Isotic Mineralogy: Criteria Review and Application in Blue Mountains, Oregon

The isotic family was established in U.S. Soil Taxonomy to create a division in the mixed mineralogy class identifying soils exhibiting properties of noncrystalline minerals. Application of the isotic class has resulted in correlation problems of soil series in various regions of the country. This study of 56 pedons in the Blue Mountains physiographic province of eastern Oregon examines isotic and andic properties of soils with the following objectives: (i) review theory and background related to the current criteria for the isotic class; (ii) examine these criteria on selected pedons in the Blue Mountains; and (iii) discuss the current approach to the use of the isotic family classification in this geographic region. The criterion, 1500 kPa water retention/clay percentage ≥ 0.6, is derived from the rationale of water adsorbed on surfaces of soil materials and is often used as a criterion for appropriate dispersion during particle-size analysis. The reaction of NaF with noncrystalline soil minerals (e.g., allophane) results in the exchange of hydroxyl ions from the mineral surface with fluoride. The nonlinear increase in pH of NaF establishes the semiquantitative nature of the test. For this study, NaF pH has correlation coefficients of 0.80, 0.72, and 0.34 for acid oxalate-extractable Al, Si, and Fe, respectively. The relationship of NaF pH with properties related to andic materials suggests that different OH sites exist in these soils and that they react at varying rates and release differing amounts of acid oxalate-extractable elements. Calculated clay based on 1500 kPa water retention and using criteria in Soil Taxonomy (2.5 rule) always exceeds measured clay for all horizons. A better relationship of calculated and measured clay was achieved with a factor of 1.7 (rather than 2.5). This factor is derived assuming a linear slope of 0.6 (rather than 0.4) between 1500 kPa water retention and clay. In the Blue Mountains survey area, pedons in the isotic versus mixed families do not correlate with specific geographic areas, moisture/temperature regimes, or parent materials. Lack of mappable differences of these two classes results in no discernible differences in properties affecting interpretations as we currently view them.