What are the hazards in plastic shed in summer?

plastic shed, commonly known as cold shed, is a simple and practical facility for carrying culture in protected areas. With the development of plastic industry, it is widely used in all countries in the world because of its easy construction, convenient use of plastic shed and less investment. Using bamboo, steel and other materials, covered with plastic film, to build an arched shed for cultivating vegetables can advance or delay the supply, improve the output per unit area, and help to defend against natural disasters, especially in the northern region, which can supply fresh and tender vegetables in early spring and late autumn and off-season. Plastic greenhouse makes full use of solar energy, which has a certain heat preservation effect, and can adjust the temperature and humidity in the greenhouse within a certain range by rolling the film. Therefore, plastic greenhouses are mainly used for heat preservation cultivation in northern China, which can be advanced by 3-35 days in spring and delayed by 2-25 days in Li Neng in autumn, but winter cultivation cannot be carried out; In southern China, plastic greenhouses are not only used for heat preservation and overwintering cultivation of vegetables and flowers in winter and spring, but also can be used for shading and cooling in summer and autumn and for facility cultivation of rain, wind and hail prevention. China has a vast territory and a complex climate. Using plastic greenhouses to cultivate vegetables, flowers and plants has played a special and important role in alleviating the contradiction between supply and demand of vegetables in the off-season, which has significant social benefits and huge economic benefits. Simple bamboo-wood greenhouse This kind of greenhouse is different in different regions, but its main parameters are basically the same as the shape of the greenhouse. The greenhouse has a span of 6-12m, a length of 3-6m, a shoulder height of 1-1.5m and a ridge height of 1.8-2.5m.: According to the width (span) direction of the shed, set up an upright post every 2 meters, the upright post is 6-8 cm thick, and the top is arched, and the buried depth is 5 cm. Pad bricks or tie crossbars, and tamp them. Fix bamboo pieces (poles) at the top of the upright post to form an arch, and add crossbars at both ends to bury them underground and tamp them. The arch spacing is 1m, and they are connected by longitudinal tie rods, forming a whole; The arches are covered with film, and the end of the film after tension is buried in the surrounding soil. The film is pressed between the arches with film pressing lines or No.8 lead wires and bamboo poles. Its advantages are convenient material picking, low cost and easy construction; The disadvantages are many columns in the shed, high shading rate, inconvenient operation, short service life and poor resistance to wind and snow load. Welded steel structure greenhouse In this steel structure greenhouse, the arch frame is a flat plastic greenhouse frame welded by steel bars, steel pipes or a combination of the two, with 16mm steel bars or 6 branches for the upper chord, 12mm steel bars for the lower chord and 9-12mm steel bars for the longitudinal tie rods. The span is 8-12m, the ridge is 2.6-3m high, the length is 3-6m, and the arch is 1-1.2m.. The longitudinal arches are connected and fixed by tie rods or oblique tie rods to form a whole body. The arch frame is covered with film, and after tension, the film is pressed with film pressing wire or No.8 lead wire, and the two ends are fixed on the ground anchor. The greenhouse with this structure has a strong skeleton, no central column, large space in the greenhouse, good light transmittance and convenient operation, and is a relatively good facility. But this kind of skeleton is painted with oil paint to prevent rust, which needs to be painted once every 1-2 years, which is troublesome. If it is well maintained, its service life can reach 6-7 years. Galvanized steel pipe assembled greenhouse The greenhouse skeleton of this structure, its arch bar, longitudinal tie bar and end column are all thin-walled steel pipes, and they are connected with special fixtures to form a whole. All the bars and fixtures are treated with hot dip galvanizing for rust prevention, which is an industrial product produced in factory, and has formed more than 2 series of standard and standardized products. This greenhouse has a span of 4-12 meters, a shoulder height of 1-1.8 meters, a ridge height of 2.5-3.2 meters, a length of 2-6 meters, and an arch spacing of .5-1 meter, which is connected and fixed into a whole by longitudinal tie rods (pipes). Film winding machine can be used for ventilation, thermal insulation curtain, sunshade curtain and cooling. This kind of greenhouse is an assembled structure, which is convenient to build, and can be disassembled and moved, with large space, less shading and convenient operation. Conducive to the growth of tussah; The components are corrosion-resistant, with high overall strength and strong ability to withstand wind and snow, and their service life can reach more than 15 years. It is the most advanced