4 DefinitionSalt-Affected Soil: Any soil containing sufficient quantities of soluble salts or sodium to cause adverse effects to plants or soilSalineSodicSaline-Sodic
5 Definition Alkaline Soil: A soil with pH >7.0. Commonly, soil alkalinity is found in areas with limited soil weathering.Moderately alkaline pH Often but not always associated with the presence of CaCO3 in soils (calcareous)Highly alkaline pH >8. Often associated with the presence of excess exchangeable Na in soils.
6 Soil SalinitySoils in arid regions commonly have “excessive” concentrations of soluble salts because:Lack of leaching to remove saltsPoor DrainageSalts added in irrigation water
7 Soluble Salts Common soluble cations found in saline soils: Mg2+Na+K+Common soluble anions found in saline soils:Cl-SO42-HCO3- /CO32-NO3-
8 Saline SoilsDefinition: Have an electrical conductivity of a saturated paste extract (ECe) of >4 dS/m and an exchangeable sodium percentage (ESP) of <15%.So, to classify a soil as saline, the EC, exchangeable Na, and CEC must be known.Note: these numbers are somewhat arbitrary.
9 MeasurementsEC is measured in a saturated paste extract. The soil is saturated, extracted and the EC is measured with a conductivity meter.
12 Properties of Saline Soils Saline soils typically:Are well-aggregated (salts flocculate clays)Have a pH from 7 to 8 (usually occur in areas of limited soil weathering)Are often calcareous (contain calcium carbonate)
13 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
14 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
15 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
16 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.
17 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
18 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
19 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
20 Dispersion Clay with only exchangeable Ca2+, clay particles can approach closely,promoting flocculationClay with exchangeable Ca2+ and Na+,clay particles cannot approach closely,causing dispersionCa2+Na+
21 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+]
22 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 (cmol(+)/kg)Na+ESP =Cation Exchange CapacitySAR and ESP are approximately equal numerically
23 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.
24 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
25 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
26 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
27 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
28 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
29 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
30 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
31 Sodic Soils Are adversely affected by the presence of excess Na Definition: Have an electrical conductivity of a saturated paste extract (ECe) of <4 dS/m and an exchangeable sodium percentage (ESP) of >15%.So, to classify a soil as sodic, the EC, exchangeable Na, and CEC must be known.Note: these numbers are somewhat arbitrary.
32 Properties of Sodic Soils Sodic soils typically:Are poorly-aggregated (sodium disperses clays)Have slow rates of water infiltrationHave a pH of 8 or above . This is due to the presence of soluble Na2CO3.
33 Saline-Sodic SoilsDefinition: Have an electrical conductivity of a saturated paste extract (ECe) of >4 dS/m and an exchangeable sodium percentage (ESP) of >15%.So, to classify a soil as saline-sodic, the EC, exchangeable Na, and CEC must be known.
34 Salts Affect Soil Structure Based on irrigation water analysisSodium level (SAR)Poor soilstructureGood soilstructureThis is an illustration of the ability of sodium to destabilize soil. On the upper and left side of the graph, there is lots of sodium (measured by sodium adsorption ratio) but not enough total salts (represented by electrical conductivity) to keep soil particles aggregated. At the lower right, there are lower levels of sodium and more total salts, so soil structure is stable. Soil aggregates are stable if sodium levels are low and/or if total salt levels are high.
35 Salt and Sodium Risks Salinity is mostly harmful to plant growth. Most plants, especially crop plants, are sensitive to salts. The properties of the soils themselves can be improved by the presence of salts (flocculation).Sodium is harmful to plants and soilsSodium causes soils to have undesirable physical and chemical properties. Sodium can also cause toxicities to plants.Alkaline pH (esp. in sodic soils) can limit nutrient availability to plants.
36 Salt Effects on PlantsExcess soluble salts can be harmful to plant growth because:Salts lower the osmotic potential energy of soil water. Water is less available to plants.Some soluble salt ions can have specific toxic effects on plants, such as:Na+, Cl- , H3BO3
39 Soil Salinity and Nutrients Some specific effects of salinity on nutrients:High Na concentrations can inhibit Ca, Mg uptake by roots.Ion toxicity limits nutrient uptake, lowering nutrient requirements.High HCO3- can limit Ca availability.
