Presentation on theme: "Soil Structure: The Roles of Sodium and Salts"— Presentation transcript:
1 Soil Structure: The Roles of Sodium and Salts Dr. Jim WalworthDepartment of Soil, Water and Environmental ScienceUniversity of ArizonaAZ 1414July 2006
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 ParticlesFlocculated Particles
3 Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.In A horizons, where organic matter levels are high and there is a lot of biological activity (earthworms, ants, termites, microbes, etc.) particles tend to be arranged in small, round aggregates or granules.This type of structure is common in the surface horizons of many forest and prairie soils
4 In all but the sandiest soils, dispersed clays plug soil pores and impede water infiltration and soil drainage.In A horizons, where organic matter levels are high and there is a lot of biological activity (earthworms, ants, termites, microbes, etc.) particles tend to be arranged in small, round aggregates or granules.This type of structure is common in the surface horizons of many forest and prairie soils
5 Most clay particles have a negative electrical charge Most clay particles have a negative electrical charge. Like charges repel, so clay particles repel one another.Negatively charged clay particleNegatively charged clay particleHere is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) contains dissolved cations and anions. The concentration of cations is much greater close to the particle surface than in the bulk soil solution. The cations are not bonded to the clay, but just attracted to the surface.Conversely anions are repelled by negatively charged clays, so the concentration of anions is greater in the bulk soil solution than close to a clay particle.
6 A cation is a positively charged molecule 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).+Here is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) contains dissolved cations and anions. The concentration of cations is much greater close to the particle surface than in the bulk soil solution. The cations are not bonded to the clay, but just attracted to the surface.Conversely anions are repelled by negatively charged clays, so the concentration of anions is greater in the bulk soil solution than close to a clay particle.Negatively charged clay particleNegatively charged clay particle
7 Relative Flocculating Power Flocculating CationsWe can divide cations into two categoriesPoor flocculatorsSodiumGood flocculatorsCalciumMagnesiumIonRelative Flocculating PowerSodiumNa+1.0PotassiumK+1.7MagnesiumMg2+27.0CalciumCa2+43.0Sumner and Naidu, 1998
8 Flocculating Power of Cations Cations in water attract water molecules because of their charge, and become hydrated.Water molecule is polar: (+) on one end, (-) on the other end(+)(-)Hydrated cation+Cations with a single charge and large hydrated radii are the poorest flocculators.CationCharges per moleculeHydrated radius (nm)Relative flocculating powerSodium10.791.0Potassium0.531.7Magnesium21.0827.0Calcium0.9643.0
9 Sodium Adsorption Ratio The ratio of ‘bad’ to ‘good’ 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/LSAR =[Na+][Ca2+] + [Mg2+]
10 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/mSoil with a high EC is salty; soil with a low EC is not.
11 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++++++++++++++++++++++SARECLower ECHigher ECFlocculated soilDispersed soil
12 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++ECFlocculated soilDispersed soil
13 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++EC+++Flocculated soilDispersed soil
14 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.Na+SARECCa2+ and Mg2+Lower ECHigher EC+++Flocculated soilDispersed soil
15 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+ECSAR+++Lower ECHigher ECFlocculated soilDispersed soil
16 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”.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++++++++++++++++++++++++++ECLower ECHigher ECCa2+ and Mg2+Flocculated soil++++++Na+SAR+EC+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.+Lower ECHigher ECDispersed soil
17 Soil Classification EC SAR Condition Normal <4 <13 Flocculated Soils can be classified by the amount of soluble salts (EC) and sodium status (SAR). This classification can tell us something about soil structure.Soil ClassificationECSARConditionNormal<4<13FlocculatedSaline>4Sodic>13DispersedSaline-Sodic
18 Observe your soil - sodic soils often crack when dry
19 Increasing soluble calcium improves aggregate stability in soils with poor structure. GypsumNa+CaSO4+Ca2+SO42-SAR++++EC++++++++++++++++Flocculated soilDispersed soil
20 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++Ca2+SO42-Here is a schematic representation of sodic soil reclamation.
21 Sulfuric acid* can be used instead of gypsum on calcareous (CaCO3 containing) soil only. Sulfuric acid dissolves calcium carbonate in the soiland makes gypsum!*Sulfuric acid is extremely dangerous and should only be handled by trained personnel.
22 Soil microbes convert sulfur into sulfuric acid Elemental sulfur can also be used as an alternative to gypsum on calcareous soilsSoil microbes convert sulfur into sulfuric acidH2SO4 dissolves calcium carbonate and makes gypsumConversion to sulfuric acid takes timeseveral weeksfaster in warm soils
23 Manage soil structureBe 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.
24 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, James A. Christenson, 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.