Solarization

Technical articles - Weeds, Weed control by soil covering

Solarization is a weed control method that consists of eliminating dormant seeds stored in the soil, and seedlings by heat. This practice can also help fight against soilborne pathogens and/or nematodes.

It is the increase in soil temperature (up to 40°C to 50°C or more under shelter) that promotes seed germination and accelerates their degradation by increasing the rates of microbial and chemical processes in the soil. It is performed before planting.

Although the use of plastic may not seem ecological, it is a temporary method that can help clean the soil of dormant seeds and then, after a few years, allow to do without it.

Principle

Where: Soil solarization is more effective in warm and sunny regions; otherwise, shading is preferred. Solarization is optimal in areas where the slope is low or nonexistent, or when the slope faces south or southwest. Solarization of areas on north-facing slopes is not as effective.
When: It is recommended to implement solarization between early May and June 15 and leave it in place for a minimum of 30 days. The highest soil temperatures occur when days are long, temperatures are high, skies are clear, and wind is low. The soil warming effect is not as strong on cloudy days. Wind disperses trapped heat and can detach or damage the plastic sheets. Shaded areas may not be effectively treated by solarization. Solarization is more effective when performed during the hottest weeks of the year.
Duration: between 30 and 60 days before sowing or planting the crop. It is recommended to perform solarization every 2-3 years for maintenance and at least two consecutive years if the soil is heavily contaminated.
Against what? This practice is useful to fight against weeds and some fungi soilborne pathogens: Olpidium, Sclerotinia, and Pythium on lettuce and ginger; Rhizoctonia; Sclerotinia and Rhizoctonia on melon; Fusarium solani on zucchini; Sclerotium on yam; Against the bacterium Ralstonia solanacearum on solanaceous plants.
Advantages:
- Technique usable in conventional farming and organic.
- No waiting period before replanting, no toxicity, no residues.
- After several years, if the weed seed bank in the soil is well reduced, it can be done without.
- Effective against many weeds and some pests of the soil (pupae, larvae, nematodes).
- Starter-type side effect observed on some vegetable crops due to mineralization of organic matter and microbial biomass on the surface.

Disadvantages:
- The main limitation remains sunlight in the implementation area at the planned period. Besides sufficient cumulative global radiation over the entire covering period, a key point is the rapid temperature rise needed within the first 3 days after laying the plastic sheet.
- Long method: it requires the availability of plots for a sufficient duration, at least 4 weeks.
- Practice adapted rotations to have free surfaces at the end of spring.
- Limited effectiveness for perennial plants: treatment periods must be extended.
- Not very effective against deeply buried weed seeds and against weeds with vegetative propagation.

Equipment

Choice of plastic:
- The plastic used must be transparent, made of polyethylene 30 to 50 μm thick, non-perforated, treated UV-resistant and resistant to 700 hours of sunlight (special solarization). The width of the plastic should be that of the tunnel plus 50 cm. In open field, tarps of 3.60 m, 4.70 m or 5.80 m can be used.
- Many other types of plastics can be used. Plastics designed for large-scale solarization are generally treated with an ultraviolet (UV) inhibitor so they do not degrade as quickly in sunlight.
- For small areas, plastic rolls from 0.025 to 0.1 mm can last the 4 to 6 weeks of the solarization period without starting to degrade. When available, choose clear and transparent films rather than cloudy, milky or translucent materials that reduce sunlight.
Plastic thickness: Thin plastic allows better heating but is also more prone to tearing due to wind or animals walking on it (0.025 mm). Slightly thicker plastic is preferable in windy areas (0.037 to 0.05 mm). Thick plastic, 0.1 mm or more, is durable and can be reused for several years. Reuse can be practiced on a small scale when it is possible to remove and fold/roll it manually. For single use, thin films are generally preferred as they are cheaper and generate less waste compared to thicker single-use films.
Recovery: It is possible to use greenhouse plastic (e.g., 6 mm polyethylene recovered from greenhouses after 3 to 6 years of use), provided it is free of holes. For covering, sturdy silage tarps are popular.

