Effects of Wind-Induced Sodium Salts on Soils in Coastal Agricultural Fields

 

High tides associated with hurricane storm surges and other wind-driven events often inundate low-lying farm fields in coastal North Carolina. Flooding with salt water affects soil fertility by elevating soluble salts, especially sodium (Na) and chloride (Cl), which can be toxic to plants. Fertile agricultural soils contain soluble salts in the form of multiple elements (potassium, calcium, magnesium, chloride, sulfur, and nitrogen), but concentrations of these salts are low enough to cause no detrimental effects on plant growth. Salt water may significantly increase sodium levels in soil, thereby elevating the level of soluble salts. This soluble salt elevation can come from multiple sources—ocean spray from hurricane winds; tidal surges in creeks, canals, and ditches; and saltwater intrusion into shallow groundwater. The severity of this potential sodium influence is directly related to the proximity to the ocean, the site elevation and hydrology, the drainage network, the amount and timing of rainfall, and the soil properties. If sufficient rains saturate soils prior to saltwater influx, little sodium may infiltrate the soil. If significant rainfall occurs during or shortly after a storm that brings an influx of salt, it may dilute and flush salt out of soils, thus minimizing sodium accumulation. Maximum sodium accumulation occurs when soils are relatively dry prior to saltwater influx since more sodium will directly contact soil particles.

 

In fall 2018, we measured water salinity levels in drainage canals and tidal streams in agricultural areas near the North Carolina coast, finding from 4 to 16 parts per thousand (ppt) in an area of Hyde County, and from 1 to 3 ppt in an area of Pasquotank County with less direct seawater influence. As a comparison, sea water is typically 32 to 36 ppt. Compared with irrigation water salinity guidelines, waters in both areas contained salt levels that might be problematic (Irrigation Water Quality Problems, NC State Extension, AG-759). During that time, water from shallow groundwater monitoring wells in a Hyde County field had salinity levels of 4.4 ppt at a depth of 4 feet and 8.3 ppt at a depth of 7 feet. The crop had failed in that portion of the field. In a section of the field where the crop was doing well, the salinity was 5.2 ppt at a depth of 4 feet and 5.4 ppt at a depth of 9 feet. As this was immediately after Hurricane Florence, it is unknown what the salinity concentrations were when the soybeans were first planted and trying to grow. Data continue to be collected from this site, as well as from sites in Pasquotank and Camden counties, to determine the concentration and movement of salt in the soil profile.

 

Substantial accumulation of sodium salts in agricultural soils can have dramatic impacts on crops (Figure 1). You can confirm the problem diagnosis by submitting soil samples to the Soil Testing Laboratory of the N.C. Department of Agriculture & Consumer Services’ Agronomic Services Division. Be sure to use the Diagnostic Soil Sample Information form for soluble salt analysis. For routine soil samples, the Soil Testing Lab reports only sodium levels, not soluble salts. A limited number of diagnostic samples can be submitted without approval from the lab; verify with the lab if submitting more than 10 diagnostic samples. Preferably submit separate samples collected from the topsoil and the underlying layer, and from both normal and damaged crop areas. Depending on the stage of plant development and rooting, it may be helpful to collect more shallow samples than you would routinely.

 

 

Table 1 presents soil test data from salt-affected and “normal” areas of a field in eastern North Carolina. When reviewing Table 1, it is important to know that background sodium levels in most North Carolina agricultural soils are around 0.1 meq/100 cm3, with an ESP of 5 percent or less. In this field, although sodium levels were elevated in areas where plant growth was unaffected, the overall salt level did not injure the plants.

 

 

Data collected from fields inundated with salt water show a strong relationship between soluble salts and sodium levels (Table 2), so the sodium value on routine reports can be used as an indicator of soluble salt levels. Typically, soils in agricultural fields at the end of the growing season will have soluble salt index levels (SS-I) of 20 or less, with sodium levels around 0.1 meq per 100 cc soil as mentioned above. It is important to remember that the SS-I is influenced by other fertilizer salts, not just by sodium. Generally, salt injury may be expected to occur on emerging seedlings and young plants growing in soils with an SS-I of about 50; however, this is greatly dependent on the crop sensitivity and growth stage, soil type, soil texture, and moisture status of the soil.

 

 

A number of factors can affect plant stress tolerance to salt injury besides salt concentration. Plants vary in their tolerance to salts. You can find information on relative differences in crop tolerances in Chapter 4, page 65, in Diagnosis and Improvement of Saline and Alkali Soils. Seedlings and young transplants are generally more sensitive than mature plants to salts. Soil type and soil moisture also impact potential salt injury. Salt stress will be more likely in sandy, low organic soils, especially under dry conditions. Over time, salts will persist longer in finer-textured soils since water infiltration is slower and leaching is less likely.

 

Soils containing high levels of sodium are undesirable for several reasons other than salt injury. When significantly altered, the balance with other cations (calcium, magnesium, and potassium) can also be affected, creating additional fertility concerns. When the ESP of the cation exchange capacity is 15 percent or more, sodium can alter soil structure, especially of clays, limiting infiltration and causing drainage or crusting concerns.

 

 

 

If salt is concentrated in the upper 2- to 4-inch depths, deep plowing may be beneficial. Another potential method for remediating fields high in sodium is to displace the Na cation with calcium, usually by applying calcium sulfate or gypsum. Since gypsum is a salt, leaching rainfall or irrigation after its application is required for benefit. The amount of gypsum needed for Na displacement is found in Table 3. In practice, this technique only has limited success in coastal North Carolina since it is often difficult to flush the dissolved gypsum through the soil profile in a manner that leaches the sodium. Flushing is impeded because many of the soils are relatively fine-textured, the elevation is low, the high water table may limit deep percolation, and sufficient fresh water may not be readily available for irrigation. This problem is further exacerbated if additional sodium is introduced during periodic storms or via groundwater intrusions. This impeded flushing was observed in a Pasquotank County field where 9,000 pounds per acre of gypsum were applied over three years. The top 6 inches of soil still had an SS-I of 350 and an ESP of 36.

 

 

Other management practices may be useful in coastal North Carolina, including practices that encourage rainfall infiltration such as tillage to roughen the surface, residue management, cover crops, or vegetated fallow periods. Additionally, leaching can be enhanced by managing water control structures to alternate periods of saturation via either rainfall or freshwater irrigation with periods of drainage. For more severely impacted fields where economical yields of current agricultural crops cannot be maintained, changes in land use practices may be required.