lec10 acca iss soil alkalinity salinity sodicity
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Soil Alkalinity, Salinity, and Sodicity
Bryant Scharenbroch
Introduction to Soil ScienceACCA & The Morton Arboretum
Laboratory notebooks will be collected on Wednesday, 10/22/08
Alkalinity, salinity, sodicity
Review questions: Chapter 10: 5, 6, and 11
Review article: Savin et al. 2004 (Sarah)
Laboratory exercise: pH and EC
Soil Ecology
Review questions: Chapter 11: 6, 7, and 9
Review article:Gregorich et al. 2006 (Kevin)
Laboratory exercise: CEC
Today Next time
Arid soils
Soils of dry regionsOrders: Aridisols, Entisols, Mollisols, Alfisols, and Vertisols
Suborders: Ustic, Xeric, and Natric (SAR>15)
Water-limited, high pH, carbonate-rich, salt-rich, and Na-rich
Arid region agriculture40% of world’s cultivated land in dry regions, but most dry lands are uncultivated deserts and rangelands
Irrigation only possible on small fraction
Tillage practices (removing unwanted vegetation, fallow periods, etc.) to increase infiltration
Stubble mulch and no-till to reduce evaporation and erosion
Islands of fertility
(serc.carlton.edu)
SOMNutrients
AggregationSurface biopores
infiltration
NPP
RestitutionOrganismsWind traps
Desert pavementThin (single) layer of rock fragments (coarse gravel to cobbles) via wind erosion or shrink-swell and uplifting
>65% surface coverage is considered desert pavement
Provides protection from wind erosion and traps additional soil particles
Increased pavement = decreased infiltration
Impacts plant water availability and salt leaching
Creates water sinks (high infiltration and low runoff) Creates water sources (low infiltration and high runoff)
(USGS)
Microbial crustsThick, dark, jagged coating
Algae, fungi, and cyanobacteria
Fragile
Reduce wind and water erosion
Rough types increase infiltration
Smooth types decrease infiltration
(serc.carlton.edu)
Calcium-rich layers
Cl: Low precipitation
P: Calcareous soil materials (with free Ca carbonates)
Conditions may induce P or micronutrient deficiency
Cementation (e.g., petrocalcic or duripans)
Depth of calcic and gypsichorizons determined by depth of leaching
Stony Gypsisol, Israel, with desert pavement;a massive petrogypsic horizon occurs near to the surface (Yermi-Epipetric Gypsisol)
Calcic horizon in a Mollisol
Saline seeps
Emergence of saline groundwater via cultivation
ReducedEVT
Increasedpercolation
Removal ofdeep-rooted
plants
Watertablerise
Increasedsalts left via E
Alkaline soils
Alkalinity
EVT > PPT
Accumulation of cations (K, Na, Ca, and Mg) released from weathering
Base cations are non-hydrolyzing and do not produce H upon reacting with water like Fe or Al (pH 7) pH > 7 as a result of OH reactions
Hydroxyl generating anions carbonate (CO32-) and bicarbonate (HCO3
-) from calicite of carbonic acid react with water to form OH ions, thus increase pH
Reactions go to left with increasing biological activity and to the right with increasing calcite dissolution
Problems with alkaline soils?
Problems with alkaline soils
1. Nutrient deficiencies2. Clay dispersion
Zn, Cu,Fe, and Mn
Insoluble at high pH
Often requires irrigation and chelation
Systemic or foliar application
Tree Fe deficiencies
http://www.canr.msu.edu/vanburen/ffc14.jpgArborjet
B, Mo,and Cu
Inner-sphere complexation of B increases with pH
Mo availability increases with pH to toxic levels
Molybdenosis in ruminant animals as a result of high Mo and low Cu
P
Dissolved Ca and Mg ions constrain dissolution of P carrying minerals
Soluble P reacts with Ca to form insoluble Ca-P
Fungi, bacteria, and Brassica plants excrete organic acids to dissolve Ca-P
What is CEC of alkaline soils?
CEC of alkaline soils
Higher CEC than acid soils (assuming similar texture and SOM)
2:1 type clays common in alkaline soils have high permanent charge (e.g., smectite)
High pH will stimulate high levels of pH-dependent charge
What is soil clay dispersion?
Soil clay dispersionSlaking and aggregate destructionReduced macroporosityReduced aerationReduced percolationSurface sealing (crusts)
http://www.agric.wa.gov.au/ikmp/images/F05790a.GIF
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
(Jim Walworth, U. Ariz)
Flocculation is important because water moves mostly in large pores between aggregates. Also, plant roots grow mainly between aggregates.
(Jim Walworth, U. Ariz)
In all but the sandiest soils, dispersed clays plug soil pores and impede water infiltration and soil drainage.
