Originally Posted by rob7233
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.
I might disagree that N delivery via rainwater is meaningful. In fact, the studies I did years back showed N from rainwater was almost nonexistent. Rainwater may dissolve soem gaseous N2 from the atmosphere, but N2 isn't plant available. Some plant available N can be made in a reaction with lightning, but this usually adds 0.02 #N/M, so it doesn't add much.
Rainwater usually perks up lawns because most irrigated lawns use treated municipal water, which usually has a pH in the mid 8s, which haxs a liming effect each time the lawn is irrigated. Lower pH rainwater helps to bring pH in range for N forms to be more easily available.
Like you, I find soil testing for N to be unnecessary and to have little value. But, I was raked over the coals on this board for suggesting that. I wonder if anyone will rake you over the coals for doing the same?