greenhouse structure at present. The following mainly introduces the structure, varieties and installation of galvanized steel pipe assembled plastic greenhouse. There are several kinds of covering materials for greenhouses: ordinary films are made of polyethylene or polyvinyl chloride, with a thickness of .1 mm and colorless and transparent. The service life is about half a year. Multifunctional long-life film Multifunctional long-life film is made by adding proper amount of anti-aging material and surfactant in the process of polyethylene blow molding. The multifunctional membrane produced by Xinguang Plastic Factory in Zhejiang Province has a width of 7.5 meters and a thickness of .6 mm. Its service life is twice as long as that of ordinary membrane, and the greenhouse temperature at night is 1-2℃ higher than other materials. Moreover, the film is not easy to form water droplets, with good covering effect, low cost and high benefit. Grass quilt and grass fan are woven from straw, which has good thermal insulation performance and is a night thermal insulation material. Polyethylene high foaming film is a white plastic film with many bubbles. It is 1 meter wide and .4-.5 cm thick. It is light and can be rolled up, and its thermal insulation is similar to that of grass quilt. Nonwoven non-woven fabric is a kind of polyester filament, which is not woven and spun. There are two kinds of black and white, and they have different densities and thicknesses. The common specification is 5g/? Besides heat preservation, it is often used as a sunshade net. A plastic woven screen. Commonly used are black and silver gray, and there are several density specifications, and the shading rate is different. It is mainly used for sunshade and rainproof in summer, and can also be used for thermal insulation and covering in winter. Choose a place that is sunny, sheltered from the wind, high in dryness, well drained and free from soil infectious diseases. Try to avoid the mechanical damage of the shed film, especially the bamboo shed. Before the film is buckled, the protruding part of the shelf surface should be flattened or wrapped with old cloth. When fixed by spring, a layer of old newspaper should be padded at the card slot. In addition, attention should be paid to avoid long-term contact between old and new films, so as not to accelerate the aging of new films. Be careful when ventilating. Freezing or exposure of the film will promote the aging. When the steel pipe is exposed to the sun in summer, the temperature can rise to 6-7℃, thus accelerating the aging and crushing of the film. In the process of using the film, it is inevitable that there will be holes, so it should be repaired with adhesive or tape in time. Second, the environmental characteristics and control greenhouse is covered with plastic film, which forms a special microclimate that is relatively closed and different from the open field. To cultivate vegetables in greenhouse, we must master the characteristics of the environment in greenhouse and take corresponding control measures to meet the conditions of vegetable growth and development, so as to obtain high quality and high yield. The plastic film has thermal insulation under temperature conditions. After the film is covered, the turbidity in the greenhouse will increase with the increase of the outside temperature and decrease with the decrease of the outside temperature. There are obvious seasonal changes and large temperature difference between day and night. The lower the temperature, the greater the temperature difference. Generally, the daily warming can reach 3-6℃ in the greenhouse in cold season, and the warming capacity is only 1-2℃ on cloudy days or at night. In the warm spring season, the temperature difference between the shed and the open field gradually increases, and the temperature increase can reach 6-15℃. When the outside temperature rises, the temperature of shed meat increases relatively, and the highest temperature can reach above 2℃, so there are high temperature and freezing hazards in the greenhouse, which need to be adjusted manually. In the high temperature season, the high temperature above 5℃ can be generated in the shed. Ventilation is carried out for the whole shed, and the outside of the shed is covered with grass curtains or set up as a "arbor", which is 1-2℃ lower than the open air temperature. On sunny days in winter, the lowest temperature at night is 1-3℃ higher than that in the open field, and on cloudy days, several subjects are the same as those in the open field. Therefore, the main production seasons of greenhouses are spring, summer and autumn. The greenhouse temperature can be kept at 15-3℃ by heat preservation and ventilation cooling. The light transmittance of plastic film with new lighting conditions can reach 8-9%, but the light transmittance decreases by 1-3% during use due to dust pollution, water droplets adsorption, film aging and other reasons. The lighting conditions in the greenhouse are greatly different due to the season, weather conditions, covering methods (shed-shaped structure, orientation, scale, etc.), film types, and the use of old and new conditions. The higher