A multi-nutrient soil-test extractant that pulls phosphorus, potassium, calcium, magnesium and micronutrients in one shot — the most common test across the eastern and central U.S.
Mehlich-3 is a chemical extractant a lab mixes with your soil to estimate how much of each nutrient is plant-available. Because it extracts many nutrients at once, it's efficient and widely adopted — but the critical levels (the test value below which you'll see a yield response) are calibrated differently from state to state and crop to crop.
A Mehlich-3 phosphorus value that's 'optimum' in one state can be 'low' in another. That's the whole reason Zone Forge reads your test against your state's published calibration instead of a generic chart.
Related: Cation exchange capacity (CEC) · Buildup & maintain