Correlation of several tests for phosphorus with resin extractable phosphorus for 30 alkaline soils

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Abstract Phosphorus was extracted from 30 alkaline soils from western Minnesota using the Olsen NaHCO3, the Soltanpour AB‐DTPA, Mehlich 2, Mehlich 3, Bray‐1, and resin methods. Values from the first six methods were compared to resin extractable phosphorus. The NaHCO3‐P values for 30 soils were most closely related to resin extractable‐P values with r ‐ 0.943, followed by the AB‐DTPA with r ‐ 0.889, and Mehlich 3 with r ‐ 0.809. Bray‐1 values from three different soil‐to‐solution ratios were less closely related to resin P. The correlation coefficient between Olsen NaHCO3‐P and Bray‐1 was r ‐ 0.693 for all 30 soils. Soils were divided on the basis of their reaction to dropwise addition of 10% HC1. Violently effervescent samples (n‐15), with one exception, were found to be those soils in the study group with calcium carbonate equivalent (CCE) ranging from 7 to 62 percent. Non‐violently effervescing soils (n‐15) were found to have CCE of less than 7 percent. The elimination of the violently effervescing, high CCE soils from the study group resulted in r values greater than 0.9 for all methods when compared to resin P. Improvement in the relationships between the acid extractants (Bray‐1 and Mehlich 2 and 3) with resin P was attributed to elimination of high CCE soils. Improvement in the relationship between AB‐DTPA and resin P was attributed to the removal of high clay soils that, by chance, also contained high CCE. Olsen NaHCO3 and Bray‐1 (1:10 soil‐to‐solution ratio) values were closely related (r‐0.973) for the non‐violently effervescing (low CCE) soils. Only the NaHCO3 values were closely related to resin P for the 15 high calcium carbonate soils, although AB‐DTPA values were closely related to resin P when the high clay (> 35%) soils were removed.