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Relation of Nonexchangeable Potassium and Magnesium to Development in Some Midwestern Soils¹

ABSTRACT

The nonexchangeable (NEx) potassium and magnesium status of the <1µm clay and other properties of the thin loess over till soils of northwestern and eastern Iowa were investigated to better understand the genesis and morphology of these soils and to provide a firmer basis for classification of this recently delineated group of soils. These soils are compared with similar soils from Illinois and Ohio to evaluate the regional effects of parent materials, vegetation, and climate.

The Sac and associated soils from northwestern Iowa have a higher NEx K and Mg content, are less texturally developed, release more K with sodium tetraphenylboron, and have a higher illite content than the Dinsdale and associated soils from eastern Iowa. These differences are attributed to the less weathered condition of the northwestern Iowa soils, primarily because of the lower mean annual precipitation (MAP) in that area. These differences support the separation of the two groups of soils at the series level.

The loess-derived upper sola of the forested Iowa soils have about the same NEx K and Mg contents, but more illite, and are less texturally developed than the upper loess portion of the forested Illinois and Ohio soil profiles. The lower-till-derived sola of the Iowa soils have a lower NEx K, NEx Mg, and illite content than the Illinois and Ohio soils. These differences in characteristics support the separation of the Iowa thin loess over till soils from the Illinois and Ohio thin loess over till soils at the series level.

¹ Journal Paper no. J-8139 of the Iowa Agric. & Home Econ. Exp. Stn., Ames, Iowa. Project No. 1992.

² Formerly Research Associate, Dep. of Agronomy, Iowa State University, and now Agriculturist, Div. of Agricultural Development, National Fertilizer Development Center, TVA, Muscle Shoals, AL 35660 and Professor, Dep. of Agronomy, Iowa State University, Ames, Iowa; respectively.