Beware of High Tunnel Salt Build Up!

Rose Ogutu, Horticulture Specialist, Delaware State University

What causes high salt build up?
The soluble salts can be nitrate nitrogen and potassium that are valuable soluble fertilizers. The salts may also be calcium and magnesium from fertilizers or from hard water. Calcium and magnesium may be present at levels that will indicate high salts but not at levels detrimental to plant growth unless the nutrient balance is off the recommended levels.  There is also the limited chance that the salts are undesired sodium and chloride from irrigation water or compost or municipal leaves.  Effects of high salt build up can show as early as the 4th year of your high tunnel (HT) production.

Remember that it does not rain nor snow inside the high tunnels and the most commonly used drip irrigation does not allow for adequate leaching. Despite the silt nature of Delaware soils, generally characterized as very deep, well-drained, moderately rapidly permeable soils, a few high tunnel growers have started noticing the problem. Prolonged high evapotranspration rates, frequent irrigation (including, perhaps, using high EC water), adding various materials to the soil (sometimes, the same material is added over and over again), short rotations, shallow tillage and shallow-rooted crops, minimal freeze-thaw mediated soil mixing, and few episodes of natural soil flooding – can allow excess salinity to become a problem. This is true for organic, sustainable and conventional HT growers.

How can you tell?
Evidence of salinity includes symptoms like necrosis of leaf margins, less vegetative growth, reduced root growth, and reduced seed germination for directly seeded plants. Carrots and beans are sensitive and will show symptoms sooner. Low pepper yields may signal high salts.

Clockwise from top right: salt injury on a bean, pepper and cantaloupeClockwise from top right: salt injury on a bean, pepper and cantaloupe

Determining the status of your high tunnel from the soluble salts results
It is important for a HT grower to request a soluble salt test if it is not included in the routine soil test. Note that a Sodium (Na) mineral soil test is different from a soluble salt test. The soil sample has to be taken carefully. Normally, the top 0.5 to 1 inch of soil in high tunnels can have twice the nutrient levels of a sample taken from the top 6″ that includes the surface layer (normal recommended sampling method).

Electrical conductivity is used to measure soluble salt concentration and is measured in millimhos per centimeter (mmhos/cm) or decisiemens per meter (dS/m).

A water extract is done on a your soil sample and soluble salts may be reported for a 1 soil to 2 water dilution/extraction or a 1 soil to 5 water dilution/extraction or for a saturated medium extract (SME) which is like a soupy paste. The SME is used for greenhouse growing media and is being recommended at times for high tunnel soils. The table below shows what the numbers mean.

Interpretation of Results Based on a SME Extract



Adapted from Ray E. Lamond and David A. Whitney, Management of Saline and Sodic Soils, Kansas State University, May 1992 and Agricultural Analytical Services Laboratory, The Pennsylvania State University.

A soluble salt result reading 4.0mmhos/cm and above should raise your eyebrows!
Whether you are using organic or inorganic sources of plant nutrition, soil soluble salt levels should be monitored. A study at the Penn State High Tunnel Research facility showed that a baseline soil soluble level at 0.15mmhos/cm increased to 0.3mmhos/cm after 2 years with application of 1 inch of compost. Applying 2 inches of compost can lead to increases of approximately 5.5 to 13 times, yearly.

Minimizing the potential for, and remediating the high salt problem
Successful approaches to minimizing the potential for salinity problems are multi-faceted and involve testing soil, irrigation water and soil additives.

  • Check the salinity of your irrigation water. Any high tunnel farmer needs to have a complete irrigation water analysis to predict possible effects on the tunnel soil. Additional information about how we manage high irrigation water hardness and bicarbonate alkalinity will be explained in a future issue.

When using trickle irrigation, apply to maintain favorable soil moisture conditions. Table C-4 on page C3 of the 2014 Mid-Atlantic Commercial Vegetable Production Recommendations lists the maximum number of hours for trickle irrigation systems to apply 1 to 1.5 inches of water based on soil texture.

• Leach your soil (Wait a minute- you are washing away plant nutrients too!). This can be done periodically by putting a lot of water through the soil over the course of a day, or can be done with each watering by watering more than you need to. One can use leaching fraction to determine the extra amount of water you need to apply on top of what’s needed for irrigation in order to send excess salinity below the rootzone. A leaching fraction of 40% is recommended. As a general guideline for leaching out soluble salts from the top foot of soil, apply 6 inches of water to leach about 50% of the salts, apply 12 inches to leach about 80% of the salts and 24% to leach about 90% of the salts.

You will need to replace nutrients that have been washed away with the salts so do not leach soluble salts if you don’t need to.

The salts that move down to the lower soil profile and may gradually return over time dependent on watering practices. That can be a good thing if the salts are excess nitrogen and potassium, not so good if sodium and chloride.

Taking the cover off the tunnel and allowing rain in is an option. You however need 3 inches or more of rain within 7 to 10 days. Overhead irrigation can also accomplish this.

Flooding seems problematic when considering soil biology and soil structure but it has been done.

  • Select fertilizers with low salt indexes and limit the use of organic nutrient sources containing animal manure. Base compost selection and rates of addition on analyses that include both the total nutrient analysis and the soluble nutrients (SME) soil test. Sometimes you need to stop fertilization for a while in a non-leaching system like a stationary high tunnel, it is very important to not over apply fertilizers and to be cautious with the use of compost.  Manure based compost, particularly dairy or poultry compost are not recommended except for initial soil building prior to construction or in very small quantities if soil tests indicate low phosphorus or potassium levels.  Compost varies dramatically in soluble nutrient content.
  • Move the HT to a different location. This would be easier with movable HTs.
  • Add biochar – but you need to know your soil. Biochar is like a sponge. It very aggressively absorbs water. Likewise, it can absorb nutrients or other constituents out of your soil. Biochar has a liming effect. This could be good or it could be bad. It really depends on your situation. Knowing how to apply it, how much of it to apply, and under what circumstances to apply it is not widely known or clearly understood.• Add peat. Peat is rich in humus and gives an acid reaction. Humic acids are effective in absorption of various metal ions.
  • Mulch your crop. Plastic mulches used with drip irrigation effectively reduce salt concentration from evaporation.
  • Plant winter greens or winter rye, which will help reduce the residual nitrogen in the soil.

Elsa Sánchez. Penn State University Dealing With High Soluble Salt Levels in High Tunnels. Assessed May, 14, 2014.

Montri. A. and J. Biernbaum. 2009. Management of the Soil Environment in High Tunnels. HortTechnology 19:34-36.

Ray E.Lamond and David A. Whitney, Management of Saline and Sodic Soils, Kansas State University, May 1992.

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