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Soil amendments for reclaiming sodic soils fall into three general categories: (1) calcium salts, (2) acid forming materials, and (3) organic materials. Calcium salts and acid-formers provide a source of soluble calcium. Calcium dissolves in the soil solution and replaces sodium (Na) on the exchange complex. As sodium is replaced, it can be removed by leaching. Organic materials promote better crop growth and faster leaching by improving soil tilth and water permeability.
Gypsum (calcium sulfate, CaSO4) is the calcium salt most commonly used to improve sodic soils, because of low cost and wide availability. Gypsum rates must be based on the amount of sodium to be removed, depth of the sodic zone, and quality of the gypsum. Gypsum rates are calculated from soil analysis results (see Crop File 4.03.051, Calculating Gypsum Requirements).
Gypsum is a neutral salt and will not change the pH of a non-sodic soil. Many sodic soils have a high pH because of sodium bicarbonate generated during soil chemical reactions. As the gypsum lowers the sodium percentage of a soil, the pH value tends to drop because less bicarbonate is being generated by soil reactions. Table 1 compares the effect of gypsum and lime on the pH of a non-sodic silt loam soil.
Some manufacturers have processed finely ground gypsum into prills or pellets to improve handling characteristics, but this does not change the agronomic effectiveness. The product must still be applied on the basis of actual gypsum content.
Gypsum, or any other soil amendment, needs to be placed as near as possible to the sodiumaffected zone in the soil profile. This is normally the surface six to 12 inches. Agricultural-grade gypsum that has been surface broadcast and lightly tilled into the surface 2 to 3 inches will move downward gradually with irrigation or rain water.
If over five tons of gypsum is required, the total should be split into two or more annual applications of five tons or less. Routine soil tests will monitor progress of the reclamation and verify the need for more gypsum.
Finely ground gypsum dissolves and reacts faster than coarse gypsum. About 1 to 1½ inches of water is needed to dissolve gypsum particles smaller than 80 mesh. Particles larger than 50 mesh may require 6 inches of water or more to completely dissolve.
Very finely ground gypsum (100 mesh or smaller) can be applied in surface irrigation water with the proper equipment. It should be fed gradually and continuously into the irrigation water supply.
If soils are tilled when wet, benefits of gypsum can be offset by compaction. Deep ripping or subsoiling helps break up sodic zones, but does not incorporate gypsum well. Research shows that the applied gypsum stays near channels formed by the ripper shank, so has little effect on the area between those channels.
Deep plowing (2 to 3 feet deep) would be the most effective method for incorporating gypsum in the subsoil.
The source of sodium may be a high sodium irrigation water which often forms a crust by dispersing the surface soil. In this case, about one-half to one ton of gypsum can be applied to the soil surface after final tillage and before crop emergence. This localizes the soluble calcium in the surface soil and allows it to react with sodium in the irrigation water before it causes crusting.
¶ Table 1. Effect of Lime and Gypsum on Soil pH After One Year |
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| Tons/acre applied* | Soil pH | |
| Gypsum | Lime | |
| 0 | 4.5 | 4.5 |
| 2 | 4.5 | 5.1 |
| 4 | 4.6 | 5.4 |
| 8 | 4.6 | 6.1 |
| *Pond Creek silt loam soil | ||
Lime or calcium carbonate (CaCO3) can be used as a soluble calcium source, but reacts slowly when compared to other materials. Lime is nearly insoluble in water, so furnishes calcium ions very slowly.
Another source of soluble calcium or an acid forming material may be required to enhance the applied lime and supplement the soluble calcium ions needed to displace the exchangeable sodium in a timely manner. Table 2 illustrates the difference between lime and gypsum for removing exchangeable sodium.
Calcium chloride (CaCl2) is a very soluble calcium salt and exchanges quickly with sodium. Calcium chloride is very expensive and is probably not economical unless it can be obtained as a waste product. The chloride component could be a problem with certain crops.
Calcium nitrate (CaNO3) or other materials may be available in some areas. However they are not usually cost competitive with gypsum.
Acid forming compounds are useful only with calcareous soils, when the soil has excess lime. Many acid-formers are sulfur compounds and they react with the insoluble lime to form more soluble calcium salts. For example the reaction of sulfuric acid (H2SO4) is shown below:
H2SO4 + CaCO3 → CaSO4 + CO2 + H2O
sulfuric acid + lime → gypsum + carbon dioxide + water
2Na-clay + CaSO4 → Ca-clay + Na2SO4
sodic soil + gypsum → non-sodic soil + sodium sulfate
Sulfuric acid is an excellent amendment for sodic soils and may be superior to gypsum in soils with extremely high exchangeable sodium percentages. It can be applied through the irrigation ditch water and reacts quickly. In some areas sulfuric acid is available at low cost as an industrial byproduct. It is very corrosive and if not handled properly will destroy concrete, aluminum, or other metallic pipes, pumps, or irrigation equipment.
Elemental sulfur (S) can be used as a soil amendment. It is oxidized in the soil by microbial action and forms sulfuric acid as shown:
2S + 3O2 → (microbial oxidation) → 2SO3
elemental sulfur + oxygen → microbial activity sulfite
SO3 + H2O → H2SO4
sulfite + water → sulfuric acid
Elemental sulfur must be thoroughly incorporated. The soil must be warm, well aerated, and moist to encourage bacterial action. Finely ground sulfur has more surface area and reacts much faster than granular sulfur. Fine sulfur is often available as a prilled 90% sulfur fertilizer material and works well as a soil amendment.
Some sulfur fertilizers, like ferrous sulfate (FeSO4), aluminum sulfate (Al2(SO4)3), and ammonium thiosulfate can react to form sulfuric acid. The cost of these fertilizers will generally prohibit their use.
Other fertilizers, like langbeinite (e.g., Sul-Po-Mag™, K-Mag™), are neutral salts and are not effective in reclaiming sodic soils. Sulfur dioxide, lime-sulfur solution, and ammonium polysulfide are used as amendments, but all require special equipment and handling. They require bacterial action and moisture to become effective. Concentrated phosphoric acid banded over the row has been shown to help reduce crusting and improve emergence.
¶ Table 2. Effect of Gypsum and Lime on Removing Exchangeable Sodium |
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| Treatment | Sodium (Na) concentration after: | ||
| 6 mo. | 1 yr. | 2 yrs. | |
| --- ppm (meq/100g) --- | |||
| Check | 1450 (6.3) | 1380 (6.0) | 1360 (5.9) |
| Lime | 1310 (5.7) | 1290 (5.6) | 1150 (5.0) |
| Gypsum | 1150 (5.0) | 620 (2.7) | 460 (2.0) |
Organic materials like manure, straw, rotted hay, sewage sludge, or crop residues can help improve sodic soils. Fibrous crop residues help improve soil tilth and water infiltration. Manure makes nutrients like zinc and iron more available to the crop.
Apply organic materials at rates from 20 to 40 tons per acre and incorporate into the top 8 to 12 inches of soil. Do not completely invert these materials if plowing. This may create a mat of organic matter under the top soil layer which slows water infiltration and drainage. Add nitrogen fertilizer to speed up decomposition and prevent crop nitrogen deficiency.
Livestock manure may have a high soluble salt content and should be tested before using. Sewage sludges and waste products should also be tested for soluble salts and heavy metals.
When picking an amendment producers must consider material cost, availability, handling equipment, corrosivity, physical form, application cost, freight charges, soil type, and availability of water.
Table 3 gives equivalent amounts of various soil amendments and reaction speed as compared to gypsum.
For example, if the gypsum requirement is 7 tons and free lime is present in the soil, then 1.47 tons of 90% sulfur could be substituted for the recommended gypsum (7 tons x 0.21 = 1.47 tons).
Tables 4 and 5 contain estimates of time required to remove exchangeable sodium from the top 6 inches and the full 4-foot profile, respectively, of a sodic soil when using the recommended amount of gypsum, given different precipitation amounts.
