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Gypsum or calcium sulfate (CaSO4) is the most commonly used material for amending sodic soils because of low cost and wide availability. Other soil amendments can be substituted for gypsum and are discussed in Crop File 4.03.052, Using Gypsum and Other Soil Amendments for Sodic Soils.
Sodic soils are corrected by replacing the exchangeable sodium (Na) on the clay particle exchange surfaces with a soluble calcium source (i.e., calcium “salt”). The soluble calcium is applied to the soil, then dissociates in soil solution to displace sodium on the exchange complex.
The displaced sodium essentially forms a soluble sodium “salt” which can then be leached below the root zone. The soil structural damage caused by excess sodium begins to reverse ss the calcium percentage on the exchange complex increases.
The general process is illustrated as follows:
Na-clay + Ca-X → Ca-clay + Na-X leaching sodic soil + calcium salt → non-sodic soil + sodium salt
Gypsum applied to the soil eventually dissolves in the soil water and dissociates into the calcium and sulfate ions (Ca+2 and SO4-2). The sulfate (SO4-2) ion recombines with two of the displaced sodium ions (Na+) when the soil dries. This precipitation reaction forms sodium sulfate (Na2SO4) which is highly soluble and can be easily leached.
Calculating a gypsum requirement
The following equation is used to calculate a gypsum requirement for a 12-inch soil depth:
GR = [((ESPi – ESPf)/100) x CEC] x (1.72 x 1.25) where:
GR = field gypsum requirement in tons per acre
ESPi = initial exchangeable sodium percentage, % Na from soil analysis report
ESPf = target exchangeable sodium percentage, % Na
CEC = cation exchange capacity, meq/100 gm)
1.72 = tons of pure gypsum needed to replace 1 milliequivalent Na per 100 grams soil
1.25 = factor to allow for field inefficiencies in applying gypsum.
Example calculation: Assume the soil analysis report shows a value of 32% exchangeable sodium and a CEC value of 21 meq/100g. We wish to lower the ESP to 10% Na.
Then:
GR = [((32 - 10) / 100) x 21] x (1.72 x 1.25)
= [(22 / 100) X 21] x 2.15
= [0 .22 X 21] x 2.15
= 4.62 x 2.15 GR = 9.93
The calculated gypsum requirement in this situation is 9.9 tons of gypsum per acre to adjust the ESP from 32% Na to 10% Na in the surface 12 inches.
A soil with 15% Na or greater is traditionally classified as “sodic”. The standard assumption is that an “average” soil shows the first symptoms of sodic soil development when the exchangeable sodium exceeds 10% Na, so amendment applications are targeted to reduce the ESP below 10% Na.
An “average” soil is assumed to have a medium texture with about 2% organic matter and mixed clay mineralogy. The target ESP should be adjusted for soils with different textures or clay mineralogy.
Fine textured soils with high clay content are likely disperse and potentially lose permeability at levels well below 10% Na. Sandy soils with low clay content may remain permeable with good infiltration rates until the ESP exceeds 20% to 25% Na.
Clay minerals differ in their natural potential to shrink or swell when dried or wetted. Certain clay minerals, like montmorillonite or smectite, swell easily when wet. Clay minerals, like kaolinite, are considered non-swelling.
As the ESP increases, swelling can become more pronounced as more sodium ions occupy the sites on the exchange surfaces of these clays. Soils with a high proportion of swelling clays typically lose their permeability at lower ESP levels than soils with nonswelling clays, given they have the same total percentage of clay particles.
As individual clay particles swell, size and diameter of individual soil pores become smaller and permeability begins to decline. These smaller-sized soil pores can then get plugged or blocked more easily by individual clay particles that have been dispersed by excess sodium accumulations. Smaller pores and blocked
pores will restrict air and water movement through the soil, so overall infiltration rates slow down.
The traditional approach has been to recommend enough gypsum to amend a sodic soil to a final target ESP of 10% Na or less. Correcting sodic soil conditions may be more successful if the target ESP (ESPf) is adjusted for the individual soil characteristics.
Table 1 lists suggested ESPf values to be used in application rate calculations to adjust for soil texture and clay mineralogy differences. The shrink-swell potential of a soil gives a general idea of the clay minerals in that soil.
A target ESPf of 10% Na is the default value. The ESPf in the table has been adjusted upward or downward with respect to these two soil properties.
For example, when calculating a gypsum rate for a finetextured soil with high shrink-swell potential, an ESPf of 4% or 5% Na is suggested, rather than 10% Na, to adjust for the different soil characteristics.
Soil characteristics for a particular site can be found in the Web Soil Survey:
(https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx). The “Soil Map” will provide the soil type name and texture. The “Soil Data Explorer”, “Suitabilities and Limitations for Use” section can provide other characteristics.
Shrink-swell ratings may be included in the limitations found under “Building Site Development”, “Dwellings With Basements”. Ratings are scored on a 0.01 to 1.00 scale. A high rating would indicate a high proportion of smectite or montmorillonite clays.
The “Sanitary Facilities”, “Septic Tank Absorption Fields” may provide a water movement rating on a 0.1 to 1.00 scale as a limitation to development. A high rating suggests the soil is more susceptible to loss of permeability from sodium accumulations.
