What are Sodium Adsorption Ratio and Sodicity?
Photo Credit: https://www.ndsu.edu/soilhealth/?page_id=1000
Soil sodicity is caused by excessive saturation of Na+ ions at the soil cation exchange sites (negative charges of clay and humus particles that attract positively charged chemical ions). Sodicity directly affects soils as high Na+ levels compared to calcium (Ca2+) in combination with low salt levels can promote “soil dispersion”, which is the opposite of flocculation. Soil dispersion causes the breakdown of soil aggregates, resulting in poor soil structure (low “tilth” qualities). Due to the poor soil structure, sodic soils have dense soil layers, resulting in very slow permeability of water, air and other contaminants through the soil profile. It is important to note If Na+ is present as a salt, it will not cause dispersion as the positive charges of Na+ ions will be neutralized by the negatively charged chemical ions such as sulfates (SO42-) or chloride (Cl–). However, due to the constant exchange of positively charged ions like Ca2+, magnesium (Mg2+) and Na+ between soil water and the soil clay and humus particle negative charges, high levels of Na+ based salts in the soil water can result in sodicity as more negative charges could get saturated with Na+.
Unlike salinity, in most cases there are no clear visual symptoms of sodicity, which could be seen at the soil surface. The best way to know about the soil sodicity issues is by sampling the affected areas and getting them analyzed from a soil testing lab. Historically, Exchangeable Sodium Percentage (ESP) test has been used to analyze soils for sodicity. However, over time, ESP has been replaced by Sodium Adsorption Ratio (SAR) in most of the regional soil laboratories. As a result, SAR values can be substituted for ESP (Oster et al., 1999).
Soil SAR is a measure of the ratio of sodium (Na+) relative to calcium (Ca2+) and magnesium (Mg2+) in the water extract (solution phase) from a saturated soil paste (Eq. 1). The units of Ca2+, Mg2+ and Na+ are milliequivalent/liter [meq/L) or mmol(c)/L].
For classification purposes, a soil is considered sodic if SAR levels are 13 or more, however, these values are just general guidelines and don’t imply that the lower numbers will not result in detrimental effects related to soil sodicity, especially for shrinking and expanding clays. Based on the findings of NDSU research for shrinking and swelling type of clays, if soil results for SAR are more than 5 and the Electrical Conductivity (EC) is less than 2 millimhos per centimeter (mmhos/cm), movement of soil water may be restricted due to swelling, dispersion or both.
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| Title | Source | Resource type and Date | Short Summary / Preview |
|---|---|---|---|
| Saline and Sodic Soils Part 1 | North Dakota State University | Video
Dec 2013 |
This two-part video explains soil salinity, sodicity, how they affect plants and soils and the reasons, which cause both of these issues in North Dakota. |
| Saline and Sodic Soils Part 2 | North Dakota State University | Video
Dec 2013 |
This video talks about the common symptoms of soil salinity, sodicity, importance of sampling the affected areas and remedial options. |
| Soil Sodicity And Its Management | North Dakota State University | Video
Feb 2016 |
This video specifically focusses on soil sodicity, how it breakdown aggregates and reduce the pore sizes through dispersion and swelling and the chemical processes initiated by amendments like gypsum that remediate sodicity by using animation. |
| Improving Soil Health on Sodium-Affected Soils | North Dakota State University | Video
Jun 2014 |
Soil sodicity is a widespread soil health issues found across the state of North Dakota. This video gives an overview of an NDSU research project started in 2014 that focuses on remediating sodic soils by using commonly available soil amendments on tiled and non-tiled soils. |
| Saline and Sodic Soils | North Dakota State University | Fact Sheet
Not Dated |
This publication outlines the causes of salinity and sodicity in North Dakota, their effects on plants and soils and general management suggestions. |
| Soil sodicity | North Dakota State University | Extension Webpage
Not Dated |
