Morphology and Genesis of Some Soils Containing Fragipans in Northern Michigan

Detailed physical, chemical, and mineralogical studies were made on profiles representative of the McBride, Isabella, and Nester series. These are soils that were formed in calcareous sandy loam, sandy clay loam, and clay loam tills, respectively, in northern Michigan. They are bisequal profiles representative of the zonal Podzol group, intergrading to Gray Wooded soils. They each contain eluvial fragipan horizons (Eq) between the overlying illuvial humus sesquioxide horizon (Ihi) and the underlying illuvial silicate clay or textural horizon (It). Genesis of the Podzol sequum is characterized by high physical, chemical, and biological activities, while the underlying Gray Wooded sequum is characterized mainly by leaching of soluble materials and high physical activity such as eluviation, illuviation, contraction, and rearrangements of soil particles. The fragipan is most strongly developed in the McBride. The fragipans occur at depths of 8 to 20 inches beneath the surface and are 6 to 24 inches thick. They are deeper and thicker in the soils formed in the moderately coarsetextured primary materials. The fragipans are grayer and more compact than adjoining layers. They have high bulk densities, are hard when dry and firm when moist. When moist, the resistance of the materials studied to the penetration of a metal cone increases with clay contents up to about 15%. With higher clay contents their resistance decreases rapidly. The clay in these horizons is largely illitic but some chlorite and kaolinite are also present. The clay and silt form a dense matrix or bridges between the sand grains. The alterations of the primary materials necessary for the formation of the pedogenetic fragipan horizons include: leaching of lime, eluviation of the finer expanding lattice clays leaving a residual concentration of the coarser nonexpanding clay, compaction of the layers by weight of the overlying layers or the tap roots of plants, and possibly some weak reversible cementation with soluble alumina. The fragipan layers interfere with the movement of water and plant roots in the soil profile. Roots branch little in these layers and tap roots of alfalfa commonly zigzag through cracks in them.