Salinity Management in High Tunnel Systems

Introduction

High tunnels are controlled environment structures that accelerate crop growth and protect the crop from environmental extremes. Since most crops are produced in soil within the high tunnel, soil health is very important for sustainable food production. Salt accumulation in high tunnels can occur from extended use of high-salt fertilizers or composts and the absence of rainfall in the structure. High salt levels in the soil can reduce water uptake, stunt plant growth and even cause death of seedlings or susceptible mature plants.

Electrical conductivity, or EC, is a test provided by the WVU Soil Testing Lab that analyzes the soluble salt concentration of a soil. The test costs $3 per sample. Growers will get the current EC status of their high tunnel soil, so they know if a correction needs made. When submitting a soil sample to the WVU Soil Testing Lab, simply check the box that you want an EC test on the soil submission form.

WVU Extension recommends using the following table to determine salt tolerance thresholds and to categorize crop sensitivity. Salt tolerance thresholds in Table 1 provide a cut-off value, where EC results above this number may result in yield loss. Table 1 categorizes crops into four levels of sensitivity – sensitive, mostly sensitive, mostly tolerant and tolerant – with the maximum cut-off EC result indicated.

Table 1. Salt tolerance threshold measured as EC in a 1:1 soil/water mixture and EC sensitivity for selected crops rated as sensitive (S), mostly sensitive (MS), mostly tolerant (MT) and tolerant (T).

My Soil Report Indicates an Elevated EC – What Can I Do?

Using the following formula, developed by the USDA Agricultural Research Service Salinity Lab in Riverside, California, we can estimate the amount of irrigation water needed to move the salts out of the rooting zone and into a deeper layer of soil below.

D _w = (k x D _s x EC _ei) ÷ EC _ef

Where:

D _w = Depth of water infiltrated

D _s = Depth of soil to be reclaimed (Common rooting zone for high tunnel crops is 3 inches.)

k = A constant (In the absence of previous information, use 0.3 for fine-textured soils and 0.1 for course-textured soils.)

EC _ei = Initial soil salinity of soil profile

EC _ef = Final soil salinity desired

Let’s Look at an Example

Lettuce is being grown in a high tunnel. According to the soil test, the EC is 2.1 with a soil texture of sandy clay loam. According to Table 1, the threshold for lettuce is 1.5. We want to reclaim a soil depth of 3 inches, and with a clay soil, the k factor would be 0.3.

D _s = 3

k = 0.3

EC _ei = 2.1

EC _ef = 1.5

D _w = (0.3 × 3 × 2.1) ÷ 1.5

D _w = 1.89 ÷ 1.5 = 1.26 inches of irrigation water

This grower would need to apply 1.26 inches of water evenly throughout the tunnel to push the salts out of the 3-inch zone to minimize risk of a yield reduction.

According to a study by WVU Extension, by applying the recommended amount of irrigation, salts can be moved down into the soil profile and out of the rooting zone preventing crop damage and yield loss. In this study, the soluble salt status of four high tunnels were evaluated and if a reduction was needed, a calculated irrigation rate, using the equation mentioned above, was applied.

The cooperator used an overhead sprinkler irrigation system to apply the recommended water to the tunnels. The high tunnel was retrofitted with a cost-effective sprinkler system (approximately $100), bisecting the tunnel to achieve adequate coverage. Emitters attached to the Orchard Hose, fitted with Senninger Irrigation Mini Wobblers, were placed down the center of the tunnel on 15-foot intervals. Application is achieved when the hose is hooked up to a water supply with full pressure and left to run until the desired irrigation level is met.

Conclusion

High tunnel growers should be mindful of soluble salts and their impact on yield reduction. It is recommended to have a protocol in place for dealing with soluble salts and correcting EC levels. Electrical conductivity for each tunnel should be monitored at each cropping cycle or, at minimum, annually. The “k factor” is an index of soil infiltration in the USDA ARS Salinity Lab equation. A consideration for the producer will be to adjust their “k factor” to fit their soils. To determine an individual high tunnel soil’s k factor, sand, silt and clay particle size distribution would need to be determined.

Irrigation is effective at moving soluble salts out of the rooting zone, allowing for maximum crop production. Another consideration a producer must be mindful of is salt inputs into the tunnel. Salts can be introduced through fertilizers, pesticides and water. Select fertilizers with low salt indexes, use water that is low in salt content and avoid the over application of nutrients. Producers are encouraged to work with their local WVU Extension offices for guidance.

Resources

• How to Purchase Electrical Conductivity & Organic Matter Determination

WVU is an EEO/Affirmative Action Employer -- Minority/Female/Disability/Veteran.
In accordance with Federal law and U.S. Department of Agriculture (USDA) civil rights regulations and policies, this institution is prohibited from discriminating on the basis of race, color, national origin, sex, age, disability, and reprisal or retaliation for prior civil rights activity. (Not all prohibited bases apply to all programs).
The WVU Board of Governors is the governing body of WVU. The Higher Education Policy Commission in West Virginia is responsible for developing, establishing, and overseeing the implementation of a public policy agenda for the state’s four-year colleges and universities.
Reasonable accommodations will be made to provide this content in alternate formats upon request. Contact the WVU Extension Service Office of Communications at 304-293-4222.