THE CHOICE OF COVERING MATERIAL FOR GREENHOUSES IN CONNECTION WITH THE GEOGRAPHICAL SITUATION
Recently we wrote a paper concerning this problem in France. Some reaction seems to prove that our meaning was incorrectly interpreted. We do not hope to prevent a priori any commercial act or to limit any commercial competition between producers of covering material. But the possibilities of the new material industry are so wide, so numerous, that it seems rational to try to use the right material in the right place. It is possible to consider this problem from different points of view. Climate is in fact the result of interaction between different factors: temperature, light, humidity, wind. But in this short report we attach the greatest importance to the factor light (solar radiation)- transmission of the material. We are now looking for the relation between light and other factors, but is it too early to speak about this today.
We have the opportunity to classify the covering materials for greenhouses which I will call: Non absorbing materials. Among those we find transparent ones (glass, some plastic types) and translucent ones, which diffuse but do not absorb light (some types of glass and plastics).
Light absorbing materials. In this group we find the "polyester" type, which diffuses and simultaneously absorbes part of incident light and the coloured materials which exhibit the normal spectrum of the corresponding uncoloured ones plus a special absorption concerning the complementary colour of their own.
The first group "Non absorbing materials" does not cause important losses of light in the greenhouse (only about ten percent loss as a result of light reflections at the material surfaces), while the second group does not permit the plants to receive the benefit of all the light and is responsible for a more or less important decrease in illumination.
The reduction is related to the kind and intensity of the "colour" of the material. The result of this light loss will vary with the country considered, that is with the importance of observed effective radiation. The opportunity exists to express the observed radiation under the form of potential radiation (that is the maximum radiation which can be received in a region, in relation to its geographical position) and of really observed sunshine (that is when we make allowance for haziness).
This radiation can be expressed in energy (g.cal/cm2/time unit) or in number of sunshine hours. Because it is very difficult to define the energetic level necessary for the best plant growth and because this energy varies from plant to plant and even from variety to variety, we have used in a first approach to this problem the monthly sum of sunshine hours really observed in various regions.