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Agriculture

Soil Solarization For Weed Control And Pest Management

13.08.2021
plastic film for soil solarization

Soil solarization proves efficient to kill microbes and control diseases, pests, and weeds. Yet, its benefits are more extensive – it boosts soil health. The soil solarization technique employs the energy of water and the sun to reach its goals and dismisses chemicals, which makes it environmentally-friendly. For this reason, soil solarization best practices have been adopted around the world, and in the U.S. in particular.

Offering advantages, the method also demands certain efforts and knowledge for proper preparation of soil before solarization and regular control. The technique implementation and management are far easier with remote sensing and AI-powered analytics.

What Is Soil Solarization?

The process suggests covering weed-clean earth with a transparent air-proof substance (typically plastic) to accumulate solar energy, which raises soil temperature. The major purpose of soil solarization is to kill weeds and all sorts of pests with the obtained heat. Depending on location, the temperature in the 5-cm (2-inch) topsoil under solarization may range from 108-140°F (42-60°C).

Additionally, soil solarization improves soil chemistry (enhancing fertility) and provides quite a decent protection from soil erosion. Land covered with plastic is not subjected to destruction by water and winds.

field solarization with dark plastic film

How Does Soil Solarization Work?

When expecting to enjoy the soil solarization advantages in full, farmers should arrange the process thoroughly and keep up with all the requirements, from earth preparation to plastic aftertreatment. It is important to bear in mind all the factors with a possible impact on the soil solarization process. The required knowledge will empower agronomists to avoid what hinders and enhance what helps. The specific factors to consider include:

  • soil type and color;
  • presence of weeds and their removal;
  • possible shade-casting objects (e.g., agroforestry);
  • season of the year;
  • soil solarization duration and timelines;
  • moisture content and irrigation plan if natural soil moisture is not enough;
  • plastic type and color;
  • bed orientation (north to south for raised beds is preferable);
  • plastic further use and recycling, etc.
field preparation for solarization

Preparation

Proper preparation of soil before solarization is the initial step to success. At this stage, it is important to complete the following steps:

  • choose the plastic to use;
  • perform soil testing to know the soil type;
  • get the field ready.

Plastic Types

The choice of plastic type, color, and thickness plays an essential role. Plastic typically stands for plastic polymers including:

  • polyethylene (PE),
  • polyvinyl chloride (PVC),
  • ethylene-vinyl acetate (EVA).

PVC and EVA have better optical properties but are more expensive. For this reason, PE is the most common option.

solarization with black polyethylene

Plastic Color

The colorless (clear) plastic is the best for the sun rays to penetrate and supply heat to the earth (compared to black or white). However, in cooler regions, black polyethylene is justified since even though it absorbs heat, it also controls weeds that may develop under transparent plastic when the heat is insufficient.

Plastic Thickness

The thinnest polyethylene (1 mil) is the best to get maximum heating, but it is damaged easily, too. In windy areas, 1.5-2 mils polyethylene is typically used. Smaller territories are sometimes covered with thicker plastic (4 mils).

Preliminary Soil Testing

Soil testing matters because the soil solarization method is not equally efficient for all types. It works better in soils with good water retention properties (containing clay or loam).

Clay and loam earths retain moisture longer, which means they can produce steam longer, too. Sandy earths are less effective in this regard but not completely unsuitable. In this case, it is possible to improve the situation through irrigation.

Preparatory Agricultural Activities

The field preparation includes:

  • cleaning from weeds and debris not to spoil the plastic and because vegetation cover hinders heat penetration;
  • making central elevations for covered strips for rainwater to run off the plastic and not to cool it down;
  • arranging beds (flat or raised);
  • tilling to break big earth pieces that may damage or raise the cover;
  • facultative soil aeration to decrease compaction since bulk earth density adversely affects water-holding capacity;
  • placing drip irrigation lines if necessary;
  • assessing moisture content and moisturizing the beds when needed.

Either excessive or insufficient soil moisture won’t allow achieving the desired results. For soil solarization success, it is necessary to wet the beds at least 12 inches deep.

Our product Crop Monitoring has a specific feature to check the soil moisture in the field (2-inch topsoil and up to 27 inches in the rhizome area). This way, farmers can make a weighted decision on whether or not a field is ready for soil solarization and what additional measures (specifically in terms of irrigation) it requires.

soil moisture data on Crop Monitoring
Soil moisture data on Crop Monitoring.

To save on moisturizing, it is optimal to lay the plastic the next day after rainfalls or irrigation events. Sometimes, natural soil moisture is enough.

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How To Lay The Plastic

There are two options to place the plastic: it may either cover the field completely or in strips, as wide as the sheet size allows. Strip coverage is more cost-effective, yet pests and weeds may re-infest the covered areas by penetrating from the uncovered ones.