(Jim Walworth, U. Ariz)
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
(Jim Walworth, U. Ariz)
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
++
(Jim Walworth, U. Ariz)
Flocculating Power of CationsCations in water attract water molecules because of their charge, 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 1 0.79
0.53
1.08
0.96
1.0
Potassium 1 1.7
Magnesium 2 27.0
Calcium 2 43.0
Water molecule is polar: (+) on one end, (-) on the other end
(+)
(-)
(+) Hydrated cation +
(Jim Walworth, U. Ariz)
Ca2+ and Mg2+Na+
SAR
EC
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.
Flocculated soil
Dispersed soil
++++++
++++
++
+ + +++++
+
+
Lower EC Higher EC
(Jim Walworth, U. Ariz)
Na+
SAR
EC
Soil particles will flocculate if concentrations of (Ca2+ + Mg2+) are increased relative to the concentration of Na+ (SAR is decreased).
Flocculated soil
Dispersed soil
+
++
Ca2+ and Mg2+
++
++++++++
++++
++++++
(Jim Walworth, U. Ariz)
Na+
SAR
EC
Flocculated soil
Dispersed soil
++
+
Ca2+ and Mg2+
++++++
Soil particles will disperse if concentrations of (Ca2+ + Mg2+) are decreased relative to the concentration of Na+ (SAR is increased).
++
+
+
(Jim Walworth, U. Ariz)
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
Ca2+ and Mg2+
Lower EC Higher EC
++
++
+
+
+
++
++++++++
++++
++++++
(Jim Walworth, U. Ariz)
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
EC
Lower EC
Flocculated soil
Dispersed soil
Higher EC
++++
++
++
+
(Jim Walworth, U. Ariz)
Why are clays in alkaline soils subject to dispersion?
Alkaline clay dispersion
Al and Fe (acid soils), which are strong flocculating/cementing agents are lacking
Monovalent cations (K and Na) are good at dispersion and not leached from alkaline soils
Salinization
What is salinization?
SalinizationProcesses that result in the accumulation of neutral soluble salts
PPT/EVT of 0.75
Low, flat areas and high water tables
Soluble salts (chlorides and sulfates of Ca, Mg, Na, and K) from weathering are moved from wetter to drier areas (up profile and across landscape)
Salts are left as water evaporates
Salinization16 million ha (increasing 10% annually)1/3 of US arid soils are salt impacted
How do salts affect plants?
Salt impacts on plants
Salts lower the osmotic potential of soil solution; thus plants must respond by lowering root osmotic potential
Na, Cl, H3BO4-, and HCO3
- toxicity
Dispersion and puddling leading to reduced aeration and water-logging
Will soluble salts increase or decrease pH?
Soluble saltsNeutral salts (CaSO4, Na2SO4, NaCl, and CaCl2) will tend to lower pH by moderating alkalizing reactions due to the common ion effect
A salt will be less soluble if one of its constituent ions is already present in the solution
More common ions will reduce dissolution of carbonates
Which is more likely to increase salts, over- or under-irrigation?
Which is more likely to increase salts, over- or under-irrigation?Over-irrigation
Input of salts > output of drainage water
Increased EVT and raise water table
90 cm of water applied may deposit 6 Mg ha-1
(3 ton ac-1)
Salinization of the Fertile crescent (SE Iraq): poorly drained and irrigated with water from the Euphrates converted productive land to be barren wastelands
How to measure salinity?
Total dissolved solids
Evaporate water and weigh solids
Irrigation water (5 to 1,000 mg L-1)
Soil extract (500 to 12,000 mg L-1)
EC
Pure water is a poor conductor of electricity, and conductivity increases with salts
Electrical conductivity is an indirect measure of salt
EC > 2 dS m-1 will adversely affect sensitive plants
EC > 4 dS m-1 will adversely affect most plants
Electromagnetic inductionMeasures electrical current in the body of soil, related to EC, thus salt
Magnetic field is generated and generated currents are measured
Measures to depth and without disturbance
(EM38 in Sudduth et al. 2003)
Exchangeable Na percentage
Relates Na on CEC
ESP = exchangeable Na (cmolc kg-1) x 100CEC (cmolc kg-1)
ESP > 15 are associated with deteriorated soil physical properties
Sodium adsorption ratio
Compares Na relative to Ca and Mg
SAR = [Na+] / (0.5[Ca2+] + 0.5[Mg2+])1/2
SAR of 13 = ESP of 15
K could be included with Na
SAR
The ratio of ‘bad’ to ‘good’ flocculators gives an indication of the relative status of these cations:
Na+++
+ + ++
+
Ca2+ and Mg2+++
++++++++++
++
(Jim Walworth, U. Ariz)
Saline soils “white alkali”EC of > 4 dS m-1
ESP of < 15 (SAR < 13)
CEC is with Ca and Mg, not Na
pH is usually below 8.5
Plant growth hindered by salts
Non-Na salts prevent dispersion
Infiltration, aggregate stability, and aeration are not problematic
Referred to as white alkali soils, due to white salt crust
http://sis.agr.gc.ca/cansis/taxa/soil/regosolic/saline_pr_sl.jpg
Saline-sodic soilsEC of > 4 dS m-1
ESP of > 15 (SAR < 13)
Plant growth hindered by salts and Na
Intermediate dispersion, infiltration, aggregate stability, and aeration
But, neutral salts contribute cations that move in close to CEC, thus reducing dispersion
If soils are leached of salts, they will become sodic
L. MacDougal
Sodic soils“black alkali”
EC of < 4 dS m-1
ESP of > 15 (SAR < 13)
pH > 8 to 10 (Na more soluble than Ca carbonate, so high carbonate, bicarbonate, OH in solution)
Plant growth hindered by excess Na, OH, and HCO3
-
Main detriment is the dispersion and subsequent decreased infiltration, aggregate stability, and aeration
SOM will disperse, dissolve, and move up in capillary flow; thus black alkali or slick spot nomeclature
soer.justice.tas.gov.au/2003/image/548/index.php
How to assess structure degradation?
