Evaluation of EC1:5 Soil to Water Extract Methods.

Conducting a 1:5 soil:water (EC_1:5) extract to measure electrical conductivity (EC) is an approach to assess salinity and is the preferred method used in Australia.  The objectives of this research were to 1) compare methods of agitation for determining EC_1:5 and 2) determine optimal times for equilibration for each method across a range of salinity levels determined 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.  For each method, nine equilibration times were used up to 48 hr.  The three agitation methods were stirring, shaking, and shaking with centrifugation (shaking/centrifuging).  Agitation methods were significantly different (p ≤ 0.05) from each other for 65% of soils and shaking/centrifuging were significantly different (p ≤ 0.05) from stirring for all soils.  In addition, 75% of the shaking/centrifuge soils were significantly different (p ≤ 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 various levels of salinity in the soils.  For soils with ECe values less than 4 dS/m, over 24 hr was needed to obtain both 95 and 98% of equilibration for the three methods.  However, less than 8 and 3 hr were needed to reach 98 and 95% equilibration, respectively, across methods for soils having ECe values greater than 4 dS/m.  These results indicate that establishing a standard method is necessary to help reduce variations across EC_1:5 measurements.