditions to contend with.

In commercial glasshouses sterilisation is a regular annual task, and soil that is to be used in potting is also sterilised. Sterilisation may be carried out either by steam, electrical heating or by chemical means. Large scale sterilisation by steam requires a considerable amount of apparatus, which makes it unsuitable for use in the home glasshouse. How- *

ever, it can be used for sterilising soil that is to be used in potting composts. Small quani'tities can be Sterilised in the kitchen using a large saucepan with a tight fitting lid. About half-an-inch of water is placed in it and brought to the boil. Dry soil (the drier the better), which has been previously seived, is then added until the saucepan is filled. The lid is replaced and the saucepan is allowed to simmer for from 15-20 minutes. At the end of this time the soil is emptied out of the saucepan on to a clean surface and spread out so that excess moisture can escape and the soil can cool quickly. Larger quantities can be sterilised in the domestic copper. A few inches of water tre poured into it and brought to the boil. A sack of dry seived soil is then suspended in the steam, but above the boiling water. Alternatively, a grid can be placed in the copper above the water level and a bucket of soil, or the sack of soil, stood on this. The soil is left in the steam for 30 to 40 minutes, after which it is spread out on a clean surface so it can cool as quickly as possible. While these methods are most suited for potting soil, the latter could be used for the soil in a glasshouse border. The soil could be excavated to a depth of 13 inches and sterilised, a sack at a time; it would be tedious and would only be suitable for small areas.

Chemicals provide an alternative method of sterilising soil, and those in common use today by commercial growers are methyl bromide and chloropicrin. These are efficient in controlling most fungi, insect pests and weed seeds, but are less efficient than steam. They have, however, the advantage of requiring less apparatus and being less expensive.

Both these chemicals are very dangerous and are not available to the 'general public. However, there are qualified contractors who will do the job for you:

There is one chemical that can be used by home gardeners for soil sterilisation and that is formalin. This is unpleasant to use and care should be taken with it. Avoil spilling it on the skin and do not breathe more of the fumes than is absolutely necessary. The soil should be well worked before application. It should be in "a fine state and relatively dry. Remove all plants from the house as the fumes are dangerous to plant growth. The formalin is watered on to the soil as a two per cent, solution at the rate of five pints to the square foot. Afterwards the surface of soil is covered with damp sacks for 48 hours. The soil should not be used for three weeks, longer if there is any smell of formalin noticeable. It is advisable to work the soil before using to ensure all fumes have dispersed.