Soil Structure: The Roles of Sodium and Salts

1. Soil Structure: The Roles of Sodium and Salts Dr. Jim Walworth Department of Soil, Water and Environmental Science University of Arizona AZ 1414 Revised 10/2011

2. Soil clay particles can be unattached to one another (dispersed) or clumped together (flocculated) in aggregates. Soil aggregates are cemented clusters of sand, silt, and clay particles. Dispersed Particles Flocculated Particles

3. Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.

4. In all but the sandiest soils, excess sodium causes clays to disperse. Dispersed clay plugs soil pores and impedes water infiltration and soil drainage.

5. Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another. Negatively charged clay particle Negatively charged clay particle

6. + A cation is a positively charged molecule. Common soil cations include sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+). Cations can make clay particles stick together (flocculate). Negatively charged clay particle Negatively charged clay particle

7. If we add the right cations to a dispersed soil the soil will flocculate and form aggregates

8. Flocculating Cations • We can divide cations into two categories • Weak flocculators • Sodium • Strong flocculators • Calcium • Magnesium Sumner and Naidu, 1998

9. (+) Water molecule is polar: (+) on one end, (-) on the other end Hydrated cation surrounded by water molecules (+) + (-) Flocculating Power of Cations Cations in water attract water molecules because of their charge, and become hydrated. Cations with a single charge and large hydrated radii are the poorest flocculators.

10. ++ ++ ++ ++ ++ ++ ++ [Na+] SAR = [Ca2+] + [Mg2+] + + + + + + + Sodium Adsorption Ratio The ratio of ‘weak’ to ‘strong’ flocculators gives an indication of the relative status of these cations: Na+ Ca2+ and Mg2+ Mathematically, this is expressed as the ‘sodium adsorption ratio’ or SAR: where concentrations are expressed in mmoles/L

11. + + + + + ++ ++ ++ ++ ++ ++ Exchangeable Sodium Percentage An alternative to SAR is ESP, Exchangeable Sodium Percentage Na+ - - - - - - - - - Ca2+ and Mg2+ Mathematically, this is expressed as the percentage of the CEC (cation exchange capacity) that is filled with sodium in units of charge per mass (cmolc/kg) Na+ ESP = Cation Exchange Capacity SAR and ESP are approximately equal numerically

12. Measuring Soil Salts Ions in solution conduct electricity, so the total amount of or soluble soil ions (salts) can be estimated by measuring the Electrical Conductivity (EC) of a soil water extract. EC is measured in units of conductance over a known distance: deci-Siemens per meter or dS/m High EC = salty soil Low EC = non-salty soil Soil salts can also be expressed as Total Dissolved Salts (TDS) in ppm TDS = EC (dS/m) x 640 - - - + + + + - + - - + + + - - - + - + + + + - + - + - - -

13. EC Lower EC Higher EC ++ ++ ++ ++ ++ ++ ++ Flocculated soil Dispersed soil + + + + + + + Aggregate stability (dispersion and flocculation) depends on the balance (SAR) between (Ca2+ and Mg2+) and Na+ as well as the amount of soluble salts (EC) in the soil. Na+ Ca2+ and Mg2+ SAR

14. + + + EC ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ Soil particles will flocculate if concentrations of (Ca2+ + Mg2+) are increased relative to the concentration of Na+ (SAR is decreased). Na+ Ca2+ and Mg2+ SAR Flocculated soil Dispersed soil

15. + + + + + + + EC ++ ++ ++ Flocculated soil Dispersed soil Soil particles will disperse if concentrations of (Ca2+ + Mg2+) are decreased relative to the concentration of Na+ (SAR is increased). Ca2+ and Mg2+ Na+ SAR

16. Na+ Ca2+ and Mg2+ SAR EC Lower EC Higher EC ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ + + + + + + + Soil particles will flocculate if the amount of soluble salts in the soil is increased (increased EC), even if there is a lot of sodium. Flocculated soil Dispersed soil

17. EC ++ ++ ++ Flocculated soil Dispersed soil + + + Soil particles may disperse if the amount of soluble salts in the soil is decreased (i.e. if EC is decreased). Ca2+ and Mg2+ Na+ SAR Lower EC Higher EC

18. If soils are close to the “tipping point” between flocculation and dispersion, the quality of irrigation water will influence aggregate stability. If irrigation water infiltrates, and rain water does not, this indicates that the soil is close to the “tipping point”. Soils irrigated with saline water (with high EC) will generally have good structure, and water will infiltrate rapidly. However, salts can accumulate and damage plants unless properly managed. Na+ If soils are irrigated with clean water (with low EC), soil EC will decrease, which can destabilize aggregates. Irrigation water will infiltrate slowly. + + + + + + + Ca2+ and Mg2+ SAR ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ EC ++ ++ Higher EC Lower EC Ca2+ and Mg2+ Flocculated soil ++ ++ ++ Na+ SAR + EC + + Higher EC Lower EC Dispersed soil

19. Soils can be classified by the amount of soluble salts (EC) and sodium status (SAR). This classification can tell us something about soil structure.

20. Observe your soil - sodic soils absorb water slowly and often crack when dry

21. + + + SAR EC ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ Controlling Na+ requires increasing concentrations of soluble Ca2+ to decrease soil or water SAR, thus favoring soil flocculation. Ca2+ and Mg2+ Na+ Flocculated soil Dispersed soil

22. + + + EC ++ ++ ++ ++ ++ ++ ++ ++ Increasing soluble calcium improves aggregate stability in soils with poor structure. Gypsum Na+ CaSO4 Ca2+ SO42- SAR ++ ++ Flocculated soil Dispersed soil

23. Ca2+ SO42- Ca++ Ca++ - - - - - - - - - Ca++ Ca++ Na+ Na+ Na+ - - - - Na+ - - - - - Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Apply gypsum before leaching salts out of soils susceptible to dispersion (the amount of gypsum needed can be determined by a soil test). Replacing sodium with calcium before leaching will stabilize soil structure.

24. Sulfuric acid* can be used instead of gypsum on calcareous (CaCO3containing) soil only. • Sulfuric acid dissolves calcium carbonate in the soil and makes gypsum! *Sulfuric acid is extremely dangerous and should only be handled by trained personnel.

25. Elemental sulfur can also be used as an alternative to gypsum on calcareous soils • Soil microbes convert sulfur into sulfuric acid • H2SO4 dissolves calcium carbonate and makes gypsum • Conversion to sulfuric acid takes time • several weeks • faster in warm soils

26. S + O2→ SO2 SO2 + H2O → H2SO3 (sulfurous acid) Alternatives to Gypsum:Sulfur burners Sulfur burners eliminate the need for handling dangerous sulfuric acid

27. Relative effectiveness of amendments for supplying calcium

28. Manage soil structure • Be aware of the quality of irrigation water. Water with high levels of sodium (high SAR) will tend to destabilize soil. • Have irrigation water analyzed for SAR and EC or ask your water provider for analyses. • If you have high sodium irrigation water, the water and/or the soil may need amendments such as gypsum or sulfuric acid. • Observe your soil. • If water infiltrates very slowly, or if rain water infiltrates more slowly than irrigation water, the soil may have a sodium problem. • Sodium impacted soils may noticeably crack when dry. • Analyze your soil. • Laboratory analysis can tell you the soil EC and SAR or ESP.

29. cals.arizona.edu/pubs/crops/az1414 Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Kirk A. Astroth, Interim Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.