Greenhouse and field evaluation of the NH 4 HCO 3 ?dtpa soil test for Fe 1

Abstract

The NH₄HCO₃‐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 NH₄HCO₃ and 0.005 M DTPA (diethylenetriaminepentaacetic acid), adjusted to pH 7.6. The soil test consists of shaking 10 grams of air‐dry soil with 20 ml of extractant for 15 minutes. The soil‐extractant mixture is filtered and the P, K, Cu, Fe, Mi, and Zn concentrations are determined in the filtrate. A greenhouse study was carried out to evaluate the ability of the NH₄,HCO₃‐DTPA soil test to separate Fe deficient from nondeficient soils. Forty Colorado soils, including 11 benchmark soils, having wide ranges in extractable levels of Fe were chosen. Results obtained frcm this study showed that the NH₄,HCO₃‐DTPA soil test effectively separated these soils into deficient and non‐deficient categories. The critical Fe level for sorghum was established at 4.8 ppn. For field study 23 Fe‐deficient sorghum fields were visually identified throughout eastern Colorado. Composite soil samples were obtained in both deficient and nondeficient areas within each field by removing soil from the root‐zones of 5 to 10 plants. No Fe chlorosis was observed in soils with more than 4.4 ppm NH₄HCO₃‐DTPA extractable Fe. The results indicated that the NH₄HCO₃‐DTPA soil test was as effective as the DTPA soil test of Lindsay and Norvell (1978) in predicting plant available Fe in Colorado soils. However, the NH₄HCO₃‐DTPA soil test is more economical because it simultaneously extracts both macro and micronutrients. The critical levels were the same for both soil tests for Fe.