Genesis, distribution, and classification of sodic soils in Oklahoma

Scope and method of study. Aspects of sodic soil spatial distribution, classification, and genesis are studied. The objectives of this study were: (1) to determine the effect of the climate and other factors on the distribution and properties of sodic soils in Oklahoma, (2) to expand sodic soil classification by using soil properties that reflect soil moisture conditions, (3) to analyze selected major types of sodic and leached sodic (Solods) soils to reveal the formation pathways. Fourteen sodic soil series from 19 Oklahoma counties were sampled in 1997. Horizon delineation and in-situ soil descriptions were made using standard techniques. Characterization of each profile was performed by conventional methods. Data on dispersion characteristics for each horizon were obtained from Oklahoma Department of Transportation Testing Laboratory. Classification of soils under study was conducted using Soil Taxonomy. Maps visualizing soil spatial distribution were generated in ArcView/GIS computer software.;Findings and conclusions. There is a gradual change in sodic soils properties along the Precipitation-Evaporation (P-E) Index and soil moisture regime gradients. Depth and value of salinity (EC), depth to carbonates, depth and values of maximum relative sodium content and SAR, and pH increased with decreasing P-E index. Variability of sodic soils on a local scale was affected by position on the landscape, which influences presence of water table close to the surface. Parent material affected soil occurrence, as well as soil color, texture, chemical composition of soil water solution, and clay mineralogy. The presence of a ground water with relatively large sodium content determines the occurrence of sodic soils in the area. Suggested classification for sodic soils in Oklahoma takes into account soil salinity, which differs significantly between soils with different moisture regimes and landscape positions and is recognized on a subgroup level. Three types of sodic soils, which differ in the amount of salts, SAR values, acidity and pH, are formed under the humid conditions of southeast Oklahoma. The first type, Natrudalfs, has SAR up to 30 and moderate to high salinity (EC > 2) and occur in the lowest position on a landscape. The second type, sodic Hapludalfs, is located on an intermediate landscape position, and has lower salinity and sodium content compared to Natrudalfs. The third type, solodic Hapludalfs, is the leached sodic soils of terraces and uplands.