In winter and spring, the temperature is low, and the air circulation inside greenhouses is often poor, leading to high humidity. This environment makes vegetables more susceptible to harmful gases, which can hinder their growth or even cause them to wither. Effective control is essential to prevent such issues.
When heating greenhouses using coal, toxic gases like carbon monoxide and sulfur dioxide are commonly released due to poor-quality coal, improper heating methods, or unfavorable wind conditions. These gases pose three main types of risks: first, hidden poisoning, where plants show no visible symptoms but experience reduced photosynthesis and lower quality; second, chronic poisoning, which causes brown spots around leaf stomata, yellowing of leaves, and gradual damage; third, acute poisoning, characterized by white cotyledons in seedlings, cell death in leaves, and eventual plant death. These gases also pose a risk to human health, making it crucial to improve heating systems and ensure proper ventilation.
To reduce gas emissions, coal should be fully burned, and flues must be checked for leaks. When temporary heating is needed, a chimney should be installed to vent harmful gases. Alternatively, biogas digesters can be used to replace coal, reducing carbon monoxide production while providing additional carbon dioxide for plant growth.
Excessive use of chemical fertilizers, especially raw manure, can release ammonia, which is toxic to vegetables. Ammonia concentrations above 1000 ppm can burn leaves, and at 4000 ppm, entire plants may die. Nitrogen-based fertilizers like urea and ammonium sulfate also release ammonia under high temperatures, potentially causing leaf damage. In low ammonia conditions, nitrite gas can form, 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 the leaves.
To manage this, farmers should use high-quality organic fertilizers, apply phosphorus and potassium appropriately, and minimize nitrogen use. Avoid using raw manure, chicken manure, or uncomposted waste. Fertilization should follow the "base fertilizer plus top dressing" principle, with a single application before covering the greenhouse. Urea should not exceed 0.6 kg per 10 square meters, and top dressing should involve deep digging, soil coverage, and timely watering.
Proper ventilation is also key. Open vents during midday when temperatures are higher, and briefly ventilate even on cloudy or rainy days. Avoid long periods without ventilation, especially in cold weather. Additionally, plastic films used in greenhouses may release toxic substances like ethylene, chlorine, and phthalates under high heat. These gases can enter plants through leaf pores, damaging cells and reducing yield and quality. For example, 2-isobutylphthalate in water droplets can cause toxicity when absorbed by vegetables. Chlorine concentrations as low as 0.1 mg/l can harm most crops within 4 hours, causing leaf discoloration and shedding. Ethylene levels above 1 mg/l may lead to yellowing and eventual plant death.
To avoid these issues, only use certified, non-toxic plastic films. Pesticides like copper-based products and organophosphates are safe in open fields but can become harmful in enclosed greenhouses due to heat and poor air circulation. Copper pesticides may release toxic ions at high temperatures, while trichlorfon can produce free chlorine ions, leading to plant damage. Careful selection and application of pesticides are essential to protect both plants and the environment.
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