Closing the Water and Nutrient Cycles in Soilless Cultivation Systems

Soilless cultivation systems are common in Dutch greenhouse horticulture, i.e., less than 20% of the greenhouse area is still soil grown. For long, it was assumed that in these so-called closed systems the emission of nutrients and plant protection products (PPPs) was close to zero. However, Water Boards have shown that in greenhouse areas with soilless cultivations the quality of surface waters is often far below the standards for chemical and ecological good water, as is demanded by the Water Framework Directive 2000/60/EC. In 2008 this lead to an agreement between the Grower’s organisation, Water Boards and others to reduce the emission of nutrients and PPPs from these systems to almost zero in 2027. To reach this goal, the sources should be clarified, and solutions should be developed to stop these emissions. We analysed the emission problem by joint fact finding with growers and other stakeholders. Since soilless cultivation systems recirculate nutrient solutions, they have the potency of a closed water cycle, and thus a zero emission of nutrients and PPPs to the water. In reality, drain water is often easily discarded when there is only the slightest doubt about the water quality. On average 10% of the nutrient solution is discharged yearly. From this follows that soilless cultivation in the Netherlands uses 6.5 million m3 year-1 fresh water, and annually emits 1,300 ton N, 200 ton P, and 1,134 kg PPPs. There are large differences in amount discarded between crops and greenhouses: from less than 50 to more than 3000 m3 ha-1 year-1. Reasons for discharge are accumulation of sodium, fear of unknown growth inhibiting factors, imbalances in nutrient composition, and fear of spreading diseases. There can also be significant discharge from filter back-flush water and technical malfunctions. This analysis resulted in a dual approach in our research program. Our long term goal is to close the water and nutrient cycle by developing solutions for the reasons for discharge. We have been focussing on growth inhibition and nutrient imbalances. Calculations show that elimination of those reasons for discharge will reduce the use of fresh water by 2.6 million m3 year-1, and reduce the water pollution by nutrients and PPPs by 60%. This can be further improved to (almost) 100% by the already known, but not yet broadly implemented solutions to prevent discharge (i.e., low sodium supply water, adequate disinfection equipment, and the reuse of filter back-flush water). Our second research goal is to develop end-of-pipe solutions through evaluating water purification tech¬niques for use in a greenhouse environment. These techniques may offer a short term solution to meet the demands of the WFD, but may also be of value on the long term, since growers will want to retain the possibility to discharge occasionally.