S06 - Session O7 - Effect of Na concentrations on yield and fruit quality of soilless tomato cultivation with closed-loop irrigation system.

Authors: Tommaso Barbagli *, Wim Voogt, Nina Oud, Li Bo, Andrea Diaz Ismael

Soilless cultivations allow high water and nutrients use efficiencies when drainage water is recycled. A full closed-loop irrigation system is however difficultly reached sometimes due to worsening of drainage water quality when recirculated. In particular, sodium (Na) accumulation due to higher Na input concentrations than the actual Na uptake concentration, is considered the main reason to discharge part of the drainage water. In the Netherlands, recycling of drain water is obligatory and in the 1980s, standards for the maximum acceptable Na have been established by the Dutch Water Authorities, allowing discharge of a certain rate of the drainage, to keep Na under control. However, as a result of recently strengthened regulations, the discharge of effluents is limited. We explored the very limits for Na concentration in the root environment, by testing the effects of Na concentrations on yield and product quality in a long-term tomato crop with closed-loop soilless cultivation. The space for increasing the Na concentration, when keeping the EC equal, was created by proportional lowering the concentrations of other cations (K, Ca, Mg), but not lower than the minimum nutrient concentrations for the crop growth, which is equivalent to 2.2 ds.m -1 . Since the recommended EC-level is 3.8 ds.m -1 , in two experiments with tomato in two consecutive years, a range of 5, 15, 20 and 25 mmol.l -1 Na concentrations were applied. It was not observed any significant yield reduction or decrease in fruit quality nor shelf life even at 3 times more Na concentrations than the law limit. The results clearly showed that it is possible to cultivate with more Na, provided that the concentrations of the other cations do not decrease below the minimum required for crop growth. This opens a discussion of whether being in control of the minimum crop nutrient requirements is more critical than the Na concentration itself.