Montana State University |
Montana State University |
By Jim Bauder
MSU Extension Soil and Water Quality Specialist
03/29/98 BOZEMAN -- Salinity, sodicity and alkalinity are often used interchangeably to describe salty soils, though they actually refer to different conditions.
Salinity is a measure of the total amount of soluble salt in the soil or soil solution -- all kinds of salts, including calcium, magnesium and sodium salts; carbonates and sulfates. The standard measurement for salinity is electrical conductivity. Salinity is rarely a problem on sandy or gravelly sites unless there is a very high water table or very inadequate leaching with poor quality water. Problems generally would occur on silt loams or silty clay loams in areas of relatively low rainfall, less than 14 inches per year.
The following table shows the plants that tolerate various degrees of salinity.
Electrical Conductivity Rating and Plant Tolerances |
|
| under 2.0 mmhos/cm | The soil is okay for all vegetables and crops. |
| 2.0 - 4.0 mmhos/cm | Slightly salty; will grow beans, foxtail, barley, some clovers, radish, celery |
| 4.0 - 8.0 mmhos/cm | Moderately salty; will grow cereals, alfalfa, clover, grass, most vegetables except radish, celery, green beans |
| 8.0 - 16.0 mmhos/cm | Strongly salty; barley, beets, wheatgrass, wildrye, trefoil, fescue |
| over 16.0 mmhos/cm | Excessively salty; will grow very little except saltgrass |
Sodicity refers to soil exchange capacity and the degree to which sites are occupied by sodium ions, as compared to the more preferred calcium and magnesium ions. Sodium, a common component of detergents and laundry soaps, is a dispersing agent. Soils saturated with sodium tend to be very difficult to work with. These soils are sometimes consolidated, blocky and poorly drained. These are the soils we often refer to as "gumbo."
The best indicator of sodicity in a soil is the sodium adsorption ratio or SAR, which compares the amount of sodium to calcium and magnesium. Generally, soils with SAR greater than 15 are considered sodic. The most common way to deal with sodic soils is to add another cation, such as calcium or magnesium, to displace the sodium. But good drainage is essential.
Alkalinity refers to the acidity of the soil.
Soils which are basic (as compared to those which are acidic) are considered to be alkaline. The parameter most often used to refer to soil alkalinity is pH. Soils with pH greater than about 8.7 are considered alkaline. Alkalinity also is commonly associated with poorly drained sites. Hence, before any action can be taken to lower the pH, good drainage must be insured. Ample additions of organic matter, increased cropping intensity or large amounts of sulfur can be used to lower the pH and create more acidic conditions.
You can have a soil with all of these conditions at the same time, though it is not likely.
Send questions or comments to Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or to Bauder and Flaherty with this link: carolf@montana.edu.
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