It is important to bury the plastic film edges thoroughly; otherwise, heat will escape outside. For this reason, the sheet should be wider than the strip. If the film is damaged, it can be patched with clear tape.

It is important to fit the plastic as tight to the ground as possible as air space reduces heating and will cause plastic “waves” on windy days, which will dissipate heat.

Double layers of plastic increase the heating effect (2-10°F) because of the air in between, compared to a single layer. However, this soil solarization technique is more cost- and effort-consuming.

How And Why To Irrigate

It would be wrong to consider that soil solarization increases soil temperature by merely trapping and harnessing the sun. The method combines solar and aquatic properties that jointly produce steam (vapor), and it is the steam to credit for the killing power. This is why, even though plastic-covered earths are more disinfected than bare ones, soil solarization without moisture is not quite effective. Vapor becomes visible when it condenses on the plastic in the form of water beads. If there are no beads, it is high time for water supply.

Solarization of the soil kills microbes when the earth is not only hot but wet because moisture conducts heat more efficiently. Irrigation may take place both before or after laying the plastic, depending on the soil moisture content. Water can be supplied via pipes or furrows, with undercover drip irrigation being a perfect choice.

Additional irrigation cools down the earth, yet it is still necessary to produce evaporation, which brings a better outcome in the end.

field irrigation under plastic film

How Long Does Soil Solarization Take?

Typically, the process lasts from four to eight weeks. It should be enough time to conduct soil solarization before planting fall crops. Provided all the requirements are met, the earth treatment spares agronomists the trouble of pests for three to four months.

For soil solarization success, it is logical to take advantage of the hottest season, which is usually the summer months of June, July, and August. If the weather permits, late May and early September will do, too.

Soil solarization in spring won’t give the same result as in summer. Cloud cover, winds, lack of heat, short days, and fogs decrease solarization efficacy.

Removing Plastic

When solarization of soil is over, the plastic can be either removed or painted:

  • black to suppress weed growth;
  • white to cool down the earth;
  • silver to repel flying pests.

However, if the plastic is not removed, it wears out and gets dirty quickly, which makes it impossible to reuse the same material in the next season. The film can be also used unpainted for crop transplantation.

growing organic green onions in open ground covered with black polyethylene film

Soil Solarization For Disease Control & Pests Management

Solarization is used as a pest control measure, but its impact on different pathogens and pests varies. Earth’s temperatures as high as 86-91°F (30-33°C) kill most earth pests. The treatment duration is different, depending on species. While some will die just in several days, only 4-6-week sun exposure will be lethal to others.

Does Soil Solarization Kill Fungi And Bacteria?

The method is effective with many fungi and bacteria species that cause plant diseases, though certain pathogens are resistant to it and require additional treatment.

In particular, solarization destroys Verticillium dahliae, Phytophthora cinnamomi, Clavibacter michiganensis, Streptomyces scabies, Rhizoctonia, Fusarium, Sclerotinia, Agrobacterium tumefaciens, which explains the role of soil solarization in disease control. However, the practice is not strong enough for Macrophomina, Pythium, Pseudomonas solanacearum responsible for stem and root rots.

cultivating tomatoes with plastic film to kill weeds

Does Soil Solarization Kill Weed Seeds?

The method proves efficient both in the short and long run for weed management. If weeds develop under plastic, it means the process is carried out incorrectly. The soil solarization technique for weed management is efficient when the temperature is hot enough to kill certain species. In this regard, it will bring more results in controlling winter weeds rather than summer ones due to higher heat tolerance in summer plants. Soil solarization for weed control of perennials is less effective compared to annuals due to the deep earth penetration of perennial seeds and roots.

The method combats such annual species as Ageratum, Amaranthus, Digitaria, Portulaca, Setaria, and others. Weeds reproducing with multiple tiny seeds are easier to control than those spreading with stolons.

Weed management through solarization also helps reduce pathogen or pest populations that may dwell on wild plants nearby, too.

Soil solarization accords with the principles of permaculture, regenerative agriculture, and organic farming since it prefers traditional knowledge to intensive farming methods. This explains the benefit of soil solarization technique for weed management in permaculture and other chemical-free practices. However, permaculturalists’ opinions differ. Some of them think that plastic does not fit into natural landscapes. Another point is that permaculture promotes perennial plants, and solarization works better with annuals.

Does Soil Solarization Kill Earthworms?

High temperatures are not favorable for earthworms, preferring damp but cool areas. The solarization impact on them is understudied. Yet, it is believed that they just escape the heat by digging deeper.

earthworms in soil

Does Soil Solarization Kill Nematodes?

