The effect of sodium accumulation on the growth and production of tomato was tested in a test in stone wool with full recirculation. Four treatments with concentrations of 5 to 20 mmol/l and two with 15 mmol/l and adapted K/Ca ratios were tested in an 8-month cultivation. No significant effects on production or quality (shelf life or taste) were found. There was a slight Blossom End Rot incidence, but it was not clearly related to Na. Based on the results, the maximum acceptable limit for Na can be increased from 8 to 15 mmol/l. The uptake concentration was linear with the Na-concentration and is with approx. 0.8 mmol/l at 10 mmol/l at a fairly high level. Consequently there are no problems to be expected when using only rainwater or water of equal quality. On the basis of these results, it may be expected that when water is applied below 0.8 mmol/l Na, full closed systems can be applied.