This website has resources that explain differences in soil sodicity versus salinity by using pictures, videos and soil analysis results. |
| Remediation of Sodic (Alkali) and Saline Soils | SARE | Book Excerpt
2010 |
This resource explain soil sodicity and salinity and the remedial practices. |
| Sodicity and Remediation of Sodic Soils in North Dakota | North Dakota State University | Fact Sheet
Not Dated |
This publication explains in detail the issue of sodicity and what it does to soil chemistry and physical properties, the phenomenon of soil dispersion and swelling and their effect on soil pore space, causes and spread in North Dakota along with remedial options involving use of soil amendments and improvement of soil drainage. |
| Effectiveness of Gypsum in the North-central Region of the U.S. | North Dakota State University | Fact Sheet
Nov 2006 |
This resource explains the beneficial uses of gypsum on remediating soil sodicity, maintaining an optimum soil pH, reducing bulk density and use as a nutrient source for the crops. |
| Title | Source | Resource type and date | Short Summary |
|---|---|---|---|
| Reclaiming Saline, Sodic, and Saline-Sodic Soils | University of California | Fact Sheet
Aug 2015 |
This publication explains soil salinity, sodicity, their effects on crop production and water and air movement through the soils along with the remedial practices including the calculation of the amendment rates per acre to remediate sodicity. |
| Managing Sodic Soils | Colorado State University | Fact Sheet
May 2012 |
Sodic soils (a) are poorly drained and tend to crust, (b) respond to continued use of good irrigation water, good irrigation methods, and good cropping practices, and (c) are often reclaimed by adding a calcium-based soil amendment. |
| Add-on Test: Salinity | Cornell University | Fact sheet
May 2017 |
Although salinity and sodicity are often mistaken as the same thing, they are in fact quite different from each other. We include the comparison between salinity and sodicity here for clarification – Problems with salts (salinity) and sodium (sodicity) may occur naturally, but are especially prevalent under irrigated agriculture. |
| Soil Sodicity | State of Victoria (Agriculture Victoria), Australia | Government Webpage
Jan 2019 |
Sodicity is a measure of the exchangeable sodium in relation to other exchangeable cations in soil and is expressed as the Exchangeable Sodium Percentage (ESP). A soil with an ESP greater than 6 is generally regarded as being a sodic soil in Australia. |
| Basics of Salinity and Sodicity Effects on Soil Physical Properties | Montana State University | Extension Webpage
2003 |
Sodicity refers specifically to the amount of sodium present in irrigation water.Irrigating with water that has excess amounts of sodium can adversely impact soil structure, making plant growth difficult. Highly saline and sodic water qualities can cause problems for irrigation, depending on the type and amount of salts present, the soil type being irrigated, the specific plant species and growth stage, and the amount of water able to pass through the root zone |
| Unavailability of Water in Saline Soils | New Mexico State University | Video
Jun 2016 |
Salts in the soil provide nutrients to plants. However, an excess of salt can interfere with water uptake. This animation demonstrates how sodium chloride in soil binds with water and impedes roots from taking up water. |
| Using Gypsum and Other Calcium Amendments in Southwestern Soils | University of Arizona | Extension publication
Aug 2012 |
This publication emphasize the role of Ca2+ in promoting chemical aggregation of soil particles and the related benefits apart from being an important plant nutrient. It provides in-depth information about the negative effects of a lower Ca2+ level versus Na+ and sources of adding Ca2+ in case of a deficiency. |
| Title | Source | Resource type and Date | Short Summary |
|---|---|---|---|
| Effects of Flue Gas Desulfurization and Mined Gypsums on Soil Properties and on Hay and Corn Growth in Eastern Ohio | Journal of Environmental Quality | Peer-reviewed publication
Jun 2014 |
This publication talks about the agricultural potential benefits of commercially produced gypsum as a byproduct, changes in soil chemistry due to its use and results of an Ohio research study that studied effects of gypsum application on the yield and quality of corn and hay. |
