Pedogenesis of Outwash-Derived Soils on Terraces of the Des Moines River.

According to Hans Jenny’s classic model, soils are a product of parent material, organisms, relief, climate, and time. If all factors except one (climate) are held relatively constant, can more be learned about the effect that climate plays in the formation of soils? This study investigates that question. Four sites were selected adjacent to the Des Moines River which stretches across the State of Iowa from the northwest to the southeast. Mean annual precipitation (MAP) and mean annual temperature (MAT) both vary across this track. The soils being compared all formed on the same landscape (high river terrace), in the same parent material (glacial outwash), with the same organisms (trees followed by dominantly prairie) over the same amount of time (about 11k - 12k years). Given that outwash is primarily composed of sand and gravel, these coarse particles make it easy to identify transformations due to chemical and physical weathering during soil formation (pedogenesis). Likewise, the presence or translocation of smaller, clay-sized particles in great abundance will hint at the degree of soil development and the history of soil processes shaping these soils. Early findings are that soil development tends to increase and the depth-to-carbonates increases as both MAT and MAP increase. This project is also investigating the origin of the loamy soil textures that are typically present in the surface horizons (A and upper B horizons -- or upper sola) that overlie the more expected and typical coarse soil textures (mostly sand) of the lower horizons (B and C horizons). This abrupt particle size change has been traditionally interpreted as evidence of loamy sediments being deposited on top of coarser sediments before the onset of pedogenesis. However, intense weathering (including dissolution of carbonate minerals) of the upper soil profile and/or the activities of burrowing organisms in the soil may explain this phenomenon.