Zinc deficiencies are generally more widespread on calcareous soils than are Cu deficiencies. The concentration and the degree of complexing of Zn and Cu were measured in the soil solution of 20 calcareous soils from Colorado. The Zn levels in solution were generally less than 2 ppb with rarely more than 75% of the Zn being complexed. Copper levels generally ranged from 5 to 16 ppb in the soil solution with 98 to 99% of the Cu present as organic complexes. Thus, complexing increased the total Cu concentration in solution by a factor of about 100, whereas it increased the total Zn in solution by a factor of 4 or less. Compared to acid soils studied previously, soluble Zn levels were much lower and complexing of Cu was somewhat higher in calcareous soils.

The double‐buffer (DB) feature of the recently‐published Yuan method was applied to the Shoemaker‐McLean‐Pratt (SMP) method at several buffer pH levels, shaking times, and soil/solution ratios. The results, based on regression analysis of buffer‐indicated vs. Ca(OH)2‐titrated acidities, indicate that the widely used SMP single‐buffer (SB) method can be improved considerably by incorporation of DB, quick‐test, and mathematical‐adjustment features. However, the SMP‐SB method is still probably the most satisfactory compromise between simplicity of measurement and reasonable accuracy of results for soils of a wide range in lime requirement (LR). The SMP‐DB method with narrow soil/solution ratio and shorter shaking time gave excellent indication of soil LR when adjusted for incomplete reaction with the soil during the shorter “quick‐test” reaction time. This adaptation which looks very promising as an improved method (especially for use on soils of low LR where the SMP‐SB method is known to lack desired accuracy) is included as a recommended improvement of the SMP‐SB method.

A soil test was developed for the diagnosis of Zn‐deficient soils. The test consists of shaking 10 g soil with 20 ml of extractant (pH 8.6) containing 0.01M EDTA (ethylenediaminetetraacetic acid) and 1M (NH4)2CO3. After 30‐min shaking period the suspension is filtered, and Zn is determined directly in the filtrate by means of atomic absorption spectrophotometry. The new soil test was evaluated on 42 Colorado neutral and high lime soils whose response to Zn was determined in the greenhouse with corn (Zea mays) as the test crop. A critical level of 1.4 ppm of extractable Zn separated the Zn‐deficient from the nondeficient soils. The new Zn soil test compares favorably with the dithizone method and is an improvement over the 0.1N HCl method. Unlike strong‐acid extractants, the new procedure suppresses the dissolution of carbonates and oxides, and thereby avoids extraction of occluded Zn.

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