Fumigation, sustainable technologies for soil pest control
The research in the sector is evolving towards the development of machines for the distribution of products characterized by the widest sustainability. Solarization, biofumigation and steam use are the solutions most frequently adopted
Fumigation disinfects the soil from pests and germs through the diffusion of biocidal gases distributed employing dedicated machinery. The use of the well-known methyl bromide has been withdrawn for many years due to its toxicity, but also the use of the main alternative substances identified (such as chloropicrin and the 1,3 dichloropropene, also distributed in mixtures) has been banned for some time. However, in the absence of real valid solutions, the application of these substances is still authorized by derogation, with temporary measures, for emergency uses and only on some selected crops. In order to make the process efficient and safe (both for the environment and for operators) it is, however, essential to spread on the surface of the treated soil a film that ensures an effective barrier, retaining the fumigant product on site.
In this situation, the research of the sector has been directed towards alternative solutions, such as solarization, biofumigation, the use of steam, and the use of microwaves, but also towards the experimentation of new chemical and microbiological products able to make the affected crop overcome biotic and abiotic stresses.
Solarization
It uses the greenhouse effect produced by solar radiation to increase the temperature of the soil's most superficial layers to eliminate some pathogens and phytophages. Basically, the soil must be covered in the hottest period of the year for at least 4-6 weeks with a suitable cloth (thus bringing it to temperatures not lower than 45-50°C). Sometimes, several geo-pest control products and/or other substances with biocidal action (ammonia, sulfuric compounds, isothiocyanates, etc.) must be distributed along with it.
The soil to be solarized must be carefully worked to a depth of at least 30 cm, brought to field water capacity, and then kept moist for the entire duration of the treatment. The most commonly used materials for covers are polyethylene, ethylene-vinyl acetate, and polyvinyl chloride films, with a thickness of 0,03-0,05 mm. In protected crops, closed greenhouse solarization allows increasing the soil temperature further, improving the effectiveness of the treatment.
Biofumigation
An interesting and fully sustainable option to reduce nematodes in the soil is the green manure of Brassicaceae and some grasses and legumes. In particular, besides providing a significant amount of organic matter to the soil, Brassicaceae - both naturally in their root system and finely chopped - can release compounds (isothiocyanates and nitriles) with biocidal properties against nematodes and fungi in the soil.
The combination with solarization also allows activating interesting synergistic effects. On a commercial level, products based on pellets or flours of deoiled seeds of various species of brassicaceae, to be distributed directly in the soil, or concentrated emulsions containing vegetable glucosinolates are currently available.
Use of steam
It is a well-known method of soil disinfection (and disinfestation), developed in particular by Celli of Forlì, which a few years ago developed the Bioflash method, which combines the action of steam with that of some fully compatible exothermic reaction substances, such as potassium hydroxide, calcium oxide and zeolites, which can also perform other useful corrective actions in the soil, such as pH stabilization and fertilization. Basically, the first 20 cm of soil are heated at 65-80°C for about 20 minutes: This treatment is highly effective against nematodes and weed seeds and at the phytoiatric level, for example, against the salad collar rot (Sclerotinia Minor).
Celli's new machine is the Ecostar, a hydrostatically driven self-propelled vehicle with rubber tracks equipped with a 60 HP diesel engine, combined with an 8 kW three-phase 380 V electric generator, which incorporates a diesel burner combined with a 600 kg/h steam generator, obtained from a 600 l water tank, and combined with a 230-l hopper for the exothermic reaction product. A hydraulically operated hoe works the soil, while a dedicated module spreads the mulching plastic film. The steam is injected into the tilled layer of soil at a depth of about 20 cm, through two bars of about 1.70 m each, while the machine advances very slowly, between 60 and 2500 m/h..
Fumigation machinery
OliverAgro from Salizzole (VR) produces several ranges of fumigators, suitable for both open field and protected crops. The New Velox range uses a series of metal wheels with frontal blades that break up the soil in the surface layer for the entire width of the pass. This also helps to eliminate the compaction caused by the tractor's wheels, which two specific track-breaking anchors anyway provide.
