In a recent visit to a high tunnel, we observed a severe salinity problem on tomatoes. Approximately one month after planting , most tomato plants in the affected area had not sent out any new leaves. Roots did not grow at all (Figure 1). After conducting a soil test, very high soluble salt level explains these symptoms. This article reviews the basics of soil salinity.
Figure 1. Stunted growth of tomato plants due to salt damage. Note the accumulated salt in the form of white crust on top of the soil between the rows (Photo by Dan Egel)
Salinity describes salt content in the soil. Virtually all fertilizer materials are salts, but they vary in their effects to increase salt concentration in soil solutions. In a field situation, precipitation in the form of rain and snow tend to leach salts. Since high tunnels exclude rain and snow, elevated salt levels are a common concern for high tunnel vegetable growers. Table 1 are the salt indexes of common fertilizers. If you are using a premixed fertilizer such as 12-12-12, check the fertilizer components on the bags. Note manure has very high salt index. For the same amount of materials, animal manures as well as manure-based compost in general have higher salt contents compared with synthetic fertilizers. Organic growers should be very careful in choosing compost with low salt levels, and should always avoid applying manures directly to soils in high tunnels and prevent running off of manures applied to adjacent field.
Seed germination and seedlings are most susceptible stages to salt damage. After plants are established, high salt concentrations may make it difficult for crops to obtain water from soil, therefore reducing plant growth and yield. Vegetable crops differ greatly in their sensitivity to salt damage. Table 2 has the soil salinity tolerance levels for several vegetable crops. Salinity of 2 ds/m may reduce the yield of salt sensitive crops such as bean, carrot, radish and onion while asparagus may tolerant a level greater than 5 ds/m. If you suspect you have a salinity problem, a soil test may help. But be sure to include test for soluble salts.
Excessive salt can be leached from the soil with water. The required water is determined by salt level of irrigation water and the soil, as well as other soil characteristics. As a general rule, 6 inches of water can leach soil salts by half; more water is needed to further leach the remaining salts in sandy soil. Considering the large amount of water needed, removing high tunnel plastic and exposing the soil under natural rain is the most efficient approach for reducing salinity problems.
Table 1. Salt indexes for common fertilizers (Adapted from: Fertilizer Salt Index, 2002)
| Fertilizer | Salt Index |
| Manure Salts | 113 |
| Ammonium nitrate | 104 |
| Ammonium sulfate | 88 |
| Urea | 74 |
| Ammonium polyphosphate | 20 |
| Monoammonium phosphate | 27 |
| Diammonium phosphate | 29 |
| Potassium chloride | 116 |
| Potassium nitrate | 69 |
| Potassium sulfate | 43 |
| Ammonium thiosulfate | 90 |
Table 2. Soil salinity tolerance levels for different vegetable crops (Adapted from Ayers and Westcot, 1976).
| Vegetable crops | Yield potential | |||
| 100% | 90% | 75% | 50% | |
| Asparagus | 5.0 | 8.0 | 11.0 | 13.0 |
| Bean | 1.0 | 1.5 | 2.3 | 3.6 |
| Beet | 4.0 | 5.1 | 6.8 | 9.6 |
| Broccoli | 2.8 | 3.9 | 5.5 | 8.2 |
| Cabbage | 1.8 | 2.8 | 4.4 | 7.0 |
| Cantaloupe | 2.2 | 3.6 | 5.7 | 9.1 |
| Carrot | 1.0 | 1.7 | 2.8 | 4.6 |
| Cucumber | 2.5 | 3.3 | 4.4 | 6.3 |
| Lettuce | 1.3 | 2.1 | 3.2 | 5.2 |
| Onion | 1.2 | 1.8 | 2.8 | 4.3 |
| Pepper | 1.5 | 2.2 | 3.3 | 5.1 |
| Potato | 1.7 | 2.5 | 3.8 | 5.9 |
| Radish | 1.2 | 2.0 | 3.1 | 5.0 |
| Spinach | 2.0 | 3.3 | 5.3 | 8.6 |
| Sweet corn | 1.7 | 2.5 | 3.8 | 5.9 |
| Sweet potato | 1.5 | 2.4 | 3.8 | 6.0 |
| Tomato | 2.5 | 3.5 | 5.0 | 7.6 |