Four major points of greenhouse watermelon cultivation techniques: air, temperature, soil and air humidity
Watermelon, listed by netizens as one of the “three treasures of summer”, shows just how much people love it. Sitting under the air conditioner in summer, playing on a phone and eating iced watermelon is simply too enjoyable for words. Watermelon tastes sweet, comes in many varieties, and has plenty of flesh, with two colors of flesh, red and yellow, the latter having an even finer texture. In recent years many farmers have taken up watermelon cultivation, so what are the key points of greenhouse watermelon cultivation techniques?
I. Light
It depends on the intensity of solar radiation outside the greenhouse, the optical characteristics of the covering material and the degree of contamination. The light transmittance of new plastic film is 80%—85%, while that of old film polluted by dust and mud is often below 40%. Water droplets condensing on the film surface, through the diffusing effect of the droplets, can reduce the light inside the greenhouse by 10%—20%. The greenhouse frame, the press line and the trellising materials of tall vegetables all shade light, so in greenhouse management one should avoid and eliminate as far as possible the factors that weaken the light inside the greenhouse.
II. Temperature
(1) Patterns of temperature change: the daily trend of air temperature inside the greenhouse is the same as in the open field, but the day-night temperature difference varies greatly. With sufficient light during the day, if the film is sealed tight the temperature inside the greenhouse rises quickly, reaching as high as 40—50℃, more than 20℃ higher than outside. On cloudy and rainy days the warming effect is poor, and at night the minimum air temperature inside the greenhouse is generally 1—3℃ higher than outside. The soil temperature inside the greenhouse is more stable than the air temperature, usually 10—20℃. (2) Temperature inversion phenomenon: in greenhouses covered with polyethylene, on breezy clear nights in winter the temperature inside the greenhouse sometimes drops lower than outside. The reasons are: at night the air temperature outside the greenhouse is higher aloft than near the ground; owing to wind disturbance, the air near the ground outside can gain heat replenishment from the upper air layer, whereas inside the greenhouse, blocked by the covering, this heat cannot be obtained; in winter the days are dim, the soil stores little heat, and moreover polyethylene film has a relatively high long-wave radiation rate and slightly poorer heat retention, so effective heat radiation from the ground is large and heat loss is great, thus causing the temperature inside the greenhouse to be lower than outside. (3) Temperature regulation: temperature regulation in the greenhouse is mainly carried out through ventilation and heating. Using film removal for ventilation is the most common method of lowering and controlling the daytime air temperature inside the greenhouse; using shading materials to reduce the amount of light the greenhouse receives can also prevent the air temperature inside from becoming too high.
III. Soil moisture and salt content
Soil moisture inside the greenhouse is distributed unevenly. The soil near the two sides of the greenhouse frame has greater moisture because more water seeps in from outside and because water droplets on the film run down there. The middle of the greenhouse is relatively dry. For cucumbers and eggplants grown in greenhouses in spring, especially those under plastic film mulch, soil moisture is often insufficient and seriously affects quality. It is best to lay soft drip irrigation tape and apply fertilizer and water as needed at any time, which is an effective measure for increasing yield. Because the greenhouse is covered for a long time and lacks rainwater leaching, salts move upward from below with the groundwater, easily causing excessive accumulation of salt in the cultivated layer and resulting in salinization. Therefore, attention should be paid to proper deep plowing, applying organic fertilizer, and avoiding long-term application of fertilizers containing chloride ions or sulfate ions.
IV. Regulation of air humidity
(1) Patterns of change in air humidity inside the greenhouse: plastic film is highly sealed, so the exchange of air inside the greenhouse with the outside air is hindered, and the water vapor from soil evaporation and leaf transpiration is hard to disperse. Therefore, the humidity inside the greenhouse is high. During the day, with the greenhouse ventilated, the relative air humidity inside is 70—80%. On cloudy and rainy days or after irrigation it can reach above 90%. The relative air humidity inside the greenhouse decreases as the temperature rises, and at night is often 100%. The humid air inside the greenhouse, upon meeting cold, condenses into a water film or water droplets that attach to the inner surface of the film or to the plants.
(2) Regulation of air humidity: excessively high air humidity inside the greenhouse not only directly affects the photosynthesis of vegetables and their absorption of mineral nutrients, but also favors the germination and infection of pathogen spores. Therefore, ventilation should be carried out to promote exchange between the highly humid air inside the greenhouse and the low-humidity air outside, which can effectively lower the relative humidity inside. Heating with ground heating wires inside the greenhouse can also lower the relative humidity. Adopting drip irrigation technology, combined with plastic film mulch cultivation, reduces soil moisture evaporation and can greatly lower air humidity (by about 20%).