The Relationship Between Mehlich-3 Soil Test Indices and Leaf Nutrient Concentrations of Irrigated Soybean.

The Mehlich-3 (M3) solution is considered a universal soil extractant, but very little information exists concerning the meaning of extracted nutrients other than P, K, and Zn.  Our objective was to determine if M3 extractable nutrients were correlated with irrigated (furrow or flood) soybean [Glycine Max. (L.) Merr.] trifoliolate leaf (TL) nutrient concentrations. Soil and recently matured TL (top three nodes) from 44 P-fertilization trials were collected and the mean data were analyzed using correlation and regression analysis (linear, quadratic and linear plateau models). Soil (0-10 cm, preplant) and TL (R1-R2 stage) were both collected from the no P control plots (n=4-6) of each trial. Correlation analysis showed significant (Pr<0.10) relationships existed between M3- and TL-P (r=0.56), M3- and TL-Ca (r=0.29), and M3- and TL-Zn (r=0.44). Regression analysis showed that the relationship was quadratic for P (r² =0.42), Ca (r² =0.29), Mg (r² =0.31), and Zn (r² = 0.23) and nonsignificant for S, Na, Fe, Mn, and Cu. Omitting the site-years where an indeterminate variety (maturity group 4) was grown changed and improved the relationship for Zn (n = 25, Linear, and r² = 0.55), nominally improved the quadratic relationships for Ca (r² =0.33) and Mg (r² =0.36), and had no influence on the nonsignificant relationships for S, Na, Fe, Mn, and Cu. For nutrients with a significant quadratic model the linear plateau model produced relationships with similar r² values. Linear plateau models predicted TL concentrations increased linearly until a plateau was reached at 0.41% P, 1.09% Ca, and 0.399% Mg for M3 values of 31 mg P kg^-1, 1886 mg Ca kg^-1,and 172 mg Mg kg^-1.  Slope coefficients were 0.005 for P, 0.00022 for Ca and 0.00185 for Mg. Results suggest that M3-P, -Ca, -Mg, and -Zn index values are related to soybean TL nutrient concentrations and the TL nutrient concentrations of determinate and indeterminate varieties may behave differently. The lack of significant relationships for S, Fe, Mn, and Cu may be related to irrigation events, lack of diversity in the range of soil and plant concentrations, and/or failure of the M3 solution to extract specific bioavailable soil nutrients.