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 0.005M DTPA (diethylenetriaminepentaacetic acid), 0.1M triethanolamine, and 0.01M CaCl2, 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 CaCl2 so that equilibrium with CaCO3 is established at a CO2 level about 10 times that of the atmosphere. Thus, the extractant precludes dissolution of CaCO3 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.

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The activity isotherms for AlPO4 · 2H2O (variscite), FePO4 · 2H2O (strengite), Ca10(PO4)6F2 (fluorapatite), Ca10(PO4)6(OH)2 (hydroxyapatite), Ca4H(PO4)3 · 3H2O (octocalcium phosphate), and CaHPO4 · 2H2O (dicalcium phosphate dihydrate) were represented on a single solubility diagram in which a function of the phosphate activity in solution was plotted against pH. This plot is useful for assessing the relative stabilities of these phosphate compounds and for predicting their transformations in soils upon the application of fertilizer or lime. For this development, the assumptions were made that Al3+ activity in soils was limited by the solubility of gibbsite, the Fe3+ activity by the solubility of goethite, F‐ activity by that of fluorite, and the Ca++ activity was arbitrarily selected as that of a 0.005M solution of a monovalent salt. Equations and graphs are included for modifying the basic diagram so as to correspond to any Al3+, Fe3+, F‐, or Ca++ activity that may be found in soils or that may be established during a particular soil extraction.