Why soil pH should usually be kept above 5.5
I almost always recommend keeping the soil pH above 5.5. This minimizes soluble aluminum in the soil, which can be toxic to roots. Get the soil pH above 5.5, and the soluble aluminum won’t be very soluble anymore—it drops almost to 0.
This week I received test results from Brookside Labs for five golf course clients. Plotting the soil pH (measured in a 1:1 solution with H2O) and the KCl-extractable aluminum, you’ll see just how dramatically that Al3+ drops as the pH gets above 5.5.
Another reason to keep the pH above 5.5 is to optimize microbial activity. Once the pH gets above 5.5, I’m not especially worried about it.
What’s the case when I don’t mind pH less than 5.5? It would be an anti-Poa measure, if one has Agrostis that is performing well, and one is trying to stress Poa. But I’d rather stress the Poa in other ways, for example by withholding P.