Data is presented that suggest high exchangeable magnesium concentrations in Mississippi Delta soils may be primarily responsible for the potash fertilizer yield response in cotton at higher than normal soil test X levels, and may cause an imbalance of plant nutrients on certain land leveled fields with magnesium base saturation levels above 30%. Mathematical equations are proposed to estimate the loss in soil test K due to exchangeable magnesium. Nutrient ratios are suggested for determining critical soil K sufficiency levels in soils based on potassium uptake in cotton plants.