With all this talk of the need for measuring soil pH in the terra preta nova setting, I'd like to recommend my favored field approach: the Hellige-Truog Soil pH Test Kit. I have used these kits in my work for 3 decades. Accurate and inexpensive, the methodology allows you to focus on exactly the pinch of soil, or char, you are interested in. It was standard issue when I was a salaried
NCSS soil mapper. The preferred NCSS field method is a now a pH meter for 1:1 (water:soil) but results from the HT kits are still acceptable.
Soil pH is so dynamic over short distances, and with this approach, you can sometimes get varied pH indicator colors in the sample, which I find particularly intriguing to view: for example all green hues, pH 5.8 to 6.4 and a point of bright yellow 4.0 from a root exudate. I wouldn't use it in lieu of a lab test, but I wouldn't hesitate to recommend it for dinking around and getting some quality face time with your soil.
Now if we only had an equivalent field kit for soil pE (aka redox potential), pH's energetic dance partner.
