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Sodium can be abundant in soil water or occupy exchange sites on clay and organic matter. When exchangeable sodium weakens attraction between particles, clay may disperse, pores can seal and hydraulic conductivity can fall. Yet the response also depends on total electrolyte concentration, clay mineralogy and wetting history. Sodicity is therefore a coupled chemical and structural problem, not a colour or taste label.
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Chapter contents
Choose the route closest to the decision, or read from Chapter 1. The chapters remain open; these grouped routes are a navigable index.
Route 1 · Define the decision
Route 2 · Work the ledgers
Route 3 · Transfer the method
Route 4 · Check limits and evidence
Route 5 · Ask and continue
CHAPTER 1 OF 12 · DEFINE THE DECISION
1. Separate sodicity from salinity
Salinity describes the concentration of soluble salts, commonly assessed through electrical conductivity. Sodicity concerns the relative importance of sodium in the soil solution or on exchange sites.
A soil can be saline without being strongly sodic, sodic without remaining highly saline, or both. Leaching salts from a sodic soil can sometimes expose dispersion by lowering electrolyte concentration before exchangeable sodium has been corrected.
CHAPTER 2 OF 12 · DEFINE THE DECISION
2. Define exchangeable sodium percentage
Exchangeable sodium percentage is ESP=100×(exchangeable Na)/(cation exchange capacity), with numerator and denominator expressed on compatible charge bases. It describes occupancy of measured exchange sites.
ESP is not the percentage of total soil mass that is sodium. The value depends on the CEC method, pH conditions and sampling depth, so report the laboratory procedure.
CHAPTER 3 OF 12 · WORK THE LEDGERS
3. Work an ESP calculation
A horizon has CEC 20 cmolc/kg and exchangeable sodium 2.4 cmolc/kg. Its ESP is 100×2.4/20=12%.
| Quantity | Value | Role |
|---|---|---|
| CEC | 20 cmolc/kg | Total measured exchange capacity |
| Exchangeable Na | 2.4 cmolc/kg | Sodium occupying exchange sites |
| ESP | 12% | Relative exchange-site sodium |
The number alone does not predict a universal structural outcome. A second soil with different clay mineralogy and organic matter can respond differently at the same ESP.
CHAPTER 4 OF 12 · WORK THE LEDGERS
4. Calculate a solution-side sodium adsorption ratio
SAR compares sodium with calcium and magnesium in the soil solution: SAR=Na/√((Ca+Mg)/2), with concentrations conventionally in mmolc/L. If Na=8, Ca=4 and Mg=4, SAR=8/√4=4.
| Solution ion | Concentration (mmolc/L) | Structural role |
|---|---|---|
| Na | 8 | Promotes diffuse-layer expansion when dominant |
| Ca | 4 | Divalent counter-ion supporting flocculation |
| Mg | 4 | Divalent counter-ion, with context-dependent effects |
SAR is convenient for water and saturation extracts; ESP describes exchange sites. They are related through equilibria but should not be substituted without a justified model.
CHAPTER 5 OF 12 · WORK THE LEDGERS
5. Trace the dispersion mechanism
Clay surfaces carry charge balanced by counter-ions. Sodium is monovalent and often permits a more expanded diffuse double layer than calcium at comparable conditions. Repulsive interactions can then overcome attractive forces, releasing clay particles.
Dispersed particles can migrate into pore necks and form surface seals. The result may be lower infiltration, lower saturated hydraulic conductivity, crusting and turbid drainage water.
CHAPTER 6 OF 12 · TRANSFER THE METHOD
6. Transfer the test across electrolyte concentration
Take two aggregates from the same sodic horizon. Immerse one in low-electrolyte rainwater and the other in a more concentrated calcium-bearing solution. The first may disperse more strongly even though its sodium inventory is unchanged.
The changed-context check is to record water chemistry. A jar test using distilled water cannot be assumed to reproduce field irrigation water, and a stable aggregate under saline water may disperse after salts are leached.
CHAPTER 7 OF 12 · TRANSFER THE METHOD
7. Connect charge capacity with sodium occupancy
Use cation exchange capacity, variable charge and pH dependence to understand why the denominator in ESP can change with method and soil chemistry.
Exchange measurements are operational. Carbonates, gypsum, soluble salts and incomplete displacement can interfere, so pair the number with mineralogical and method information.
CHAPTER 8 OF 12 · TRANSFER THE METHOD
8. Design a field and laboratory evidence chain
Sample by horizon or management layer. Measure EC, pH, soluble cations, SAR, exchangeable cations, CEC, texture and organic carbon; observe dispersion, aggregate stability, infiltration and hydraulic conductivity.
A strong diagnosis connects chemistry to function: elevated exchangeable sodium, a dispersion response under relevant water chemistry and a matching pore or infiltration problem. Any one item alone has alternatives.
CHAPTER 9 OF 12 · CHECK LIMITS AND EVIDENCE
9. Know treatment logic and its limits
A calcium amendment can replace exchangeable sodium when it dissolves and water can transport the displaced sodium away. Drainage and water quality therefore matter as much as the amendment rate.
Adding gypsum without confirming calcium need, dissolution and drainage can waste material. Rapid leaching before replacement may reduce salinity and temporarily worsen dispersion.
CHAPTER 10 OF 12 · CHECK LIMITS AND EVIDENCE
10. Use authoritative checks and sources
Report depth, moisture history, EC method, extract ratio, SAR units, exchange method, mineralogy, amendment history and water chemistry. Repeat functional measurements after treatment rather than assuming chemical correction restored pore continuity.
The USDA NRCS National Soil Survey Handbook, Part 618 defines ESP and notes that elevated ESP can increase clay and organic-matter dispersion, with clay mineralogy affecting the response. The NRCS Soil Survey Field and Laboratory Methods Manual provides the standard SAR relationship and explains its use in salt- and sodium-affected soils.
CHAPTER 11 OF 12 · ASK AND CONTINUE
11. Frequently asked questions
Is every salty soil sodic?
No. Salinity and sodicity measure different chemical features and can occur separately.
Does one ESP threshold fit every soil?
No. Thresholds are useful screening conventions, while mineralogy, electrolyte concentration and organic matter modify structural response.
Will leaching alone solve sodicity?
Only if sodium is displaced and drainage can remove it without causing damaging dispersion along the way.
CHAPTER 12 OF 12 · ASK AND CONTINUE
12. Continue with a transfer check
Recalculate ESP after exchangeable sodium falls from 2.4 to 1.0 cmolc/kg while CEC stays 20. Then suppose the measured CEC rises to 25 under a different method and explain why method comparability matters.
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