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Well since nobody else answered.....If I understand EC correctly it measures how much stuff is in the water! We know that water does not conduct Electricity, so if the EC is too high we need to remove some of the nutes from the water, so either don't add nutes or flush with fresh water.>)
Hee hee never saw this one my man! EC is another way of measuring the nute strength (more salts in the soln the better the conductivity - electrical conductivity) in the States we use TDS (total dissolved solids) which is a different way of measuring the same thing being "nute strength". What you said is true if the EC is too high add water to lower it. Now that would be for the liquid soln itself not the soil. If you have over fertilized and need to lower the amount in the soil follow the Hairy One's advice "flush" but try watering without nutes for a bit. If you indeed have a Mg deficiency pH adjust the water to 6 -6.5 and add a teaspoon of Epsom Salts per gallon of water (or pop for General Hydro's Micronutes). -B