Evaluation of the NH4HCO3‐DTPA Soil Test for Iron and Zinc

The NH 4 HCO 3 ‐DTPA soil test of Soltanpour and Schwab (1977) was developed to simultaneously extract plant‐available P, K, Cu, Fe, Mn, and Zn. The extractant consists of 1.0 M NH 4 HCO 3 and 0.005 M DTPA (diethylenetriaminepentaacetic acid) adjusted to pH 7.6. The soil test consists of shaking 10 g of air‐dry soil with 20 ml of extractant for 15 min. The soil‐extractant mixture is filtered, and the P, K, Cu, Fe, Mn, and Zn concentrations are determined in the filtrate. Greenhouse studies were carried out to evaluate the ability of the NH 4 HCO 3 ‐DTPA soil test to separate Fe and Zn deficient from nondeficient soils. Forty Colorado soils, including 11 benchmark soils, having wide ranges in extractable levels of Fe and Zn were chosen. Results obtained from these studies show that the NH 4 HCO 3 ‐DTPA soil test effectively separated these soils into deficient and nondeficient categories. The critical Fe level for sorghum was established at 4.8 ppm, whereas the critical Zn level for corn was 0.9 ppm. The results indicated that the NH 4 HCO 3 ‐DTPA soil test was as effective as the DTPA soil test of Lindsay and Norvell (1978) in predicting plant available Fe and Zn in Colorado soils. However, the NH 4 HCO 3 ‐DTPA soil test is more economical because it simultaneously extracts both macronutrients and micronutrients. The critical levels were the same for both soil tests for Fe but were different for Zn.