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

The accumulation of substances can limit the continued use of nutrient solution in recirculation systems in greenhouse production. Under commercial conditions, particularly sodium (Na) accumulates in the drainage solution. In the Netherlands, discharge is only allowed if Na concentrations in the drainage solution (Na(d)) have reached concentrations of e.g. 4 mM (cut rose) or 8 mM (tomato). To determine salinity-sensitivity of rose c.v. 'Madelon' in a rockwool system, Na-concentrations in the recirculation tank (Na(t)) were allowed to accumulate 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 relatively 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 output of the system were calculated and compared with the experimental Na(t). Na uptake concentrations (Na(u), in mM) based on output data (using Na concentrations in the tissue and production data) were far lower than based on input data (using Na concentrations in the irrigation water Na(iw)). Calculations showed that the higher Na(u) based on input data corresponded better with the experimental Na accumulation possibly due to higher Na-accumulation in the growing media than anticipated. Possibilities to decrease discharge of nitrogen from (semi-)closed nutrient systems are quantified, including decreased N concentrations in the nutrient solution.