Development of a DTPA Soil Test for Zinc, Iron, Manganese, and Copper

Abstract A DTPA soil test was developed to identify near‐neutral and calcareous soils with insufficient available Zn, Fe, Mn, or Cu for maximum yields of crops. The extractant consists of 0.005 M DTPA (diethylenetriaminepentaacetic acid), 0.1 M triethanolamine, and 0.01 M CaCl 2 , with a pH of 7.3. The soil test consists of shaking 10 g of air‐dry soil with 20 ml of extractant for 2 hours. The leachate is filtered, and Zn, Fe, Mn, and Cu are measured in the filtrate by atomic absorption spectrophotometry. The soil test successfully separated 77 Colorado soils on the basis of crop response to Zn, Fe, and Mn fertilizers. Critical nutrient levels must be determined separately for each crop using standardized procedures for soil preparation, grinding, and extraction. The critical levels for corn using the procedures reported herein were: 0.8 ppm for Zn, 4.5 ppm for Fe, and tentatively 1.0 ppm for Mn, and 0.2 ppm for Cu. Development of the soil test was based, in part, on theoretical considerations. The extractant is buffered at pH 7.30 and contains CaCl 2 so that equilibrium with CaCO 3 is established at a CO 2 level about 10 times that of the atmosphere. Thus, the extractant precludes dissolution of CaCO 3 and the release of occluded nutrients which are normally not available to plants. DTPA was selected as the chelating agent because it can effectively extract all four micronutrient metals. Factors such as pH, concentration of chelating agent, time of shaking, and temperature of extraction affect the amount of micronutrients extracted and were adjusted for maximum overall effectiveness.