Methods and requirements for chemical disinfection of greenhouse soil

The ideal chemical for soil disinfection should completely eliminate pests and pathogens without harming beneficial organisms, such as nitrifying bacteria and antagonists. From this perspective, fumigants that spread evenly through the soil layer and penetrate sufficiently deep are the most effective.

A chemical must meet a number of requirements:

• Effectiveness in autumn and winter greenhouses at a temperature of 8—10 °C.
• Elimination of pathogens over a short period (preferably within 1—2 days or faster).
• Rapid evaporation from the soil or decomposition into non-toxic products by the end of the treatment period.
• Absence of toxic residues in the soil for plants and humans.
• Safety during application and absence of unpleasant sensations.
• Economic efficiency.

However, only a small number of products even partially satisfy these requirements.

Chemical disinfection yields the best results if the soil is well prepared for treatment and the choice of product corresponds to the set goal. Application methods for chemicals are very diverse. Forms of products used for treatment include:

• gases;
• liquids, where the active ingredient is dissolved in an organic solvent;
• aqueous suspensions;
• dusts or granules.

Before beginning work, the soil is loosened to a depth of 30—35 cm. It must be in good tilth and leveled. An important parameter is soil moisture, the level of which is presented in the table:

This value is higher than the usual optimum for seedling compost and moisture at the time of transplanting. (Moisture reaches the level of field capacity when the soil is saturated with water and the excess has been removed by drainage.)

If a chemical is applied with a large amount of water, the soil must actively and evenly absorb the working solution, thereby eliminating the risk of chemical accumulation in poorly drained areas. Before chemical treatment, peat or well-decomposed manure should be incorporated into the soil. It is not recommended to apply undecomposed manure, as it is difficult to process and may release toxic ions. Sources of organic nitrogen in the soil decompose into ammonium and nitrite ions (as mentioned above), although to a lesser extent than during steaming. Therefore, this factor must be taken into account when planning the timing of transplanting. PRODUCT SELECTION Product selection is often determined by the needs of the farm. In a greenhouse with crop rotation and minimal intervals between crops, fast-acting products that easily evaporate from the soil after treatment should be used. Currently, only methyl bromide meets such requirements. If the interval between crops can be prolonged, for example, 10 days from treatment to planting, one can use pesticides based on methyl isothiocyanate. Many products are approved for soil disinfection, but their application rules vary by country, so before choosing a chemical, it is necessary to consult the relevant list of authorized pesticides. In Europe, products based on methyl isothiocyanate, formaldehyde, chloropicrin, and DD are primarily used. 3.4. PERFORMANCE INDICATORS OF PESTICIDES FOR SOIL DISINFECTION [Table content omitted for translation accuracy] Note: very effective, „ — weakly effective. Methyl bromide. This pesticide is highly toxic to all living organisms, and in many countries, including the UK, it is permitted only for the treatment of soil, mushroom compost, or premises (in the latter case — by contract with an official organization). The boiling point of liquid methyl bromide is 3.6 °C, i.e., under the conditions of greenhouses, it is a gas. Methyl bromide vapors are heavier than air; they usually move according to the slope of the greenhouse and have a high degree of penetration, moving freely through the pore spaces of the soil. The movement of methyl bromide vapors often creates problems, as it can penetrate into nearby residential buildings through the greenhouse drainage system. As a fumigant, methyl bromide is odorless; however, small quantities of chloropicrin — a tear gas — are added to most of its formulations during manufacturing, which allows for the rapid detection of vapor leaks.

The operator applies methyl bromide from a cylinder placed in the center of the greenhouse or from small canisters. In some countries, mounted tractor injectors are used for soil treatment in field conditions. The preparation and application process includes the following stages:

• Preliminary soil preparation: the surface must be moist but without waterlogged areas.
• Covering the area with gas-impermeable polyethylene film (recommended gauge 150; thinner films must not be used).
• Placement of the fumigant cylinder in the center of the area to be treated.
• Introduction of vapors under the film via a heating coil, in which methyl bromide evaporates.
• Distribution of vapors under the film through plastic pipes.

The canisters are placed on the area before laying out the film and are punctured to allow the release of methyl bromide vapors; the film should be slightly lifted above the soil surface during this process. Fumigant distribution occurs uniformly when the soil temperature is above 10 °C (ideally at 15 °C).

1. Methyl bromide disinfection: rapid turnaround and concentration control

Methyl bromide allows for minimizing greenhouse downtime between crop rotations. When treating under a film, a gas concentration sufficient to kill most pests and pathogens quickly builds up in a 20–30 cm deep soil layer. The effectiveness of disinfection depends on the CTP value (concentration-time product). A high vapor concentration with a short exposure time works as effectively as a low concentration over a long period.

