The Impact of Soil Chemical and Physical Properties on the Amount of Sequentially Extracted P by the Mehlich 3 Method

Phosphorus is one of the main plant nutrient and plants can obtain this nutrient only from soil. The total P content in soil ranges from 1.5 to more than 9000 mg kg⁻¹, with the global average being approximately 550–600 mg kg⁻¹. Phosphorus exists in soil in many different forms. Most phosphorus compounds in soil are insoluble in water. Plants can uptake only the soluble form of phosphorus, which constitutes approximately 0.5% of the total phosphorus in the soil.

To determine the plant-available or so-called soluble P, numerous extraction methods have been developed over more than a century. In Europe alone, more than ten different methods are used for determining plant-available P content. Typically, these are simple extraction methods using low-concentration salts, mineral acids, water solutions, or their mixtures. Depending on the solution composition, the amount of extractable P may vary significantly. Not only does the solution composition influence the amount of P extracted from soil, but the soil's chemical and physical properties also play a significant role.

Many studies have explored the relationship between different methods and the impact of soil chemical properties (such as pH and organic matter) and physical properties (like clay content) on soil P analysis results. All these methods are extraction methods, and the extracted P represents only part of the soluble P fraction in soil.

The amount of P in soil extract is in equilibrium with P in solid form. This equilibrium is determined by both the properties of the solution and the properties of the solid phase (soil). The aim of our work was to investigate how the amount of extracted P changes over three consecutive extractions and what soil chemical and physical properties affect this process. For the extraction, the Mehlich 3 method was used. Our results indicate the influence of soil carbon, calcium, magnesium, and clay content on the relative quantities of sequentially extracted P using the Mehlich 3 method.