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Salt‐affected soils can be identified by appearance, but are classified by laboratory measurements. Lab analysis results are used to determine both the total amount of salt and the chemical composition or type of salt. The following measurements are used to place a soil into one of four basic categories: normal, saline, sodic, or saline‐sodic (Table 1).
¶ Table 1. Classifying Salt‐Affected Soils |
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| Category | Extract electrical conductivity (ECe) | Exchangeable sodium percentage (ESP) |
| Normal | < 4 mmho/cm | < 15% Na |
| Saline | ≥ 4 mmho/cm | < 15% Na |
| Sodic | < 4 mmho/cm | ≥ 15% Na |
| Saline‐sodic | ≥ 4 mmho/cm | ≥ 15% Na |
Electrical conductivity (also called “specific conductance”) refers to the ability of a water solution to conduct an electric current. The EC reading indicates the total ion concentration and is related to total dissolved salt content. The conductivity increases as the concentration of the ions in solution (dissolved salts) increases. This measurement can be expressed several different ways, but the term “millimhos per centimeter” (mmho/cm) will be used in this Crop File.
In routine soil analysis, the EC reading is obtained from a slurry prepared by adding one part soil to one part water (“one‐toone” or “1:1”). This value appears as “1:1 Water‐Soil, Sol. Salts, mmho/cm” on a routine soil analysis report. This is a quick test to identify potential salt problems.
The ratio of soil to water will be much greater than 1:1 under field conditions. An EC obtained from a “saturated paste extract” give a more accurate idea of salinity in the field. A saturated paste extract is prepared by adding water to a soil sample just until it reaches the saturation point. The soil water is drawn off by vacuum and the conductivity is measured. This result is referred to as “ECe” or electrical conductivity of the extract. Another term for this reading is “extractable salts” to distinguish it from the 1:1 soluble salt reading. The extractable salt reading is about 1½ to 2½ times the soluble salt reading.
As the dissolved salt content of the soil water increases, the EC also increases. At some point, the salt content becomes enough that it interferes with the plant’s ability to extract water from the soil. Table 2 gives general guidelines for crop response to increasing EC (i.e., increasing soil salinity levels). Individual crops react differently to soil salinity. The effect on a crop at a particular EC value depends on the crop salt tolerance, variety, soil moisture, stage of growth, etc. (See Crop File 4.03.021, How Saline Soils Affect Crop Growth.)
A soil is considered “saline” when the EC exceeds 4 mmho/cm. Yields of moderately sensitive crops may be restricted at this point, but most crops will yield satisfactorily. There may be no visible soil symptoms at this point. When visual symptoms of salinity do appear, yield loss has already occurred. The ECe value is a good early warning sign of a developing salinity problem.
¶ Table 2. General Crop Response to Soil Salinity |
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| Electrical conductivity (ECe mmho/cm) |
Crop response |
| <1 | Salinity effects mostly negligible |
| 1 – 2 | Yields of very sensitive crops may be restricted |
| 2 – 4 | Yields of moderately sensitive crops may be restricted |
| 4 – 8 | Yields of many crops restricted |
| 8 – 16 | Only tolerant crops yield satisfactorily |
| >16 | Only a few tolerant crops yield satisfactorily |
The exchangeable sodium percentage (“ESP” or “% Na”) results from calculating cation exchange capacity (“CEC”). The CEC is a soil analysis value determined by adding a solution with a high cation concentration, like ammonium acetate, to a soil sample. The cation in the solution (i.e., ammonium) displaces other cations from the exchange surfaces on the clay and organic matter. The concentrations of sodium (Na), calcium (Ca), magnesium (Mg), and potassium (K) ions are determined from the extract when displacement is complete. These concentrations appear in the “Lab Results” section of the soil test report and are expressed as “parts per million (ppm)”. The buffer pH value (BpH) is used to estimate the hydrogen ion (H) concentration. These five results are then used to calculate the CEC, expressed as “milliequivalents per 100 grams of soil (meq/100g)”. The percentage of each exchangeable cation is calculated from the CEC result. The two key formulas are:
CEC meq/100g = (ppm Na/230) + (ppm Ca/200) + (ppm
Mg/120) + (ppm K/390) + [(7 – BpH) * 12]
ESP or % Na = (ppm Na / 230) / CEC
The ESP value is significant because soil structure deteriorates as the ESP increases. This deterioration results when soil aggregates (or “granules”) break apart or disperse into their individual clay, silt, or sand particles. Soil pore spaces that are necessary for air permeability and water infiltration begin to collapse. The individual soil particles begin to plug or block the remaining pore spaces. Soils begin to crust and seal, resulting in excess runoff. Soils become “blocky” as the ESP increases, restricting water movement and root growth.
A value of 15% Na is used to classify a soil as “sodic”. This is the point where a “typical” soil has the symptoms described above are severe enough to affect crop growth and yield. An ESP of 10% is used as a warning point, where a soil is in the early stages of becoming sodic.
The “typical soil” is defined as having a silt loam texture, 2% organic matter, and a mixed clay mineralogy. Clay content, clay type, and organic matter level can all affect the point where dispersion is a problem and sodic symptoms are obvious.
Fine‐textured soils with a high clay content tend to disperse at lower ESP than will coarse‐textured. For example, dispersion in a soil with clay or silty‐clay texture may become a problem when the ESP is 7% to 10%. Sandy soils, with low clay content, may not exhibit sodic soil symptoms until the ESP is 20% to 25%.