After removing the film and ventilating the greenhouse, it is mandatory to test for chemical residues. Once safe levels of the fumigant in the soil and air are reached, preparations for planting can begin immediately. The use of methyl bromide reduces the intervals between crops to one week, ensuring rapid crop rotation and maximum utilization of the greenhouse area.

The sensitivity of harmful organisms to methyl bromide decreases in the following order (from most susceptible to resistant):

• Seed and plants: germinating seed → large seed;
• Insects and mites: active larvae → inactive stages;
• Nematodes: active larvae and adults → dry cysts and eggs;
• Fungi: active mycelium → asexual spores → resting spores → sclerotia (Pythium, Phytophthora, Rhizoctonia, Botrytis, Didymella, Pyrenochaeta, Verticillium, Phialophora, Fusarium species);
• Bacteria: active cells → resting spores.

• Time under film — 4 days
• Treatment depth — 20–30 cm
• CTP value — 1000–5000
• Interval before planting — 7 days
• Soil temperature — from 7 °C
• Application rate during irrigation — 27 l/m²

2. Methyl isothiocyanate application: soil requirements and phytotoxicity testing

Methyl isothiocyanate (MIT)-based products are less toxic to humans than methyl bromide and are convenient for use in protected soil. However, in practice, MIT efficiency is often reduced due to uneven distribution in the treated layer. To improve the distribution of the active ingredient, various formulations are used: MIT concentrate in xylene is introduced by injector, an aqueous solution of metam sodium (carbam) is used for irrigation, and dazomet granules and capsules are incorporated into the soil. To broaden the spectrum of action, mixtures of MIT with chloropicrin and DD (dichloropropane-dichloropropene) are also used.

The result of MIT treatment directly depends on soil structure, humidity, and temperature. The soil must be finely loosened to a depth slightly exceeding the disinfection horizon. The optimal soil humidity is about 70% of field capacity (when applied via irrigation, the product can also be added to dry soil). Light sandy soils are best suited for MIT application, whereas in soils with an organic matter content of more than 5%, the product's effectiveness decreases significantly.

After applying MIT by any method, the soil surface must be sealed. This is achieved by rolling, flood irrigation, or covering with polyethylene film. After approximately 4 weeks, the greenhouse is ventilated and the soil is tilled to remove gas residues. Before planting transplants, it is mandatory to conduct a bioassay to determine MIT residues in the soil using lettuce seed.

1. Take a soil sample from a depth of 10–12 cm so that the sample reflects the state of the entire treated area.
2. Place the soil in a container with an airtight screw cap.
3. Sow several lettuce seed in this container.
4. Prepare a control sample: fill a similar airtight container with untreated soil of the same type and also sow lettuce seed.
5. Seal both containers tightly and place them in a warm location at a temperature of 20 °C.

In addition to methyl bromide and methyl isothiocyanate, other agents are used for the chemical disinfection of greenhouse soil — formaldehyde, chloropicrin, and the nematicide DD. Each has its own technological features, safety limitations, and strict requirements for waiting periods.

Formaldehyde is being gradually replaced by more modern fumigants but remains a reliable agent for general greenhouse disinfection. Its effectiveness in the soil depends directly on the volume of the solution applied — the higher the dose, the more reliable the result. Besides the soil, this agent is used to disinfect nursery equipment and greenhouse structures themselves.

Chloropicrin is valued for its rapid suppression of soil-borne pathogenic fungi. However, this agent is extremely inconvenient to handle due to its lachrymatory properties. After treatment, the soil must be covered with plastic, and the greenhouse must be ventilated for a long time before planting the transplants.

Regulations for the use of nematicide DD

Nematicide DD, which helped control root-knot and potato cyst nematodes, is now practically unused in its pure form. Today it is used primarily in a mixture with methyl isothiocyanate (MIT). The agent requires spot subsurface application and strict adherence to a two-stage treatment scheme.

For safe and effective application of DD, follow this sequence of actions:

1. Apply the mixture of DD and MIT using a manual injector to a depth of 20 cm, maintaining a distance of 30 cm between injection points.
2. Immediately after injections, tightly seal all resulting holes in the soil.
3. Wait 6 weeks and perform the first loosening of the treated soil.
4. Immediately before planting the transplants, perform a second soil treatment.

• Concentration of initial formalin — 38 %
• Concentration of working solution of formaldehyde — 2–5 %
• Application rate of working solution of formaldehyde — 50 l/m²
• Depth of application of the DD and MIT mixture — 20 cm
• Grid spacing for DD application — 30 cm

Technological parameters of agents for planning disinfection work: