The changing law of the air humidity in the greenhouse:
The plastic film has a strong sealing property, the exchange between the air in the shed and the outside air is hindered, and the moisture of soil evaporation and leaf surface evaporation is difficult to disperse. Therefore, the humidity in the shed is large. During the day, under the ventilation of the greenhouse, the relative humidity of the air in the shed is 70-80%. It can reach more than 90% on rainy days or after irrigation. The relative humidity of the air in the shed decreases with increasing temperature, and it is often 100% at night. When the humid air in the shed is cold, it condenses into a water film or water droplets and attaches to the inner surface of the film or plants.
Control of air humidity:
In single span plastic tunnel greenhouse, the air humidity is too high, not only directly affects the photosynthesis of vegetables and the absorption of mineral nutrients, but also conducive to germination and infection of germ spores. Therefore, ventilation should be performed to promote the exchange of high-humidity air in the shed with low-humidity air outside, which can effectively reduce the relative humidity in the shed. Heating the geothermal line in the shed can also reduce the relative humidity. Using drip irrigation technology, combined with mulching film cultivation, to reduce soil moisture evaporation can greatly reduce air humidity (about 20%).
Air composition in the shed:
Due to the film covering, the airflow and exchange in the shed are restricted. In the case of tall vegetable plants and lush foliage, the carbon dioxide concentration in the air in the shed changes drastically. Before sunrise in the morning, due to crop respiration and soil release, the carbon dioxide concentration in the shed is 2 to 3 times higher than that outside the shed (about 330PPM); after 8 to 9 o'clock, with the enhancement of leaf photosynthesis, it can be reduced to below 100PPM. Therefore, after sunrise, it is necessary to ventilate and ventilate as appropriate to replenish the carbon dioxide in the shed in time. In addition, artificial carbon dioxide fertilization can be carried out at a concentration of 800-1000PPM, which can be used after sunrise and before ventilation. When carbon dioxide is applied artificially, the effect of increasing production is very significant under weak light and low temperatures in winter and spring.
In the low-temperature season, the greenhouse is often sealed and insulated, and it is easy to accumulate toxic gases, such as ammonia, nitrogen dioxide, sulfur dioxide, ethylene, etc., causing harm. When the ammonia in the greenhouse reaches 5PPM, the leaf tip of the plant will produce water-like spots, and then it will become black and die; when the nitrogen dioxide reaches 2.5─3PPM, the leaves will have irregular green and white spots, except for the veins. The leaves are bleached. The production of ammonia and nitrogen dioxide is mainly due to the improper use of nitrogen fertilizer. The production of carbon monoxide and sulfur dioxide is mainly caused by heating with coal fire, incomplete combustion, or poor coal quality. As the aging of the film (plastic tube) can release ethylene and cause premature plant senescence, excessive use of ethylene products is also one of the reasons.
Single Span Plastic Tunnel Greenhouse
In order to prevent the accumulation of harmful gases in the shed, it is not possible to use fresh manure as the base fertilizer or use the unripe manure as the top dressing. Cover the soil in time; the amount of fertilizer should be appropriate and not excessive; the low-temperature season should also be properly ventilated to eliminate harmful gases. In addition, the quality of the coal used is better and must be fully burned. If necessary, use hot air or hot water pipes to heat up and discharge the exhaust gas after burning out of the shed.
Soil moisture and salinity:
The soil moisture distribution in the greenhouse is uneven. The soil near the two sides of the scaffold, due to the greater water penetration outside the shed, and the greater humidity of the water droplets on the shed membrane. The middle of the shed is relatively dry. Cucumbers and eggplants planted in greenhouses in spring, specially cultivated with plastic film, often suffer from a serious impact on soil quality due to insufficient soil moisture. It is best to be able to lay hose drip irrigation tape and apply fertilizer water at any time according to actual needs, which is an effective production increase measure. Due to the long-term coverage of the greenhouse and the lack of rain leaching, the salinity moves up and down with the groundwater, which can easily cause excessive accumulation of salinity in the soil of the farming layer and cause salinization. Therefore, it is necessary to pay attention to proper deep cultivation, apply organic fertilizers, and avoid long-term application of fertilizers containing chloride ions or sulfate ions. Topdressing should be light, and soil testing and fertilization are best. There is a certain period of time each year without covering the film, or in the summer, only the shading net is covered for shading cultivation, so that the soil is leached by rain. When the soil salinization is severe, flooding can be used to press salt, the effect is very good. In addition, the use of the indoor hydroponic system is a fundamental measure to prevent soil salinization.