Sodium affects different soil clay minerals differently. Certain clay types (like montmorillonite or smectite) tend to swell and shrink more when wetted and dried than other clay minerals (like kaolinite). This swelling squeezes soil pores and reduces their size. The smaller‐sized pores are more easily blocked by dispersed clay particles. Thus, soils with high shrink‐swell potential tend to lose permeability at lower ESP levels than soils with a low shrink‐swell potential.
Sodic soils can be classified more precisely if the soil texture (percent clay) and the shrink‐swell potential (clay type) are known. Table 3 has guidelines to better identify sodic soil problems. For example, assume a fine‐textured soil with medium shrink‐swell potential and 11% Na. This soil would be considered “non‐sodic” by traditional guidelines for the “typical” soil. This soil may actually be a sodic soil because the degree of dispersion may be greater because of the individual soil characteristics.
The sodium adsorption ratio (“SAR”) is calculated from the saturated paste extract results, as follows:
SAR = (mg/L Na/23) / [square root {(mg/L Ca/20)+(mg/L Mg/12)) / 2}]
The SAR expresses the relationship of sodium ions to calcium and magnesium ions in the soil solution. It indicates the degree to which sodium ions are competing with calcium and magnesium ions for sites on the exchange surfaces of soil clays. Calcium and magnesium tend to offset the damaging effects of sodium on the soil structure.
A lower SAR means a lower proportion of sodium ions in soil solution compared to calcium and magnesium ions. This means there is a smaller chance for sodium ions in solution to displace other ions on the exchange complex and increase the sodium percentage.
The SAR is sometimes used instead of the ESP to identify sodic soils. The two values are often correlated, but there is a difference. The SAR indicates the proportion of sodium ions in the soil solution. The ESP indicates the proportion of sodium ions on the soil exchange complex. If the soil solution and the exchange complex are in equilibrium, the SAR value is typically 70% to 85% of the ESP value. The two values can differ greatly and should not be used interchangeably.
In the early stages of sodic soil development, the SAR value may be larger than the ESP value. This means that the sodium in the soil water has not yet reached equilibrium with the sodium on the exchange complex. As the soil moves closer to equilibrium, the ESP will increase. A soil with a high SAR and a low ESP needs corrective actions to be applied quickly to minimize potential damage from dispersion.
Research comparing lab measurements of ESP and SAR show the SAR value tends to underestimate the actual ESP, especially in sandy soils. Correlation between the two values was generally best when the CEC was 30 meq/100g or greater. The SAR underestimated the ESP by two‐fold to twenty‐fold in soils with a CEC less than 10 meq/100g. Sodic soils are best identified using the ESP measurement obtained by extraction with an active cation, like ammonium.
The soil pH was formerly used to identify salt‐affected soils, because the pH is often over 7.5 in sodic soils. This gave rise to the term “alkali” soil to describe salt‐affected soils. There are a few sodic soils with a pH of 6.5 or lower, called “degraded sodic soils”. These soils have sodium and hydrogen as dominant cations on the exchange complex. The terms “saline” and “sodic” define the properties of a salt‐affected soil more precisely than does the term “alkali”.
The “excess lime” test can help pick the soil amendment needed to deal with a sodic soil. If no excess lime is present, a soluble calcium source (like gypsum) is needed to replace sodium ions on the clay surfaces. If excess lime is present, either a soluble calcium source or an acid‐forming material can be used. The acid‐forming amendment dissolves the soil carbonates, providing a source of soluble calcium and magnesium.
¶ Table 3. Exchangeable Sodium Percentages Used to Classify Sodic Soils as Adjusted for Basic Soil Characteristics |
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| Shrink‐swell potential | |||
| Soil Texture | High | Medium | Low |
| Fine | 6 – 8 | 9 – 12 | 11 – 14 |
| Medium | 9 – 12 | 12 – 15 | 15 – 20 |
| Coarse | 12 – 15 | 18 – 20 | 23 – 25 |
| Fine‐textured: silty clay loams, clays; Medium‐textured: loams, silt loams, clay loams; Coarse‐textured: sands, loamy sands, sandy loams | |||
Some crops are sensitive to sodium, chloride, or boron. If these ions are present at excess levels in the soil solution, certain crops may show symptoms of nutrient disorders or toxicity. (see Crop File, 4.03.014, Toxic Ions in Salt‐Affected soil.
The EC, CEC, ESP, and SAR can be used to identify various characteristics of salt‐affected soils. The relationship of these values are as follows:
Electrical conductivity (EC): Estimation of total ions found in soil solution (i.e., “dissolved salts”). Cations primarily include Na, Ca, Mg, and K. Determined from a soil‐water mixture of a specific ratio. Used to assess the ability of plants to extract water from the soil.
Cation exchange capacity (CEC): Sum of total cations found on the soil exchange complex. Cations include Na, Ca, Mg, K, and sometimes H. Increases with increasing clay and organic matter content. Used to calculate the percentage of exchangeable cations.
Exchangeable sodium percentage (ESP, % Na): Exchangeable sodium ions as percentage of total exchangeable ions on exchange surfaces. Used to assess the potential effect of sodium on soil permeability.
Sodium adsorption ratio (SAR): Proportion of sodium ions compared to calcium and magnesium ions found in soil solution. Determined from saturated paste extract. Used to assess further potential for the exchangeable sodium percentage to increase.