Automated composition control of nutrient solution in closed soilless culture systems

A computer algorithm was developed to perform replenishment and reuse of the drain nutrient solution in close hydroponic systems on the basis of dispensing as many nutrients and water in it as were needed to maintain a target electrical conductivity in the irrigation solution. To achieve this in each watering application, dilution ratios of the concentrated fertilizer solutions are automatically adjusted through a computer program based on this algorithm, in relation to the volume and the electrical conductivity of the drain solution. It was proved that the returning nutrient solution could be corrected and reused efficiently through this method. However, the concentration of P, Mn and Zn in the irrigation nutrient solution was extremely low in samples taken after continual reuse of the drain solution for a fortnight. Since the analysed irrigation solution was prepared using a large quantity of drain solution, it seems likely that reduced P, Zn and Mn concentrations in the former were due to the high pH of the latter, and this may have caused immobilization of these elements. (C) 1999 Silsoe Research Institute.