Closed soilless growing systems in the Netherlands: the finishing touch

This paper gives an overview of the latest developments in closed soilless growing systems in the Netherlands, where growers are obliged to invest in environmently friendly cropping systems, in order to comply with new legislation. Subjects of discussion will be the reasons for regulation, the growers' reaction to the proposed legislation, the methods to comply with legislation and related problems such as the availability of good water, the systems applied and the developments in the disinfection of nutrient solutions. For the latter, the interest of growers in the possibilities of slow sand filtration to disinfect the nutrient solution will be mentioned. In the last ten years there has been an awakening of environmental consciousness in society. Agriculture and horticulture have had to face their polluting aspects, too, such as the leaching of nutrients, the emission of pesticides and the waste of materials such as plastics and substrates. Initial legislation was rather rough (“all growers should change to closed systems”) and not based on research. Later, forced by court judgements and social pressure, new regulations (the Waste Water Disposal Decree) have set a timetable for all nurseries to adopt specific measures in order to decrease the leaching of water and fertilizers to the environment. The recirculation of drain water and the collecting of rainwater have become compulsory for substrate growers. On the other hand, recirculation via the soil and the traditional drainage system has been approved as a “closed” substrate system. For soil-bound crops, maximum water supply and collecting rainwater will be compulsory. In the last few years, low market prices have shown very clearly which closed systems were profitable and which were not. As a consequence, a discussion arose about the benefits for the environment of the new regulations and the costs for the growers. There were few new developments in growing systems, as the growers' first point of attention was economic survival. This year, 1996, prices have been better and the growers are again looking for possibilities to improve the quality of their systems. In the Netherlands, it has always been said that disinfection of the recirculating nutrient solution is a must for long-term crops to avoid a disaster due to an outbreak of root diseases. However, existing sterilizing systems are not always applied, mainly because of the high costs. Now, developments in disinfection equipment focus on the removal of pathogens without a complete sterilization of the nutrient solution. Investigations of the prospects of slow sand filtration as a cheap, robust disinfection method have proved its feasibility only on a small scale. Now, it can be seen that the development and application of closed soilless growing systems is approaching maturity in the Netherlands. On the other hand, developments i