COMPARISON OF SOIL ZINC EXTRACTANTS FOR DETECTION OF APPLIED ZINC ANDPREDICTION OF LEAF ZINC CONCENTRATION

Many soil extractants have been developed for determination of zinc (Z n) availability to plants. The optimum soil Zn extractant should be us eful not only for prediction of plant Zn concentration but also for de tection of applied Zn levels. The objectives of this study were: i) to compare soil Zn extractants for detecting applied Zn and for predicti ng peanut leaf Zn over a range of soil pH levels, and ii) to correlate other soil-extractable Zn levels with Mehlich-1. Soil and peanut leaf samples were taken from a field study testing pH levels as the main p lots and Zn application rates in the sub-plots. Extractable Zn was det ermined on soil samples using Mehlich-1, Mehlich-3, DTPA, MgNO3, and m any dilute salt extractants of varied strength and pH. Correlation of extractable soil Zn to cumulative applied Zn levels revealed Mehlich-1 , Mehlich-3, DTPA, and AlCl3 extractants to be among the best indicato rs of applied Zn. Leaf Zn concentration was best correlated with soil Zn extracted by dilute salts, such as KCl, CaCl2, NH4Cl, CaSO4, and Mg Cl2. Including soil pH as an independent variable in the regression to predict leaf Zn considerably improved R-square values. The DTPA-extra ctable soil Zn levels were very well correlated with Mehlich-1-extract able Zn. Mehlich-3 extracted about 20% more soil Zn than Mehlich-1, bu t Mehlich-3 soil Zn was not as well correlated to Mehlich-1 soil Zn as DTPA soil Zn. Lower pH solutions extracted more of the applied Zn, bu t more neutral solutions extracted Zn amounts which were better correl ated with Zn uptake. On the other hand, Mehlich-1, which had a lower p H, had better correlations with both applied Zn and leaf Zn than did M ehlich-3. Shortening the DTPA extraction time to 30 minutes resulted i n better correlations than the standard two hour extraction time. Chlo ride (Cl) was the best anion tested in relation to soil applied Zn rec overy in combination with potassium (K), calcium (Ca), and aluminum (A l), and Cl optimized leaf Zn correlations for ammonium (NH4), K, Ca, a nd magnesium (Mg). The larger the valence of the cation, the better th e correlation with applied Zn and the poorer the correlation with leaf Zn.