Originally Posted by Keith
I think I remember reading the ph of rainwater is typically 5-6. In some areas lower than that. I believe this explains why you can water a lawn with an inch of groundwater and it looks nowhere as good as what an inch of rain will do.
The reason that a lawn irrigated with rainwater looks better after verses tap or groundwater is that Nitrogen is very mobile and changes form readily from a gaseous state to being very water soluble being carried by rainwater. A good rain always contains Nitrogen(N) which is taken up by the lawn's root zone. I'm not sure how much but that's likely one reason any given lawn looks better after a good rain. Additionally, I also recall vaugely something about ion exchange with rainwater but can't remember anything else regarding that.
Also that with soil testing, N is not measured since it's so mobile.