1. Field of the Invention
The invention relates to the greenhouse cultivation of bulbs. More precisely, the invention relates to a technique and to an artificial substrate for the cultivation of bulbs.
2. Background of the Related Art
It is known, for the greenhouse cultivation of bulbs, to place the latter under conditions which make possible a first stage of development of the plants, their transport and finally, the completion of the cultivation. These various stages, which meet the needs of the market, are preferably conducted, for practical reasons, on the same substrate.
Thus, it is traditional to use a layer consisting of peat of a thickness on the order of 5 cm for a cultivating substrate of the bulbs, for example tulip bulbs. The bulbs are placed on this material in the arrangement fit for leaving them a suitable space for their growth. They are held in this arrangement by covering the peat with a layer of sand, or with a material of the same nature, which surrounds the bulbs by covering them to a certain height. In this way, the bulbs which are only placed on the peat are held in position during various handlings to which they can be subjected.
This arrangement makes possible a satisfactory cultivation of the bulbs but comes up against some difficulties. Of the latter, it is necessary to mention particularly those difficulties which arise due to transportation of the bulbs. At present, the initial development of the cultivation is often very distant from the final consumption site. For example, the greenhouse cultivation of tulips is very widely developed in the Netherlands, which exports throughout the entire world the bulbs from which flowers are obtained in the country of destination. This trade, which is carried out in sensitive products, largely resorts to means of air transport. For this reason, the weight of the cultivating substrate is of very great importance in the cost. The peat and the sand are very heavy elements and constitute the essential part of the transported weight, the bulbs and the trays representing only a small part.
Another difficulty, linked to the use of the sand, is its instability. The layer of sand that is attached to nothing can be moved in the handling of the trays carrying the bulbs causing a poor distribution of the bulbs in the tray and an unsuitable position of at least some of these bulbs (their position being no longer "vertical").
Other types of substrates have been proposed, in particular synthetic materials such as polymer foams (polystyrene, polyurethane, phenolic resins). In use, however, these materials prove not very satisfactory for various reasons. First, they exhibit less suitable hydrous characteristics. The water retention is lower and consequently necessitates a much more careful monitoring of the conditions under which the conservation and the development of the plants is performed. It also seems that these materials result in a poorer "root colonization." This feature is probably linked to the preceding one. On another plane, the use of substrates is very dependent on their cost. The synthetic foams prove appreciably more costly than the products such as peat and sand. For all these reasons, the synthetic foams are not used in practice by the professionals concerned by the applications considered by this invention.