Distribution and Origin of Argillic Horizons Across Iowa.

Distribution and origin of argillic horizons across Iowa

M. A. Ibrahim and C. L. Burras

Department of Agronomy, Iowa State University, Ames, IA 50011

ABSTRACT

Argillic horizons occur across 33.5% of Iowa's land area.  Two-thirds of the area has Udalfs and Aqualfs.  The remaining third has Argiudolls, Argiaquolls and Argialbolls.  Soils in eastern and southern Iowa commonly have argillic horizons (e.g., argillic horizons cover almost 90% of some counties) while central and northwestern Iowa have the fewest (e.g., 0% in some counties). This general distribution has variously been attributed to trends in - and combinations of - rainfall, native biota, and parent material.  Merit exists for each of these hypotheses.  Conversely, uncertainty exists for each especially after argillic horizon distribution is evaluated regionally.  That is, Nebraska, South Dakota and Minnesota have argillic horizons in soils where “Iowa” thinking would predict they should not form. This study investigates the relationship between argillic horizon occurrence and location across Iowa. We hypothesize the presence and magnitude of argillic horizon expression is a function of whole catena characteristics. The data undergirding our study comes from a variety of NRCS and Iowa Cooperative Soil Survey databases and pedon descriptions. Our analyses are via GIS. Our preliminary interpretation is open drainage systems generally have upland soils with well-expressed argillic horizons.  The exception is when the average slope of the watershed exceeds between 5 and 10%. Closed catenas rarely have any soil with an argillic horizon. We attribute this to catena-wide destabilization of silicate minerals that occurs in response to depressional hydrophytes demand for silica as a nutrient.