The Micromorphology of Some Loessial Soils of Iowa

There are some fundamental differences in the loessial soils of Iowa other than those resulting from slight variations in the nature of their parent materials. In the Knox and in the Marshall there is not the differentiation of horizons as in the Tama, Clinton, and Fayette. The Knox is a lighter colored soil than the Marshall and is found in the steeper regions next to the bottomlands of the larger streams. Unlike the Marshall, the Tama, Clinton, and Fayette contain no lime in their profiles. The Clinton, often found in hilly country overlying streams in eastern Iowa, is much lighter than the Knox of western Iowa, being gray instead of light brown and has a more compact subsoil with no lime present. The Fayette has a grayish-brown surface but has a looser, more friable subsoil than does the Clinton. In order better to characterize the exact nature of these profile differences in the loessial regions of Iowa and to better understand the nature of the soil-forming processes involved, a microscopic study was made of undisturbed samples of the Marshall, Knox, Tama, Fayette, and Clinton silt loams. All five of these soils show, under the microscope, the fine, equigranular skeleton particles that characterize the loess. A slightly more detailed description of these soils is given. The Marshall silt loam is derived from the loess of the Missouri River valley. The surface soils are dark brown to black and the subsoils are light brown to yellow. The substratum is even lighter colored and is high in calcareous material. The Knox silt loam is formed from upland loess occurring near the bottomlands of the Missouri River and its major tributaries. It is a light colored, friable, calcareous soil, showing little profile development. The Tama silt loam, a rolling loessial upland type found extensively throughout northeastern Iowa to the west of the main Clinton and Fayette areas and east of the drift soils and the Marshall region, is dark brown to black with ochre to yellowish-brown subsoils. The substratum is.compact and is mottled with gray, yellow, brown, and rusty brown. The surface soil of the Clinton silt loam is gray or dark gray and overlies a yellowish-brown, compact subsoil. The lower substratum is lighter with some iron stains. The series is typically developed in the loess belt on the west bank of the Mississippi River north of Missouri. The soil contains no lime in the subsoil, being acid throughout the profile. The topography is rolling to broken and drainage is well established. The Fayette silt loam also occurs in the Mississippi loess soil area of northeastern Iowa. It is a gently rolling to hilly, well-drained soil with a grayish-brown to brown, surface and a light brown or yellowishbrown friable, silty subsoil. It is derived from loess. It is an acid soil. In debris preparations of'the Marshall silt loam, shown in Figure 1, naked skeletonal grains on which humus deposits occur can be seen. Few traces of the original plant materials are visible even under the microscope and some humic substances appear in the form of minute granules and fragments. Some of the humic material is soluble and is deposited in the peptized state. There was a difference in the amount of dissolved humus present depending on the soil reaction, the samples from less basic areas of Marshall containing less dissolved humus material than those from the more basic locations. Samples of Knox from virgin timber areas also contained less dissolved humus material than did the typically basic Marshall. In this respect the less basic Marshall, lower in dissolved humus material, resembles the degraded chernozem of Russia and the more basic Marshall is more like the true chernozem. In comparing the Knox and Marshall silt loams with the Tama silt loam, it is seen that the brown color of the Knox and Marshall is due to the humus and not to the