Which extraction method is used to estimate both phosphorus and potassium in soil?

• A. Olsen
• B. Bray 1
• C. 0.5 M NH4Cl
• D. Mehlich 3

Answer: (D)

Mehlich 3 is a versatile extractant designed to pull a broad range of nutrient forms out of soil, so it can release both phosphorus and potassium into solution. Because of this capability, it provides simultaneous estimates of plant-available P and K across many soil types, making it a convenient and widely used choice for fertility testing. In contrast, the Olsen method is tailored specifically for available phosphorus, especially in calcareous soils, and doesn’t measure potassium. Bray 1 targets phosphorus in acidic soils and isn’t the right tool for potassium. A 0.5 M ammonium chloride solution extracts exchangeable cations like potassium but does not release phosphorus in a way that reflects total available P. So, Mehlich 3 is the method that best estimates both phosphorus and potassium in a single extract, which is why it’s the correct choice.