Even following the protocol recommended by the method of analysis, variations in analytical results can be caused by the way samples, or the kind of materials used in the analysis, are handled. The objective of this study was to evaluate the influence of soil particle size, the type of extraction container, and the container void volume on soil P content extracted by Mehlich-1 and Mehlich-3. In the first experiment, soil samples were ground to pass through sieves with 2.000, 1.700, 0.850, 0.600, and 0.300 mm diameter openings. In the second experiment two 50 mL container types (Erlenmeyer and "snap-cap") were used. In the third experiment the void volume of the container was changed while maintaining the soil:extraction solution ratio within the container at 1:10, 1.5:15, 2.5:25; 3:30 and 4:40 cm³ cm-3, respectively. Soil particle size of the samples did not affect the extraction capacity of Mehlich-1; however, the extraction capacity of Mehlich-3 was affected, especially in clay soils. Extracted P was significantly higher with "snap-cap" type containers than Erlenmeyer containers for both methods. The void volume of the container affected the content of P extracted by Mehlich-1 and Mehlich-3 in 100 and 64 % of the samples, respectively. We recommend standardizing soil particle size for P extraction by the Mehlich-3 solution. A single container type should be adopted by laboratories for P analysis, regardless of the extraction method, always maintaining the sample volume (cm³) to extraction solution volume (cm³) contained in the bottle constant.
soil analysis; phosphorus extraction method; analytical procedures
