I used to keep tropical fish and would check the amount of Total Dissolved Solids in the water to ensure a healthy tank. The TDS of my (hard) tap water here is around 0.9 The TDS of rainwater from my butt is around 0.03 So rain water has far less stuff dissolved in it than tap water.
Water a pot plant,you know you've just bought one home,give it a really good soak,then remove it from its pot,a large amount with be bone dry. You can't physically water everywhere like rain. I watered the hanging baskets last night,hubby poked his finger in round the edge,- bone dry,soaking them in the weed trug.
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The TDS of my (hard) tap water here is around 0.9
The TDS of rainwater from my butt is around 0.03
So rain water has far less stuff dissolved in it than tap water.