Soil sterilization techniques

Several machines have been developed, based on chemical or physical solutions, which can eliminate pathogens, parasites, and even viruses and bacteria in agricultural soil, particularly in protected crops. Promising new alternatives are also being studied, including the use of UV light, or ozone

In Italy, greenhouse cultivations are gradually expanding, both for the higher income they can assure compared to open field production, and because of the market need to have at disposal for extended periods of the year products that would otherwise not be possible to cultivate in an open field, basically for climatic reasons. A hot and humid climate is typical in the greenhouse, crop rotation is usually minimal (if not absent), and the planting density is always extremely high. These conditions promote the proliferation of numerous pests and diseases. The application of strict hygiene measures thus becomes an essential step in the cultivation cycle since they are the only ones allowing to fight viruses and bacteria. Greenhouse disinfection is not always strictly necessary, and in recent times techniques have been developed to deliberately make microorganisms antagonistic to pathogens increase in the soil. In most cases, however, periodic sterilization cycles of the ground are still an obligatory practice. In some cases, it can be highly counterproductive because eliminating all organisms residing in the medium may leave the door open to the first pathogens that will reappear after the treatment.

 

Implementation modes

Soil sterilization can be carried out in different ways, which can be classified into two macro-categories:   chemical" methods, which involve the use of specific molecules to more or less selectively eliminate the pathogens (and possibly also the seeds and rhizomes of weeds), and "physical" methods, which mainly use heat to inhibit microorganisms. This second category includes, for example, the traditional technique of "solarization", which stands out for some qualities, i.e., economy, absence of residues and deficiency times. However, it requires longer execution times than other solutions, and, above all, it is effective only with conspicuous insolation.

Until the end of 2004, the use of "chemistry" was an almost obligatory option outside the summer period, almost universally using the well-known methyl bromide. With the banning of this (dangerous) compound, mainly for environmental reasons, some alternative formulations have been developed, as well as new physical techniques of soil sterilization.

 

Biocide or phytosanitary

However, the chemical solution is still preferred because it does not depend on weather conditions, and the procedure is usually rapid. Basically, it involves adding disinfectant products with a biocidal or phytosanitary action to the surface layer of the soil. According to a note drafted by the European Commission, when the target organism is harmful to plants or their derivatives, the product is considered a phytosanitary, as long as it is applied directly on the plants or derivatives or indirectly on the empty structure. However, if the disinfectant product used is also harmful in other areas (such as to humans or other organisms), it is considered a biocide. The current list of chemical products authorized as biocides or phytosanitary is in constant progress:  most of them have a fungicidal, nematicidal and bactericidal action, but some also act against viruses. In any case, in order to increase their efficiency, it is imperative to adopt the correct distribution techniques. In many cases, a drip line is placed above the ground, or, more simply, the same irrigation sprinklers are used (then watering immediately afterward to bring the active principle in depth). However, only with the help of specialized machinery is it possible to maximize the effectiveness of the intervention for uniform incorporation in the layer of soil concerned.

 

Sterilizers

Most modern sterilizing machinery has been designed to optimize the distribution of the so-called "fumigants," i.e., products in liquid form that gasify when introduced into the soil, "suffocating" the pathogens. Sterilizers are usually equipped with stainless steel tanks and hydraulic fittings in order to resist to the high chemical aggressiveness typical of most fumigant products. Through a dense network of anchors (similar to those of the standard ripper, but hollow inside), they inject the active principle under the soil's surface employing specific pumps to avoid harmful dispersions of the product in the gaseous state into the atmosphere. A compacting roller is usually placed just behind the perforated anchors in order to slightly compress the surface layer of the soil to decrease its permeability. A device that spreads a plastic film mulch completes the entire system to keep most of the distributed product at the ground level for as long as possible. Finally, some models can be equipped with special toothed wheels to break up the soil before the intervention of the anchors for distributing the product. With its New Velox range, Oliver Agro of Engazzà from Salizzole (VR) is one of the two Italian companies that are particularly active in this sector. The other is Roter Italia of Ostiglia (MN) with its Deeper-into series.  A further alternative, developed in even more recent times (and especially widespread in the field of organic crops), provides that fumigation is conducted by incorporating brassicaceous plants, such as rape or rapeseed, into the soil. It has been proved that when they are introduced in the soil at the right level of maturity, during their maceration, isothiocyanate develops, a completely biological fumigant poisonous gas. In order to speed up the process and avoid cultivating brassicas directly on the land to be sterilized, some varieties of rapeseed and rape that produce more isothiocyanate have been selected and harvested at the optimal point of maturity, then dried and pelleted. Traditional fertilizer spreaders are used to distribute this product, followed immediately by a pass with a hoe to incorporate the pellets. Lastly, it comes watering and/or spreading a plastic mulching film.

 

Solarization

It consists of using the sun's heat to sterilize the soil. Usually, a black plastic film is laid in colder climates and transparent in warmer ones. In addition to sterilization, the lengthy period of exposure to the sun also promotes the decomposition of plant matter, which benefits the availability of nutrients for the following crop. More than actual sterilization, solarization produces a sort of pasteurization of the soil. It purifies the soil from most pathogens, but sometimes it is not highly effective on some herbaceous perennial weeds.  Another promising technique is the anaerobic disinfestation of soil, already studied in Holland and Japan since the nineties. It is, in fact, an integrated solarization, which involves the preventive creation of solid anaerobic conditions. Plastic sheets cover the soil saturated with water and organic material to cause a significant reduction in oxygen content.

Steam

For the same purposes (but in a much shorter time...), it is also possible to use sterilizing machinery that injects water vapor. These solutions are also allowed in the biological specifications, but rather expensive from the energy point of view, for the use of fuels such as diesel or LPG, which with combustion are the heat source to generate steam, which is injected up to 20-25 cm deep, and incorporated into the soil with a hoe. Again, a plastic sheet is placed on the sterilized surface immediately after treatment. An example of this solution is the Biovap of Ingauna Vapore of Castelbianco (SV), while a variant on the theme is proposed by Celli of Forlì, which with the well-established self-propelled Ecostar wich uses the Bioflash method, in addition to steam, previously inserts common quicklime in the soil. Once moistened by steam, it produces an exothermic reaction that keeps the first 20 cm of soil between 65 and 80°C for about 20 minutes. In this way, a significant energy saving is achieved compared to the use of steam alone. Even in this case, the treatment performed is conceptually more similar to pasteurization rather than total sterilization, although it is remarkably effective on most parasites with a lethal threshold below 80°C.

 

Other techniques

 An alternative solution for sterilizing involves using microwaves, already studied almost twenty years ago by the University of Padua, and taken up more recently, for example, by the Dutch Koppert Machines BV, with the Agriton model. The problem (still not solved) is not related to the effectiveness of the sterilizing action but rather to the orientation of the microwaves only on the treatment area, without dispersing them in the surrounding environment. A further alternative is the exploitation of UV rays, with which, however, the sterilizing effect is only superficial, without the possibility of going deeper.  Conversely, the use of massive quantities of ozone dissolved in water is undoubtedly promising. This is an opportunity on which numerous experiments have been conducted, and some commercial solutions are already available. However, at present, there is no European legislation regulating this technique in agriculture, except for limited use in the conservation of products in cold rooms, which are, nonetheless, closed environments.