Solarisation, false sowing and soil cultivation during periods of frost

Solarization

Many fungal and bacterial diseases are caused by pathogens that live in the soil tens of centimetres deep, making them difficult to eradicate. Without proper care, these pathogens can survive in the soil for many years and lead to an inability to grow crops susceptible to these pests. In addition, intensive agriculture, characterised by multiple crops on the same plots, results in an increase in soil-borne diseases. Soil-borne pathogens are controlled by various methods, including disinfection with chemicals such as methyl bromide (a) or metam sodium. Methyl bromide increases the imbalance of biodiversity and depletes the ozone layer. Solar disinfection has the advantage of avoiding these pitfalls.

Solarisation is a method of pest control that involves heating the soil by solar radiation. Solar disinfection is based on a combination of several thermal, biological and biochemical mechanisms that have numerous deleterious effects on the pathogens responsible for microbial and fungal infections. It is certainly one of the most effective methods for controlling soil pathogens.

The idea and development of this environmentally friendly technology was the work of Professor Yaakov Katan of the Hebrew University of Israel and his colleagues. To date, 1,400 scientific studies have been published on the subject of solar disinfection (b). Today, solar disinfection is used as an alternative in more than 70 countries in a variety of cropping systems (traditional or organic) and in different climatic zones.

The principle of solarisation is to cover a moistened soil with a transparent polyethylene cladding sheet to create a greenhouse effect. Its main function is to retain heat in the soil for a long period of time, resulting in evaporation of water from the soil and deep fumigation. It is obvious that the soil should not be compacted so that water vapour can circulate between the interstices. In the dry season, watering is necessary before the covering is laid (the soil should be wetted to a depth of at least 30 cm).

Solarisation sail installed at a private home. Photo Mme Mickal Eliad, Zichron Yaacov Israël

Solarisation is most effective in summer in sunny regions, i.e. in the south of France. A lack of sunlight can lead to a failure of solarisation with weed growth.

During solar disinfection, physical, chemical and biological changes take place in the soil leading to the death or weakening of the bio-aggressors living in the soil. The higher the temperature, the more effective the disinfection and the better the pest control. A high temperature increase can also destroy weed seeds and harmful nematodes. Soil solarisation also improves soil structure and increases the availability of nitrogen and other essential plant nutrients.

If solarisation is carried out under greenhouses or insect netting, it is imperative to remove the protective coverings before solarisation. After the solarisation process, the soil should be worked as little as possible, except for transplanting or sowing. This is to avoid bringing seeds or microorganisms to the surface that have not been affected by the disinfection. In Israel, the installation of the solarisation equipment is carried out after the installation of the drip system, which allows the soil to be moistened during solarisation to a depth of 30 cm to improve heat conductivity.

Some studies have shown that many phytopathogens are destroyed by a treatment at 55° for only 30 minutes (1). Compared to the saprophytic fungal flora, pathogenic fungi are found to be the most sensitive to heat treatment (2).

To target the most resistant pathogens, the coating should be left for 5 to 12 weeks, depending on the disinfection period, the type of floor and the type of coating. The ideal date for installation is during the hottest part of the summer.

Mulches should be removed so that the surface temperature can reach 55°C. It is important that the soil remains moist with a pH above 7 if possible. The survival of Fusarium Avenaceum is best in dry soils with a pH between 3.8 and 4.6. A constant temperature of 56° for 30 mm is necessary to eliminate Fusarium spores (3).

To target the most resistant pathogens, the coating should be left for 5 to 12 weeks, depending on the disinfection period, the type of floor and the type of coating. The ideal date for installation is during the hottest part of the summer.

Mulches should be removed so that the surface temperature can reach 55°C. It is important that the soil remains moist with a pH above 7 if possible. The survival of Fusarium Avenaceum is best in dry soils with a pH between 3.8 and 4.6. A constant temperature of 56° for 30 mm is necessary to eliminate Fusarium spores (3).

Transparency and cleanliness of the cover are essential to allow solar radiation to heat the soil without interference. The wind must not enter the cover. For this purpose, the edges of the plastic sheet are sealed with wooden boards and/or some soil. It is useful to check the temperature with a laboratory thermometer with a probe to measure the temperature at different soil thicknesses. Of course, if possible, the treated plot should not be permanently shaded by buildings, walls and trees.

Many parasitic worms are destroyed during this warming, including nematodes that parasitise roots, rhizoctones, pythium that causes damping off of seedlings, sclerotinia known to produce white rot of carrots, lettuce, tomatoes and beans, and botrytis (a grey rot that occurs on tomatoes, strawberries and grapes), or verticillium wilt of tomato, aubergine and potato leaflets.

It is the most effective treatment for verticillium. It does not seem to affect the useful soil microflora, which regenerates quite easily at the end of the summer after the plastic sheeting is removed.

Soil preparation :

Solarisation is most effective when the plastic film is placed as close as possible to a smooth soil surface. Clods from a plough should be broken up. Any objects or debris that could lift or puncture the sheeting should be removed with a rake.

It is strongly recommended that fresh manure and/or compost be scattered before laying the cover. The decomposition of these biodegradable amendments increases the temperature and releases volatile biotoxic compounds, thus enhancing the biological destruction mechanisms of pathogens and weed seeds.
The addition of organic matter also increases microbial antagonistic activity in the soil and weakens the pressure of pathogenic mycelia. Earthworms are not affected by the treatment, especially since in summer, and particularly in the PACA region, they are used to going down to more than 30 cm to protect themselves from the heat..

Irrigation:

The soil should be irrigated in order to conduct the heat to the deeper layers (moist soil conducts heat better than dry soil). The soil must be irrigated again during solarisation if the soil is very light and sandy, or if the soil moisture is less than 50%.

Solarisation is well known in Israel where vegetable growers use very thin polyethylene sheets, between 30 and 40 microns (not reusable, the sheets are UV resistant between 6 and 12 months depending on their quality). Some sheets can be thinner or thicker to suit a particular use. For example, coatings between 30 and 70 microns can be found that contain an anti-fogging agent to prevent the formation of large drops on the film and maximise light transmission (the life of these films is shorter, around 5 to 7 months depending on the region). Some additives added during the manufacture of the film increase the temperature by 4 to 5° and reduce heat loss at the end of the day. Other additives reduce the loss of infrared radiation during the night (solarisation tarpaulins containing these additives are manufactured by POLITIV).

In France, professional market gardeners can buy solarisation films (e.g. « film trips » from the Barbier group or solarisation films from the Guérin Plastiques company) from their cooperatives, the ACTURA network, or even VITIVISTA, AGRALIA, etc.

Beware, black solarisation films are sometimes offered on internet sites, such as mulching cloth for strawberry and salad plants. The expression “solarisation” to designate this weeding technique is abusive. The solarisation technique invented by Israeli agricultural engineers is intended to treat the soil before planting and requires transparent films, not black ones.

For want of anything better, some websites in France offer transparent polyethylene tarpaulins with properties similar to those used in Israel. For example, HaGa tarpaulins available in various sizes for tomato greenhouses are UV resistant and are claimed by their manufacturer to be very light permeable. Another option is Nortene’s 50µ climafilm which is UV resistant and is available in some garden shops.

For more information on the solarisation technique, see this example from Guyana by clicking here.

a) Methyl bromide is prohibited in soil disinfection by the 1985 Vienna Convention and the 1987 Montreal Protocol.
b). For his research, Professor Katan has won numerous awards and international recognition. In 2014, he won the Israeli Research Award.