the greenhouse is, the greater the difference of vertical irradiance in the greenhouse, and the difference of irradiance between the upper layer and the ground in the greenhouse is 2-3%. In winter and spring, the light condition of the greenhouse extended from east to west is better than that of the greenhouse extended from north to south, and the local light condition is almost the same. However, the irradiance difference between the north and south sides of the east-west extended greenhouse can reach 1-2%. Different shed structures have a great influence on the light reception in the shed. Although the double-layer film cover has good thermal insulation performance, the light reception conditions can be reduced by about half compared with the single-layer film cover shed. In addition, multi-span greenhouses and the use of different shed materials also have a great impact on the light (Table 4-2-3). As can be seen from the table, the greenhouse with single steel and hard plastic structure receives better light, which only reduces the light transmittance by 28% compared with the open field. The lighting condition of multi-span shed is poor. Therefore, when the materials used to build the shed can bear a certain load, we should try our best to choose light materials and simplify the structure, which can not affect the light, but also protect the solid, economical and practical. Table 4-2-3 Light intensity of different shed types Lux (ten thousand) Light transmittance (%) Single steel structure Single bamboo-wood structure Single hard plastic structure Multi-span reinforced concrete exposed contrast 7.67 6.65 7.65 5.99 1.6472. 62.5 71.9 56.3 1 film will greatly reduce light transmittance due to dust pollution during the covering period. After two days of using the new film, It will be reduced by 25% after 1 days and below 28% after half a month. In general, the light transmittance can be reduced by 1-2% due to dust pollution. When it is seriously polluted, the light received in the shed is only 7%, which makes it unusable. Generally, the film is easy to absorb water vapor and condense into water droplets on the film, which reduces the light transmittance of the film by 1-3%. Therefore, it is important to prevent film pollution and condensed water droplets. Moreover, during the use of the film, due to high temperature, low temperature and the influence of sunlight and ultraviolet rays, the film "aged". After aging, the light transmittance of the film decreased by 2-4%, and even lost its use value. Therefore, the film covered in the greenhouse should be a long-life film with temperature resistance, anti-aging and dust removal, so as to enhance the light reception, increase the temperature and prolong the service life in the greenhouse. Under the humidity condition, the air tightness of the film is strong, so the air temperature in the shed is high due to soil moisture evaporation and crop transpiration after covering. If there is no ventilation, the relative humidity in the shed is very high. When the greenhouse temperature increases, the relative humidity decreases, and the relative humidity increases. In sunny and windy days, the relative temperature is low, and in cloudy and rainy (foggy) days, the relative temperature is high. In the absence of ventilation, the relative humidity in the shed can reach 6-8% during the day, often around 9% at night, and the highest is 1%. The suitable air relative humidity in the shed varies according to different crops, and it is generally required to be maintained at 5-6% during the day and 8-9% at night. In order to reduce the harm of diseases, the humidity at night should be controlled at about 8%. When the relative humidity in the shed is saturated, increasing the shed temperature can reduce the humidity. For example, when the humidity is 5℃, the humidity will be reduced by about 5% for every 1℃ temperature increase, and when the temperature is 1℃, the humidity will be reduced by 3-4% for every 1℃ temperature increase. When the water vapor content in the air in the shed is not increased, the relative humidity is about 7% when the shed temperature is 15℃. When the temperature is raised to 2℃, the relative humidity is about 5%. Because the air temperature in the shed is high and the evaporation of soil is small, the irrigation amount should be reduced in winter, spring and cold season. However, when the temperature in the greenhouse rises, or when the temperature is too high, it needs ventilation, which will cause the humidity to drop, accelerate the transpiration of crops, cause the transpiration rate of plants to decrease due to water shortage, or cause physiological disorders. Therefore, it is necessary to keep proper humidity in the shed according to the requirements of crops. Fourth, the cultivation season and conditions The cultivation of plastic greenhouses is mainly in spring, summer and autumn. The area with the lowest temperature of-15℃--17℃ in winter can be used for cold-resistant crops to prevent cold and overwinter in the shed. In cold and arid areas, greenhouse can be used for cultivation early. In the northern region, seedlings are raised in greenhouses in winter, so