Range of Fragipan Expression in Some Michigan Soils

The evolution of fragipans in the Great Lakes region is poorly understood despite the common occurrence of fragipans in the region. To help resolve this problem, three soils with fragipans in northern Michigan were studied to evaluate the pedogenic pathways for fragipan evolution, and the results extended to soils forming under similar conditions. Soil characterizations were made via field, physical, chemical, and micromorphological observations and analyses and were evaluated in terms of the major models of fragipan genesis. Because it appears that a combination of pedogenic models best explains many of the fragipan properties of our soils, while also allowing for their variable expression, we developed a new model—one that integrates and synthesizes existing models and our findings—to explain fragipan evolution for many soils in Michigan and the Great Lakes region. The Michigan Model of Fragipan Evolution (MMFE) is therefore a synthesizing model that involves the self‐weight collapse of a wet soil or parent material, followed by physical ripening of the collapsed zone. Ripening helps to retain the closely packed fabric and intergrain bridging in the collapsed zone, creating a protofragipan. Later, amorphous bonding agents precipitate in the protofragipan due to its position near a weathering‐front discontinuity. The resulting fragipan develops progressively and becomes better expressed with further pedogenesis. Fragipan degradation is eventually initiated by an increasingly prominent, perched zone of saturation that forms seasonally above the fragipan. With time, processes associated with fragipan degradation and translocation of materials to lower parts of the profile exceed processes associated with progressive development, and the fragipan is destroyed.