If you have a heavy acid rain it might. Last year during one of the Cali forest fires we got a rainstorm and I happened to check the run-off for the hell of it and found it to be barely 3.0! I noticed no harm to my plants from it but am sure if they got a daily dose of such off-balanced PH I would have had some probs! But for outdoor grows I usually don't fix the PH and use it straight from the faucet/sprinkler as it doesn't seem to affect them like it does in an indoor setting...but maybe the ocassional acid rain balances it out?