Diagnosing Salinity Problems
The symptoms and causes of salinity, high pH, specific ion toxicity, and sodicity are frequently confused. These conditions can all have adverse affects on plant growth, but they differ significantly in their cause and relative impact. Effective management of these problems varies considerably and requires correct diagnosis of the problem. Proper diagnosis is critical to successful problem correction.
Simply stated, salinity problems are caused by accumulation of soluble salts in the root zone. These excess salts reduce plant growth and vigor by altering water relations or by causing ion-specific toxicities or imbalances.
Soil salinity is typically described and characterized in terms of concentrations of soluble salts. While a general classification scheme based upon soil EC and SAR has been accepted, be aware that salinity problems occur on a continuum rather than at a given threshold. Appropriate management strategies are evaluated from the following and other measurements:
- Saline = high salt content (EC > 4 dS/m)
- Sodic = high sodium content (SAR > 13)
- Saline-sodic = high salt, high sodium
- Basic = high pH (problems usually start at pH > 7.8)
|Classification||Electrical conductivity (dS/m)a||Soil pH||Sodium adsorption ratio (SAR)b||Soil Physical Condition|
adS/m = mmhos/cm
bIf reported as exchangeable sodium percentage (ESP), use 15% as threshold value.
Some visual symptoms can be used to help diagnose these problems, but ultimately soil and irrigation water analyses are the best way to make an accurate diagnosis. When salinity is suspected to be caused by a high water table, you may be able to measure ground water depth by boring holes with an auger. If free water collects in holes less than 4 to 5 feet deep, a drainage problem is indicated.
Plants growing in saline soils may appear water stressed. In some cases, a white crust is visible on a saline soil surface. Salt-affected soils may inhibit seed germination and cause irregular emergence of crop seedlings, particularly in sensitive crops such as beans or onions. Plants sprinkler irrigated with saline water often show symptoms of leaf burn, particularly on young foliage.
Saline soils can be found in many areas of the United States, but are especially common
in the west. These salts originate mainly from the natural weathering of soil minerals
or from fossil salt deposits left from ancient sea beds. Salts accumulate in soils
of arid environments as irrigation water or groundwater seepage evaporates, leaving
minerals behind. Irrigation water often contains salts picked up as the water moves
across the landscape, or the salts may come from man-induced sources such as municipal
runoff or water treatment. As water is diverted and rediverted in a basin, salt levels
increase as the water is consumed (transpired or evaporated). Knowing the source of
salt problems can be helpful in diagnosing and managing salt-affected fields.
Soils with high levels of exchangeable sodium (Na) and low levels of total salts are called sodic soils. Sodic soils may impact plant growth by 1) specific toxicity to sodium sensitive plants, 2) calcium deficiencies or nutrient imbalances caused by excessive exchangeable sodium, 3) high pH, and 4) dispersion of soil particles, resulting in poor physical conditions in the soil. If a soil is highly sodic, a brownish-black crust sometimes forms on the surface due to dispersion of soil organic matter. By the time darkened crusts are visible on the soil surface, the problem is severe, and plant growth and soil quality is significantly impacted. Dispersion of soil particles often results in crusting and poor emergence. Plants growing on sodic soils may appear stunted and often show a burning or drying of tissue at the leaf edges, progressing inward between veins.
High pH, or basic, soil doesn't usually look any different from soil with neutral
pH, although sometimes the soil may have a powdery substance on the surface. Other
times it is obvious that the fields have lost topsoil due to erosion or leveling,
leaving subsoil exposed or near the surface. Plants growing in high pH soils sometimes
give clues about the problem. High pH reduces the availability of some nutrients (zinc,
iron, phosphorus). Yellow stripes on middle to upper leaves (signs of zinc and iron
deficiency) or dark green or purple coloring of the lower leaves and stems (signs
of phosphorus deficiency) can be signs of high soil pH. In particular, looking for
symptoms can be useful when growing high pH sensitive plants such as dry beans, sorghum,
or silver maples. Corn and wheat are moderately susceptible to high pH and may also
suffer from nutrient deficiencies on these soils.
Table 2. Diagnosing high pH, salinity or sodicity problems in the field.
|high pH||nutrient deficiencies manifesting as
|saline irrigation water||
Soil testing labs typically evaluate pH and EC (electrical conductivity) as part of a routine package. If the pH is high (>8.5), sodium adsorption ratio (SAR) should also be calculated. There is no standard procedure for obtaining soil samples to diagnose saline conditions; proper sample depth depends upon cropping system and the nature of the situation. However, crop salinity thresholds are based on the average salinity level of the active root zone.
