Humidity Control Technology in Greenhouse Vegetable Production

Humidity Control Technology in Greenhouse Vegetable Production

Crop production in greenhouses is carried out in a relatively closed state. As a result, most of the evaporated water vapor on the surface stays in the shed, which leads to a much higher relative humidity than open-air cultivation. At the same time, the high humidity environment is the direct cause of many diseases. For this reason, it is very important to understand the correct dehumidification techniques.

1 Greenhouse humidity change rules

In a sealed state, the greenhouse is airtight and watertight, and the humidity in the greenhouse can often be maintained at 80% to 90%. Analysis of the change regulation of the humidity in the greenhouse has a great relationship with the change of temperature. Typically, the higher the temperature in the shed, the lower the relative humidity. The lower the temperature in the shed, the higher the relative humidity. The relative humidity is low on sunny days and high on rainy days. In spring, with the rapid rise of the shed temperature after sunrise every day, plant transpiration and soil evaporation will intensify. If no ventilation is performed, the amount of water vapor (absolute humidity) in the shed will increase greatly. After ventilation, the humidity in the shed will drop until the afternoon wind shuts down. , the humidity drops to the lowest point; at night, when the temperature drops, the shed film will condense a large number of water droplets, and the relative humidity will reach a saturated state. When the relative humidity in the shed reaches a saturated state, increasing the shed temperature can reduce the humidity. Studies have confirmed that when the temperature in the shed is 5 to 10 °C, the relative humidity will decrease by 3% to 4% for every 1 °C temperature increase.

2 Humidity control technology in greenhouse vegetable production

2.1 Dehumidification technology

2.1.1 Cover with mulch. By mulching, the loss of water in the soil can be greatly reduced, and the purpose of controlling the air humidity in the shed can be achieved. Studies have confirmed that: covering with plastic film can increase the ground temperature (at 10cm) by 2 to 3 °C. Therefore, it is recommended to widen the width of the mulch film, and cover the cultivation areas and ditches with mulch film.

2.1.2 Ventilation and air exchange. The best way to dehumidify the shed is to ventilate. In the early stage of greenhouse construction, ventilation openings and ventilation walls should be designed in advance. If necessary, add ventilation holes in the middle of the greenhouse. Through the communication with the outside air, the purpose of dehumidification is achieved. The design ensures that the film can be buckled down to block the insect net when necessary. For even-arched sheds, skylights should be designed to increase ventilation. Within a day, the best time for ventilation and dehumidification is noon, because the humidity inside and outside the shed is very different during this period, and the moisture is easier to discharge; in other periods, the ventilation time should be extended as much as possible under the premise of ensuring the temperature requirements. In addition, special attention should be paid to strengthening ventilation and dehumidification 2 to 3 days after watering in the greenhouse, 1 to 2 days after foliar spraying of fertilizer (medicine), rainy (snow) days before and after sunset.

2.1.3 Reduce spraying. If there are pests and diseases in the shed, use medicine to prevent and control them. The choice of medication method also affects the humidity in the shed. In consideration of dehumidification, it is recommended to use the smoke method and dust method. It must be treated by spraying. It is recommended to choose it in the morning on a sunny day, and leave enough time to ventilate and remove moisture after spraying.

2.1.4 Manual dehumidification. If the humidity in the shed is too high, the general dehumidification effect is not good, and the purpose of humidity control can be achieved by laying hygroscopic materials manually. For example: quicklime, wheat straw, plant ash, fine dry soil, etc., all have a good effect of absorbing moisture, which is conducive to controlling the humidity in the shed.In addition, the use of industrial dehumidifiers can achieve efficient and fast dehumidification.

2.1.5 Cultivation and loosening of soil. The purpose of dehumidification is achieved by cultivating and loosening the soil. The principle is that by cultivating the furrows and ridge backs, the capillaries in the soil are cut off to reduce the upward movement of water in the lower layer, so as to control the evaporation of water and reduce the humidity in the shed. It is best to arrange cultivating and loosening the soil at noon on a sunny day, and the effect of dehumidification will be better.

2.1.6 Temperature and humidity reduction. The humidity in the shed can be controlled by heating. For this reason, it is recommended not to ventilate in the morning, and to heat up without harming farming. In this way, as the temperature increases, the humidity naturally decreases. When the temperature reaches a suitable temperature, start ventilation and dehumidification.

2.1.7 Use drip-free film. The use of drip-free film can effectively reduce the amount of water accumulated on the surface of the film, overcome the disadvantage of a large number of water droplets attached to the inside, and effectively control the humidity in the shed. Moreover, the light transmittance is better, which is conducive to increasing the temperature in the shed. If a normal film is used, a layer of de-dropping agent can be applied to the surface. Alternatively, regularly smear soybean powder, milk powder, etc. on the surface of the film to reduce the amount of water accumulated on the surface and achieve the purpose of controlling humidity.

2.1.8 Ridge cultivation. Raise high ridges to increase surface area. The long exposure time during the day will absorb excess heat from the air, which is conducive to raising the ground temperature and better evaporating the water in the soil, so as to ensure that the humidity is not too high.

2.1.9 Control water and reduce humidity. Control the amount of irrigation water, reduce the number of watering, and play the purpose of reducing the humidity in the shed. On the basis of meeting the water demand of crops, the frequency of water use should be minimized, so that it is better to be dry than wet. Watering should be done with "five watering and five no watering": water on sunny days, but not on cloudy days; water before noon, but no water in the afternoon; water a little, but not flood; in winter, when the water temperature is low, watering should be done first. Preheat in the shed, and then pour it when the water temperature is close to the ground temperature; pour dark water, not open water. At the same time, we must vigorously promote new irrigation technologies such as drip irrigation and sub-film irrigation.

2.2 Indoor humidification technology

In some cases, in order to meet the needs of crop growth, the shed needs to be humidified. For example: newly grafted seedlings, new cutting crops, etc., are more conducive to survival in a high humidity environment. In winter, the heating system is installed, the temperature is high and the relative humidity is low, and humidification is also required.

The most common humidification methods are ultrasonic humidification and high pressure fog humidification. The micro-fog humidifier is to atomize and suspend the water vapor in the air under high pressure, and achieve the purpose of increasing the humidity through rapid evaporation. Duokai's high-pressure micro-fog humidifier has been widely used in greenhouses and mushroom cultivation.

When the greenhouse is sealed, the humidity in the greenhouse can often be maintained at 80%-90% because it is airtight and watertight. In a high temperature and high humidity environment, it is not very conducive to the growth of crops, and it provides convenience for the breeding and reproduction of certain pests and diseases. To this end, according to the needs of crop growth and development, it is particularly important and critical to adjust the temperature and humidity in the shed properly.