41 Soil Alkalinity and Nutrients Soil pH >7.5Alkalinity is specifically associated with:Sodic soilsCalcareous soilsSoils high in soluble carbonatesSaline soils may or may not be alkaline.
42 Soil Alkalinity and Nutrients Specific Effects:pH dependent AEC decreases, and CEC increases as pH increases.Nintrogen: NH3 volatilization increases as pH increases.Phosphorus: P availability decreases at pH>6 due to Ca-P reactions.Fe, Mn, Cu, Zn: solubility decreases x for every 1 pH unit increase.B: availability decreases at pH >7.
43 Treatment for Saline Soils Amendments for removing salts from soils:NothingManagement PracticesAdequate LeachingMaintain soil drainage through proper tillage
45 Soil Amendments for Salinity and Sodium Control Soil amendments will not help with salinity control unless a sodium problem also exists.Amendment additions are necessary to correct sodium problems. Leaching alone is not enough.
46 Should Alkaline Soils be Acidified? It is rarely advisable to acidify soils to significantly lower pH:Amounts required may be enormous:A soil with 2% CaCO3 in the top 30 cm will contain kg CaCO3/ha. This would require about 93 tons H2SO4/ha to neutralize the CaCO3 .There is rarely an economic benefit to such large application rates.
47 So, what to do about alkaline soils? If soils are sodic and highly alkaline, use of gypsum and leaching will usually lower pH to <8.4.When pH is <8.4, micronutrient deficiencies in most crops are rare and manageable with foliar applications.
49 Soil Amendments (1) Gypsum (CaSO4.2H2O) the amendment most commonly used for controlling sodium problems. Can be soil-applied or water-run.Gypsum application rates for removing sodium are commonly 1 to 10 tons/acre, depending on soil and irrigation water properties.Gypsum will normally lower the pH of sodic soils, by replacing exchangeable Na+ and allowing Na2CO3 to be leached from soils.
50 Increasing soluble calcium improves aggregate stability in soils with poor structure. GypsumNa+CaSO4+Ca2+SO42-SAR++++EC++++++++++++++++Flocculated soilDispersed soil
51 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.Ca2+SO42-Ca++Ca++---------Ca++Ca++Na+Na+Na+-Here is a schematic representation of sodic soil reclamation.---Na+-----Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+Na+
53 Soil Amendments (2) Sulfuric Acid (H2SO4) In soils with free lime (calcareous), sulfuric acid is an effective amendment for correcting or preventing sodium problems: CaCO3 + H2SO4 --> Ca2+ + SO42- + H2O + CO2Can be applied to soil or water-runRates are commonly 1-3 tons/acre
54 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.
55 Constant H2SO4 injection keeps water pH low andprevents formation of CaCO3in the drip lines, and alsodissolves some CaCO3 inthe soil, helping to maintainhigh exchangeable Ca2+ andlow exchangeable Na+.
56 Soil Amendments (3) Elemental Sulfur 97% Sulfur Reaction: Effective acid-forming amendment: soil microorganisms use S to produce sulfuric acid. The sulfuric acid reacts with CaCO3 to release Ca.Requires microbial activity to react. May take months to react completely.Reaction:2 S + 3 O2 + 2 H2O 2 H2SO4
57 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
58 Soil Amendments (4)Nitro-Sul (Ammonium Polysulfide) 20% NH4-N, 40-45% sulfurCauses release of acidity after microbial oxidationIn some, but not all, cases applications increase rate of water infiltrationRelatively expensive as a source of N or S
59 Soil Amendments (5) Thio-Sul (Ammonium Thiosulfate) 12% NH4-N, 26% S Releases small amounts of acidity. Is used mostly as a fertilizer.Remember--a need for S is rare in Arizona soils
60 Soil Amendments (6) N-Phuric (urea + sulfuric acid) 10-28% N, 9-18% S Safer way to use sulfuric acidReleases acidityGood for drip or microsprinkler systems