Variabilidade do fósforo extraído causada pela variação analítica nos métodos Mehlich - 1 e 3 ; Variability of extratected phosphorus caused by analytical variation on Mehlich - 1 and 3 methods

Performing a soil analysis requires a series of operations so a specific element may be conveniently extracted to be then detected or measured. Protocol methods have been adjusted to facilitate the execution, however here may be variability in analytical results by different laboratories even if they follow the same general principles of the method. This study aimed to identify sources of variability that affect analytical precision and accuracy of phosphorus extracted by Mehlich-1 and Mehlich-3 under variations in protocol procedures. Eleven soil samples in the surface layer (0 - 20 cm) were selected and collected in the geomorphologic Plateau and Central Depression of Rio Grande do Sul state, Brazil, which have wide variation of physical, chemical and mineralogical composition, including different clay contents and phosphorus availability. In the laboratory, seven experiments were conducted to determine the effect of different factors: sample particle size (<0.30, <0.60, <0.85, <1.70 and <2.00 mm), extraction container type (Erlenmeyer and snap-cap ), the void volume of the container (6.0, 17.0, 22.5, 33.5 and 39.0 ml), the shaking frequency (80, 100, 120, 140 and 160 oscillations per minute - opm), time to rest solution after shaking (8, 12, 16, 20 and 24 h), reaction time of the phosphomolybidic complex used as a reducing agent (15, 30, 45, 60 and 120 minutes) and environmental temperature (15, 20, 25, 30 and 35 °C). The results indicate that the particle size of the soil samples did not significantly affect the amount of phosphate extracted by the Mehlich-1 solution. However, for Mehlich-3 solution, particle size of the samples had an effect being more pronounced in soil phosphorus with medium and high availability. The use of snap-cap type containers optimized the quantities of phosphorus extracted by Mehlich-1 solution when compared with Erlenmeyer container. For the Mehlich-3, there is a combined effect of the type of container with the soil mineralogy and phosphorus content in the sample. ...