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Item Sodic Soil Swelling and Dispersion and their Implications for Water Movement and Management(North Dakota State University, 2014) He, YangboNorth Dakota has over 1.9 million ha of sodium-affected soils, influencing water movement and crop production. This dissertation consists of four studies examining different aspects of sodic soils. The first study surveys sodium adsorption ratio (SAR) methods to determine which is the most reliable. The second and third studies investigate the dispersion and swelling functions of sodic soils. The final study examines field spatial distribution of Na in order to propose management strategies. Analytical approaches for converting alternative to standard approaches are needed. The SAR was determined from many non-standard techniques. One hundred soils were used, SARe and 1:5 soil/water SAR1:5 determined using shaking, stirring, and a USDA-NRCS method were compared. Three of the methods influenced the SAR1:5 values. Electrical conductivity (EC), SAR, and Ca/Mg ratios influence dispersion. Three pure clay minerals (montmorillonite, kaolinite and illite) were pretreated by variable Na and cation ratios and absorbance was determined using spectrophotometer for dispersion. Calcium-Mg ratios across the same SAR did not influence clay dispersion. Dispersion increased with higher SAR and reduced EC whereas no dispersion for kaolinite. Swelling is associated with hydration of clays, which forces clay tactoids to separate. Four soil series from North Dakota field sites were used. To assess swelling, field capacity (FC) was used as proxy. The study found that soil Na and soluble salt concentrations were two important chemical factors influencing FCW. The FCW increases with increased SAR and lower levels of EC. These results indicate that maintaining an EC level above 4 dS m-1 may mitigate swelling, which is an issue considered in tile drainage. Over- and under-application of amendments in sodic soils was studied in a 8.1 ha sodic soil field. At each site, samples were taken from two depths; electromagnetic (EM38) and elevation readings were done. Elevation was significantly correlated with soil variables except for Na%. The EM38 was reliable to express soil EC and was correlated with Na% and dispersion. Therefore, conducting the EM38 and RTK may allow site-specific management of Na. Improved knowledge of sodic soils dispersion, swelling, and field distribution will benefit researchers and farmers in managing their fields.Item Evaluation of 1:5 Soil to Water Extract Electrical Conductivity Methods and Comparison to Electrical Conductivity of Saturated Paste Extract(North Dakota State University, 2011) He, YangboConducting a 1 :5 soil:water extract to measure electrical conductivity (EC) is an approach to assess salinity and is the preferred method used in Australia. However, the influence of salinity on plant growth is predominantly based on saturated paste extract electrical conductivity (ECe) and ECe is recommended as a general method for estimating soil salinity internationally, so it is necessary to convert EC1:s to ECe, The objectives of this research were to 1) compare methods of agitation (shaking plus centrifuging (shaking/centrifuging), shaking, and stirring) for determining EC1: 5; 2) determine optimal times for equilibration for each method across a range of salinity levels determined from saturated paste extracts (ECe) (objectives 1 and 2 are for paper 1); and 3) develop predictive models to convert ECu data to ECe based on four different 1 :5 extraction methods listed above and a USDA-NRCS equilibration technique ( objective 3 is for paper 2). The soils evaluated for the two studies were from north central North Dakota, USA, where 20 soil samples having ECe values ranging from 0.96 to 21 dS m-1were used for the first study (objectives 1 and 2), and 100 samples having ECe values ranging from 0.30 to 17.9 dS m-1were used in the second study (objective 3). In the first study, for each method, nine equilibrium times were used up to 48 hrs. In the second study, a uniform agitation time (8 hrs) was applied to the first three agitation methods, and 1 hr was also used for the USDA-NRCS method. For the first study, significant relationships (p < 0.05) existed between values ofEC1:s and agitation time across the three methods. Agitation methods were significantly different (p S 0.05) from each other for 65% of the soils and shaking/centrifuging was significantly different (p < 0.05) from stirring for all soils. In addition, for 75% of the soils, shaking/centrifuging was significantly different (p :S 0.05) from shaking. Based on these results, methods were analyzed separately for optimal equilibration times. The agitation times required for the three methods to reach 95 and 98% of equilibration were a function of the level of soil salinity. For soils with ECe values less than 4 dS m·1, over 24 hrs was needed to obtain both 95 and 98% of equilibration for the three methods. However, less than 3 and 8 hrs were needed to reach 95 and 98% equilibration, respectively, across methods for soils having ECe values greater than 4 dS m·1. These results indicate that establishing a standard method is necessary to help reduce variation across EC1:s measurements. In the second study, the value ofECe was highly correlated with EC1:s (p < 0.0001) across four agitation methods in non-transformed, log10- transformed, and dilution ratio models through regression analysis. The values of coefficient of determination (r2 ) were greatly improved and average about 0.87 using log10- transformation compared to other two models (r2 values of about 0.68 for the nontransformed models and 0.69 for the dilution ratio models). Since agitation methods were determined to be highly correlated with each other, any regression model determined under the four agitation methods were applicable for the estimation of ECe from another method. The results from this research indicate that comparing data across studies should be done with caution because both agitation method and time can influence results. Also, estimation ofECe from EC1:5 can be done with confidence, but models may not be transferrable across different soil orders or across various salt types.