How Reliable Is Mehlich 3 for Estimating Plant-Available Phosphorus? Evidence from Sequential Soil Extractions

Soil fertility is a key determinant of agricultural productivity and sustainable land management. Among essential nutrients, phosphorus (P) plays a critical role in crop growth and yield formation, yet its availability in soils is highly variable and strongly controlled by soil physicochemical properties and management practices. Reliable assessment of plant-available P is therefore essential for accurate fertilizer recommendations and environmentally sound nutrient management. The Mehlich 3 (M3) extraction method is widely used as a multi-element soil test to estimate plant-extractable nutrients, including P, K, Ca, Mg, and micronutrients, across a broad range of soil types. However, interpretation of M3-extractable P remains challenging due to differences in extraction efficiency among soils with contrasting texture, mineralogy, pH, and organic carbon content.

Phosphorus in soil solution is in dynamic equilibrium with solid-phase P pools, and the size and replenishment of extractable P depend on both solution chemistry and soil solid-phase properties. The aim of this study was to investigate changes in extractable P over six consecutive Mehlich 3 extractions and to evaluate how soil texture and chemical properties influence this process. Sequential extractions were conducted on soils differing in texture and chemical composition, allowing assessment of P release dynamics beyond a single extraction event.

The results demonstrate that Mehlich 3–extractable P does not always fully represent the pool of plant-available phosphorus in soils. Sequential extraction patterns revealed substantial differences in P release among soils, indicating varying capacities of solid-phase P to replenish the soil solution. Soil organic carbon, calcium, magnesium, and clay content were identified as key factors controlling the relative proportions of sequentially extracted P. These findings highlight the importance of soil-specific controls on P extractability and suggest that a single Mehlich 3 extraction may be insufficient to characterize soil P availability in certain soil types.

From a practical perspective, the results indicate that assessment of soil P reserves, in addition to standard Mehlich 3 P measurements, could improve fertilizer recommendations and support more efficient and environmentally sustainable phosphorus management.