Comparative Analysis On The Environmental Performance Of Two Large-span Plastic Greenhouses And The Overwintering Cultivation Effect Of Leafy Vegetables In Ningxia

In order to improve the thermal insulation performance of the plastic greenhouse and operate mechanically,this paper designs a plastic greenhouse with low walls.There are 1.2m high low walls on the east and west sides of the greenhouse..Analyze the environmental advantages of T1 greenhouse by measuring the environmental factors such as illumination intensity,temperature,and relative humidity in the plastic greenhouse with low walls(T1 greenhouse),and using the same span but no low-wall plastic greenhouse(T2 greenhouse)as a control;On the basis of T1 greenhouse,two different environmental control methods(increasing the inner insulation screen and the mode of uncovered insulation)were adopted to analyze the thermal insulation and energy-saving properties of the inner insulation screen,and screened out a more superior insulation uncovered mode:at the same time,with Five leaf vegetables(spinach,rapeseed,lettuce,Chinese cabbage,chrysanthemum)were used as test crops to compare the overwintering effects of two plastic greenhouses.The test results are as follows:(1)The difference in light intensity in the two plastic greenhouses is small.The average daily illumination intensity of T1 greenhouse and T2 greenhouse were 201601x and 195921x,and the outside of the greenhouse was respectively 468961x.The uniformity of light distribution in the north-south direction of the greenhouses is above 95%,and the uniformity of light distribution in the east-west direction is below 58%.(2)The temperature of T1 greenhouse was significantly higher than that of T2 greenhouse and outside the greenhouse.In winter,when the extremely low temperature outside the greenhouse is-22.7℃.the extremely low temperature inside T1 greenhouse and T2 greenhouse are-1.1℃ and-3.3℃respectively.From mid-December to early March,the average temperature inside T1 greenhouse was 2.0℃ and 16.1℃ higher than that in the control greenhouse and outside the greenhouse respectively,and the ground temperature was 1.3℃and 11.5℃ higher than the control greenhouse and outside the greenhouse respectively.On sunny days,the temperature in T1 greenhouse was 2.3℃ and 18.8℃higher than that in T2 greenhouse and outside the greenhouse,and the ground temperature was 1.2℃higher than that in T2 greenhouse;on cloudy days,the temperature in T1 greenhouse was 1.6℃ and 11.9℃ higher than that in T2 greenhouse and outside the greenhouse respectively,and the ground temperature was 1.0℃ higher than that in T2 greenhouse.The temperature at 02:00、08:00、20:00 rose with the increase in altitude,and the temperature in the north-south direction showed north>central>south.The fluctuation of ground temperature decreased with the increase of depth.(3)The relative humidity of T1 greenhouse is higher than that of T2 greenhouse and outside the greenhouse.From mid-December to early March,the ten-day average relative humidity of T1 greenhouse was 91%,which was 3%and 37%higher than that in T2 greenhouse and outside the greenhouse respectively.The relative humidity of T1 greenhouse was higher than that of T2 greenhouse in different weather.(4)The impact of two environmental regulation methods on the environment in the greenhouse.Installing an inner thermal screen can remarkably raise the temperature in the greenhouse and decrease the relative humidity.On sunny and cloudy days,the mean night air temperature in the greenhouse increase by 1.3℃ and 1.0℃ respectively,while the mean night relative humidity decrease by 9.8%and 8.1%respectively,and the fractional energy saving rates are 18%and 4%,The air temperature in the greenhouse can be effectively increased by adopting the mode of exposing the thermal insulating coverings on the east and west sides at intervals.The mean daily air temperature in T1 greenhouse and T2 one increase by 1.6℃ and 0.7℃ respectively,and the mean night air temperature increase by 0.5℃and 0.3℃ respectively.When increasing the heat storage of soil in the greenhouse during the day,the cumulative heat absorption of soil in T1 greenhouse and control greenhouse during the day increased respectively0.091MJ·m-2、0.089MJ·m-2.(5)The impacts of two greenhouses on growth and development of leafy vegetables.Compared with T2 greenhouse,the growth period days of spinach,rape,lettuce,pakchoi and crowndaisy chrysanthemum in T1 greenhouse were shortened by 8 days,10.8 days,12.6 days,9.6 days and 10.6 days,respectively.The plant height,leaf number and leaf area of leafy vegetables were all higher than those in T2 greenhouse,and the chlorophyll content was lower than that in T2 greenhouse.The average yields of spinach,rape,lettuce,pakchoi,and chrysanthemum in T1 greenhouse were 101%,83%,114%,114%,and 119%of T2 greenhouse,respectively.(6)The best sowing date for different leafy vegetables in the two plastic greenhouses were obtained using the membership function analysis.The best sowing date of spinach and chrysanthemum in T1 greenhouse is October 30,lettuce and Chinese cabbage is November 9,and rape is November 19;in T2 greenhouse,spinach,lettuce and chrysanthemum are October 30,Chinese cabbage is November 19,and rape is November 29.Regression analysis was used to show that leafy vegetables in the plastic greenhouses in winter were greatly influenced by average temperature,average geothermal and average humidity.In summary,plastic greenhouse with stub walls can significantly increase temperature and geothermal,and have better thermal insulation capacity,more suitable for promotion in Ningxia;increasing inside thermal screen and adopting the mode of separating insulation on the east and west sides can effectively improve the superiority of the greenhouse environment.This study can provide theoretical basis and reference for the use of plastic greenhouses for vegetable production in winter in Ningxia.