Soil Solarization in Greenhouses: Pest and Disease Control

Repeated cropping in the same soil block allows soil-borne pathogens — Fusarium, Sclerotinia, Phytophthora — and root-knot nematodes to build up season after season. Chemical soil disinfestation can knock these populations down quickly, but cost, handling complexity and environmental concerns make alternatives worth pursuing. Soil solarization is a low-input, chemical-free approach that uses solar energy to do much of that work.

The biological control in greenhouses guide covers solarization as one layer of a wider integrated pest management programme. This article focuses specifically on how solarization works, the conditions it needs, what it reliably suppresses and where its limits lie.

What exactly is soil solarization?

Soil solarization means covering moist soil with transparent polyethylene film and letting accumulated heat suppress soil-borne organisms. The scientific basis is well-established: near-surface soil temperatures under clear film during peak summer regularly exceed the lethal threshold for a range of fungal pathogens and nematode stages.

The method was first studied in Israel in the late 1970s and has since spread across Mediterranean, tropical and subtropical production regions. In our field work in Turkey’s southern and western coastal greenhouses, we have consistently observed meaningful suppression of Fusarium wilt incidence and root-knot nematode pressure where solarization was applied correctly.

What conditions does solarization require?

Reaching pathogen-lethal temperatures depends directly on solar radiation and ambient air temperature. In Turkey’s Mediterranean belt, early June through late July is the most reliable window; in the Aegean and Marmara coastal zones that window narrows somewhat.

Successful treatment requires direct solar radiation for an average of 8–10 hours per day with minimal cloud cover. Under clear film, daytime surface temperatures can reach 50–55 °C; this drops with depth — typically 40–45 °C at 10 cm and 35–40 °C at 20 cm. What matters is that these temperatures are sustained for at least four weeks; a single hot day provides no lasting suppression.

Keeping the soil moist under the film is critical. Dry soil conducts heat poorly; wetted soil increases both thermal conductivity and pathogen sensitivity to lethal heat. Irrigate thoroughly before laying the film, allowing 24–48 hours for moisture to reach lower layers.

Which soil-borne organisms does solarization suppress?

The most reliable effects are on organisms in the top 0–15 cm:

• Soil-borne fungi: Fusarium oxysporum, Sclerotinia sclerotiorum, Rhizoctonia solani and some Phytophthora species are suppressed. Durable resting structures in deep layers — Sclerotinia sclerotia, for instance — may survive.
• Root-knot nematodes (Meloidogyne spp.): Egg masses and second-stage juveniles in upper layers show marked suppression. Deep-layer populations are less consistently affected.
• Weed seeds: Shallow-layer seeds lose viability. Seeds buried below the heated zone are unaffected; deep tillage after solarization would bring untreated seeds back to the surface.
• Some bacterial pathogens: Partial effects on Verticillium, some Pythium species and Agrobacterium have been reported; results are more variable than fungal suppression.

The key limitation: meaningful heat accumulation rarely occurs below 20–25 cm. Deep root pathogens, deeply positioned nematode populations and durable structures remain largely untouched.

How is solarization applied, step by step?

No specialised equipment is needed, but preparation matters.

Step 1 — Remove crop residues

All previous-crop debris — especially infected material — must be collected and removed from the greenhouse before solarization. As covered in greenhouse hygiene and cultural measures, leaving infected residues defeats the purpose: solarization works on living organisms in the soil, not on surface debris. Break up large soil clods with shallow tillage if needed.

Step 2 — Saturate the soil

Irrigate until the soil is fully moist through the top 25–30 cm. A drip system or overhead irrigation can both work; allow 24–48 hours for the moisture to distribute evenly.

Step 3 — Lay the film

Stretch transparent polyethylene over the bed or planting area and secure edges tightly with soil, sandbags or weighted planks to prevent air entry. Overlapping strips must seal without gaps — air pockets reduce heat accumulation under the film.

Step 4 — Treatment period

Leave the film in place for 4–6 weeks. Extending to 6 weeks improves suppression in deeper layers; beyond 6 weeks the incremental gain is usually minor. Repair any tears promptly.

Step 5 — Remove film and prepare for planting

After removing the film, avoid deep tillage: inverting the soil moves surviving inoculum from deep layers back to the surface, undoing part of the work. Prepare the top layer minimally and wait for soil temperature to return to planting range (usually 1–2 weeks after film removal) before transplanting.

Which film type should you choose?

Colour and thickness both affect performance:

• Clear (transparent) polyethylene: Transmits solar radiation directly to the soil surface and delivers the highest heat accumulation. This is the standard choice for solarization.
• Black polyethylene: Blocks weed germination but absorbs rather than transmits radiation — it cannot build the soil temperatures needed for effective solarization.
• Double-layer or bubble film: Some trials report up to 20 °C additional heat accumulation with a double clear layer. Cost-benefit depends on specific conditions and film availability.

On thickness: 25 micron transmits heat best but is fragile in large or windy structures. In our field experience, 40–50 micron provides a workable balance between heat transmission and handling durability.

What are the limitations of solarization?

Deep-layer protection is limited: Lethal temperatures rarely penetrate below 20–25 cm consistently. Pathogens with deep root involvement, deeply positioned nematode populations and thermally durable structures are unlikely to be fully suppressed.

Seasonal constraint: Effective treatment requires the greenhouse to stand empty during peak summer for 4–6 weeks — a scheduling conflict for growers with year-round production cycles.

Re-contamination risk: After the film is removed, clean soil is easily re-contaminated through soil-carrying transplants, irrigation water, tools or worker movement. The hygiene and monitoring practices described in integrated pest management basics remain just as important after solarization as before.

Impact on beneficial microflora: Competing microorganisms are also suppressed. This is partly why post-solarization inoculation with beneficial agents can be advantageous — but it also means that solarization is not simply beneficial for all soil biology.

Should solarization be combined with biological agents afterwards?

This combination is one of the most practically interesting questions from our field observations. The logic is straightforward: solarization reduces competing microflora, creating openings that subsequently applied beneficial organisms — Trichoderma harzianum, Bacillus subtilis group — can colonise more effectively than in an undisturbed soil. International trial data support improved Trichoderma colonisation following solarization.

The recommended sequence:

1. Complete solarization and remove the film.
2. Prepare the top layer minimally — no deep inversion.
3. Apply Trichoderma or Bacillus formulations at or just before transplanting, once soil has cooled to planting temperature.

This combination carries real potential for long-term Fusarium suppression in greenhouses with a history of that pathogen. Local validation matters: results vary by crop-pathogen combination and soil type, so treat it as a hypothesis to test rather than a universal recipe.

How does the approach change by greenhouse structure?

Soil beds and direct-soil production: Solarization is straightforward to apply and delivers its strongest results here. Wide beds may need multiple overlapping strips — seal the overlaps carefully.

Bag or pot substrate systems (coir, perlite, etc.): Solarization can be applied to the floor soil beneath the bags, but does not treat the substrate inside the bags. Season-to-season substrate disinfestation requires a different protocol.

Concrete or hard-floor sections: Solarization is not meaningful where there is no exposed soil. Steam pasteurisation or biochemical approaches are alternatives for these areas.

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Soil solarization is a low-cost, practical biological control strategy in greenhouse regions with ample summer sun and a production calendar that allows a 4–6 week empty period. It is one layer, not a standalone solution — combined with hygiene, monitoring and biological agents, as outlined in the greenhouse biological control programme, it delivers its strongest results. Knowing which pathogens dominate your soil is the essential first step to judging whether solarization will pay off.