The following Tables 2, 3, and 4 list a series of gypsum application rates for different soil textures and clay mineralogy groups. The exchangeable sodium concentration (Na ppm) and cation exchange capacity (CEC meq/100g) from the soil analysis report are needed to locate the appropriate range of application rates.
¶ Table 1. Soil Property Adjustments to Final Target Exchangeable Sodium Percentage (ESPf) |
|||||
| Shrink-swell potential | |||||
| High | Medium | Low | |||
| Class | Soil Texture | Clay Content | ---------- Target Na % ---------- | ||
| Fine Texture | Clay | 50% – 80% | 4 – 5 | 6 – 8 | 7 – 9 |
| Silty clay | 45% – 50% | ||||
| Sandy clay | 40% – 45% | ||||
| Silty clay loam | 35% – 40% | ||||
| Medium | Clay loam | 30% – 35% | 6 – 8 | 8 – 10 | 10 – 13 |
| Sandy clay loam | 25% – 30% | ||||
| Silt clay | 10% – 25% | ||||
| Loam | 10% – 20% | ||||
| Coarse | Sandy loam | 5% – 15% | 8 – 10 | 12 – 14 | 15 – 17 |
| Loamy sand | 3% – 10% | ||||
| Sand | 3% – 7% | ||||
¶ Table 2. Estimated gypsum rates required for soils with mixed clay mineralogy |
|||||||
| (Soils with mixed clay types typically have a "low" to "moderate" shrink-swell potential.) | |||||||
| General soil texture class → | Coarse (sandy) | Medium (loamy) | Fine (clayey) | ||||
| Typical CEC, meq/100g → | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| ppm Na | ------------ Gypsum rate, ton/ac to 12-inch depth ------------ | ||||||
| 100 | 0.0 - 0.1 | --- | --- | --- | --- | --- | --- |
| 200 | 0.1 - 0.2 | 0.2 - 0.3 | 0.0 - 0.1 | --- | --- | --- | --- |
| 300 | 0.8 - 1.9 | 0.5 - 0.7 | 0.3 - 0.4 | 0.0 - 0.2 | 0.0 - 0.1 | --- | --- |
| 400 | 1.6 - 3.6 | 1.0 - 1.5 | 0.6 - 0.7 | 0.3 - 0.5 | 0.2 - 0.3 | 0.1 - 0.2 | --- |
| 500 | 1.6 - 3.6 | 2.0 - 3.0 | 1.1 - 1.5 | 0.6 - 0.8 | 0.5 - 0.6 | 0.3 - 0.4 | 0.2 - 0.3 |
| 600 | 1.6 - 3.6 | 3.0 - 4.5 | 2.2 - 2.9 | 1.6 - 1.9 | 1.3 - 1.5 | 1.0 - 1.1 | 0.5 - 0.7 |
| 700 | 1.6 - 3.6 | 4.0 - 6.0 | 3.3 - 4.3 | 2.7 - 3.3 | 2.4 - 2.8 | 2.1 - 2.4 | 1.6 - 2.1 |
| 800 | 1.6 - 3.6 | 5.0 - 7.5 | 4.3 - 5.7 | 3.8 - 4.6 | 3.6 - 4.2 | 3.3 - 3.7 | 2.7 - 3.5 |
| 900 | 1.6 - 3.6 | 5.0 - 7.5 | 5.4 - 7.1 | 4.9 - 6.0 | 4.7 - 5.5 | 4.4 - 5.1 | 3.8 - 4.9 |
| 1000 | 1.6 - 3.6 | 5.0 - 7.5 | 6.5 - 8.5 | 6.0 - 7.4 | 5.8 - 6.9 | 5.6 - 6.4 | 4.9 - 6.3 |
| 1500 | 1.6 - 3.6 | 5.0 - 7.5 | 6.5 - 8.5 | 11.6 - 14.2 | 11.6 - 13.6 | 11.4 - 13 | 10.3 - 13.3 |
| 2000 | 1.6 - 3.6 | 5.0 - 7.5 | 6.5 - 8.5 | 11.6 - 14.2 | 17.3 - 20.3 | 17.2 - 19.7 | 15.8 - 20.3 |
¶ Table 3. Estimated Gypsum Rates Required for Soils With Smectite or Montmorillonitic Clays |
|||||||
| (Soils with these clays typically have a "high" shrink-swell potential.) | |||||||
| General soil texture class → | Coarse (sandy) | Medium (loamy) | Fine (clayey) | ||||
