In winter and spring, the temperature is low, and the air circulation inside greenhouses is poor, leading to high humidity. This environment makes vegetables susceptible to harmful gases, which can hinder growth or even cause them to wither. It's crucial to manage these conditions effectively. When heating with coal in greenhouses, toxic gases such as carbon monoxide and sulfur dioxide are often released due to low-quality coal, improper heating techniques, or unfavorable wind directions.
These gases pose three main types of harm. First, hidden poisoning: plants show no visible symptoms, but their ability to photosynthesize decreases, and quality declines. Second, chronic poisoning: gas enters through leaf stomata, causing brown spots around them, and yellowing of the leaf surface. Third, acute poisoning: similar to nitrous acid damage, seedlings turn white, and cells in leaves, margins, and veins die, eventually killing the plant. These gases also endanger human health, so improving heating systems and ensuring proper ventilation is essential.
When using coal for heating, it must be fully burned, and flues should be checked for leaks. During temporary warming, a chimney should be installed to vent harmful gases. Biogas digesters can be built near greenhouses to replace coal heating during cold months. This reduces carbon monoxide emissions while increasing temperature and providing COâ‚‚ as a natural fertilizer.
Excessive use of chemical fertilizers or improperly applied farmyard manure can release ammonia, which is toxic to vegetables. Ammonia absorbed by plants can lead to leaf burns, browning, and wilting. At 4000ppm, ammonia can kill entire plants. Nitrogen-based fertilizers like urea and ammonium sulfate may also release ammonia under high temperatures, causing leaf damage. In some cases, nitrite gas forms, leading to nitrous acid poisoning. Sensitive crops like eggplants, tomatoes, and peppers may show symptoms 10–15 days after application, starting with water-like burns on lower leaves, followed by white veins and severe leaf loss.
To reduce ammonia risks, use high-quality organic fertilizers, apply phosphorus and potassium appropriately, and minimize nitrogen use. Avoid raw manure, human waste, and chicken manure in greenhouses. Follow the "base fertilizer, supplemented by top dressing" principle. Apply all fertilizers once before covering the greenhouse in October, mix thoroughly, spread evenly, and avoid over-application. When applying urea, limit it to 0.6 kg per 10 square meters. Deeply bury the fertilizer, cover with soil, and water promptly to prevent toxicity.
Proper ventilation is key. Open vents during midday when temperatures are higher, and even on cloudy or rainy days, ventilate briefly. Don’t skip ventilation just because it’s cold. Plastic films used in greenhouses may emit harmful gases like ethylene, chlorine, or phthalates when exposed to heat. These gases can enter plants through leaf pores, damaging cell structures and reducing yield and quality. For example, 2-isobutylphthalate in water droplets can cause poisoning when absorbed by vegetables. Chlorine at 0.1 mg/l can harm most crops within 4 hours, causing white or brown spots on leaves. Ethylene above 1 mg/l can lead to yellowing and eventual death of leaves.
Use only tested, non-toxic plastic films approved by relevant authorities. Pesticides like Bordeaux mixture and organophosphorus compounds are safe in open fields but can be dangerous in enclosed greenhouses. High temperatures may cause copper pesticides to release toxic ions, while trichlorfon may produce free chlorine, leading to plant damage. Always take precautions to avoid these risks.
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