that seedlings can be planted in greenhouses in early spring for early cultivation. Sow in summer, postpone cultivation after autumn, and plant two crops a year. Due to the advance of spring and the delay of autumn, the cultivation period in greenhouse is extended for two months. In some frozen areas of Northeast China and Inner Mongolia, planting in spring, pulling seedlings after autumn, and planting one crop all year round, the yield per mu of cucumber is 2-4 times higher than that in the open field. Watermelon planting in greenhouse in Heilongjiang Province was successful. In the windy, sandy and arid areas in the northwest and the border areas of Inner Mongolia, greenhouse is used to achieve year-round production, and cold-resistant vegetables are planted in greenhouse in winter, which creates a precedent for greenhouse planting in winter. In order to improve the utilization rate of greenhouses, planting in early spring and late autumn are often carried out by temporary heating in greenhouses, adding two-layer curtains for cold protection, building sunny beds in greenhouses, adding small arch sheds or middle sheds, covering with plastic films, and covering the periphery of greenhouses with straw curtains, so as to prolong the growth period, increase the number of planting stubbles and increase the yield. Because of the film covering, the air flow and exchange in the shed are limited. Under the condition of tall vegetable plants and lush foliage, the concentration of carbon dioxide in the air in the shed changes dramatically. Before sunrise in the morning, due to crop respiration and soil release, the concentration of carbon dioxide in the shed is 2-3 times higher than that outside the shed (about 33PPM); After 8-9 o'clock, with the enhancement of leaf photosynthesis, it can be reduced to below 1PPM. Therefore, after sunrise, ventilation should be carried out as appropriate to replenish carbon dioxide in the shed in time. In addition, artificial carbon dioxide fertilization can be carried out with a concentration of 8—1PPM, which can be used after sunrise and before ventilation. Artificial application of carbon dioxide, in the case of weak light and low temperature in winter and spring, has a very significant effect on increasing production. In the low temperature season, greenhouses are often sealed and insulated, and it is easy to accumulate toxic gases, such as ammonia, nitrogen dioxide, sulfur dioxide, ethylene and so on. When the ammonia gas in the greenhouse reaches 5PPM, water-soaked spots will appear at the tip of plant leaves, and then turn black and die; When nitrogen dioxide reaches 2.5-3 ppm, irregular green and white spots appear on the leaves, and in severe cases, all the leaves are bleached except the veins. The production of ammonia and nitrogen dioxide is mainly caused by 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 quality of coal. Because the aging of plastic film (plastic pipe) can release ethylene, causing premature senescence of plants, so excessive use of ethylene products is also one of the reasons. In order to prevent the accumulation of harmful gases in the shed, fresh manure can not be used as base fertilizer, nor can it be used as topdressing; It is forbidden to use ammonium carbonate as topdressing. When urea or ammonium sulfate is used as topdressing, it should be watered or covered with soil in time after hole application. The amount of fertilizer should be appropriate and not excessive; Proper ventilation is also needed in low temperature season to eliminate harmful gases. In addition, the quality of coal used is better and it should be fully burned. If possible, use hot air or hot water pipes to heat up and discharge the burnt waste gas outside the shed. The distribution of soil moisture and soil moisture in salt greenhouse is uneven. The soil near the two sides of the shed has more water infiltration outside the shed and higher humidity of water droplets flowing on the shed film. The middle part of the shed is relatively dry. Cucumber and eggplant planted in greenhouse in spring, especially those cultivated with plastic film, often suffer from insufficient soil moisture, which seriously affects the quality. It is best to lay hose drip irrigation belt and apply fertilizer water at any time according to actual needs, which is an effective measure to increase production. Due to the long-term coverage of the greenhouse and the lack of rainwater leaching, the salt moves from bottom to top with the groundwater, which is easy to cause excessive accumulation of salt in the cultivated soil and cause salinization. Therefore, it is necessary to pay attention to proper deep tillage and application of organic fertilizer to avoid long-term application of fertilizers containing chloride ions or sulfate ions. Topdressing should be light, and it is best to test soil and fertilize. Every year, there should be a certain period of time without covering the film, or in summer, only covering the sunshade net for shading cultivation, so that the soil can be leached by rain. When soil salinization is serious, flooding can be used to suppress salt, and the effect is very good. In addition, use none.