Category: Cooperative Extension
Reading Soil Tests for Coastal Salinity
September 08, 2025 Written by Jarrod Miller, Agronomy Extension Specialist
Many nutrients in soil exist as salts, soluble and available for plant uptake. But when salts accumulate in excess, crop growth can decline. With more frequent surface flooding in coastal fields and communities (Figure 1), it is increasingly important to interpret soil tests correctly. These tests can help guide management decisions, including soil amendments and crop selection.
Standard Soil Salinity Testing
The standard test for soil salinity is the saturated paste extraction, though it is not commonly used in the Mid-Atlantic. In this method, soil is wetted to a paste-like consistency, then the extracted water is tested for salts. Results include electrical conductivity (ECe) and the sodium absorption ratio (SAR), which compares sodium (Na) to calcium (Ca) and magnesium (Mg). ECe is widely used to measure total soil salts and to predict crop response. These tests can be performed by labs in the western USA.
Mid-Atlantic Soil Tests
There are various types of tests performed on soils to estimate their fertility (salt) contents across the USA, but the most common in the Mid-Atlantic is the Mehlich-3. This double-acid extractant measures nutrient salts such as Ca and Mg along with micronutrients, but it has not traditionally been used to assess salinity. A recent University of Delaware study (https://doi.org/10.1002/saj2.70075) showed good relationships between Mehlich-3 sodium and both ECe and SAR. From this work, we developed a table estimating sodium thresholds where yields begin to decline (Figure 2). For example, corn yields may decline when Na exceeds 340 ppm, while sorghum is more tolerant, with reductions beginning near 1400 ppm. Most Delaware soils test between 20–50 ppm Na, but values can be higher following saltwater flooding.
Comparisons in Delaware Fields
In one Delaware coastal corn field, Na levels ranged from 24 ppm in higher areas to 2600 ppm in lower, flood-prone zones (Figure 3a). Areas below 340 ppm (shown in green) supported healthy corn growth (Figure 3b). Declines began around 500 ppm, and no corn survived at the highest concentrations. These results support the estimated thresholds for corn in Figure 2. For farmers and homeowners, comparing Na values between good and poor-performing areas can reveal how crops and landscape plants respond to flooding and help guide decisions on crop choice or soil management. We will continue to update recommendations as we gather more information.
