Soil Testing, Part 2: Potassium

Potassium is the nutrient where Midwest thinking gets Southeast farmers in trouble. Our soils don't bank K the way theirs do, and the numbers on your report don't mean what a lot of folks think they mean. We work through the soil chemistry, the plant physiology, and the arithmetic behind reading an NCDA&CS report for potassium. Our soils don't hold a K reserve — Midwest 2:1 clays trap and slowly release potassium between the layers. Piedmont kaolinite doesn't. There's no savings account here, which means the crop is living on what's on the exchange complex right now. Yes, potassium leaches in the Southeast — Low CEC means few places to hold a K ion. Add sand and rainfall and applied K moves. Calcium and magnesium from lime actively push K off the exchange, which is why fall-banking K on a Coastal Plain sand is a losing proposition. How to read the K index — and why base saturation misleads you — The K-I is a rescaled ppm number with no CEC in the equation. We run the conversions on air. And we work the math that matters: 3% K saturation on a CEC 3 sand is about 75 lb K₂O total, while 3% on a CEC 12 clay is 300 lb. Same percentage, four times the potassium. Chase the index, not the ratio. Some soils physically can't hold a season's worth — We calculate maximum K holding capacity by CEC class and compare it against what corn, cotton, soybeans, and wheat actually pull. On the lightest ground, splitting isn't a refinement — it's the only way the K stays there. Cotton is the problem child — Roughly 70% of cotton's K comes in by first bloom, on a small root system, right when bolls take over as the sink. That's why you see late-season deficiency on the upper leaves and Stemphylium on fields that soil-tested fine. We cover UGA's foliar K work and where it actually pays. Crop-by-crop rates, removal, and litter credits — NC's actual K₂O equations by index, removal figures for grain versus silage and grain versus baled straw, and why poultry litter K credits nearly pound-for-pound while litter N doesn't.