Manganese solubility due to compaction in soils under corn and soybean
Abstract: Manganese has a complicated redox chemistry which plays an essential role in the uptake of soil manganese by the plants. Mn is the only element that can be both deficient and toxic in the same site during a growing season. Manganese is present in different redox forms in soils Mn(II) Mn(III) and Mn(IV), where Mn(II) (as Mn2+) is the most available form for plants. In this study Mn solubility in two different soils was investigated, one came from a field cultivated with corn and the other from a field cultivated with soybeans. The objectives of the study were to evaluate if extractable soil manganese was reliable on the bulk density in the soils and if there was any difference in extractable soil Mn concentrations between the two different species. It was also investigated whether the water content of the soil correlated with the plant available Mn. In a first experiment cylinder piles with soils from the two crops was set up with different bulk densities. In the cylinder piles a gradient in water content was supposed to be established. In a second experiment, the effect of bulk density was investigated with single cylinders. The manganese solubility was obtained from a sequential extraction procedure consisted of: (i) 1 M ammonium acetate (pH 7), (ii) 1 M ammonium acetate (pH 3) and (iii) 0.018 M Hydroquinol in 1 M pH 7 NH4OAc. The manganese solubility was higher in the samples with the highest bulk density. This was probably a result of a more rapid depletion of oxygen at the higher bulk densities. The Mn concentration was consistently higher in the corn than in the soybean soils. One possible explanation is that this is due to a special fungus that exists in the soil under soybean, which would be able to decrease the availability to plant hosts. No relationship between the Mn solubility and water content was found at constant bulk density. This could be due to the fact that the water content only was measured at the end of the experiment. Practical problems in some cases when measuring the water content at high degree of saturation could have contributed to the lack of correlation.
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