The gas density of the plastic film is strong, so the soil moisture evaporation and crop transpiration in the greenhouse result in high temperature, such as without ventilation, the relative humidity in the greenhouse is very high.
When the greenhouse temperature rises, the relative humidity decreases, and the greenhouse temperature decreases and the relative humidity increases. When the weather is sunny and windy, the relative temperature is low, and the relative temperature increases in cloudy and rainy days. Under the condition of no ventilation, the relative humidity in the shed can reach 60 – 80% during the day, and 90% at night, the highest relative humidity can up to 100%.
The suitable relative humidity of air in the greenhouse varies according to the type of crops. Generally, it is maintained at 50 – 60% during the day and 80 – 90% at night. In order to reduce the harm of disease, night humidity should be controlled at around 80%. When the relative humidity in the shed reaches saturation, raising the greenhouse temperature can reduce the humidity. If the humidity is 5℃, the humidity is reduced by about 1 ℃, and the humidity is reduced by about 5%. When the temperature is 10℃, the temperature is increased by 1℃ and the humidity is reduced by 3 to 4%.
When the water vapor content in the air is not increased, the relative humidity of the greenhouse temperature is about 7% when the temperature is 15℃. When the temperature is increased to 20℃, the relative humidity is about 50%. Because the air temperature in the greenhouse is large and the evaporation capacity of the soil is small, it is necessary to reduce the amount of irrigation in winter and spring. However, the increase of temperature in the greenhouse, or the need for ventilation when the temperature is too high, will cause the decrease of humidity, accelerate the transpiration of the crops, and cause the decrease of the transpiration rate of water shortage in the plant, or the physiological maladjustment. Therefore, the suitable humidity must be kept in the greenhouse according to the requirements of crops.
In order to prevent high humidity in the greenhouse, eight ways can be adopted:
1. use non dripping film covering: no drip film can overcome the disadvantages of a large amount of water droplets attached to the inside, and improve the transmittance of the film, which is 10% to 15% higher than that of the general agricultural film, which is beneficial to temperature increasing and humidification.
2. plastic mulching film: Mulching Film can greatly reduce the evaporation of soil moisture, especially the whole mulch film, which is an important measure to reduce the humidity inside the greenhouse.
3. use drip irrigation under mulch: mulch drip irrigation combines the common advantages of plastic film mulching and drip irrigation, which is the most effective measure to reduce humidity in the greenhouse. The method is to put plastic drip irrigation tapes into the middle of the border and cover the plastic film.
4. reasonable watering: water is the main factor that leads to the increase of humidity in the greenhouse. In winter and spring, the water can be poured in sunny days. Water should be strictly controlled and the humidity in the shed is too high. Every time let in fresh air properly and the soil is released in time, it can reduce the soil humidity and reduce the humidity of the air.
5. ventilation and dehumidification: ventilation is the simplest way to reduce the air humidity inside the greenhouse. Ventilation must be carried out at high temperature, otherwise the greenhouse temperature will drop. If the humidity decline is too fast in ventilation, we should close the air vent in time to prevent the temperature from falling suddenly and harming the crops.
6. increasing the light transmittance: increasing the light transmittance can increase the humidity of the shed, and the humidity of the air will decrease after the humidity of the greenhouse rises. Generally speaking, air humidity decreases with increasing temperature.
7. hang the reflective screen: the reflective screen in the shed can not only increase the light intensity, but also improve the ground temperature and air temperature. Because the relative humidity of the air decreases with the increase of temperature, therefore, hanging the reflecting screen in the shed also has the effect of reducing the humidity of the air.
8. the use of dust method or smoke method: the humidity of the air in the shed is large, and the use of conventional spray method will increase the humidity in the greenhouse, which is unfavorable to the prevention and cure of the disease. The use of dust or smoke method can avoid increasing air humidity, due to spray and improve control effect.