Evaluation of 1:5 soil to water extract electrical conductivity methods

Conducting a 1:5 soil:water extract to measure electrical conductivity (EC) is an approach to assess salinity and has been the preferred method in Australia, but not commonly used in the United States where the 1:1 soil to water ratio is preferred. The objectives of this research were to 1) compare methods of agitation for determining EC1:5 and 2) to determine optimal times for equilibration for each method across a range of salinity levels determined from EC values achieved from saturated paste extracts (ECe). Soils evaluated for this study were from north central North Dakota (USA) and had ECe values ranging from 0.96 to 21.2 dS m(-1). For each method, nine agitation times were used, up to 48 h. The three agitation methods were shaking plus centrifuging, shaking, and stirring. Agitation methods resulted in significantly different EC1:5 values for 13 out of 20 soils across the three agitation methods, and shaking plus centrifuging was significantly different (p=0.05) from stirring for all soils. In addition, 75% of the shaking plus centrifuging soils were significantly different 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 below 4 dS m(-1), over 24 h was needed to obtain both 95 and 98% of equilibration for the three methods. However, less than 3 and 8 h 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 investigating the effect of agitation methods and times is important to help reduce variations across EC1:5 measurements. (C) 2012 Elsevier B.V. All rights reserved.