Process

Soil preparation: soil preparation is equivalent to a false seedbed.
- Avoid deep loosening by plowing or rototilling, but favor toothed tools (cultivator, ridge former, actisol) that maintain seed positioning in the soil profile. The goal is to keep the maximum number of seeds to germinate near the surface.
- A very smooth soil (use a roller), with few clods and surface litter, will allow the plastic sheet to lie firmly on the soil, producing fewer air pockets, reducing the risk of tearing by wind and reducing heat dissipation risks.
- If the plastic sheet is applied on a prepared seedbed, it allows later planting with little soil disturbance, minimizing chances of bringing up buried weed seeds that survived the treatment. Preparing a seedbed can also improve soil water retention capacity.
Moisten the soil for better results.
- Wet seeds are more sensitive to thermal destruction than dry seeds, and moisture can stimulate germination. Water also increases conductivity, allowing heat to be transported deeper into the soil profile to reach more deeply buried seeds.
- The soil must remain moist throughout the solarization period to ensure good heat conduction at depth. Sprinkler irrigation providing 50 to 80 mm (or more depending on soil) to fully saturate the soil to 50 cm depth (amount depending on soil type) should be done before laying the plastic. Irrigation or waiting for rain can increase solarization effectiveness.
- If the soil then becomes dry during solarization, do not irrigate again, as this lowers soil temperature and would prolong the time needed for successful solarization.
Apply the plastic: the closer the plastic is to the soil surface, the better the heating.
- Wait until the next day or the day after, when the soil is a bit dry enough, before laying the plastic sheet. It must be well stretched and pressed to the soil (to prevent weed growth). A light sprinkling after laying improves plastic adhesion.
- Laying can be done only on planting strips, but applying it over the entire field can make solarization more effective, as heat loss at edges is reduced. This avoids leaving bare soil where weeds can survive.
- Tight fixing of the plastic edges leads to better results. A success factor will be the sealing between joints. To ensure good holding of the sheet, dig 10 to 15 cm along the edges to bury the plastic.
- During installation, it is imperative to have at least 3 consecutive sunny days for rapid temperature rise and to prevent development of some weeds.
- In case of crops under shelter, to have a rapid temperature rise, it is recommended to keep greenhouses closed for a few days while avoiding too high temperatures that could damage equipment, especially some irrigation installations. For this, it is recommended to leave, for example, a ridge ventilation (about 20 cm) to create a "chimney" effect.
- The first days of solarization are crucial to eliminate germinating weeds, especially purslane. Check the weather forecast to ensure at least 3 consecutive days of full sun from the moment of mulching to achieve rapid temperature rise under the plastic film.
- Alternative: to treat small areas in cooler climates, it may be useful to use a double layer of plastic with an air space created by objects such as plastic bottles or PVC pipes between layers. This method has been shown to increase soil temperature by 1 to 5°C more than temperatures obtained with a single layer of transparent plastic.
Wait: The plastic film must remain in place for a minimum duration of 45 days for crops under shelter and 60 days for open field crops. The lower the soil temperature, the longer the plastic must remain to raise the temperature to desired levels. The goal is to maintain daily maximum temperatures in the top 15 centimeters of soil at about 43 to 52°C or higher. Using a soil thermometer or thermal probe allows checking that these temperatures are reached.
The cropping period:
- Solarization should be stopped as late as possible before replanting by first removing the plastic sheet. The soil, if necessary, is then worked superficially (maximum 10 cm) to avoid bringing up soil layers with pests and weeds that could not be destroyed by the technique.
- Solarization stimulates mineralization of organic matter, so it is recommended to monitor nitrogen content in the soil and adjust nitrogen fertilization accordingly.
- Plant when soil temperature has dropped to 20°C and plan for recycling or storage of the plastic if it can be reused.
- Minimize soil disturbance after removing the plastic to avoid bringing up new weed seeds from below.

Option

Addition of organic matter: it is possible to increase the effects of solarization by incorporating organic materials, such as crop residues and composts, into the soil before solarization. During decomposition of organic matter, chemical changes occur, releasing some natural products, such as organic acids, which are toxic to soil organisms. However, care must be taken not to incorporate excessive amounts of organic matter as the treated soil may be impacted for a prolonged period by these natural toxins. In this case, planting should be delayed until soil conditions are adequate. Moreover, treated soil can be detoxified by irrigation, which leaches organic acids and other toxins below the root zone.

Comparison with the shading method

Solarization uses a transparent sheet and is more effective in a warm and sunny climate, while shading uses a black sheet and is more effective in colder regions or with more shade and wind.

Temperature: it is higher with solarization. During solarization, light waves penetrate the transparent plastic and directly heat the soil underneath. The heat is then trapped under the plastic by a greenhouse effect. With a black sheet, solar energy is absorbed by the plastic, some heat is transferred to the soil and some is lost to the surrounding air. Applying a double layer of transparent plastic, or transparent over black, can further increase temperatures and effectiveness.
Light-dependent germination: the black sheet could block germination of some seeds. Light is an important signal for germination of many species, and for photosynthesis of plants.

Possible combinations

Shading can be combined with solarization in conditions requiring optimization of the weed control technical itinerary. This may be the case in anticipation of early carrot sowing or for establishing a leek nursery. One would then use summer solarization (early August – late September) followed by shading to keep the plot clean until planting after winter. If the plot is available the summer before shading, it may be interesting to plant a frost-sensitive summer green manure, such as buckwheat (Fagopyrum esculentum), which can develop quickly and secrete, through its roots, toxins limiting weed development. However, care must be taken that this green manure does not go to seed, generating a new weed flora the following year.

