ProCrop 1999

SOIL TESTING FOR SALINITY

Soil areas that are severely affected by salts often have a bright white, crusty appearance when dry. The extent of severity of the saline area usually extends well beyond the obvious area. In areas lacking a surface crust or obvious vegetation loss, the salts are dissolved in the soil water and cannot be seen. Therefore, the extent of the problem can only be identified with a soil test.

Soil testing laboratories use the electrical conductivity (ECe) of a soil extract to measure salt concentrations. There are handheld conductivity meters available which, when properly calibrated, can be used to make field measurements quickly and help define saline area boundaries. These field determinations are important, because often when a composite soil sample is taken, areas of high and low salt are mixed, giving an unrepresentative picture of the field.

Several measurements should be taken in the suspected saline areas just outside the area, and at some distance surrounding the area in order to properly map the field. Field ECe levels can be extremely variable within short distances. Knowing what the salinity patterns are in the field and how extensive they are can greatly influence a management strategy.

Electrical conductivity is a low cost analysis. The results are either reported as decisiemens/meter (dS/m) or as millimohs/cm (mmohs/cm). One dS/m equals one mmoh/cm, so the terms are equivalent. Data, charts and papers can be found which use both terms.

Laboratories measure ECe on different soil to water extracts because of their convenience to the laboratory. The most common measurements are made on extracts from either a saturated paste or a 1:1 by weight soil-to-water slurry. The saturated paste extraction is more precise but is time consuming and expensive. The 1:1 soil water paste is a simple, rapid, low cost and excellent procedure for screening problem soil sites and is the procedure used by the NDSU soil laboratory.

Results can roughly be converted back and forth from a 1:1 slurry to a saturated paste, using the following formulas where y = ECe of a 1:1 soil-to-water slurry and x = ECe of a saturated paste extract. These formulas are not well calibrated and should only be used as a rough guide.

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