Hydraulic conductivity
Readiness of water movement
KSAT is so low for sodic soils that infiltration rate is zero
Causes of low KSAT of sodic soils
Exchangeable Na increases tendency for aggregates to slake; and slaked particles clog pores
Na increases swelling of expandable 2:1 clays; thus macropores are squeezed shut
High Na and low salt contents leads to dispersion;thus clay particles exist in a gel-like condition
Which soil will be more puddled?
EC of 5 dS m-1 and ESP of 16
EC of 1 dS m-1 and ESP of 16
Puddling
Low salt concentration and high Na content will encourage dispersion and increase puddling
Soil dispersion (high Na, low salt)
Na has a single charge and large hydrated radius; thus Na swarm
Soil dispersion (high Na, low salt)
High salts: cationsstay closer and anions come closer to clay; ionic swarm is compressed; flocculation
Low salts: cationsdiffuse away from clay, anions move from clay; ionic swarm is expanded; dispersion
De-icing saltsImpact is usually temporary as long as drainage is adequate
KCl or sand are better options than NaCl
“halophyte exotic” seaside goldenrod
Reclamation of saline soils?
Reclamation of saline soils
Leach soils with low salt irrigation water
Leaching ratioLeaching ratio indicates minimum amount of water to be leached through wet soil and meet plant EVT needs; it is multiplied by the water added
If EC in water is high (high ECiw) and the plant has a low salt tolerance (low ECdw); it will require more water to leach
LR = ECiw / ECdw
Leaching ratio is figured from a salt balance (Siw + Sp + Sf + Sm = Siw + Sp + Sf + Sm)
(irrigation water, deposition, fertilizers, mineral weathering = drainage water, crop removal, chemical precipitation of carbonates and sulfates)
Reclamation of saline soils
LR is problematic because it does not take into account water table rise, etc.; thus, direct measurements of EC or EM throughout the profile are preferred
Reclamation of saline soils
Deep-rooted vegetation to lower the water table and reduce upward movement of salts
Should we leach saline-sodicsoils for reclammation?
Reclamation of saline-sodic soils
If simply leach a saline-sodic soil, will reduce soluble salts and they will likely become sodic (Na and pH increase)
First reduce Na, then deal with salts
Reducing NaReplace with Ca (gypsum, tons ha-1)
Deep-rooted plants will increase gypsum percolation
Reducing NaS yielding sulfuric acid, changes sodium bicarbonate to leachablesodium sulfate and also decrease pH
Review question 10.5
Review question 10.5Sounds like a sodic soil
Irrigation water likely has SAR indicating high levels of sodium compared to calcium and magnesium; thus stimulating a SAR in the soil solution, a higher pH, and a higher exchangeable Na level
Consequently aggregate stability was likely reduced when colloids become highly sodium-saturated. The high pH and Na levels, along with poor soil structure, result in reduced plant growth
Review question 10.6
Review question 10.6
Additions of gypsum, elemental sulfur or H2SO4 can replace the exchangeable Na+ with Ca2+ or H+ ions, and irrigation water can be used to leach the Na+
ions from the soil
Growth of Na- tolerant plants such as barely, rye and clover can help open up root channels
Review question 10.11
Review question 10.11Change in ESP needed is from 30 to 4%, or a reduction of 26 percent
Calculate amount of Na+ ions to be replaced:
Multiply CEC by change in ESP = 25 cmolc/kg x 26/100 = 6.5 cmolc/kg soil
Calculate gypsum (CaSO4·2H20) needed to replace 6.5 cmolc of Na+ ions
Calculate mass of 1 cmolc gypsum required172 g gypsum x 1 mole x 1 molc = 172 = 0.86 g/cmolc
Mole 2 molc 100 cmolc 200
For 6.5 cmolc/kg: 0.86g/cmolc x 6.5 cmolc = 5.6g CaSO4 2H2O/kg soil
Express gypsum needed for 1 hectare 30 cm deep:
One hectare 15 cm deep weighs 2 x 106 kg5.6g gypsum/kg x 4 x 106 kg/hectare = 22.4 x 106g gypsum/haThis is 22.4 x 103 kg/ha or 22.4 Mg gypsum/ha
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