Chemical Soil Sterilization
Various chemicals are used for this purpose, and in commercial spheres these have become extremely popular for economic reasons. They are also useful for the amateur gardener, but great care must be taken to follow the directions implicitly. The fumes, however, can be toxic to young plants, which raises problems inwhere there is mixed cropping.
The chemicals, to be effective, must give off gas which is sufficiently toxic to ensure the destruction of pests, diseases and weeds, yet without harming the beneficial organisms in the soil. The efficiency of chemical sterilization varies greatly, not because of the precise chemicals involved, but because of the condition of the soil, its moisture content and, most important, the prevailing temperature. You should also ensure even distribution in the soil to be sterilized.
This is reasonably effective on some fungal diseases, but relatively ineffective against many pests. It is used at a strength of 1 part formalin 38 — 40% to 49 parts water (approximately 0.5 litre in 25 litres or 1 pint to 6 gallons) and the soil is thoroughly drenched — approximately 23 litres (5 gallons) being required per m2 (square yard) to 23cm (9in) depth. There is a wait period of 20-40 days according to temperature (which must be sufficiently high otherwise the formaldehyde will become polymerized) before the soil can be used.
Phenols, Cresylic Acid etc.
Various forms are available such as ‘Sterizal’ (phenol), Brays Emulsion and Armillatox (cresylic acid), also Jeyes fluid (tar oil plus vegetable oil). They should be used according to directions, noting the ‘wait’ period and using enough of the mixed material to thoroughly drench the soil or growing media. These materials vary in their effectiveness against pests and diseases and are possibly more effective against pests with some fungal action.
Other chemicals used commercially or by approved contractors are Chloropicrin, Chloropicrin + Methyl Bromide, Dazomet, Basamid (granular Dazomet), D-D Fumigant (mainly for eelworm), 1, 3 — Dichloropropane Metham Sodium. Of these, Basamid (Dazomet Granular) is probably the most widely used as it is in a convenient dust-free prill (a coarse powder) in 5kg packs. It is used at the rate of 1kg per 28-30m2 (1-1/2lb per 20 sq yd). It must be thoroughly mixed with the top 20cm (8in) of damp soil, ideally by rotary cultivator or in heaps of damp soil or growing media spread out on a clean surface in shallow layers. Note the rates of applications can vary. Soil or compost ingredients should be moderately moist and at a temperature of around 9.5°C (49-50°F).
While a form of sealing can be carried out by applying water to the surface, it is better to cover the area or material being sterilized with a clean plastic sheet for several weeks, as this contains the fumes at a toxic level. All these chemicals must be used strictly according to directions, which generally state that the soil should be cultivated, the sterilants applied and, with the powdered or prilled form, rotovated in before ‘sealing’ with water.
To ensure that these chemicals are effective, the temperature of the soil should ideally be above 9.5°C (49-50°F). The soil should be rotary cultivated or forked to release the fumes after a few weeks. Heaps of potting soil can be treated with chemicals in the same way as border soil. Methyl bromide has found favour in recent years because of its effectiveness on pests and diseases and also its rapid dissipation. It must be applied by a contractor. There are recent complaints about water pollution where methyl bromide is used and restrictions on use may be pending.
Checking for chemical sterilants
Cress tests using duplicated wide mouth glass jars are useful techniques for checking whether chemically treated soil is free of sterilizing chemicals. Fill one jam jar with untreated soil and another with treated soil. Scatter cress seed lightly in both jars and seal them, or suspend wet balls of cotton wool dipped in cress seed on wires above the soil. In both cases compare germination.
It is stressed that most of the chemicals mentioned are not intended for use by amateur gardeners. Certain chemicals require a certificate of competence on the part of the user before they can be obtained and used.