A Zinc Soil Test Calibration Based upon Mehlich 3 Extractable Zinc, pH, and Cation Exchange Capacity

The soil Zn critical level generally decreases as organic matter content increases. Certain other properties, notably cation exchange capacity (CEC) and optimum soil pH, also vary with organic content. These properties and Mehlich 3 extractable Zn (M3Zn) were used to calibrate a Zn soil test from greenhouse and field data. Soils were collected from four lime treatments in field experiments conducted on six Ultisols. An Inceptisol and a Histosol were also sampled and four lime treatments established, providing a range in pH and organic matter content. Three levels of Zn as ZnSO₄ · 7H₂O were added to each soil. Soybeans [Glycine max (L.) Merr., cv. Centennial] and corn (Zea mays L., cv. Pioneer 3369A) were grown successively in the greenhouse, each for 5 weeks. Mehlich 3 extractable Zn decreased with an increase in soil pH on only one of the eight soils, but Zn concentration in plants grown on all soils decreased with an increase in soil pH. Mehlich 3 extractable Zn alone accounted for little of the variation in the plant Zn concentration of the two crops. Including soil pH and CEC in the model increased the R² to 0.56 and 0.50 for soybeans and corn, respectively. When the squared and first-order interaction terms of these variables were considered, the R² increased to 0.76 for soybeans and 0.66 for corn. For corn grown in the field, these same variables were found to be important in Zn soil test calibration. The form of the regression equation proved to be illogical, however, because the range in soil properties was not sufficient to produce a reliable prediction equation. Regression equations from the greenhouse were then adjusted to predict the leaf Zn concentrations of crops grown in the field. For corn, an equation that gave reasonable estimates of leaf Zn concentration in the field was developed. This equation may be used to interpret the need for Zn fertilization over a continuum of extractable Zn concentration, pH, and CEC in soils.