The University of Arizona Cooperative Extension 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 ParticlesFlocculated Particles

The University of Arizona Cooperative Extension Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.

The University of Arizona Cooperative Extension In all but the sandiest soils, dispersed clays plug soil pores and impede water infiltration and soil drainage.

The University of Arizona Cooperative Extension Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another. Negatively charged clay particle

The University of Arizona Cooperative Extension A cation is a positively charged molecule. Common soil cations include sodium (Na + ), potassium (K + ), magnesium (Mg 2+ ), and calcium (Ca 2+ ). Cations can make clay particles stick together (flocculate). Negatively charged clay particle +

The University of Arizona Cooperative Extension Flocculating Cations We can divide cations into two categories –Poor flocculators Sodium –Good flocculators Calcium Magnesium Ion Relative Flocculating Power SodiumNa PotassiumK+K+ 1.7 MagnesiumMg CalciumCa Sumner and Naidu, 1998

The University of Arizona Cooperative Extension 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. Cation Charges per molecule Hydrated radius (nm) Relative flocculating power Sodium Potassium Magnesium Calcium Water molecule is polar: (+) on one end, (-) on the other end (+) (-) (+) Hydrated cation +

The University of Arizona Cooperative Extension Sodium Adsorption Ratio The ratio of bad to good flocculators gives an indication of the relative status of these cations: Na Ca 2+ and Mg Mathematically, this is expressed as the sodium adsorption ratio or SAR: where concentrations are expressed in mmoles/L SAR = [Na + ] [Ca 2+ ] + [Mg 2+ ]

The University of Arizona Cooperative Extension Electrical Conductivity Ions in solution conduct electricity, so the total amount of soluble soil ions 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 Soil with a high EC is salty; soil with a low EC is not.

The University of Arizona Cooperative Extension Ca 2+ and Mg 2+ Na + SAR EC Aggregate stability (dispersion and flocculation) depends on the balance (SAR) between (Ca 2+ and Mg 2+ ) and Na + as well as the amount of soluble salts (EC) in the soil. Flocculated soil Dispersed soil Lower ECHigher EC

The University of Arizona Cooperative Extension Na + SAR EC Soil particles will flocculate if concentrations of (Ca 2+ + Mg 2+ ) are increased relative to the concentration of Na + (SAR is decreased). Flocculated soil Dispersed soil Ca 2+ and Mg 2+ ++

The University of Arizona Cooperative Extension Na + SAR EC Flocculated soil Dispersed soil Ca 2+ and Mg Soil particles will disperse if concentrations of (Ca 2+ + Mg 2+ ) are decreased relative to the concentration of Na + (SAR is increased)

The University of Arizona Cooperative Extension 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 Na + SAR EC Ca 2+ and Mg 2+ Lower ECHigher EC

The University of Arizona Cooperative Extension Soil particles may disperse if the amount of soluble salts in the soil is decreased (i.e. if EC is decreased). Ca 2+ and Mg 2+ Na + SAR EC Lower EC Flocculated soil Dispersed soil Higher EC

The University of Arizona Cooperative Extension 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. Ca 2+ and Mg 2+ Na + SAR EC Lower EC Dispersed soil Higher EC Higher EC Flocculated soil Na + SAR EC Ca 2+ and Mg 2+ Lower EC If soils are irrigated with clean water (with low EC), soil EC will decrease, which can destabilize aggregates. Irrigation water will infiltrate slowly. 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.

The University of Arizona Cooperative Extension Soil ClassificationECSARCondition Normal<4<13Flocculated Saline>4<13Flocculated Sodic<4>13Dispersed Saline-Sodic>4>13Flocculated Soils can be classified by the amount of soluble salts (EC) and sodium status (SAR). This classification can tell us something about soil structure.

The University of Arizona Cooperative Extension Observe your soil - sodic soils often crack when dry

The University of Arizona Cooperative Extension Na + SAR EC Increasing soluble calcium improves aggregate stability in soils with poor structure. Flocculated soil Dispersed soil Ca Gypsum CaSO 4 SO 4 2-

The University of Arizona Cooperative Extension 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. Na Ca ++ Ca 2+ SO Na +

The University of Arizona Cooperative Extension Sulfuric acid * can be used instead of gypsum on calcareous (CaCO 3 containing) 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.

The University of Arizona Cooperative Extension Elemental sulfur can also be used as an alternative to gypsum on calcareous soils Soil microbes convert sulfur into sulfuric acid –H 2 SO 4 dissolves calcium carbonate and makes gypsum Conversion to sulfuric acid takes time –several weeks –faster in warm soils

The University of Arizona Cooperative Extension 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.