Closed soilless growing systems: A sustainable solution for Dutch greenhouse horticulture

Dutch greenhouse growers are obliged to invest in environmental friendly cropping systems, in order to comply with new legislation. Closed soilless growing systems may lead to environmental friendly systems and thus to a sustainable horticultural sector. This paper encompasses the change from general to specific legislation, the availability of good water and the developments in 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 discussed. 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 discharge of nutrients, the emission of pesticides and the waste of materials such as plastics and substrates for cultivation. Initial legislation was rather rough and not based on research. Later, forced by court judgements and social pressure, new regulations have set a timetable for all nurseries to adopt specific measures in order to decrease the leaching of water and fertilisers into the environment. A recent covenant between horticultural employers and the Dutch government for a sustainable development of greenhouse horticulture between 2000 and 2010 enables the growers to achieve the environmental targets in their own way, if they make a specific plan for their nurseries. Closed growing systems prevent the leaching ofchemicals into the environment; in Dutch situations leaching into ground and surface water. Studies showed savings of up to 30% of water and up to 40% of fertilisers in closed systems compared to traditional open systems. A 100% efficiency of water and nutrients often cannot be achieved, because of the relatively poor water quality of the supply water. Mostly the sodium content of the water eventually becomes too high, and, consequently, part of the nutrient solution should be discharged. Closed growing systems bear the risk of dispersal of pathogens all over the nursery. Therefore, it is always said in the Netherlands that disinfection of the recirculating nutrient solution is a necessity for long-term crops to avoid a disaster due to an outbreak of root-borne diseases. However, existing, active, sterilisation systems are not always applied, mainly because of the high costs. Now, developments in passive disinfection equipment focus on the removal of pathogens without a complete sterilisation of the nutrient solut