Importance of Sterilizing Soil
Sterilization of Soil
The advantage of using a growing medium free of damaging pests, diseases and weeds cannot be over-emphasized in all spheres of greenhouse culture.
Soil sickness is a term which is difficult if not impossible to define accurately. Organic gardening adherents claim thatsickness can be avoided by practising organic husbandry principles, so maintaining a soil ‘in balance’. While this is an admirable solution, it gives rise to practical problems. Copious and regular supplies of properly made compost must be available, and even then there can often be a ‘wait’ period before the considerable forces of nature exert their influence, this taking longer than many gardeners have patience for.
Crop rotation principles for border grown crops would obviously do much to prevent the soil becoming sick in the .first case, yet without a mobile or easily portable greenhouse, this cannot readily be accomplished. The use of a new soil or growing medium does of course achieve the same object, and provide the plants with an ideal start. This is why, under many circumstances, a strong case can be made out for container-growing of crops such as.
The other answer is to pasteurize the soil, endeavouring to rectify the ills, while leaving behind beneficial elements to function to full advantage. The term sterilization is most frequently used in this connection, whereas partial sterilization or pasteurization would be more accurate.
Sterilization implies rendering the growing media completely sterile, which would give rise to many problems with a soil-containing medium. Nevertheless it is a term which will be used for convenience.
Sterilization of soil or soilless media can take several different forms, all of which have their limitations and drawbacks. The sterilized medium is not rendered immune from future trouble; it may in fact be more prone to invasion because competing organisms will have been killed off in the sterilizing process. But heat sterilization in most of its forms does help to render plant toxins soluble, so that they can be washed but, the same being true of toxic fertilizer residues; organic matter is usually broken down by heat sterilization, rendering it more palatable to micro-organisms as a ready source of food. Perhaps the most significant advantage is in the survival of the thicker-walled spore forming ammonifying bacteria which, in the absence of competition, re-establish themselves completely uninhibited, resulting in a great flood of ammonia.
One vital aspect of sterilization is that there are limits to the degree of penetration achieved over the growing medium. In the case of a greenhouse border, it is only practical to sterilize the top 25-30cm (10-12in) of soil; if the lower depths contain pests and diseases they can act as a potent source of trouble for deep rooted crops. Virus disease of, for example, can remain in the lower depths of a greenhouse border, ready to infect a subsequent tomato crop, even when the top 25-30cm (10-12in) is sterilized, as the roots soon penetrate into the subsoil. Sterilization can only be considered as 100% efficient in connection with culture in troughs and containers, preferably when the ingredients of compost are being prepared for formulation — and even then contamination from airborne or pests cannot be either ensured or ignored.
Sterilization by heat
Most pests, diseases and weeds in both vegetative and seed form have a temperature at which they are organically destroyed, the same being true of micro-organisms. The temperature aimed at during partial sterilization, while taking into account the destruction temperatures of various organisms, also bears some relation to the sheer practicality of applying a certain temperature range.
The boiling point of water is 100°C (212°F) and it is neither practical nor desirable greatly to exceed this. It is in this temperature range that most sterilization processes operate, and recourse to a detailed reference book on fungal and virus diseases will show that such a temperature range is highly effective. Indeed many of the weaker parasitic diseases are destroyed at temperatures as low as 54-60°C (130-150°F); but many virus diseases require fully 92°C (200°F) to be completely destroyed, the same being true of the eelworm cysts Globodera rostochiensis.
One of the greatest problems when applying a heat process to a soil or growing media is achieving a uniform temperature throughout the bulk. It merely requires the centre of a lump of soil to remain at low temperature for it to act as a potent source of re-infection for the sterilized area of the growing medium. Over-sterilization also raises problems of sterility and formation of toxic substances. Obviously too it is undesirable to incorporate doubtful bulky organics, especially farmyard manure, until they also have been sterilized although it is common practice not to sterilize them.
The moisture content of a growing medium will greatly affect the passage of heat and the degree of penetration achieved in a given time.