| Efficiency of Sulfuric Acid, Mined Gypsum, and Two Gypsum By-Products in Soil Crusting Prevention and Sodic Soil Reclamation | Agronomy Journal | Peer-reviewed publication
May 2005 |
This study compares the effects of four different soil amendments on reducing soil crusting sodic and non-sodic soils by using sulfuric acid, mined gypsum and gypsum produced as a byproducts; coal-gypsum and lacto-gypsum. |
| Reclamation of Salt-Affected Soils (Chapter 19 of Agricultural Drainage) | American Society of Agronomy, Crop Science Society of America, Soil Science Society of America | Book Chapter
Jan 1999 |
This publication extensively explains effects of salinity and sodicity on soil chemical and physical properties and reclamation of saline and sodic soils under tillage, crops and irrigation systems and by using soil amendments. |
| Potential Agricultural Uses of Flue Gas Desulfurization Gypsum in the Northern Great Plains | Agronomy Journal | Peer-reviewed publication
Jul 2009 |
This publication talks about the potential uses of Flue Gas Desulfurization Gypsum (FGDG) in Norther Great Plains produced by the coal combusting plants as a byproduct for producing electricity and the conversion of toxic calcium sulfite (CaSO3.0.5H2O + CO2 + 0.5H2O) into the agricultural grade gypsum (CaSO4.2H2O). |
| Field Capacity Water as Influenced by Na and EC: Implications for Subsurface Drainage | Geoderma | Peer-reviewed publication
May 2015 |
This research focused on studying the effects of excessive Na+ saturation at the soil CEC sites leading to sodicity and EC on soil swelling, soil water movement and potential impacts on reducing the efficiency of subsurface drainage systems like tiling.
Note: This links to an abstract of the article. You may need to contact your library to request a copy of this article as a free electronic version is not available. |
| Gypsum, a Tricky Material | Soil Science Society of America Journal | Peer-reviewed publication
Nov 2009 |
This publication differentiate various forms and uses of gypsum including the agricultural grade gypsum and its benefits in order to clarify some misconceptions.
Note: This links to an abstract of the article. You may need to contact your library to request a copy of this article as a free electronic version is not available. |
| The Chemistry of the Reclamation of Sodic Soils with Gypsum and Lime | Soil Science Society of America Journal | Peer-reviewed publication
Jan 1980 |
This publication explain the chemical changes application of gypsum stimulates for remediating soil sodicity.
Note: This links to an abstract of the article. You may need to contact your library to request a copy of this article as a free electronic version is not available. |
Sodium Adsorption Ratio
| Title | Source | Resource type and Date | Short Summary / Preview |
|---|---|---|---|
| Sodium Adsorption Ratio (SAR) | USDA – NRCS | Governmental Fact Sheet
Apr 2017 |
Sodium adsorption ratio is a measure of the amount of sodium (Na) relative to calcium (Ca) and magnesium (Mg) in the water extract from saturated soil paste. It is the ratio of the Na concentration divided by the square root of one-half of the Ca + Mg concentration. |
| Exchangeable Cations and Clay Dispersion: Net Dispersive Charge, A New Concept for Dispersive Soil | European Journal of Soil Science | Peer-reviewed publication
Jul 2016 |
Historically soil dispersion has been studied in light of sodicity, however, this publication looks at the potential dispersive nature of magnesium and potassium. It presents a new concept of CROSS (cation ratio of structural stability) to determine clay dispersion.
Note: This links to an abstract of the article. You may need to contact your library to request a copy of this article as a free electronic version is not available. |
| Processes Involved in Sodic Behavior (Chapter 3 of Sodic Soils: Distribution, Properties, Management, and Environmental Consequences) | Oxford Press | Book Chapter
1998 |
This publication outlines in-depth information about processes that cause soil dispersion in sodic soils.
Note: This links to the summary of the book. You may need to contact your library to request a copy of this book chapter as a free electronic version is not available. |
Technical review: March 31, 2020