Spectro-radiometrical analysis of plastic nets for greenhouse shading under arid conditions.

The use of plastic nets in agriculture is now a consolidated reality all over the World. Plastic nets are used to protect orchards, flowers and vegetables thanks to the direct effect of crop protection against atmospheric agents (hail, wind, snow, heavy rain), birds and insects, as well as sand and saltiness, avoiding damages and/or dirtiness of the crop. Plastic nets perform indirect effects as well, when they are employed to close greenhouses and tunnels: due to their influence on the values of the main microclimatic parameters (temperature, relative humidity, carbon dioxide concentration, solar radiation, etc.), they could play a fundamental role on creating more favourable microclimatic conditions during the crop growth. The considerable success that plastic nets have registered in mild climate countries, as those located within the Mediterranean area, is currently pushing towards similar interesting application even in areas characterized by different climates - like the arid conditions usually found in Northern Africa and the Middle East - where greenhouse shading is essential in summer to reduce the solar radiation load. In the present paper, the spectro-radiometrical properties of some different nets - white, black and thermal-screen - employed in Saudi Arabia to shade the roof and side-walls of a polycarbonate ventilated greenhouse, were measured in laboratory in the solar/PAR range and in the thermal Infra-Red wavelength. From the obtained results, it is clear that the absorbance of the black plastic net is very high, both in the solar (nearly 70%) and in the IR (over 75%), so their use inside the greenhouse should be avoided. On the other hand, the level of transmittance of the white plastic net (over 60%), joined to its significant reflectance – that generates mutual progressive reflections with the greenhouse cladding sheet, if it is installed inside the greenhouse – confirms once more that shading nets should be employed only outside the greenhouse, in order to fully express their potential of shading the incoming solar radiation. Finally, the thermal screen seems very effective in blocking the IR radiation. This characteristic, joined with a high reflectance in the solar wavelength, makes this material very powerful for an effective contribution to an improvement in the energy balance of a greenhouse