NUTRIENT MANAGEMENT IN CLOSED GROWING SYSTEMS FOR GREENHOUSE PRODUCTION

Closed growing systems are seen by the Dutch government as the solution to emission of nutrients from glasshouse horticulture. However, inherent to these systems are problems like accumulation of Na and Cl, or other ions, and an unbalanced nutrient solution. So standards for water quality and nutrient input have been developed. In general, the input of ions by means of the water and the fertilisers, should be equal to the plant uptake. The water quality acceptable (I max) is limited by the uptake capacity of the crop and is determined by R max (The maximum acceptable concentration in the root environment). R max ranges from 4 mM for salt sensitive crops like bouvardia to 10 mM, for salt tolerant crops like tomato. The maximum uptake (U max) of Na and Cl at R max ranges from Umax, partial leaching of the recirculating nutrient solution is required. The fraction to be leached out could be calculated from R max, U max and I max. The restrictions for water quality are valid for all ions. The nutrient supply should be equal to the demand of the crop, to avoid depletion or accumulation. Since the nutrient supply is connected with EC controllers, it means that the mutual ratios of the nutrients in the input should be equal to the ratios absorbed. For this purpose, standard nutrient solutions are established together with guidelines for adjustments according to plant stage, to keep the nutrients on the required levels.