SW—Soil and Water: Automated Replenishment of Recycled Greenhouse Effluents with Individual Nutrients in Hydroponics by Means of Two Alternative Models

Two alternative models enabling complete automation of individual nutrient replenishment in closed hydroponic systems were developed and tested. Both models require target electrical conductivity and pH values regarding the irrigation solution supplied to the crop as well as target nutrient ratios and some additional input data. The electrical conductivity and the volume of the drainage solution to be recycled, as well as the pH of the solution obtained after mixing the reused leachate with fresh water are variables, which are on-line measured when irrigation solution is automatically prepared. These data are used to automatically calculate and control the fertiliser input by varying the injection time of dosing pumps working at constant injection rates. According to the first model, the target macronutrient ratios refer to the irrigation solution and the computation of the individual nutrient doses is based on deduction of the nutrient input via drainage solution and fresh water from the recommended total supply of the corresponding nutrient. In the second model, the target macronutrient ratios refer to the input of nutrients via fertilisers and fresh water, and are considered to be equal to the corresponding mean uptake ratios as determined in previous studies. Both models are capable of supplying nutrients at the desired ratios, provided that the concentrations of Ca2+, Mg2+, and SO42− in the fresh water are not excessive.