Boring stuff:
Plants can be trained to take a pretty high EC. In order for the plant to get water from outside its roots (the coco) to inside its roots, there needs to a higher concentration of dissolved solids inside the water of the root cells than outside the roots because water molecules flow from lower dissolved solids to higher dissolved solids (osmosis) to reach equilibrium in dissolved solids on both sides.
How a plant manages this is it produces sugars to increase the dissolved solids inside the roots so that water can flow from the high EC water into the roots. If the EC outside the roots gets too high too quickly and it can't manufacture enough sugars quickly enough, then water will actually get sucked out of the roots because water travels from lower dissolved solids to higher dissolved solids, even if it's not in the direction we want.
Relevant stuff:
All of that is to say that if one gradually keeps increasing the EC, the plant will continue to manufacture more and more sugars to process the high EC feed.
This is why some people can feed as high as 3.0 EC and why some cannabis nute lines will show pretty high EC schedules in their charts - that, and because it sells more nutes
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However, when it's manufacturing more and more sugars to ensure it can get water from high EC feed, it's expending energy on doing that. It only has so much energy per day and we want as much of that energy as it can to be put toward growth. If the EC is kept at the EC that the plant has come to expect, then it can focus its energy toward growth.
The nice thing about checking your runoff EC is it will tell you. If you bump it up to 1.7 and the runoff starts coming out at 2.2, then you're getting into a danger zone. The plant will likely not exhibit any problems visually yet, so if one isn't monitoring runoff EC, they might be inclined to keep bumping it up rather than taking remedial action and then be surprised when the plant goes to shit.
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