Determination of Iron in Manure - Comparison of Digestion/Dissolution Methods (HCl/HNO3; HNO3/H2O2)

Iron is one of the basis plant nutrients. Only a trace amount of iron is needed for a plant growth and its metabolism. An increased iron becomes toxic for a plant. Animal manure can be rich in iron; therefore their application affects the amount of this metal in the soil/soil solution and in plants, too. We measured iron content in manure samples (cattle, dairy) after digestion/dissolution by HCl/HNO3 (aqua regia) and H2O2/HNO3. Iron concentration was also monitored according to a breeding system (milk production, non-market milk production, conventional and ecological/organic farm). In February, March, June and July 2018, 18 samples of dairy manures and 18 samples of cattle manures were collected from companies with different breeding system. The amount of iron was determined by atomic absorption spectrophotometry with atomization in an acetylene-air flame at a wavelength of 248.3 nm. The average iron concentration after digestion/dissolution by HCl/HNO3 (aqua regia) was 1 154 mg/kg DM, a median 926 mg/kg DM, and after extraction by H2O2/HNO3 the average was 1 119 mg/kg DM and a median 915 mg/kg DM. From the point of view of breeding system (milk production, non-market milk production), the average values of iron concentrations varied only within measurement uncertainty. Between results obtained after decomposition/dissolution by different methods (HCl/HNO3; H2O2/HNO3) was found significant relationship (r = 0.9525; alpha =0.05). Higher iron concentrations were detected in the samples taken in warmer months (June, July) by comparison with the samples get in colder months (February, March). The effect of the extraction reagents on the amount of iron in cattle manure samples was not observed, in contrast with dairy manure samples collected in February and March.