A high tunnel where the farmer is using a soilless media system to grow crops. Photo: Tom Ford, Penn State
At low levels, soluble salts generally do not harm plants. However, they can accumulate, when leaching is inadequate, to levels that do cause harm. Applying excess nutrients from inorganic fertilizers or organic nutrient sources, repeated applications of nutrients without sufficient water for leaching, and high soluble salts levels in irrigation water can cause soluble salts levels to accumulate in soils.
In the field, soluble salts levels generally do not reach levels of concern in Pennsylvania because rain, snow, and other precipitation events leach salts out of the root zone. We have heard of issues with high soluble salts levels underneath plastic-covered beds in the field; however, generally, this is not an issue. Soluble salts levels can reach levels of concern in stationary, 4-season high tunnels (covered with plastic year-round) and it is a good practice to have this tested periodically.
Leaching is limited in stationary, 4-season high tunnels. The plastic covering high tunnels excludes precipitation. Additionally, high tunnels are commonly equipped with drip irrigation systems, which also limits leaching. As a result, less water enters the soil, less leaching occurs, and soluble salts can accumulate in the root zone.
At Penn State’s Agricultural Analytical Services Laboratory (the Lab), soluble salts levels can be measured as an optional test, currently costing $5. When interpreting soluble salts levels, it is important to make sure that the interpretation matches the testing procedures used. For example, the Lab uses a 1:2 dilution method (one part soil to two parts water by volume; 1:2 soil:water). Other testing methods exist, including 1:5 and 1:1 dilution methods and a saturation extraction method. Below is a table from the Lab for interpreting values for 1:2 dilution method results.
Soluble Salts (Conductance) Interpretation for Soils
|mmhos/cm 1:2 Soil:Water||Effects|
|< 0.40||Salinity effects mostly negligible, excepting possible beans and carrots.|
|0.40-0.80||Very slightly saline; but yields of very salt sensitive crops such as flax, clovers (alsike, red), carrots, onions, bell peppers, lettuce, sweet potatoes may be reduced by 25 to 50%.|
|0.81-1.20||Moderately saline. Yield of salt-sensitive crops restricted. Seedlings may be injured. Satisfactory for well drained greenhouse soils. Crop yields reduced by 25 to 50% may include broccoli and potato plus the other plants above.|
|1.21-1.60||Saline soils. Crops tolerant include cotton, alfalfa, cereals, grain sorghum, sugar beets, Bermuda grass, tall wheat grass and Harding grass. Salinity higher than desirable for greenhouse soils.|
|1.61-3.20||Strongly saline. Only salt-tolerant crops yield satisfactory. For greenhouse crops leach soil with enough water so that 2-4 quarts (2-4L) pass through each square foot (0.1 m2) of bench area, or one pint of water (0.5 L) per inch (15 cm) pot; repeat after 1 hour. Repeat again if readings are still in high range.|
|> 3.2||Very strongly saline. Only salt-tolerant grasses, herbaceous plants, certain shrubs and trees will grow.|
High tunnel crops respond differently to soluble salts levels. For example, beets are considered salt-tolerant, while peppers are moderately salt-sensitive. Tomatoes are considered moderately salt sensitive, but with more tolerance than peppers and potatoes. A common symptom of salt damage is dead tissue (black or brown in color) on leaf margins. Yield will also be reduced. When salt levels are too high, most plants have difficulty taking up water. This is known as a chemical induced drought. Eventually, the plant can wilt and die.
This lettuce is showing early symptoms of soluble salts stress. Note the brown tissue on the leaf margin. Photo: Tom Ford, Penn State
This coleus is showing symptoms of salt stress. Photo: Tom Ford, Penn State
Soluble salts levels can also accumulate to levels where you can see the salts on the soil surface.
High tunnel soil is showing salt (white areas) on the surface. Photo: Elsa Sánchez, Penn State
In our project, soluble salts levels ranged from 0.14 mmhos/cm to 9.27 mmhos/cm, with an overall average of 1.48 mmhos/cm. In many cases, soluble salts levels resulted in plant growth problems. The average from organic high tunnels or those using organic methods was 1.56 mmhos/cm, which falls in the saline soil category. The average was 2.39 mmhos/cm for conventional tunnels, which falls in the strongly saline soil category. At these average levels, many high tunnel crops are negatively affected.
This bar is on the bottom of soil test reports from the Agricultural Analytical Services Laboratory at Penn State University. The orange box highlights the soluble salts level.
This article includes ways to avoid high soluble salts levels in high tunnels: Dealing With High Soluble Salts Levels in High Tunnels .
If your high tunnel soil has high soluble salts levels, there are things you can do. If possible, relocate the tunnel to a site with low soluble salts levels. We realize this option is not practical for most farmers but included it because it would be a solution to this problem.
Another option is to leave the plastic off the tunnel for a while when it needs to be replaced. At Penn State University’s high tunnel facility, we managed four high tunnels organically in the mid-2000’s. In the fall of 2007, the soluble salts level was on average 0.40 mmhos/cm. That November, the tops of the tunnels ripped off due to high-speed winds and because the plastic was getting old. We decided to leave the tops off until the spring. In April of 2008, we put new tops on and had the soluble salts level of the soils analyzed. On average, the soluble salts level decreased to 0.09 mmhos/cm or about 78%. Between November of 2007 and April of 2008, the site received about 11.5 inches of rain.
Farmers, from one of the farms participating in our project, remove the plastic from their tunnel every winter and reinstall it in the spring. They use organic nutrient sources, including animal manure, which has a high soluble salts content. By removing the plastic, they have been successful in keeping their soluble salts level <0.40 mmhos/cm, where soluble salts are not a concern.
At another farm, the farmer also removes the plastic over the winter. Conventional fertilizers and animal manure are used annually, and the soluble salts level is slightly above <0.40 mmhos/cm.
Removing the high tunnel plastic over the winter every year or when it needs to be replaced is not an option for farmers using their tunnels for year-round production. If this describes your situation, leaching may be an option for you.
Salts can be leached out of the soil. A general guideline for leaching out soluble salts from the top foot of soil is to apply 6 inches of water to leach about 50% of salts, apply 12 inches to leach about 80% of salts, and 24 inches to leach about 90% of salts (California Fertilizer Association, Western Fertilizer Handbook, 8th Ed.). This article has a table including how many hours to run a drip irrigation system to apply 1 inch of water: Determining How Long to Run Drip Irrigation Systems for Vegetables . Multiply the value in the table corresponding to your situation by the number of inches you wish to apply. Leaching can also be accomplished using sprinkler or flood irrigation.
Another option is to add top soil to the tunnel. On one of the farms participating in our project, the farmers had noticed issues associated with high soluble salts levels in the past. They added about 4.5 inches of topsoil to the tunnel and indicated that it helped. If you go this route be sure that the topsoil you use is low in soluble salts.
It would be useful to retest the soil after using any of these options.
If your soluble salts levels are high, hopefully, one of these options will work for your situation. For one farmer we worked with none of these options was practical. In a case such as this, in order to keep using the high tunnel, grow bags or compost socks filled with a soilless media could be used to grow crops in a soilless media hydroponic system until the soluble salts level can be lowered.
We are continuing our series examining soil test reports for high tunnels. This is based on results of soil tests from 27 Pennsylvania farms. Farmers sent soil samples from their high tunnels to Penn State’s Agricultural Analytical Services Laboratory (“the Lab”) for analysis. In this article we looked at soluble salts levels. In past articles, we have examined soil pH and nutrient levels: