Strategies to manage chemical water quality related problems in closed hydroponic systems

In the Netherlands since the late nineties reuse of drainage water is obligatory for all soil less growing systems, to reduce the environmental pollution. However, this strategy has some important bottlenecks. Apart from technical and phytopathological aspects, accumulation of Na, Cl or other residual ions could be a problem. Accumula-tion will occur if the uptake rates are lower than the concentrations in the input. Recently a database was established with the maximum acceptable concentrations in the root environment and water sources, for a number of crops. In all cases Na showed to be the bottleneck element. A high tolerance for Na not necessarily means a high uptake rate for this ion, as was found for sweet pepper. Water sources differ highly in Na concentrations. In general, for completely closed growing systems only rainwater or desalinated water is suitable. For some crops (rose, chrysanthemum, sweet pepper) the natural background concentrations of Na in rainwater is sometimes even higher than the uptake rate. Accumulation above the maximum acceptable concentrations inevitably should be followed by discharge of a fraction of the nutrient solution to prevent yield reduction or decline in produce quality. This will result in lower water and nutrient use efficiencies than required. However, the water and nutrient losses depend highly on water management strategies accomplished by the grower. Smart strategies were developed for discharge of drainage water as low as possible for N and P. These are based on the uptake dynamics of Na and Cl and minimal required N and P levels observed with different crops. These strategies have been tested for rose and resulted in significant reduction in the nutrient losses.