Salinity in Soil

Conductivity is a measure of the ability of a substance to conduct an electrical current. A solution’s conductivity is affected by the presence of inorganic dissolved salts such as chloride, nitrate, sulfate, and phosphate anions (negatively-charged ions) or sodium, magnesium, calcium, iron, and aluminum cations (positively-charged ions). Organic compounds like oil, phenol, alcohol, and sugar do not conduct electrical current very well and therefore have a low conductivity when in aqueous solution. Because conductivity is dependent upon solute concentration, conductivity measurements are a good indication of the concentration of dissolved solids in aqueous solution. Conductivity is also affected by temperature: the warmer the solution, the higher the conductivity.

In natural settings, salt content may be high in both soil and water. River waters, for example, show a wide range of salinities due to different soil types, geological structures, and the extent to which they are influenced by inflows of saline groundwater. Problems arise when the natural balance of salinity in the landscape changes. Salinity is a major threat to surface and groundwater resources. Depending on the level of salt content in soil, plant growth may be impacted. High levels of salinity in rivers may limit water use for irrigation, agriculture, stock watering and domestic water supplies. Salinity can also affect freshwater aquatic flora, fauna and riparian vegetation. In urban areas, salinity reduces the lifespan of domestic and industrial equipment, leading to higher maintenance costs and greater use of cleaning products.

For aqueous solutions, the most commonly used units of measurement for conductivity are microsiemens/centimeter (µS/cm) and millisiemens/centimeter (mS/cm). See "Teacher’s Hints" for more information about these standard units.

For each lab group:

• Conductivity Sensor with soaker bottle:
     PASPORT (PS-2116)   |   ScienceWorkshop (CI-6739A)
• Computer Interface:
     PASPORT: Xplorer GLX (PS-2002)  |  Xplorer (PS-2000)  |  USB Link (PS-2100A)  |  PowerLink (PS-2001)
     ScienceWorkshop: 500 Interface (CI-6400)
• Distilled water (or tap water if distilled water not available)
• Sample of dirt from school yard
• Sample of potting soil from local nursery or retail store
• Five 500-mL beakers, and a stirring rod
  
• Two coffee pot filters
• Wash bottle for rinsing sensor

1. Click on one of the links below to download a pre-configured DataStudio file for this conductivity experiment, and then open the file.
   
   PASPORT users: Windows (.zip file) or Macintosh (.sit file)
   
   ScienceWorkshop 500 users: Windows (.zip file) or Macintosh (.sit file)
   
   When the file is opened, you should see a graph display and a table display of Conductivity vs. Solution #, as well as a digits displays of conductivity.
   
   
2. Connect the Conductivity sensor to an Xplorer or USB link (PASPORT users), or plug the sensor into the 500 Interface (ScienceWorkshop 500 users).
   If you are using the ScienceWorkshop 500 Interface, be sure the sensor is associated correctly in the Experiment Setup window when you open the DataStudio file.
   

Sensor calibration:

1. PASPORT users: Refer to the "Setup and Calibration" instructions on the Quick Start card for calibration procedures.
   
   
2. ScienceWorkshop 500 interface users: Refer to the Conductivity Sensor Instruction Manual and Experiment Guide, (pages 7-8).
   
   
3. Use the middle sensor range setting (flask symbol ) of the Conductivity Sensor.

Data Recording:

1. Pour 250 mL of distilled water into one of the 500-L beakers.
   
   
2. Using one of the beakers, fill with the dirt sample up to the 50-mL mark. Add 250 mL of distilled water, stir well, and let sit for approximately two minutes.
   
   
3. Repeat step #2 above using the potting soil.
   
   
4. Place a coffee pot filter over one of the empty 500-L beakers, and slowly pour in the dirt/water mixture to filter out the solid material.
   
   
5. Repeat step #4 above for the beaker containing the potting soil and water. Set aside the two beakers containing the wet dirt and wet potting soil.
   
   
6. Place the Conductivity Sensor into the first beaker of distilled water.
   
   
7. Click the Start button ( ) to begin collecting data. Because the pre-configured DataStudio file has been prepared for manual sampling, the Start button will change to a Keep ( ) button.
   
   
8. Watch the digits display for the conductivity reading to stabilize, then click the left side of the Keep button.
   
   
9. A dialog box like the one shown below will appear and allow you to enter a value for solution #. Enter "1" and click OK. Notice the graph and table will update automatically.
   
   
   
10. Remove the Conductivity Sensor from the beaker and thoroughly rinse it with distilled water before measuring the next solution.
    
    
11. Place the Conductivity Sensor into the second beaker of water filtered from the dirt/water mixture, and repeat steps 8-10 described above to record the second conductivity measurement.
    
    
12. Repeat steps 8-10 for the last beaker of water filtered from the potting soil/water mixture, then click the right side ("Stop" ) of the Keep button to end the experiment.
    

1. Which of the three test solutions had the highest conductivity? The lowest?
   
   
2. What factors may have caused different conductivity readings for the test solutions?
   
   
3. If the water/soil mixtures had been allowed to stand for a longer period of time, what kind of conductivity measurements might you expect? Why?
   
   
4. If the same volume of water had been added to a larger amount of soil, what kind of conductivity measurements might you expect? Why?