Nowadays there is much interest in 'closed systems'. However, it is often forgotten that there is a 'leak' of some 70% of water supply: that is the water taken up by the crop. A small fraction of that is accumulated into fresh weight; the rest disappears altogether in transpiration. All minerals that the plants do not use will stay behind. When the water used to re-fill the system contains salts, these will accumulate in the 'closed' loop. That shows up as ever increasing EC, and that costs production.
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