I have no idea really. I use living soil and I don't feed anything when I water, so the CalMag was my best guess for the burning. Then again the rainwater could also be what's causing it, contaminants from the air and such. He can try the rainwater without the CalMag first, just to see, but I'm almost willing to bet it's one or both causing it. I've never used rainwater myself, just straight tap water, and haven't had a problem yet.