¶ Table 3. Equivalent Values of Various Sodic Soil Amendments |
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| Amendment | Tons equivalent to 1 ton of: | Relative speed of reaction | |
| Gypsum (95%) | Sulfur (90%) | ||
| Gypsum (95%) | 1 | 5.37 | Rapid |
| Sulfur (90%)* | 0.21 | 1 | Moderate |
| Sulfuric acid* | 0.61 | 3.2 | Very rapid |
| Ferrous (iron) sulfate* | 1.09 | 5.85 | Very rapid |
| Aluminum sulfate* | 1.29 | 6.34 | Very rapid |
| Ammonium thiosulfate* | 0.71 | 3.85 | Moderate |
| Calcium chloride | 0.86 | --- | Very rapid |
| Calcium nitrate | 1.06 | --- | Rapid |
| Limestone | 0.58 | --- | Very slow |
| Organic materials | --- | --- | Variable |
¶ Table 4. Estimated Number of Years Required to Remove Excess Sodium (Na) From the Surface 6 Inches by Adding Recommended Amounts of Gypsum |
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| Sodium to remove: | Annual total precipitation (inches) | ||||||||||||
| meq/100g | ppm | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 24 | 36 | 48 | 72 | 96 |
| ------------------------------ years ------------------------------ | |||||||||||||
| 1 | 230 | 5 | 3 | 2 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| 2 | 460 | 11 | 6 | 4 | 3 | 2 | 2 | 2 | 1 | 1 | 1 | 1 | 1 |
| 5 | 1150 | 27 | 14 | 10 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 1 | 1 |
| 8 | 1840 | 44 | 23 | 15 | 12 | 9 | 8 | 7 | 5 | 3 | 2 | 1 | 1 |
| 10 | 2300 | 55 | 29 | 19 | 15 | 12 | 10 | 8 | 7 | 4 | 3 | 2 | 1 |
| 15 | 3450 | 82 | 43 | 2 | 22 | 18 | 15 | 13 | 10 | 6 | 4 | 3 | 3 |
| 20 | 4600 | 109 | 57 | 39 | 29 | 24 | 20 | 17 | 13 | 8 | 6 | 4 | 3 |
| 30 | 6900 | 163 | 86 | 58 | 44 | 35 | 30 | 25 | 20 | 12 | 9 | 5 | 4 |
| These estimates based on assumption that: 1) gypsum has a solubility of 0.25%, 2) efficiency is assumed to be 50%, 3) a quantity of water is utilized by plants within the profile and is unavailable for reclamation, and 4) four inches of water evaporates before entering the soil. | |||||||||||||
¶ Table 5. Estimated Number of Years Required to Remove Excess Sodium (Na) From a 4-Foot Depth by Adding Recommended Amounts of Gypsum |
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| Sodium to remove: | Annual total precipitation (inches) | ||||||||||||
| meq/100g | ppm | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 24 | 36 | 48 | 72 | 96 |
| ------------------------------ years ------------------------------ | |||||||||||||
| 1 | 230 | ------------------- thousands -------------------- | 8 | 2 | 1 | 1 | 1 | 1 | |||||
| 2 | 460 | --- | --- | --- | --- | --- | --- | 15 | 4 | 1 | 1 | 1 | 1 |
| 5 | 1150 | --- | --- | --- | --- | --- | --- | 38 | 11 | 3 | 2 | 1 | 1 |
| 8 | 1840 | --- | --- | --- | --- | --- | --- | 60 | 17 | 5 | 3 | 2 | 1 |
| 10 | 2300 | --- | --- | --- | --- | --- | --- | 75 | 21 | 7 | 4 | 2 | 2 |
| 15 | 3450 | --- | --- | --- | --- | --- | --- | 113 | 32 | 10 | 6 | 3 | 2 |
| 20 | 4600 | --- | --- | --- | --- | --- | --- | 150 | 43 | 14 | 8 | 4 | 3 |
| 30 | 6900 | --- | --- | --- | --- | --- | --- | 225 | 64 | 20 | 12 | 7 | 3 |
| These estimates based on assumption that: 1) gypsum has a solubility of 0.25%, 2) efficiency is assumed to be 50%, 3) a quantity of water is utilized by plants within the profile and is unavailable for reclamation, and 4) four inches of water evaporates before entering the soil. | |||||||||||||
Steigler, J. and G. Johnson. OSU Extension Facts No. 2200, Farm & Home Use of Gypsum. Oklahoma State Univ., Stillwater, Okla.
Schafer, W. 1982. Saline and sodic soils in Montana. Bulletin 1272. Coop. Ext. Serv., Montana State Univ., Bozeman, Montana. 56 pg.
Branson, R. and M Fireman. 1980. Gypsum and other chemical amendments for soil improvement. Leaflet 2149. Univ. of California, Berkley, Cal. 7 pg.
Oster, J. and H Frenkel. 1980. The chemistry of the reclamation of sodic soils with gypsum and lime. Soil Sci. Soc. Am. Journ. 44:41- 45.
Hira, G. and N. Singh. Irrigation water requirement for dissolution of gypsum in sodic soil. Soil Sci. Soc. Am. Journ. 44:930-933.