Sodium accumulation and nutrient discharge in recirculation systems: A case study with roses

The accumulation of substances can limit the continued use of nutrient solu tion in recirculation systems in greenhouse production. Under commercial co nditions, particularly sodium (Na) accumulates in the drainage solution. In the Netherlands, discharge is only allowed if Na concentrations in the dra inage solution (Na-d) have reached concentrations of e.g. 4 mM (cut rose) o r 8 mM (tomato). To determine salinity-sensitivity of rose c.v. 'Madelon' in a rockwool syst em, Na-concentrations in the recirculation tank (Na-t) were allowed to accu mulate to 6 or 12 mM; electrical conductivity (EC) of the recycled solution increased with Na-t and was 2 - 4.8 mS/cm. Cut rose appeared to be relativ ely salinity-tolerant with a yield decrease of only 2% per mS/cm increase. Flower quality and vase life were not affected by salinity. Na in- and outp ut of the system were calculated and compared with the experimental Na-t. N a uptake concentrations (Na-u, in mM) based on output data (using Na concen trations in the tissue and production data) were far lower than based on in put data (using Na concentrations in the irrigation water Na-iw). Calculati ons showed that the higher Na-u based on input data corresponded better wit h the experimental Na accumulation possibly due to higher Na-accumulation i n the growing media than anticipated. Possibilities to decrease discharge o f nitrogen from (semi-)closed nutrient systems are quantified, including de creased N concentrations in the nutrient solution.