Comparison of some methods for determination of copper, zinc, manganese and iron in soils

The soil extraction procedure using DTPA extracting solution for the determination of copper, zinc, manganese and iron was introduced by Lindsay, Norvell (1978). The method proved to be relatively sensitive to experimental conditions especially to the temperature during extraction, extraction time and extraction intensity (Soltanpour et al., 1976; Lindsay, Norvell, 1978). Since 1978 the extraction procedure according to Lindsay, Norvell (1978) has become very popular and widely used in many countries of the world and international standardization of the procedure is now in the final stage. The internationally adopted procedure (DTPA-ISO) uses lower extraction temperature and higher extraction intensity than our national procedure (DTPA-CZ). The criteria for evaluation of the results for DTPA-CZ method were based on the field and pot experiments. The main reason for this study was to find whether recalculation of these criteria defined for DTPA-CZ method was possible for DTPA-ISO method and to avoid expensive field and pot experiments. The second goal of this study was to find a relationship between the DTPA-ISO (resp. DTPA-CZ) method and the Mehlich III method (Mehlich, 1984). The Mehlich III method (M III) is very suitable for simultaneous determination of macronutrients (K, P, Mg, Ca) and micronutrients and the method is very popular in many countries of the world. The M III method is very similar to the Mehlich II method, extensively used in the Czech Republic for nutrient determination and fertilizer recommendation. 95 soil samples from soil monitoring plots were collected for the study. The samples represented all major soil types, climatic regions and proportion of agronomic cultures in the Czech Republic (arable soil 79, grassland 9, orchards 4, hopgardens 1, vineyards 2). Samples were air-dried and a fraction under 2 mm in diameter was used for analysis. Samples were extracted and analyzed by all three methods (DTPA-ISO, DTPA-CZ, M III) in triplicates. Regression analysis clearly showed for DTPA-ISO and DTPA-CZ that the results achieved by both methods for all four elements are very near, and no recalculation of the criteria is needed. That means the expected increase in the results for the DTPA-ISO method by higher extraction intensity was compensated by the decrease due to the lower extraction temperature. It means from a practical point of view that laboratories in the Czech Republic can adopt the international standard procedure without changing the criteria for the evaluation of the results. We can conclude from regression analysis of the results for the DTPA-ISO and M III methods that the correlation for copper and zinc is very good and the recalculation of the criteria is possible according to the equations: Cu_M III = 0.426 + 1.514 Cu_DTPA-ISO; Zn_M III = 0.408 + 1.802 Zn_DTPA-ISO. For iron and manganese the correlation is not suitable for the recalculation of the criteria. The determination of iron and manganese by the M III extracting solution would require more precise calibration of the method based on field and pot experiments. The M III method is very suitable for simultaneous determination of nutrients, copper and zinc, where the criteria for evaluation can be recalculated. When only micronutrients are to be determined, or iron and/or manganese are required, the DTPA-ISO method should be used for the determination.