Soil Testing, Part 1: pH

Every fertility decision you make sits on top of soil pH. Get it wrong and you’re paying for phosphorus the soil won’t release, nitrogen the crop can’t use efficiently, and soybean nodules that never form. In the first episode of our soil testing series, we get into why Piedmont soils fight us on acidity, how often you really need to pull samples, and what your NCDA&CS report is actually telling you. Why we test in the Southeast — Piedmont Ultisols are old, weathered, kaolinitic, and low-CEC. Bases leach out, nitrogen fertilizer drives pH down, and there’s not much buffering to hold the line. NC State data show 37% of row-crop samples come back below pH 5.8 — and about 9% are already over-limed. How often to pull samples — Once every three years works on stable Piedmont clay. We talk through when that’s not enough: sandy low-CEC ground, cotton, heavy nitrogen programs, and after a big lime application. What pH actually is — The difference between active acidity (what the meter reads) and reserve acidity sitting on the exchange sites. That gap is the whole reason a pH number alone can’t tell you a lime rate. Aluminum is the real yield thief — Below about pH 5.5, aluminum goes into solution and attacks the root tip directly, shutting down cell elongation. You get short, stubby, brittle roots that can’t chase water. Manganese toxicity rides along with it. What we shoot for — pH 6.0 on mineral soils, 6.2 for cotton, and why chasing 7.0 costs you manganese, zinc, and boron. Buffer pH — and why North Carolina doesn’t report one — Most states run a buffer (Adams-Evans, Mehlich, Sikora) to estimate reserve acidity. NC skips that step and calculates lime straight from exchangeable acidity, or “Ac,” on your report. We walk the actual equation. This also means NC’s CEC number isn’t comparable to a private lab’s. Tennessee dolomite vs. South Carolina calcitic — The geology behind each, why dolomite reacts slower, and how to decide. Short version: let your magnesium index make the call, not a Ca:Mg ratio. Buying lime on the tag, not the sticker — Calcium carbonate equivalent and fineness combine into effective neutralizing value. We run the arithmetic so you can compare two quotes on delivered tons of actual neutralizing power. Gypsum is not lime — It’s calcium sulfate. It won’t move your pH. We cover what it’s actually good for, including subsoil aluminum.