Soil is one of the main resources for most terrestrial life. Soil Sciences study soils from many different perspectives. They are very important for agriculture, construction and mining. This dissertation has two overarching aims: (1) to better understand how and where the argillic horizon forms; and, (2) to better understand how the clay-sized fraction of soil behaves within the sola and catenas of Iowa. As pedologists, we are interested in studying argillic horizons because they have unique characteristics that formed by different mechanisms of clay accumulation. Formation of the argillic horizon has been investigated for decades.;In this dissertation I investigated the occurrence and distribution of argillic horizons across the contiguous US in order to capture a general concept about the factors that affect its formation. My data showed that argillic horizons can occur almost everywhere across the contiguous US. Forty percent of the US soils have argillic horizons. The places lacking argillic horizons tend to be areas which have sand dunes, rocky mountains, steep slopes and places with plenty of prairie potholes. Therefore, I focused on the Midwest States because there is a large spot lacking argillic horizons and yet it is surrounded by them. More specifically, I investigated the origin and distribution of argillic horizons across Iowa and the relationship between drainage systems and the formation of argillic horizons.;For the second part of the study, I investigated the possible movement directions of clay-sized particles. Four demonstrations were performed in this work. First, I tested the likelihood of upward movement of clay in sponges. The results demonstrated the possibility of upward movement. Second, I tested for upward movement of clay in sand columns. Basically, I set a number of clean and transparent plastic bottles that have holes in them at the bottom in aluminum pans and then filled them with pure medium quartz sand. Next, I supplied clay suspension to the aluminum pans so that clay suspension gets into sand columns from the bottom. Clay suspension moved upwards through sand columns and then accumulated at the top of them.;To apply the results of the second step in real soils, I tried to mimic nature by establishing a water table level in sand columns. Clean sand layers were interspersed with sandy loam layers in transparent plastic bottles that stood in glass beakers. Next, I added water to the beaker so that water got into the sand columns from the bottom. The results of this step demonstrated both downward and upward movement of clay-sized particles. Also, these results demonstrated the likelihood of clay movement by diffusion. I used the results of this work to introduce an alternative mechanism for the formation of lamellae.;Last, I investigated some pedons that have argillic horizons in them. One pedon was used from the following eight counties---Mahaska, Monroe, Davis, Jefferson, Van Buren, Keokuk, Lucas and Marion. This area was selected because it has soils that have shale in their sola. All of the pedons were completely described and analyzed. All of these study pedons were formed in layered parent materials such as Pennsylvanian Shale, Peoria Loess, glacial till or local colluvium. The results of field descriptions and laboratory analyses were used to classify study pedons down to the family level. These study pedons were classified to two main soil orders, Alfisols and Ultisols. Moreover, the occurrence of lithologic discontinuities in these pedons was tested using different methods such as total sand, sand: silt ratio as well as total silt as a depth functions.
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