The principle of sunshade net mulching cultivation is to optimize the growth environment of covered crops, to resist and mitigate the impact of disastrous weather, and to achieve high, stable and high-quality cultivation of crops by utilizing the functions of sunshade net in summer, cooling, freezing-proof in autumn and winter, and mechanical wind, rain and insect-proof. The main effects of sunshade net mulching cultivation are as follows:
1. Shading and Cooling
In the southern part of China, the daily light intensity in summer is usually as high as 80,000-120,000 lux, accompanied by absolute high temperatures above 30, or even above 35 degrees Celsius. For general vegetables, including leafy vegetables, root vegetables and thermophilic fruits and vegetables, the light saturation point is more than 30,000-50,000 lux. Therefore, in hot summer, when the light intensity exceeds the light saturation point of vegetables, it often inhibits the photosynthesis of crops. Under the influence of high temperature, plant chlorophyll was destroyed, respiration increased sharply, transpiration was blocked and physiological metabolism was imbalanced, which led to plant decay.
According to different latitude density and color, the shading rate of sunshade net is usually between 25% and 75%. Under the condition of high light intensity in summer, light intensity can be adjusted by covering to meet the needs of crop photosynthesis. At the same time, the shading of the sunshade net also plays a cooling role. According to observation, the cooling effect is remarkable after covering with sunshade net in summer and autumn. Under the hot summer sun, the general temperature can be reduced by 4-6 degrees C, the ground temperature can be reduced by 5-8 degrees C, and the surface temperature can be reduced by 8-14 degrees C, which can make the temperature and ground temperature in the covering facilities fall below the highest temperature of general vegetables, and is conducive to promoting the growth of summer vegetables. Different sunshade net with different colors and different covering forms have different cooling effects. The cooling effect of covering black sunshade net outside plastic greenhouse is better than that of silver-gray sunshade net; the cooling effect of covering silver-gray sunshade net inside plastic greenhouse is better than that of black. Floating surface covering is the best way to reduce ground temperature with black sunshade net.
2. Windbreak and Rain Resistance
After covering with sunshade net, it can effectively withstand the attack of summer typhoon rainstorm and play a better role in protecting vegetables. When rainstorm occurs, raindrops are scattered through sunshade net weaving, and become light raindrops scattered into the net. Heavy rain outside the net becomes light rain inside the net, and light rain outside the net becomes light rain inside the net, which effectively reduces the direct impact of raindrops. At the same time, the rainwater in the net is also reduced due to the water distribution of some Raindrops Through sunshade net. Therefore, sunshade net coverage can effectively avoid the direct impact of Rainstorm on plants, prevent the surface soil from compacting and reduce the degree of damage caused by rainstorm impact. According to the measurement of Zhenjiang Meteorological Station in Jiangsu Province on August 4, 1986, under the condition of 34.6 mm rainfall within 100 minutes on that day, the rainfall at the edge and middle of the greenhouse covered by sunshade net was 30.0 mm and 26.7 mm, respectively, which decreased by 13.3% and 22.8%. At the same time, the impact force of water droplets covered by sunshade net on the ground is only 1/50 of that on the open ground.
The sunshade net has high mechanical strength and good permeability, so it has good wind-proof effect. When encountering strong wind, it can slow down wind speed, reduce evaporation of soil moisture, play a role in picking, and also prevent direct damage to crops caused by strong wind. According to the measurement, the windproof effect of the sunshade net can reach 60%. When the wind speed is 6.6 m/sec, the wind speed at 10 cm away from the net is 2.2 m/sec, while the wind speed at 25 cm away from the net is only 1.4 m/sec, while at 50 cm away from the net, there is basically no wind.
3. Warming and Antifreezing
Sunshade net covering in winter in southern China has better effect of warming and freezing protection. After covering with sunshade net, it can slow down the evaporation of water, increase humidity and slow down the loss of surface heat. In winter, the lowest temperature in the greenhouse can be increased by 1.4-5.1 and the daily average lowest temperature can be increased by 2.3. The lower the outside temperature, the more obvious the effect of temperature increase. Covering the open ground with sunshade net can generally increase the minimum temperature under the net by 1-2.8 C and the daily average minimum temperature by 1.6 C. According to the measurement of Zhenjiang Meteorological Station in Jiangsu Province, the surface humidity of open land directly covered by sunshade net is lower or equal between daytime and 7pm than that of uncovered land, but higher from 10pm at night to 7am than that of uncovered land.
In the southern region where the absolute temperature is not very low in winter, the effective radiation of the ground at night can be reduced by covering with sunshade net, which has a good frost-proof effect. During the night of frost occurrence, the sunshade net is directly covered on the surface of crops, dew or fog are condensed on the net to form frost, so as to avoid the direct harm of frost to plants. At the same time, when the plant was frozen at low temperature, the sunshade net coverage could slow down the sudden excessive temperature rise at sunrise, thus reducing the damage caused by cell dehydration.
When short-term snowfall occurs in winter in some areas of the south, the sunshade net covers the crops and objects, which can block the snowfall on the net and form a good "protective layer" to protect the microclimate environment under the snow layer from freezing damage. This kind of coverage is also very conducive to vegetable harvesting and marketing after snow, so long as the net is rolled up, snow can be removed, and the trouble of picking vegetables in snow can be avoided.
4. Reduce Pests and Diseases
Summer and autumn are the season of frequent occurrence of aphids, diamondback moths, cabbage caterpillars, noctuid moths and other pests. According to the principle that aphids can resist silver gray, a silver gray sunshade net is made, which has a good Aphid-avoiding effect. Silver-grey sunshade net closed covering cultivation can effectively avoid the invasion of aphids and other major pests, reduce the spread and occurrence of viral diseases, and reduce the use and pollution of pesticides. At present, when plastic insect-proof nets begin to be used in vegetable production, new products of sun-shading insect-proof nets have emerged, which not only have the mechanism of sunshade nets, but also play a role in preventing vegetable pests.