TEMPERATURE CONDITIONS UNDER PLASTIC PROTECTION
Total greenhouse area was about 7,000 hectares, out of which 86 per cent was covered with PVC; 8 per cent with glass, 5 per cent with polyethylene, and 1 per cent with other materials (table 1).
Cucumbers and tomatoes are the most widely grown vegetable crops, followed by pepper and strawberries. Eggplant, watermelons, muskmelons, squash, lettuce, celery, vines and decorative plants are also grown under protection.
The extensive use of PVC in Japan is due to the fact that this material is more resistant to mechanical damages and is more effective in forcing the plants under its protection than polyethylene and some other materials, though it is more expensive.
The tests were conducted with a view to comparing temperature conditions created under various protection coverings. The materials used in this experiment are shown in table 2.
With the ordinary and draining PVC and polyethylene sheets, water drops formed upon the inner side of the covers are held continuously there, reflecting the sun rays and making the plants in the tunnels quite invisible through the covers. With the non-retaining PVC sheet type, the condensed water drops flow easily downwards, allowing more light to penetrate through the covers.
Tunnels provided with such type of covers were erected in parallel lines in east-west direction, each one of them having a semi-circular cross-section, 6 metres long, 110 cm wide and 50 cm high. The sides of the covers were fixed to the ground with earth, serving as fixing weight.
An automatic thermometer was used to record the temperature. The measurements were made in the centre of each tunnel. The ambient temperature was measured at a height of 25 cm over the soil surface, a considerable part of the thermometer being covered with cardboard in order to protect it from the direct sun irradiation. Other temperature recordings were made at a depth of 5 cm in the soil. Temperature recordings at the same height and depth were made in the open in order to supply data for the control. The measurements continued from 21–25 January.