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Abstract
Mehlich 3, Mehlich 2, CAL, Olsen, Egner, 0.01 M CaCl2 and aqua regia were compared as soil phosphorus extractants. Comparison was focused on the relationships between Mehlich 3 as a Czech official soil testing method and the extractants used for this purpose in the past (Mehlich 2, Egner) or frequently studied in some other countries (CAL, Olsen, 0.01 M CaCl2, aqua regia).
1173 agriculture soil samples were collected for the study (157 light soils, 960 medium soils, 56 heavy soils, 463 soils with carbonate content higher than 0.3%). The samples represented all major soil types, climatic regions and proportions of agronomic cultures in the Czech Republic. The percentage of soils with higher carbonate content was increased to improve the statistical evaluation.
When compared to Mehlich 3 (100%) the amount of phosphorus extracted by the chosen methods were as follows: aqua regia (800%), CAL (95%), Mehlich 2 (80%), Egner (80%), Olsen (40%), 0.01 M CaCl2 (2%).
Statistically highly significant linear relationships between Mehlich 3 and Mehlich 2, Olsen, CAL, and 0.01 M CaCl2 were found. No relationship was found between aqua regia and the other extractants. Coefficient of determination (R2) for the linear regression between Mehlich 3 and the other methods for soils with carbonate content less or equal 0.3% were 0.928 (Mehlich 2), 0.742 (Olsen), 0.515 (CAL), 0.492 (Egner) and 0.556 (0.01 M CaCl2), respectively. For soils with carbonate content higher than 0.3% the coefficients of determination were usually lower: 0.879 (Mehlich 2), 0.630 (Olsen), 0.732 (CAL), and 0.490 (Egner).
The hypothesis that Olsen extractant should be used instead of Mehlich 3 for soils with higher carbonate content was not proved because a highly significant relationship between the methods was found for soils with higher carbonate content. The results of the study enabled us in many cases to recalculate the criteria for the evaluation of the chosen methods but recalculation of the individual measurements cannot be recommended.