Comparison of extractants for testing coastal plain soils

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Abstract
Mehlich 3, proposed as the “universal extractant”; for the Eastern U. S., has not been adequately evaluated for the major agricultural soils of the Coastal Plain of Georgia. A greenhouse study was conducted to compare soil P, K, Ca, and Mg extracted by Mehlich 3, Mehlich 1, and Bray/Kurtz P1. Rye (Secale cereale L.) was grown oh six soil series with five P and five K rates to develop plant data for Mehlich 3 extractable P and K. Rye dry weight, P concentration, and P uptake were significantly related to applied and soil P. Extraction of soil P was significantly correlated among the extractants, but slopes varied from 1.22 to 1.84 in the order: Bray/Kurtz P1 > Mehlich 3 > Mehlich 1. Responses of soil tests to applied P were in the same order and for five of the soils were 0.87, 0.71, and 0.51 kg P extracted/kg P added for the three extractants, respectively. A sixth Coastal Plain soil with a relatively high clay content gave much smaller soil‐test responses of 0.32, 0.26, and 0.16 kg/kg for the three extractants, respectively. Sorption isotherms indicated a high P sorption for the high clay soil relative to the other soils. Isotherms or clay content may be useful in extrapolation among soils. Rye was less responsive to K than to P and its response to K was curvilinear. Mehlich 3‐K, ‐Ca and ‐Mg values were closely related to their respective Mehlich 1 values with slopes close to one. The results indicate that Mehlich 3 is a suitable extractant for P, K, Ca, and Mg in Coastal Plain soils. Samples stored for 27 mo under prevailing temperature conditions and then retested had increased Mehlich 1‐P and ‐K at low soil test levels and decreased Mehlich 1‐P and ‐K at high test levels.

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