In general, when sampling for salinity or sodicity problems, collect a composite sample of several soil cores from the active root zone or the zone you plan to manage, concentrating on the areas that appear most impacted. High pH, salt or sodium levels are rarely uniformly distributed across fields. Areas of the field suspected of these problems should be mapped and sampled separately to fully understand the severity of the problem. In many cases, comparison of soil samples from the affected area and surrounding normal appearing areas will be beneficial in diagnosing the problem. Be sure to tell the laboratory that you suspect a problem and ask that they analyze for SAR or ESP and perform a gypsum test, if necessary.
If the site in question is irrigated, be sure to collect a water sample for analysis
as well. High levels of salts and sodium may come from irrigation water, a high water
table, manure or fertilizer inputs, or from the soil parent material. To manage the
problem effectively, you need to know the source of the salts. Although 4.0 dS/m is
used as a threshold EC to define saline soils, many crops may show symptoms and reduced
yields at much lower ECs.
Other information, including soil texture, cation exchange capacity, type of clays present, calcium carbonate content, organic matter, depth to ground water, and soil profile information, will help in planning a reclamation program. Some of this information may be obtained in the county soil survey, available through your local USDA-Natural Resources Conservation Service (NRCS) office.
Tests frequently employed in diagnosing salinity/sodicity problems:
- Electrical Conductivity (EC)
- Sodium Adsorption Ratio (SAR)
- Exchangeable Sodium Percentage (ESP)
- Cation Exchange Capacity (CEC)
- Lime Estimate
- Total Dissolved Solids (TDS - water only)
- Anions and cations: eg. Ca2+, Mg2+, Na+, Cl-, SO42-, CO32-, HCO3-
- Available gypsum (CaSO4.2H2O) and gypsum requirement
- Soil texture estimate
What lab test do you need to run?
- If you are unsure but suspect a salinity or sodicity problem?
Soil EC, pH, and SAR
- If you suspect poor quality irrigation water?
TDS, EC, SAR, Na+, Cl-, B, HCO3-
- If you know that a salinity problem exists and you want to monitor or calculate leaching
Soil EC, irrigation water EC, consider spatial mapping of field EC
- If you know that a sodicity problem exists and you want to calculate gypsum requirement?
Lime estimate, CEC, ESP and/or SAR
Effective management of salinity, high pH, specific ion toxicity, and sodicity varies considerably and requires correct diagnosis of the problem to ensure successful problem correction. Some visual symptoms can be used to diagnose these problems, but ultimately, soil and irrigation water analyses are the best way to make an accurate diagnosis. In general, when sampling for salinity and sodicity problems, collect a composite sample of several soil cores from the active root zone you plan to manage. If the site in question is irrigated, be sure to collect a water sample for analysis as well. Other information including soil texture, cation exchange capacity, types of clays present, calcium carbonate content, organic matter, depth to ground water, and soil profile information will help you in planning a reclamation program.
Alkalinity: Soil pH >7.0; basic soil; problems usually appear at pH >7.8 as nutrient deficiencies
Calcareous: A soil containing significant amounts of naturally occurring calcium carbonate, which fizzes when dilute acid is added.
Cation Exchange Capacity (CEC): The amount of exchangeable cations that a soil can adsorb at a specific pH, expressed as milliequivalents per 100 grams of soil.
Gypsum: Calcium sulfate (CaSO4.2H2O) used to supply calcium and sulfur to improve sodic soils.
Electrical Conductivity (EC): EC is a measure of salinity resulting from all the ions dissolved in a water sample or saturated paste. This includes negatively charged ions (e.g. Cl-, NO3-) and positively charged ions (e.g. Ca2+, Na+). The preferred unit is decisiemens per meter (dS/m); however millimhos per centimeter (mmhos/cm) and micromhos per centimeter (µmhos/cm) are frequently used.
Permeability: Ease with which water enters the soil and percolates down through the profile. Usually reported as infiltration rate, where 2.5 mm/hr =low and 12 mm/hr = high.
Ion specific effects: Effect of chloride (Cl-), sodium (Na+), or boron (B) on plants due to osmotic stress
Salinity hazard: Total soluble salt content
Sodium hazard: Relative proportion of exchangeable sodium (Na+) to calcium (Ca2+) and magnesium (Mg2+) ions
Sodium Adsorption Ratio (SAR): The sodium hazard of soil usually is expressed as the sodium adsorption ratio (SAR). This is the proportion of water soluble Na+ to Ca2+ plus Mg2+ in the soil. The following formula is used to calculate SAR:
SAR = (Na2+ meq/L) / √ [(Ca2+ meq/L) + (Mg2+ meq/L) / 2]
Ions in the equation are expressed in millequivalents per liter (meq/L) and are obtained from a saturated paste soil extract. To convert ppm or mg/L Na+ to meq/L, divide by 23, for Ca2+ divide by 20, and for Mg2+ divide by 12.2.
Soluble salts: Major dissolved inorganic solutes