Multiple correlations were determined between liquid limit, plastic limit, and plasticity index and various combinations of standard physical and chemical determinations (cation-exchange capacity, percent of organic carbon, percent of <0.002-mm. clay, percent of montmorillonite in the clay separate, percent of illite in the clay separate, and percent of 0.05-to 0.002-mm. silt) on a wide range of Illinois soils.Multiple correlation coefficients of 0.959, 0.887, and 0.938 were obtained between liquid limit, plastic limit, and plasticity index, respectively, and three soil properties (percent of organic carbon, percent of clay, and percent of montmorillonite in the clay separaate), indicating that there is a very close relationship between Atterberg limits and these three soil properties. If additional soil properties, such as percent of silt and percent of illite in the clay separate, are included in the analysis the correlation coefficients are increased only slightly. Lower, but highly significant correlations were also obtained between each of the Atterberg limits and cation-exchange capacity, and a combination of percent of organic carbon and percent of clay.
Oxidation‐reduction potentials in situ have been measured in a hydro‐sequence of soils in the Podzol region of Michigan with variations in time and space. The soils were well‐drained Kalkaska sand, imperfectly drained Saugatuck sand and poorly drained Roscommon sand. The distribution of citrate‐oxidizing organisms and iron‐oxidizing organisms in horizons of Kalkaska and Saugatuck sands was also studied. Redox potentials were observed to vary with moisture, temperature and soil horizons. The population of citrate‐oxidizing organisms decreased with depth and iron‐oxidizing organisms were isolated from the Bir horizons of both Kalkaska and Saugatuck sands. The results indicate that certain mechanisms of Podzol formation may be active in the soils studied.
Redox potentials have been measured in situ, with variations in time and space, in two well drained Podzols, McBride fine sandy loam, and in a Humic Gley Soil, Parkhill loam. In addition, redox potentials were studied in sand columns under an organic layer in a laboratory experiment. Extremely low potentials persisted late into the growing season in the Parkhill loam, while low potentials were not observed in McBride fine sandy loam or in Marlette loam during the period from May to November. In McBride fine sandy loam and in Marlette loam, the surface horizon gave lower readings than the lower horizons during the summer months. In the Parkhill loam, the Ag horizons gave the lowest readings in the spring. A redox profile was also observed in a study using sand columns under a humus layer. This was associated with a relatively greater amount of reduced iron in the upper part of a well drained sand column. It is postulated that this ferrous iron could be dissolved in the upper part of the solum, translocated, and then precipitated when it reached a higher oxidation zone lower in the soil profile. Additional studies are under way to test this hypothesis. The measurement of redox potentials may be useful in studies of the development of Podzols, Ground Water Podzols, and Humic Gley soils.
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