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.