THE PHYSICAL PROPERTIES OF THE SUBSTRATES IN HORTICULTURE
Among the different cultures which contribute to the total production of inedible horticultural products, ornamental plants occupy the most important place. The area around Ghent, covering approximately 50, 000 ha, is a well-known centre where this development takes place.
It is obvious that the demand for organic matter is rapidly growing because of this great increase in plant production; consequently this puts a pressure on the price! Apart from the fact that resources in the country are practically exhausted and that the supply has to come from remote places (North-Germany and Scandinavia), the problem is whether one can or cannot gain by making the substrates last longer. In fact, it is customary for horticulturists to replace, every year one third of the media in which they raise their plants. Twenty years ago, when wages as well as prices for forest litter and good peat were low, substrates amounted to no more than 5% of the total production cost in plant production. Now this has changed.
The practice of replacing used compost with new, as well as applying the necessary fertilizer, now seems to have a significant effect on production costs. In Belgium, one cubic metre of fertilized peat costs US $ 12. To take out the old compost and put the new one back on the benches in the greenhouses requires two hours of work. At US $ 1. 5 per hour, the whole operation or renewing the substrate, amounts to 12 + 2 x 1. 5 = US $ 15 per m3. Hence the relative importance of substrate in production costs now amounts to ± 10%. This is twice as much as what it represented 20 years ago. With the price of classical organic substrates (litter and peat) as well as the cost of working hours rising constantly it is believed that their relative importance will still increase in future.
The other incentive for scientific research on substrates is the consideration that up to now this part of plant production has been neglected. Great advance has been made in plant genetics, light and heat requirements, physiological requirements, in major and minor elements etc. On the other hand there has been little research carried out on physics