Total soluble Fe in soil solutions of chemically different soils

Until now not much about Fe concentrations in soil solutions is known because of difficulties to extract the soil solution and to determine the low Fe concentration in them. Therefore a technique was elaborated by which a mixture of 1: 1 soil-quartz sand sample with a soil moisture of 80% of maximum water holding capacity was spread in a Buchner funnel and after an incubation time of 3 days extracted under vacuum (- 50 kPa). In comparison with other water extracts for the determination of soluble Fe our method had a low water/sample ratio and therefore anaerobic conditions were avoided. Anaerobiosis leads to the reduction of Fe (III) and thus to a high release of soluble Fe2+. The Fe(III) in solution obtained by our method was reduced by hydroxylamine hydrochloride to Fe2+ and bound to ferrozine both reacting to form a colored complex with a maximum absorption at 562 nm. The reliability of the method was tested by adding a solution with known Fe concentration to extracted samples and by mixing samples with known Fe concentrations. These tests proved a high reproducibility. By this technique 32 soils differing in texture and pH were analyzed. The Fe concentrations found differed in a range from few mu M Fe to about 200 mu M Fe. These concentrations were not related to any soil characteristics. Calcareous soils had high Fe concentrations. All Fe concentrations found were high enough to meet the plants' demand. The high Fe concentrations found in the calcareous soils prove that the Fe chlorosis of plants on such soils is not a question of the Fe availability in soils. Fifty to 90% of the Fe found in the solutions was complexed by organic molecules (i.e., siderophores). The percentage of organic Fe complexes was correlated with the soil pH (r = 0.93). The relative Fe buffer power in the soil solution differed much and was high in calcareous soils. In the discussion it is argued that the Fe concentrations found in the extracts from soil-quartz sand mixture reflect the actual Fe concentration in the soil solution at 80% of maximum water holding capacity. From the results it is concluded that rather soil organic matter and its turnover than inorganic Fe relationships in soils are decisive for the bioavailability of Fe in soils. This is in line with not yet published results obtained by us showing that the Fe concentrations in soil solution depend much on the microbial activity in soils. (c) 2006 Published by Elsevier B.V.