COMPARISON OF EXTRACTANTS FOR TESTING COASTAL-PLAIN SOILS

Mehlich 3, proposed as the “universal extractarit” 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 Pl. Rye (Secale cereale L.) was grown on 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. © 1990, Taylor & Francis Group, LLC. All rights reserved.