By Dorivar Ruiz Diaz and DeAnn Presley
An analysis of the soil for soluble salts and sodium accumulation will identify the specific problem and its severity. To see if a problem exists, take a composite sample of several soil cores, 6 to 8 inches deep, from the affected area, with a final sample volume of at least a pint of soil. In many cases, comparison soil samples from the affected area and surrounding area with a normal appearance will be beneficial. If a saltwater spill occurred and the water stood on the area for several weeks or a natural seep exists, depth increment samples to 3 feet should be taken to assess the depth of salinity. Profile information will help in planning a reclamation program. If you are unsure how to sample, consult the lab where you’re submitting the samples or your county Extension agricultural and natural resources agent.
Lab analysis methods vary, but most labs that run a specific salt-alkali test use about the same methods. The methods and interpretation presented here are used in the KSU Soil Testing Lab. Before applying these interpretations to other lab results, you should be certain similar methods were used.
The amount of soluble salts present is measured by determining the electrical conductivity (mS/cm). The electrical conductivity of a solution is proportional to its soluble salt content. The general interpretation of the results is found in Table 1.
A second important measure is the amount of exchangeable sodium. The results of this extraction must be corrected for soluble sodium measured but not exchangeable. Once this correction is made, the results are expressed as percent exchangeable sodium. The general interpretation used by the KSU Soil Testing Lab is found in Table 2.
Table 1. Interpretation of Electrical Conductivity
Saturation Extract (mS/cm) | Salt Rank | Interpretation |
0-2 | Low | Very little chance of injury on all plants. |
2-4 | Moderate | Sensitive plants and seedlings of others may show injury. |
4-8 | High | Most non-salt tolerant plants will show injury; salt-sensitive plants will show severe injury. |
8-16 | Excessive | Salt-tolerant plants will grow; most others show severe injury. |
16+ | Very Excessive | Very few plants will tolerate and grow. |
Table 2. Interpretation of Exchangeable Sodium Percentage
Exchangeable Sodium (%) | Alkali Rank | Interpretation |
0-10 | Low | No adverse effect on soil is likely. |
10+ | Excessive | Soil dispersion resulting in poor soil physical condition and plant growth is likely. |
Detrimental effects of excess exchangeable sodium on plant growth occur because of poor soil physical condition. Some plants, however, begin to show some injury at levels as low as 5 percent exchangeable sodium. The commonly grown agronomic crops in Kansas are not among those sensitive to sodium. In the general discussion of a sodic soil, greater than 15 percent is the level of exchangeable sodium for poor physical condition to develop. To alert land owners of a potential problem, the KSU Soil Testing Lab interprets anything above 10 percent exchangeable sodium as excessive. Most well-drained, normal soils in Kansas will have less than 1 to 2 percent.
The occurrence of sodic or saline-sodic soil problems on a field basis nearly always can be traced to irrigation with marginal or poor quality water. Irrigators should determine the potential salinity and sodium hazard of their water. Irrigation water quality tests are available through the KSU Soil Testing Lab. In addition to the results, the KSU Soil Testing Lab report shows how to interpret the irrigation water test results for overall salinity and sodium hazard.
Salt effects on plant growth
Crops differ in the ability to tolerate salt accumulation in soils, but if levels are high enough, (> 16 mS/cm) only tolerant plants will survive.
Crop selection can be a good management tool for moderately saline soils. The following list serves as a general guide of salt tolerance ratings for crops, realizing that management practices, irrigation water quality, environment, and crop variety also affect tolerance.
- Highly tolerant: barley, rye, bermudagrass, crested wheatgrass, asparagus
- Tolerant: wheat, oats, triticale, sunflower, alfalfa, tall fescue, sweet clovers
- Moderately tolerant: corn, grain sorghum, soybean, bromegrass, sudangrass, sorghum-sudan
- Sensitive: field beans (dry), clover (red, ladino, alsike), strawberry, onion, pea, carrot, lettuce, pepper
Crops differ in their ability to tolerate sodic soil, but if sodium levels are high enough, all crops can be affected. Generally, soybeans are quite sensitive, corn and grain sorghum are intermediate and wheat and alfalfa are more tolerant. Crested and tall wheatgrass and a few sorghum-sudan hybrids are very tolerant and are able to grow on soils with exchangeable sodium percentages above 50 percent.
Reclamation of soils summary*
Abbreviated step-by-step procedure for reclamation:
Step 1. Collect a soil sample and submit to a soil testing laboratory for a salt-alkali soil test to determine the specific problem.
Step 2. Identify source/cause of the problem.
Step 3. Eliminate the source of salt contamination if possible and establish drainage if necessary.
Step 4. Add chemical amendment (gypsum) to sodic or saline/sodic soils.
Step 5. Incorporate residue to improve water intake.
Step 6. Apply irrigation water (if available).
Step 7. Allow time for leaching and consider planting tolerant crops.
Source : ksu.edu