What Is Sodic Soil and Why It Damages Plant Growth?
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What is sodic soil? It is a soil defined by how much sodium is attached to the soil particles. The two NDSU sources used for this article give the definition, name the measures used to test for it, and describe the physical changes that follow. The Franzen chapter also notes that “salinity and sodicity can often occur together or be mistaken for each other when working with soils.” This explainer covers the definition, the mechanism, the measures, and the limits of what the sources say.
Quick answer
- A sodic soil is one where, in the words of the NDSU Langdon extension page, “Sodic soils have 15 percent or more sodium ions attached to their cation exchange sites.”
- The Franzen chapter defines the same condition as “a soil where the exchange sites on the clay particles are dominated by excessive levels of adsorbed sodium.”
- The Langdon page describes damage to soil structure, and it links the resulting conditions to “poor seed germination, very slow water infiltration and restricted root growth.”
- Sodicity is measured with the sodium adsorption ratio (SAR) or the exchangeable sodium percentage, according to the Langdon page.
What the term means
The word “sodic” appears alongside other names. The Franzen chapter opens by listing them: “Sodic soils, alkali soils, natric and solonetz soils.” It then notes a practical difference in usage: “Although the term sodic is used for soil management, the term natric is more frequently used in soil classification.”
The chapter also separates sodicity from salinity. It says that “Soil sodicity means the degree to which sodium has affected the soil.” It adds that “salinity and sodicity can often occur together or be mistaken for each other when working with soils.”
The exchange sites are the key feature. The Langdon page describes the condition as sodium occupying “15 percent or more” of those sites. The Franzen chapter uses the same idea when it defines a sodic soil as one “where the exchange sites on the clay particles are dominated by excessive levels of adsorbed sodium.”
How sodium affects soil particles
The mechanism begins with the size of the hydrated sodium ion. The Franzen chapter explains that “The large hydrated shell of an Na ion results in a swelling and dispersion of the clay particles.” It compares sodium with calcium: “the hydrated radius of the Na ion is still much larger than that of calcium.” The chapter then links the size difference to particle spacing: “The larger hydrated radius of Na ions results in greater distance between clay particles, and decrease in clay aggregation.”
The term for this loss of aggregation is dispersion. The chapter states that “The term for a lack of aggregation due to presence of Na ions is dispersion.” It describes the result for water: “Dispersion produces wet soils with very slow water infiltration.”
The Langdon page describes a similar sequence in more practical terms. It states that “Excessive dispersion of the soils (due to the formation of sodium carbonate salts), resulting in damage to the soil structure by causing disintegration of soil particles.” The page does not give a separate explanation for the carbonate step, so this article reports it only as the page states it.
What sodicity does to plants
Because the soil structure changes, the effects reach the roots and the crop. The Langdon page lists the effects of dispersion, beginning with “Soil dispersion causing poor physical conditions,” and continuing with “in soils difficult to till, poor seed germination, very slow water infiltration and restricted root growth.” It also lists “Disintegration of soil aggregates leading to wind and water erosion” as a further consequence.
The Franzen chapter links dispersion and swelling to plant growth in a specific region. It says that “In North Dakota, dispersion and soil swelling can be problematic for plant growth in a soil with a SAR greater than 5 and an EC less than 1 dS/m (a ratio of 5:1).” The threshold is regional, and the chapter treats it as a North Dakota guideline rather than a universal one.
The chapter also explains why the soil’s electrical conductivity (EC) matters. It states that “The most critical of the two previously listed factors that affect the structural stability of the soil is EC.” It adds that “the properties of sodic soils that limit plant growth” may be reduced “with an increase in the overall soil EC,” but that “the increase should not be so great as to induce salinity stress in the plants.”
How sodic soil is measured
The Franzen chapter gives the main measure: “The Sodium Adsorption Ratio (SAR) is the overall measure of soil sodicity.” The chapter also says that the classic formula has limits: “The classic SAR formula assumes that dispersion is controlled in the numerator by Na alone.” The Langdon page names two measures: “Soil sodicity is measured through soil sodium adsorption ratio or its exchangeable sodium percentage.”
The soil’s clay type also changes how the measure behaves. The chapter states that “Sodium Adsorption Ratio (SAR) was found to be the most important factor influencing dispersion across all ratios tested.” It also notes that clay type matters for the threshold, since the text describes a case where “if the soil is dominated by montmorillonite clay” swelling or dispersion are likely.
Common causes
The Langdon page lists causes in two groups. The natural causes include “Underlying sodium-rich shale in the bedrock below the soil sediments” and “Shallow groundwater table with high sodium levels.” The man-made causes include “Excessive use of irrigation water high in sodium” and “Excessive leaching of divalent cations (Ca++ and Mg++) leading to domination of Na+ in the soil.”
The Langdon page also describes the approach for managing the condition. It says to “Analyze the soil for percentage of cation exchange sites occupied by sodium,” and to “Supplement calcium-based soil amendments to replace excessive sodium in the soil.” The page presents these as management steps, and this article does not give rates or procedures beyond what the page states.
How we chose
The two sources were chosen because both are extension publications from North Dakota State University, one a chapter and one a fact sheet, and together they cover the definition, the measures and the causes. Each factual sentence in this article is tied to a quote from one of them. The sources were opened on 2026-10-27 and returned HTTP 200 before use.
What we could not verify
Claims that neither source states are left out. The threshold values in the Franzen chapter are described as North Dakota guidance, so they are not presented as a general rule for every region.
What the sources do not say
The sources do not give a single sodium percentage that applies to every crop. The Franzen chapter ties its threshold to a region and to clay type. The Langdon page gives the 15 percent figure for the exchange sites without listing crop-specific tolerances. Readers who want to judge their own soil will need a test result, which the Langdon page describes only as an analysis of “the percentage of cation exchange sites occupied by sodium.”
For general soil background, the garden soil improvement guide covers soil conditions in general terms, and the guide to acidic garden soil covers a related soil chemistry topic.
Frequently asked questions
What is sodic soil in simple terms?
According to the Langdon page, it is a soil with “15 percent or more sodium ions attached to their cation exchange sites.” The Franzen chapter describes it as a soil where the exchange sites on the clay particles are dominated by excessive levels of adsorbed sodium.
Is sodic soil the same as saline soil?
No, according to the Franzen chapter, which says that “salinity and sodicity can often occur together or be mistaken for each other when working with soils.”
Why does sodium make soil hard for water to enter?
The Franzen chapter explains that “Dispersion produces wet soils with very slow water infiltration,” and the Langdon page lists “very slow water infiltration” among the effects of dispersion.
How is sodicity measured?
The Franzen chapter names the sodium adsorption ratio (SAR) as “the overall measure of soil sodicity.” The Langdon page also names the exchangeable sodium percentage.
Can sodic soil be improved?
The Langdon page says to “Supplement calcium-based soil amendments to replace excessive sodium in the soil” and to “Improve soil drainage.” It does not claim a guaranteed result, and this article does not give a timeline.
Sources
- Franzen, D. et al., “Chapter 8: Soil Sodicity,” NDSU Extension ebook, opened 2026-10-27: https://www.ndsu.edu/agriculture/sites/default/files/ebook/Franzen/PDFs/Franzen_Ch8.pdf
- NDSU Extension, “Sodic Soils,” Langdon Research Extension Center soil health page, opened 2026-10-27: https://www.ag.ndsu.edu/langdonrec/archive/soil-health/sodic-soils
