Soil Salinity/Sodicity/Alkalinity and Nutrients Section E SWES 316

Definition Salt-Affected Soil: Any soil containing sufficient quantities of soluble salts or sodium to cause adverse effects to plants or soil Saline Sodic Saline-Sodic

Definition Alkaline Soil: A soil with pH >7.0. Commonly, soil alkalinity is found in areas with limited soil weathering. Moderately alkaline pH 7-8. 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.

Soil Salinity Soils in arid regions commonly have “excessive” concentrations of soluble salts because: Lack of leaching to remove salts Poor Drainage Salts added in irrigation water

Soluble Salts Common soluble cations found in saline soils: Mg2+ Na+ K+ Common soluble anions found in saline soils: Cl- SO42- HCO3- /CO32- NO3-

Saline Soils 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 saline, the EC, exchangeable Na, and CEC must be known. Note: these numbers are somewhat arbitrary.

Measurements EC is measured in a saturated paste extract. The soil is saturated, extracted and the EC is measured with a conductivity meter.

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)

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

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

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

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 particle Negatively charged clay particle 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.

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 particle Negatively charged clay particle

Relative Flocculating Power Flocculating Cations We can divide cations into two categories Poor flocculators Sodium Good flocculators Calcium Magnesium Ion Relative Flocculating Power Sodium Na+ 1.0 Potassium K+ 1.7 Magnesium Mg2+ 27.0 Calcium Ca2+ 43.0 Sumner and Naidu, 1998

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. Cation Charges per molecule Hydrated radius (nm) Relative flocculating power Sodium 1 0.79 1.0 Potassium 0.53 1.7 Magnesium 2 1.08 27.0 Calcium 0.96 43.0

Dispersion Clay with only exchangeable Ca2+, clay particles can approach closely, promoting flocculation Clay with exchangeable Ca2+ and Na+, clay particles cannot approach closely, causing dispersion Ca2+ Na+

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/L SAR = [Na+] [Ca2+] + [Mg2+]

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 Capacity SAR and ESP are approximately equal numerically

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.

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 EC Lower EC Higher EC Flocculated soil Dispersed soil

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 ++ 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 + + EC + + + Flocculated soil Dispersed soil

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+ SAR EC Ca2+ and Mg2+ Lower EC Higher 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+ EC SAR + + + Lower EC Higher EC Flocculated soil Dispersed soil

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 ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ EC Lower EC Higher EC Ca2+ 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 EC Higher EC Dispersed soil

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 Classification EC SAR Condition Normal <4 <13 Flocculated Saline >4 Sodic >13 Dispersed Saline-Sodic

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.

Properties of Sodic Soils Sodic soils typically: Are poorly-aggregated (sodium disperses clays) Have slow rates of water infiltration Have a pH of 8 or above . This is due to the presence of soluble Na2CO3.

Saline-Sodic Soils 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 saline-sodic, the EC, exchangeable Na, and CEC must be known.

Salts Affect Soil Structure Based on irrigation water analysis Sodium level (SAR) Poor soil structure Good soil structure This 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.

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 soils Sodium 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.

Salt Effects on Plants Excess 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

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.

Soil Alkalinity and Nutrients Soil pH >7.5 Alkalinity is specifically associated with: Sodic soils Calcareous soils Soils high in soluble carbonates Saline soils may or may not be alkaline.

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 10-100x for every 1 pH unit increase. B: availability decreases at pH >7.

Treatment for Saline Soils Amendments for removing salts from soils: Nothing Management Practices Adequate Leaching Maintain soil drainage through proper tillage

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.

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 84000 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.

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.

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.

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

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+

Gypsum Application

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 + CO2 Can be applied to soil or water-run Rates are commonly 1-3 tons/acre

Sulfuric acid* can be used instead of gypsum on calcareous (CaCO3 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.

Constant H2SO4 injection keeps water pH low and prevents formation of CaCO3 in the drip lines, and also dissolves some CaCO3 in the soil, helping to maintain high exchangeable Ca2+ and low exchangeable Na+.

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

Soil microbes convert sulfur into sulfuric acid 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

Soil Amendments (4) Nitro-Sul (Ammonium Polysulfide) 20% NH4-N, 40-45% sulfur Causes release of acidity after microbial oxidation In some, but not all, cases applications increase rate of water infiltration Relatively expensive as a source of N or S

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

Soil Amendments (6) N-Phuric (urea + sulfuric acid) 10-28% N, 9-18% S Safer way to use sulfuric acid Releases acidity Good for drip or microsprinkler systems