Effectiveness and impact

Effectiveness against pests

It all depends on the intensity, depth and duration of high soil temperatures, as well as the sensitivity of each pest species to the treatment. Some pests can be killed in a few days, but 4 to 6 weeks of full sun exposure during summer are needed to ensure control of many others. Although many soil pests are killed by solarization, many beneficial organisms can survive solarization or recolonize the soil very quickly afterward.

Fungi and bacteria

Solarization was originally developed to help farmers control soilborne diseases. Solarization controls many important plant fungal and bacterial soilborne pathogens that can cause: Verticillium wilt, Fusarium wilt, Phytophthora root rot, downy mildew, damping-off, crown gall, tomato canker, potato scab...

Some heat-tolerant fungi and bacteria are more difficult to control with solarization, such as those causing melon decline and charcoal rot.

Nematodes

Soil solarization can be used to reduce nematode populations. However, soil solarization is not always as effective against nematodes as against fungal diseases and weeds because nematodes live deeper, are relatively mobile, and can move within the soil profile to escape heat, quickly recolonizing soil and roots.

Organism	Impact of the technique	Type	Details
Olpidium brassicae		pathogen (pest)	Technique applied for lettuce.
Fusarium solani		pathogen (pest)	Technique applied for zucchini.
Nematode (pest)	MEDIUM	pest, predator or parasite	Partial effectiveness
Pythium		pathogen (pest)	Technique applied for lettuce
Rhizoctonia solani		pathogen (pest)	Technique applied for lettuce and melon.
Sclerotinia		pathogen (pest)	Technique applied for lettuce and melon.

Effectiveness against weeds

Some seeds or plant parts of weed species are very sensitive to solarization, others are moderately resistant and require optimal conditions for control, i.e., good soil moisture, well-fitted plastic sheets, and high solar radiation. Solarization generally does not control perennial weeds as well as annual weeds because perennial plants often have deeply buried underground vegetative structures such as roots, tubers, corms, and rhizomes, which have more resources and can survive longer. Control of yellow nutsedge, yellow nutsedge, field bindweed from rhizomes, and some clover species can be irregular even under favorable conditions.

Effectiveness low	Johnson grass, field bindweed, purslane, field buttercup, clover.
Effectiveness medium	Bermuda grass, crabgrass, wild oats, panic grass, amaranth, Chickweed or birds.
Good effectiveness	Kentucky bluegrass, green foxtail, shepherd's purse, lamb's quarters, jimsonweed, galinsoga, field sowthistle, red dead-nettle, mayweed, Black nightshade, nettle, oxalis, knotweed, persicaria, ragwort, speedwell.

Impact on soil fertility

Accelerates degradation of organic matter in the soil, often resulting in the additional benefit of releasing soluble nutrients such as nitrogen (from nitrate and ammonium), calcium, magnesium, potassium, and fulvic acid, making them more available to plants.
Agronomic effectiveness: Plants often grow faster, with higher yields and better quality when grown after soil solarization. This can be attributed to improved disease and weed control, increased nutrient availability, and higher proportions of beneficial microorganisms.

Negative impact on auxiliaries

Trichoderma (useful saprophytic fungi because they prevent the development of certain diseases of the roots of vegetable crops) are preserved, however the following auxiliaries are impacted:

Organism	Impact of the technique	Type	Details
Spiders	STRONG	Natural enemies of pests	All auxiliaries that carry out part of their biological cycle in the soil can be impacted by this technique (carabids, spiders, rove beetles…), as well as some hymenopteran pollinators that live in the soil such as mason bees.
Predatory and granivorous ground beetles	STRONG	Natural enemies of pests	All auxiliaries that carry out part of their biological cycle in the soil can be impacted by this technique (carabids, spiders, rove beetles…), as well as some hymenopteran pollinators that live in the soil such as mason bees.
Fungi (auxiliary)	STRONG	Natural enemies of pests	Antagonistic fungi naturally present in the soil are impacted by the technique (Coniothyrium sp. for example)
Rove beetles	STRONG	Natural enemies of pests	All auxiliaries that carry out part of their biological cycle in the soil can be impacted by this technique (carabids, spiders, rove beetles…), as well as some hymenopteran pollinators that live in the soil such as mason bees.

Economic impact

There is a labor saving (weeding by hand), treatments or mechanization costs (mechanical weeding).
Working time: for watering, installing and removing plastics about 5 h/100 m2.
Necessary investment of about €1600/ha: special PE film for solarization treated with anti-UV/35 µm (0.70 to 0.80 €/m²) = €800/ha; installation about 70 hours/ha = €800/ha.
The financing of the recycling sector set up by Adivalor must also be supported. It is financed by two means: an eco-contribution at source amounting to €240/t of new plastic (2020) for mulching films, and collection fees amounting to €145/t of used agricultural film (UAF) for mulches when the contamination rate is greater than 50% (2020). A bonus is also possible and returned for UAF with a contamination rate below this threshold: €95/t for clear plastics, €50/t for colored plastics (2020).