Is Salinity Harming Your Vegetable Crops?

How do you know if the salt level is too high in your field? You may see white precipitate on the soil surface that is indicative of salts. You might see symptoms on the margins of the leaves that look like salt damage, or some plants may be stunted or growing slowly.

Advertisement

If you can rule out disease or insect pests, or other potential problems, you may want to evaluate if salts are harming your crop.

Collect Samples

The first step is to collect samples of your irrigation water and soil for salinity analysis. Be sure that no fertilizer is being injected into the irrigation water when it is sampled. It is useful to compare soil salinity in symptomatic and asymptomatic areas of the field.

Also, soil samples should be collect-ed from several depths within the root zone and below the root zone. This strategy will allow you to assess if leaching of salts is adequate.

If salinity increases with depth there is likely sufficient leaching. But if salinity is highest near the surface where roots are concentrated, then leaching was probably not adequate.

Top Articles
Have a Plan For Climate Change? Why Fruit Growers Need To Act Now

For drip-irrigated crops, you may want to compare the salinity level of the soil near and away from the drip line. Water applied by drip usually pushes salts out to the sides of the beds as well as downward.

Measure

The next step is to measure the salinity of the water and soil samples. The most common method is to measure the electrical conductivity (EC), which can be related to the concentration of all dissolved salts in a sample.

This is simple to do in water using a conductivity meter that measures EC in units of deciSiemens per meter (dS/m). The EC of the water will be greater as the concentration of dissolve salts increases.

The same is true for soil. However, to remove the effect of water content on electrical conductivity, measurements should be made on a saturated paste extract. To create a saturated paste, distilled water is added to a known weight of air-dry soil to the point where all pore spaces are filled.

After the soil equilibrates with the added water, extract the solution from the soil paste by applying suction using filter paper and a vacuum pump. The EC of the extract solution is then measured using a conductivity meter. Many laboratories will measure soil EC based on an extract of a saturated soil paste, but other laboratories may add water to soil at a predetermined ratio of 1:1 or 2:1.

The water-to-soil ratio is critical to know because extra water will dilute the salts in the extract and give lower soil salinity values. Also, published thresholds for salt tolerance of crops have been established using saturated paste extracts.

Chart showing tolerance of selected vegetable crops to salinity in soil

Check the Results

You can find salinity tolerance thresholds for many crops in the online document, FAO (Food and Agricultural Organization) Irrigation and Drainage Paper 29 (water quality for agriculture), as well as in several university Extension publications. Values for 100% potential yield of selected vegetable crops are shown in the table to the left. Expected yield declines when soil salinity is above these thresholds.

For some crops such as lettuce, which are salt sensitive, the decline in yield with increasing salinity is relatively steep. For other species (tomato) the decline in yield is relatively shallow.

By comparing the average soil salinity in the root zone with these thresholds, one can determine if salinity is potentially harming your crop and how much yield loss would be expected. Keep in mind that these thresholds are most applicable to soluble salts such as sodium chloride.

Vegetables can often tolerate greater levels of salinity if the salts are less soluble, such as calcium and sulfate (gypsum).

4