Cores 11 through 26 by Gary B. Dr - University of Illinois at Urbana ...

in the clay-size fraction (Lindsay et al., 1989). Phosphorus-containing fertilizers are the most commonsource of phosphorus in agricultural and residential soils.The P 2O 5content in the cores generally decreased or remained relatively constant with depth. Where theP 2O 5content of the uppermost sample or two was much higher than the lower samples, there might havebeen a recent application of P-containing fertilizer.Manganese The principal source minerals for manganese in soils are amphiboles, pyroxene, biotite, andrhodonite (MnSiO 3) (Sparks, 1995). Although manganese occurs in the bulk of the soil as coatings onother minerals (Kabata-Pendias, 2001), it is also commonly found concentrated in nodules (concretions)accompanied by iron. These nodules seem to form in soil horizons that periodically become waterloggedso that reducing conditions prevail, and the soil then dries, restoring oxidizing conditions (McKenzie,1989). In some soils a microscopic layered structure of alternating bands of iron-rich and manganeserichmaterial have been observed (McKenzie, 1989). However, the lack of correlation between MnO 2andFe 2O 3in the samples from these cores suggests no such intimate association.In most of the cores the MnO 2content generally decreased with depth. One possible reason for this isthat manganese is sensitive to oxidation and reduction. Oxidized species, such as MnO 2, precipitatewhere oxygen is readily available, as it normally would be near the surface of the soil, and decrease incontent with depth. The solubility of manganese increases as the pH and Eh (the oxidation-reductionpotential) decrease (the soil becomes more acidic and less oxidizing) (Lindsay, 1979). Under conditions ofincreasing acidity and decreasing oxidation potential, manganese would be somewhat easily leached.In a few cores (12, 14, 15, 16, and 18), however, the MnO 2content increased immediately below thesurface to a depth of 1 to 2 feet, then decreased, and in other cores (12, 14, and 16) the behavior ofmanganese was opposite to that described above. Possibly in these cores the Mn was present in colloidalsizedparticles that migrated downward from the surface before becoming lodged.Sulfur The probable source of sulfur in these soils was the oxidation of the minerals pyrite and marcasite(they have the same chemical composition, FeS 2). Pyrite in Illinois soils was oxidized long ago, andthe S now is in the form of sulfate, as in gypsum, or as organic sulfur compounds. Biological activity isresponsible for producing organic sulfur compounds (Stevenson, 1964).The content of SO 3in the sixteen cores was either below (cores 11 and 12) or near the analytical detectionlimit. The SO 3content was uniform both with depth and from core to core. Sulfate forms compounds oflow solubility with both barium and strontium.Secondary sources of S are fertilizers and, perhaps more importantly, fallout of particles from emissionsgenerated by coal-fired power plants (Hoeft, 1986). As clean-air legislation has increasingly forced powerplants to reduce the concentration of S in their flue gases, fertilizer will likely become a more importantsource of S for growing crops. Prior to enactment of clean-air legislation much of the crops’ need for Swas satisfied by rainfall that transported sulfur dioxide from the atmosphere to the soil (Hoeft, 1986).Barium Micas and feldspars are sources of barium in soils. These minerals contain potassium whichis commonly replaced by barium, because of the similar size of the two atoms (Kabata-Pendias, 2001).Barium is strongly adsorbed on clay minerals in soils and, therefore, is not very mobile. Barium has beenfound in soils as barite (BaSO 4,Allen and Hajek, 1989) and hollandite [Ba(Mn 4+ ,Mn 2+ ) 8O 16, McKenzie,1989]. Barium also is concentrated in manganese and phosphorus concretions, and is specifically adsorbed20