The depth anchors are located just behind the vane wheels, and the fumigation nozzles are placed close to them. Both anchors and nozzles take on different conformations depending on whether light or heavy soils are being treated. On sandy soils, each anchor opens a tunnel into which the product is sprayed using a fan-shaped nozzle. On clayey soils, the nozzle is equipped with two opposing side holes from which the same number of jets are emitted.
The fumigant is taken from the main tank (which can have a maximum capacity of 880 l), where the product in the liquid formulation has been previously loaded with a specific transfer pump by a dedicated volumetric pump, which can adjust the flow according to the speed of progress. As an option, it is possible to install both a flowmeter, which displays the instantaneous flow rate and the total quantity distributed and a series of flowmeters to monitor the regularity of distribution on each locating anchor.
A smooth compactor roller is mounted behind the depth anchors. It is equipped with active rotation using a hydraulic motor powered by a circuit independent of the tractor, whose peripheral speed can be adjusted up to 50% higher than the actual forward speed of the machine. It also provides a particular creeping action so as to better "seal" the soil on the surface (i.e., in the first 2-3 cm), limiting fumigation losses and making the action of the product more effective.
Oliver Agro also produces the Brava 2003 range, a machine for fumigation with liquid products, which, unlike the New Velox, is equipped with a tiller (with a maximum of 36 hoes) that makes it possible to mix better the product in the entire layer worked, followed by a cage roller that evens out the soil and with its rotation moves the volumetric metering pump. The equipment is completed by a second roller, the smooth compactor roller, driven by a hydraulic motor. The fumigant is distributed through 10 fan-shaped nozzles protected by a particular welded sheet metal structure, which operate over the entire working width (up to a maximum of 3 m) and are equipped with anti-drip devices so as not to spill the liquid when the machine is not working on the ground.
On the other hand, the Mirrow range is available for solid fumigants in granular form, with working widths up to 4 m. The product is loaded into a special hopper. Through a series of micro-granulators, it is buried by flexible ducts in the entire processed layer (up to about 20 cm deep) by the underlying cutter. The machine is completed by the two rollers already described cage and smooth, with the same functions described above.
Also, Forigo Roter Italia of Ostiglia (MN) offers a wide range of sterilizing machines with liquid or solid products with fumigant action, with or without a contextual working of the soil, in the Mix-Tiller, Deeper, and Co. Modus ranges. For a correct control of the doses and a proportional distribution to the progress, all the distribution modules are managed by a computerized control unit.
To make the sterilizing action of the fumigant more effective, the machine can be equipped with a module for spreading mulching films. Particular attention must be paid to the sealing of the film, which is usually done by burying the edges employing plowshares or tamping discs. Conversely, for total coverage of the soil, the individual sheets are coupled with a specific band of overlapping, which in some cases is even better sealed with the application of special glues.
Nematodes
They are soil pests that mainly attack horticultural crops. They are tiny worms that can be detected only under the microscope but that can cause significant damage to vegetable production. In Italy, the most common and harmful nematodes belong to the genus Meloidogyne. In particular, the galls nematode (Meloidogyne incognita) attacks the plant roots causing galls, i.e., bullous formations that can reach up to 1 cm in diameter. The damage caused by nematodes results in slow plant growth, yellowing and crumpling of the leaves, and a disproportionate formation of secondary roots.
The plant species affected by nematodes attacks are very many: first of all, beets and strawberries, as well as solanaceous plants (tomatoes, peppers, potatoes, and eggplants), cucurbits (zucchini, cucumber, melon), the crucifers (cauliflower, cabbage, black broccoli), liliaceae (onions, leeks, garlic), as well as green beans, salads, spinach, celery, parsley, etc.
The nematodes' spread, both in protected crops and in open fields, depends mainly on the so-called "soil fatigue" due to intensive crops repeated in time, without adequate rotations, and with no significant (and continuous) contribution of organic substance. More specifically, nematodes develop in very exploited soils and intensive crops, both in the greenhouse and open fields.
Correct crop rotations and proper intercropping are the main remedies to limit their spread, together with careful organic fertilization with manure, compost, or even the solid fraction of digestate.