| Typical CEC, meq/100g → | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| ppm Na | ------------ Gypsum rate, ton/ac to 12-inch depth ------------ | ||||||
| 100 | 0.0 - 0.1 | --- | --- | --- | --- | --- | --- |
| 200 | 0.6 - 1.5 | 0.2 - 0.3 | 0.1 - 0.3 | 0.1 - 0.2 | 0.0 - 0.2 | 0.0 - 0.1 | --- |
| 300 | 1.4 - 3.2 | 1.2 - 1.8 | 0.7 - 0.9 | 0.5 - 0.7 | 0.4 - 0.6 | 0.4 - 0.5 | 0.2 - 0.3 |
| 400 | 2.1 - 5.0 | 2.2 - 3.3 | 1.7 - 2.3 | 1.4 - 1.7 | 1.2 - 1.4 | 1.0 - 1.2 | 0.7 - 1.0 |
| 500 | 2.1 - 5.0 | 3.2 - 4.8 | 2.8 - 3.7 | 2.5 - 3.1 | 2.4 - 2.8 | 2.2 - 2.5 | 1.8 - 2.4 |
| 600 | 2.1 - 5.0 | 4.2 - 6.3 | 3.9 - 5.1 | 3.6 - 4.4 | 3.5 - 4.1 | 3.4 - 3.8 | 2.9 - 3.8 |
| 700 | 2.1 - 5.0 | 5.2 - 7.8 | 5.0 - 6.5 | 4.7 - 5.8 | 4.7 - 5.5 | 4.5 - 5.2 | 4.0 - 5.2 |
| 800 | 2.1 - 5.0 | 6.2 - 9.3 | 6.1 - 7.9 | 5.9 - 7.2 | 5.8 - 6.8 | 5.7 - 6.5 | 5.1 - 6.6 |
| 900 | 2.1 - 5.0 | 6.2 - 9.3 | 7.1 - 9.3 | 7.0 - 8.5 | 6.9 - 8.1 | 6.8 - 7.8 | 6.2 - 8.0 |
| 1000 | 2.1 - 5.0 | 6.2 - 9.3 | 8.2 - 10.7 | 8.1 - 9.9 | 8.1 - 9.5 | 8.0 - 9.1 | 7.3 - 9.4 |
| 1500 | 2.1 - 5.0 | 6.2 - 9.3 | 8.2 - 10.7 | 13.7 - 16.7 | 13.8 - 16.2 | 13.8 - 15.8 | 12.7 - 16.3 |
| 2000 | 2.1 - 5.0 | 6.2 - 9.3 | 8.2 - 10.7 | 19.3 - 23.6 | 19.5 - 22.9 | 19.6 - 22.4 | 18.2 - 23.3 |
¶ Table 4. Estimated Gypsum Rates Required for Soils With Illitic or Kaolinitic Clay |
|||||||
| (Soils with these clays typically have “very little” or “no” shrink-swell potential.) | |||||||
| General soil texture class → | Coarse (sandy) | Medium (loamy) | Fine (clayey) | ||||
| Typical CEC, meq/100g → | 5 | 10 | 15 | 20 | 25 | 30 | 40 |
| ppm Na | ------------ Gypsum rate, ton/ac to 12-inch depth ------------ | ||||||
| 100 | --- | --- | --- | --- | --- | --- | --- |
| 200 | 0.1 - 0.2 | --- | --- | --- | --- | --- | --- |
| 300 | 0.8 - 1.9 | 0.0 - 0.1 | --- | --- | --- | --- | --- |
| 400 | 0.9 - 2.0 | 0.2 - 0.4 | 0.0 - 0.1 | --- | --- | --- | --- |
| 500 | 0.9 - 2.0 | 0.5 - 0.8 | 0.2 - 0.3 | 0.0 - 0.2 | --- | --- | --- |
| 600 | 0.9 - 2.0 | 1.5 - 2.3 | 0.6 - 0.7 | 0.3 - 0.4 | 0.0 - 0.1 | --- | --- |
| 700 | 0.9 - 2.0 | 2.5 - 3.8 | 1.1 - 1.5 | 0.6 - 0.7 | 0.2 - 0.4 | 0.0 - 0.1 | --- |
| 800 | 0.9 - 2.0 | 3.5 - 5.2 | 2.2 - 2.9 | 1.2 - 1.5 | 0.7 - 0.9 | 0.2 - 0.3 | 0.2 - 0.4 |
| 900 | 0.9 - 2.0 | 3.5 - 5.2 | 3.3 - 4.3 | 2.3 - 2.9 | 1.9 - 2.2 | 1.4 - 1.6 | 0.8 - 1.0 |
| 1000 | 0.9 - 2.0 | 3.5 - 5.2 | 4.3 - 5.7 | 3.5 - 4.2 | 3 - 3.5 | 2.6 - 2.9 | 1.9 - 2.4 |
| 1500 | 0.9 - 2.0 | 3.5 - 5.2 | 4.3 - 5.7 | 9.1 - 11.1 | 8.7 - 10.3 | 8.4 - 9.6 | 7.3 - 9.4 |
| 2000 | 0.9 - 2.0 | 3.5 - 5.2 | 4.3 - 5.7 | 9.1 - 11.1 | 14.5 - 17 | 14.2 - 16.2 | 12.7 - 16.4 |
Schafer, W. 1982. Saline and sodic soils in Montana. Bulletin 1272. Coop. Ext. Serv., Montana State Univ., Bozeman, Montana. 56 pg.
James, Hanks, & Jurinak. 1983. Reclamation of Poorly Drained and Salt-Affected Soils in Modern Irrigated Soils. Wiley Inter-Science publication. pg. 204 – 210.