Soil Testing & Assessment

Soil Testing & Assessment

If you suspect saltwater intrusion or inundation is affecting a field, soil testing can help determine whether salts or Na have accumulated and where the problem occurs. Field symptoms can identify areas of concern, but testing is needed to determine whether salinity is actually present.

Salt concentrations can also vary substantially across a field. Sampling affected and unaffected areas separately can be more useful than collecting a single composite sample from the entire field.

In Field Examination and Soil Sampling

Multiple samples should be obtained from a field where SWI is suspected, including from bare spots (probably near tidal waters), transition zones where reduced crop growth may be observed, as well as areas of the field that appear to not be affected. This can help determine where management options may be required to alleviate salinity. Bare parts of the field may also have poor crop production, but still have weed pressure later in the season.

Using field probes, such as an electrical conductivity (EC) meter or compaction probe may help reveal where soil samples should be taken. Use a consistent sampling depth when comparing samples. If possible, record the location of each sample so soil test results can be compared with elevation, crop growth, yield, and other field information.

What Soil Tests Do You Need?

The standard analyses is saturated paste, which is not typically performed in the Mid-Atlantic region. This would provide you with electrical conductivity (ECe) where dissolved salts conduct electricity, so that ECe increases as the concentration of salts increases. The sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) are also used to evaluate sodium-related soil problems. Saline soils have an ECe > 4, while sodic (high Na) soils have a SAR > 13.

There are two useful ways to think about soil test results:

• Salinity: How much soluble salt is present and how might it affect plant water uptake?

• Sodicity: How much sodium is present and could it affect soil structure and water movement?

What other salts may be present?

While Na is a dominant ion in seawater, multiple ions have been observed in soils undergoing SWI, including chloride, magnesium, sulfate, bicarbonate, and boron. Elevated levels of these ions may also indicate SWI if your soil testing lab does not test Na.

What can I ask for from a Mid-Atlantic soil lab?

Most agricultural soil testing in the Mid-Atlantic uses the Mehlich-3 (M3) extraction to evaluate soil fertility. Research on Delmarva soils has shown that M3 sodium (M3-Na) can be used to estimate soil salinity and sodium-related risk when saturated paste testing is not available. M3-Na can therefore provide a practical screening tool for identifying salt-affected areas and comparing areas within a field. Some labs may also report EC(1:2), which measures the conductivity of a soil water mixture and can be related back to ECe from the saturated paste analyses.

Research with Delmarva soils has established preliminary relationships between M3-Na and estimated ECe and SAR. As a general screening guide, M3-Na values above 60 ppm indicate the start of a salinity issue, greater than 300 ppm can indicate reduced crop growth, while samples with greater than 600 ppm Na typically being bare soil with limited crop survival. Values between these levels may indicate a transitional condition.

These values are screening thresholds, not crop-response thresholds. Crop growth can be affected at lower sodium concentrations, and the response will depend on the crop, soil texture, drainage, flooding, and other site conditions.

Need more detail?

The Salt Measurements and Soil Classifications (SWI Series 2) factsheet explains saturated paste testing, ECe, SAR, ESP, and traditional soil classifications.

The Regional Soil Tests to Evaluate Coastal Soil Salinity factsheet provides additional information on using Mehlich-3 sodium to estimate salinity and sodicity risk in Mid-Atlantic soils, including preliminary relationships between M3-Na, ECe, SAR, and crop response.