Use of a decision support system for management of the drainage solution in semi-closed hydroponic systems under different drainage fractions
Soilless cultivation, especially in closed-loop hydroponic systems offers a great option for water saving in greenhouses. Capture and recycling of the excess irrigation water that drains out of the root zone is possible and might considerably improve water use efficiency in greenhouse crops. However, irrigation water in Mediterranean countries is characterized by high salt concentrations with typical EC levels of about 2 dS m-1, including mainly Na+ and Cl-. Hence, in most cases, the quality of the available irrigation water in terms of mineral composition is incompatible with the application of completely closed hydroponic systems. Consequently, to enable Mediterranean growers to adopt recycling of the excess irrigation water, efficient solutions have to be found to minimize salt accumulation. Objective of this work was to test a decisionsupport- system developed for management of the drainage water in semi-closed hydroponic systems, based on a Na+ mass-balance model and measurements of plant water consumption. For this purpose, experiments were carried out from February to July 2012 in a polyethylene covered greenhouse located at the experimental farm of the University of Thessaly in Greece. To test the system under different case studies, a tomato crop grown in an open system with a drainage fraction of about 30% was compared with crops grown in semiclosed systems with drainage fractions of 30% and 60%. The results showed that the model included in the decision support system of the greenhouse, for management of hydroponic systems fertigation, could be successfully used for the control of Na+ concentration in the nutrient solution. The increase of drainage fraction in the semiclosed system from 30% to 60% helped to keep better conditions in the root zone, as indicated by the lower EC of the drainage solution as compared to the semiclosed system with 30% drainage, and resulted to higher crop production. However, the higher water use efficiency was observed under the semiclosed system with 30% drainage treatment, while the fertilisers discharged were also less under the same treatment than under the open or semiclosed system with 60% drainage treatments.