Fifty years of agricultural soil change in Iowa
Many scientists have studied agricultural soil change. Most of these studies focus on changes in the surface 0 to 30 cm soil. Despite a whole body of scientific research that shows that soils change on relatively short time scales under different management regimes, classical pedological theory states that we should expect these changes to occur only in the surface few centimeters and that they are not of adequate magnitude to suggest fundamental changes in pedon character over short periods of time. Nevertheless, when we resampled sites that were initially described by the soil survey between 1943 and 1963 and compared current and historical descriptions and laboratory data, we found fundamental changes--often to more than 100 cm depth in one or more of the following--soil color, texture, structure, depth to carbonates, depth to redoximorphic features, bulk density, organic matter content, and pH. These sites have been under predominantly row-crop agriculture during that time and are located in Iowa, United States. In the surface 0 to 45 cm of soil, we found degradation of soil structure, loss of organic matter, lower cation exchange capacity and lower pH. When we look at the whole soil profile (sampled to 150 cm) the story becomes more complete and interesting. For example, organic C is moving deeper in the soil profile. This results in modern pedons that have an overall accumulation of organic C relative to their originally sampled counterparts, even though epipedons today contain less organic C than they did in 1959. Likewise, grey redoximorphic features are found deeper in the profile. This reveals a deepening of the water table, suggesting a statewide lowering of base level. Overall, anthropedogenesis accelerates melanization, erosion, cumulization, oxidation and braunification, while dissolving and translocating carbonates and organic matter and altering soil structure.
From the changes in soil properties we also 1) document changes in classification and 2) estimate erosion rates. First, in each of three taxonomic systems, the classification of 60% or more of the sampled pedons was different from the original in at least one level of the hierarchies. Fifteen to 32% of sampled pedons changed at the Order (or equivalent) level with 11% to 33% of the pedons originally classified as Black Soils -- Mollisols, Chernozems or Phaeozems -- no longer classified as Black Soils. Second, we estimate actual erosion rates on our sites to be two to three times the tolerable rate. Given our sampling protocol on relatively low stable slopes, we surmise that average rates across all of Iowa--and indeed much of the Corn Belt--should, in fact, be higher, potentially much higher.