Two Methods for Monitoring Soil Salinity… Which One to Choose?

By: Heather Dose, Research Soil Scientist, ARS Morris MN

Soil salinity is a major issue for producers resulting in $12 billion annual losses around the globe.  In the northern Great Plains the number of acres affected by salinity is increasing.  But why?  Salts are naturally found in the soil of the region due to the geologic history.  However, increasing rainfall patterns in the last 20 or so years has caused many of the salts to move toward the surface of the soil.  Driving around in the spring, you can notice many areas of white soil in fields.  What makes salts a problem in soil?  Salts basically make plants think they are suffering from drought.  Just for fun, try putting some table salt on cabbage and see what happens.  In addition to having the start of sauerkraut, you will notice the salt draws the water from the cabbage resulting in a goopy mess.  The same thing happens in soil.  Salts draw water away from plant roots which is stressful for your crop. Salinity can lower yields or even result in a complete failure of crops.  Managing your fields for soil salinity will depend on the amount of salts you have.  But, how do you know how much salt you have in your soil?  What soil test options are there?

The gold standard for determining the amount of salt in your soil is the saturated paste extract method.  The saturated paste method requires a lab technician to add water to the soil until the soil shines and plops off a spoon without leaving a muddy mess.  The paste then must sit overnight before the water and salts are extracted.  Although the method is fairly straightforward, it does have many drawbacks.  For instance, each sample needs a different amount of water added to achieve saturation.  This means that automation is nearly impossible as the saturated paste method requires a trained technician to ensure the amount of water added is perfect.  Finally, the method takes over 24 hours so this is a special test at many soil testing labs.  Although the saturated paste method is very accurate for evaluating soil salinity, this method is expensive, ranging from $15 to $20 per sample.

An alternative method to measure soil salinity is using the routine 1:1 soil to water ratio method.  In this method, a standard volume of water (10 ml) is added to a standard weight of soil (10 g). After 30 minutes the electrical conductivity is measured and the levels of salt in the soil is determined.  This method is fast and inexpensive (between $1.65 and $5.00 per sample) and can allow you to get a good idea of the salinity levels in your field.  Due to the lower cost of this routine 1:1 method, you can split fields into parts for sampling to get a better idea of the range of salinity in all parts of each field.  One issue with the 1:1 method is it underestimates salinity levels compared to the saturated paste method (The test value for the 1:1 method will be lower than the test value for saturated paste method on the same sample).

The good news is the test results from both methods for testing salinity are highly related.  Over 2,300 soil samples from the northern Great Plains were tested with both methods (AGVISE Laboratories provided data from over 1000 samples to this research)  Researchers determined that you can multiply the results of the 1:1 method by 2.26 and get an accurate estimation of the salinity level measured by the saturated paste method (see figure).  Monitoring the test level of salts in your fields is a key part in managing salinity.  Using the routine 1:1 salinity method provides growers valuable information at reasonable cost.  This article is based on work conducted by Dose, H.L., Y. He, R. Kerns, D. Hopkins, B. Deutsch, J. Lee, D.E. Clay, C. Reese, D. Malo and T.M. DeSutter

Soil salinity levels measured by the 1:1 soil to water ratio method can be multiplied by 2.26 to convert to salinity levels measured by the saturated paste method.