To produce reliable analytical data in a laboratory, all the stages of the analysis protocol must be properly understood and controlled. Extraction of soil P may thus be compromised by the conditions of analysis, and this may explain variability in results. Therefore, the objective of this study was to verify the effect of deviations in agitation speed, time of soil/ solution contact, and environmental temperature on extraction of soil P by Mehlich-1 (M-1) and Mehlich-3 (M-3). The study was conducted through three experiments performed simultaneously with 11 soil samples from the tilled layer of the following soils: Latossolo, Neossolo, Argissolo, Cambissolo, and Planossolo (Oxisol, Entisol, Ultisol, Inceptisol, and Alfisol) which are representative of the geomorphological regions of the Plateau and the Central Depression of the State of Rio Grande do Sul, Brazil. For all experiments, a completely randomized experimental design was adopted in a 2 × 5 × 11 factorial arrangement, consisting of two extractors, five speeds, temperatures, or times of soil/solution contact, and 11 soil types, with four replications. In the first experiment, the soil/solution mixture was shaken at 80, 100, 120, 140, and 160 oscillations per minute (opm). In the second, after shaking for 5 min., the soil/solution mixture was left at rest for periods of eight, 12, 16, 20 and 24 h before extract removal. In the third, the temperature for performing the entire analysis protocol was kept constant at 15, 20, 25, 30, and 35 °C. The increase in soil/solution contact time increased the amounts of P extracted by M1, but the amount extracted by M3 decreased. Variations in temperature did not significantly affect the M1 extractor; however, M3 was more sensitive to variations. Agitation speed greater than 120 opm increases the amounts of P extracted by the M1 and M3 extractors. There was no consistence between resting periods and the amounts of P extracted by the M1 and M3 extractors. Standardizing the agitation speed and the resting periods would minimize differences in the amount of P extracted by M1 and M3 from soil samples.
soil analysis; analytical variability; standardization
