Present concepts of sodic soil formation do not adequately account for observed variability of sodic soil properties on many landscapes in North Dakota. A conceptual model was developed to explain sodic soil genesis related to water and salt movement on a coarse‐loamy till landscape in central North Dakota. The relationship between subsurface water flow and sodic soils was determined by monitoring soil water using neutron attenuation and water wells at three contiguous landform positions. Electrical conductivity, major ions, and water‐dispersible clay were determined on extracts from selected soil samples from each landform position. Typic Argiaquolls, characterized by low salinity (electrical conductivity [EC] < 2.0 dS/m), occurred at the wetland position and were areas of recharge to groundwater. Typic Natraquolls also occurred at the wetland position but were characterized by high salinity (EC > 5.0 dS/m) and were areas of groundwater discharge. Udic and Leptic Natriborolls were areas of groundwater discharge at the intermediate and upland positions. Udic Haploborolls were hydrologically inactive areas at the upland position. Variations in sodic soil development were attributed to the concentration of water flow (focused flow) into specific areas of groundwater discharge or recharge. The most important factors that affected the direction and intensity of focused flow were stratigraphy and landform position.
Statistical estimates of soil variability within and among delineations of taxonomic units are useful in designing and checking classification systems. We compared soil classification and spatial differentiation of a landscape dominated by sodic soils. Both statistical methodologies and standard soil‐survey techniques were used to differentiate soils. Forty‐nine pedons at three landform positions were compared with canonical discriminant analysis, principal component analysis, and kriging. Statistical analyses improved soil classification and allowed a clearer view of the field distribution of soil properties, compared with standard soil‐survey techniques. Soils with properties indicative of strong leaching (Solods) are recognized as significantly different from other soil taxonomic components at the intermediate and wetland positions. Solods were extensive at the wetland position, but could not be differentiated from Typic Natraquolls solely by morphologic observation. Although Solods are related to other leached soils, Argiaquolls and Argialbolls, they also possess soil properties that are similar to associated sodic soils. Leptic Natriborolls and Udic Natriborolls were similar except for salinity. The salinity difference by itself does not produce significant statistical differentiation between the soils of the two natric subgroüps. High salinity would be better recognized as a soil‐series phase, not a separate subgroup of Natriborolls.
Sodic soils in central North Dakota are related to landform position in relation to the water table and plant communities. Classification problems are commonly encountered when field observations are compared with established taxa. This study focused on relating soil and plant characteristics to differences in soil water regime. Soils at three landform positions, designated as upland, intermediate, and wetland, were sampled at regular intervals. The sampling pattern at each position formed a grid (49 pedons/grid). Each pedon was described and classified. Selected pedons were analyzed for major cations, electrical conductivity (EC), and dispersible clay. Water‐table observations were made from wells with perforated casings at each area sampled. Plant composition was determined by the point‐frame and quadrat methods. Landform position and plant communities reflected variations in sodic soil properties and soil water regime. The highest amounts of dispersible clay and lowest Ca/Mg ratios were in the sodic soils at the two lowest positions. The highest sodium adsorption ratios (SAR) were in soils at the intermediate position. These soils were associated with a plant community dominated by inland saltgrass [Distichlis spicata ssp. stricta (Torrey) Thorne] in the intermediate position. Sodic soils from the upland position were drier than the sodic soils at the two lower positions and were not so severely affected by Na.
This study focuses on differences in the soil water regime in a coarse‐loamy, closed, till landscape with sodic soils. Problems of classification of sodic soils are difficult to resolve in the field. Our basic hypothesis was that greater emphasis on soil water regime would improve classification of sodic soils. Three landscape positions were studied that reflect internal drainage differences: (i) a well‐drained upland position; (ii) somewhat poorly drained intermediate position; and (iii) a poorly drained seasonal wetland. We classified and studied more than 50 profiles at each position. Six pedons were selected from trenches for detailed study. Pedons were analyzed for extractable ions, electrical conductivity (EC), pH, exchangeable sodium percentage (ESP), dispersible clay, and particle size. The upland position had soils with Na‐induced clay illuviation Natriborolls. They lacked glossic morphology. The nonsodic soils at the upland position were Haploborolls. The soils at the intermediate and wetland positions were classified as Natriborolls and Natraquolls, respectively. A shallow water table exerted a stronger influence on soil development at the two positions than at the upland position. Field or laboratory evidence does not justify using the Glossic Udic Natriboroll taxon at the upland position. B‐horizon structure and substrata colors and mottling are better criteria to recognize that the upland sodic soils are different. Sodic soils at the intermediate position should be recognized in a new subgroup, Aquic Natriborolls. The aquic subgroup would effectively discriminate between the moderately well‐drained upland soils and the somewhat poorly drained intermediate soils. Sodic soils with high‐surface salinity were an integral part of the sodic soils complex at all three landscape positions. The Leptic subgroup recognizes these soils; however, it does not differentiate between sodic soils with different soil water regimes. We suggest that sodic soils with high‐surface salinity be differentiated as saline phases of soil series rather than a separate subgroup.
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