الفهرس 
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Abstract
This research work was directed towards discovering and fixing the optimum parameters of small plastic greenhouse (size, orientation, cover thickness and opening areas) in order to get the optimum performance of natural ventilation system which leads to less fungal infection and better crop productivity. Trails were made on the most popular greenhouses sizes (9 x 60 x 3.1 m. ), (6 x 36 x 2.6 m.) and research suggestion size (6 x 30 x 3 m.), for:(1)Two planting seasons (Winter and early Summer)(2)Two crops (cucumber and sweet pepper)(3)Two directions for the greenhouses: (North – South) direction and (East – West) direction(4)Two kinds of ventilation opening areas (5)Three plastic cover thicknesses(6)With and without mulch (soil covering)(7)With and without shading. All tests had been done in Egypt (Buhaira Governorate) during two seasons 2005\06 and 2006\07.The obtained results showed that the relation between air temperature and water vapor carrying capacity provides a way to lower the relative humidity in a greenhouse when outside temperatures were lower than those inside. Using the suggested model of the greenhouse with the following parameters: (1)Size: 3m. high x 6 m. wide x 30 m. long(2)Orientation: (East-West) Direction(3)Cover thickness: Polyethylene cover, 130µm thickness.(4)Using plastic (mulch) for soil covering and clay for shading.(5)Opening areas: one roof and 2 sides opening areas, within core area, Leaded to better crop in both quality and quantity as a direct result of maximizing the performance of natural ventilation