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I use rainwater by choice, because my tap water is high in sodium bicarbonate and has none of the macronutrients, N, P, K, Ca, Mg, or S. The most immediate thing rainwater needs is some lime, ground limestone or oyster shells to get Ca and Mg levels up. I want the calcium content to get up to the 50 to 100 ppm level. I then add KCl or some kind of potassium salt to get the K level up to 20 to 50 ppm. This will last a long time. I also add some magnesium sulfate just to make sure of Mg levels. I add a trace element mixture, and that takes care of everything except N and P.

N and P should be added frequently enough to keep levels measurable with test kits. This can be once a week or once every two weeks. Iron should be added at around the same time. I don't find that nitrate levels have to be maintained higher than 10 ppm as some people are saying, although it doesn't seem to hurt anything to do that if you want to.

I don't worry about Ca, Mg, K. and S and the micronutrients after my initial additions. After one or two major harvests of plants and seveal months, I check K levels, and usually they are still up there. I don't do water changes very frequently, and it has been my experience that the K, Mg, Ca, SO4, and micronutrients last for a very long time.
 
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