PERFORMANCE TEST OF A NA⁺ ACCUMULATION MODEL AS PART OF A DECISION SUPPORT SYSTEM FOR CLOSED HYDROPONIC SYSTEMS MANAGEMENT

The recycling of greenhouse effluents in soilless cultivation systems is environment-friendly, since the waste of water and the disposal of nutrients to the environment is drastically reduced or even eliminated. However, a long-term recycling of the effluents may result in accumulation of Na⁺ and Cl^-, in the recycled nutrient solution. Previous research has indicated that the concentration of Na⁺ and Cl^-, in the recycled nutrient solution may be predicted by employing mass-balance models and monitoring the plant water consumption. The major aim of the present research was to test the performance of this model when operating as part of a decision support system in a tomato crop grown in a closed-cycle hydroponic system. An additional objective of this research was to compare tomato crops grown in fully closed, semi-closed and open hydroponic systems in terms of nutrition and yield performance. The crops were planted in January and scheduled to be terminated in June 2011 and irrigated with nutrient solutions, which contained 2 mM NaCl originating from the irrigation water. During the vegetative growth stage, the young tomato plants exhibited much higher uptake concentrations of nutrients than those suggested for North Europe. As a result, the electrical conductivity (EC) in the root zone of the crops grown in closed and semi-closed systems was maintained at lower levels than in the crop with free drainage for several weeks after planting. However, progressively the EC in the crops grown in closed and semi-closed systems was increased to higher levels than in the open system due to Na⁺ and Cl^- accumulation. The efficiency of the model to minimize discharge of nutrient solution effluents in semi-closed hydroponic systems thereby maximizing water use efficiency in hydroponic tomato crops is discussed.