Soil solarization for nematode control is most beneficial for shallow-rooted, fast-growing crops. It does kill many nematode species (pin, sting, cyst, spiral, root-knot, etc.). In particular, studies report a positive impact of soil solarization on root-knot nematodes in transplanted tomatoes.

Root-knot nematodes and their eggs are killed when exposed to 125°F for 30 minutes or 130°F for 5 minutes. Nonetheless, nematodes are worms, so they are relatively mobile. Therefore, they can escape high temperatures and return when circumstances are favorable – unlike their eggs. Besides, temperature decreases under the plastic down the earth profile, which reduces the effect on nematodes dwelling deep in the subsoil.

For this reason, nematodes may be incompletely controlled with soil solarization only. In this case, the combination of chicken manure and solarization provides better control.

The purpose of soil solarization is to disinfect but not sterilize the earth. Even though it may kill many pathogens and pests, solarization cannot remove them completely.

Does Soil Solarization Kill Beneficial Bacteria And Other Microorganisms?

Solarization kills many bacteria and may threaten beneficial biota, too. This is why it is important to restore the balance by adding organic matter after completion.

The good news is that beneficial microorganisms can either resist the soil solarization process or re-establish their populations faster soon after the crucial impact. In particular, this refers to:

  • microorganisms parasitizing bacteria and fungi that are pathogens to plants;
  • symbiotic mycorrhizal fungi that colonize plant roots and trade moisture and nutrients (particularly phosphorus) for photosynthetic products.

The method may also destroy nitrogen-fixing Rhizobium. In this case, nitrogen fixation can be encouraged by planting N-fixing leguminous crops to be inhabited by the symbiotic Rhizobium.

How Soil Solarization Affects Soil Chemistry

A rise in soil temperature also means a rise in the split of organic matter that contributes to soil fertility. In particular, solarization helps increase the content of N (NO3, NH4), K, Mg, Mn, Ca, Fe, Cl, and Cu. Thus, the technique is effective in pest control but still beneficial to soil health. With the increased availability of nutrients, plants become stronger, which makes them more resistant to pathogens and boosts yields. However, each crop type requires a specific balance of nutrients for healthy development.

solarization changes soil chemistry

Improving Solarization Efficacy With Other Control Methods

Despite the current soil solarization adoption worldwide is still relatively small, it is expected to gain more popularity as a chemical-free measure because of the methyl bromide bans due to ozone depletion. Under proper conductivity and heat exposure time, solarization kills many microbes that are thermosensitive enough or cannot dig deep into the earth.

However, in cooler regions or when the obtained temperatures are not lethal but sublethal, soil solarization efficacy can be enhanced with biological, physical, or chemical methods of control and organic amendments.

Biological control efficacy presents a certain interest for researchers since its possibilities are understudied. Yet, it is already known that the fungal genera of Talaromyces yavus, Trichoderma harzianum, Glomus fasciculatum are effective biological agents of plant disease control.

Organic amendments also increase solarization efficacy. For example, chicken manure slightly raises the earth’s temperature during solarization. When combined, chicken manure and solarization kill Orobanche crenata seeds at all depths compared to unamended solarization efficacy only at the surface.

Warmer temperatures in solarized earths typically increase the effect of chemical amendments but may also speed up their volatility or degradation. On the other hand, chemicals eliminate the soil solarization benefits in pest and disease control, which may negatively affect the crops in the long run.

Crop Monitoring’s Assistance In Further Weed and Pest Control

After the films are removed and solarization is complete, there is a further risk of weeds development and pest or pathogen infestation. That’s why Crop Monitoring is effective from the earliest stages of crop growth, thanks to its MSAVI index that helps monitor crop development in the earliest stages.

MSAVI map of a bean field on Crop Monitoring
MSAVI map of a bean field on Crop Monitoring.

Timely identification of problem zones allows early fertilizer application specifically in the areas that are in trouble but not in the entire field with the variable rate technology (aka VRT). After detecting and scouting the hot spots, farmers decide on what to apply – be it fungicides, fumigants, or herbicides.

adding a scouting task to check a problem spot in bean field
Adding a scouting task to check a problem spot in bean field on Crop Monitoring

Furthermore, all agricultural operations can be easily logged and tracked on Crop Monitoring, including soil solarization, to know what exactly is going on with a field and always be aware of any ag practices processed. This way, the Field activity log helps avoid missed or mistakenly repeated treatments.

logging ag activities in bean field on Crop Monitoring
Logging ag activities in bean field on Crop Monitoring.

The potential of Crop Monitoring is not limited to the above-mentioned features. It can be used throughout the season to monitor crops development from sprouting up to harvesting.