STPAL Procedure for Analyzing Salinity in Soil

Soil samples submitted to the Soil Testing & Plant Analysis Laboratory (STPAL) for analysis of salt contamination are processed in the following manner.   Samples are dried, ground (passing a 2 mm sieve), and analyzed electrical conductivity (EC) to estimate salinity, and sodium adsorption ratio (SAR) to assess sodium hazard on soil condition.   

Soil EC and soluble salt analysis:  To determine soil EC, a 20 gram sample is mixed with 40 mL deionized water in a 125-mL erlenmeyer flask.  The container is sealed and the mixture is agitated for 1 hr in a mechanical shaker.  The mixture is filtered through Whatman 42 filter paper.  EC (dS/m) of the filtrate is determined immediately using a standard conductivity meter. The directly-measured EC is converted to saturated extract-equivalent EC by multiplying by 2.  Total soluble salts (TSS) concentration in mg/L is calculated by multiplying EC (dS/m) by 640.

 Soil SAR determination:  The filtrates from EC analysis or separately prepared extracts based on 1:2 soil to water ratio are used for analysis. The concentrations of Na⁺, Ca²⁺, and Mg²⁺ in the filtrates are measured by inductively coupled plasma (ICP) spectroscopy. The SAR is calculated as follow:

      SAR = [Na]/([Ca]+[Mg])^1/2. 

 Cation concentrations for the SAR calculation are in mmols/L.

 References:

Rhoades, J.D. 1996. Salinity: Electrical conductivity and total dissolved solids. p. 417-436. In D.L. Sparks (ed.). Methods of Soil Analysis, 2^nd Edition. Soil Science Society of America, Madison. WI.

U.S. Salinity Laboratory Staff. 1954. Diagnosis and improvement of saline and alkali soils. Agric. Handbook no. 60, USDA. U.S. Government Printing Office, Washington, DC.

 Please address question on laboratory analysis of salinity in soils to:

 Dr. Jim Wang, Associate Professor

Agronomy and Environmental Management Department

LSU AgCenter, Baton Rouge, LA.  70803

Phone: 225-578-1360

Email: jjwang@agctr.lsu.edu