Upload
brooklyn-shackley
View
271
Download
4
Tags:
Embed Size (px)
Citation preview
Class evaluationsClass evaluations
Soil ChemistrySoil Chemistry
Ion ExchangeIon Exchange
Ions adsorbed to soil surfaces can be Ions adsorbed to soil surfaces can be exchanged with ions in soil solution.exchanged with ions in soil solution.
Cations and anionsCations and anions
Ion exchangeIon exchange
Organic colloids and Organic colloids and inorganic micelles inorganic micelles (clays) (clays) are sites of are sites of ion exchangeion exchange
Where do ions in soil come from?Where do ions in soil come from?Release from organic matterRelease from organic matterRainRainWeathering of parent materialWeathering of parent material
Ion exchangeIon exchange
Exchangeable cationsExchangeable cations (on soil surfaces) (on soil surfaces) cannot be removedcannot be removed by leaching. by leaching.
Soluble cationsSoluble cations (in solution) (in solution) can be removed can be removed by by leaching.leaching.
When soil is dried…When soil is dried…
……exchangeable cations hold to adsorption sites exchangeable cations hold to adsorption sites on soil surfaces.on soil surfaces.
… …soluble cations (and anions) precipitate or soluble cations (and anions) precipitate or crystallize as salts.crystallize as salts.
Examples of soluble cations precipitatingExamples of soluble cations precipitating
Ion exchangeIon exchange
Exchangeable Exchangeable ions on soil surface ions on soil surface trading trading places with places with ions in solutionions in solution..
On soil surfaces, there are:On soil surfaces, there are:Exchangeable and Nonexchangeable IonsExchangeable and Nonexchangeable Ions
::
ExchangeableExchangeable: weakly held, in contact with soil : weakly held, in contact with soil solution, ready for quick replacement.solution, ready for quick replacement.““outer sphere complexouter sphere complex””
NonexchangeableNonexchangeable::““inner sphere complex”inner sphere complex”
adsorbed by strong bonds or held in inaccessible adsorbed by strong bonds or held in inaccessible places places (e.g., the K(e.g., the K++ between layers of illite) between layers of illite)
not part of ion exchange !not part of ion exchange !
Cation exchange capacity (CEC)Cation exchange capacity (CEC)
Sum total of exchangeable cations that a Sum total of exchangeable cations that a soil can adsorbsoil can adsorb. ( prevents nutrients from . ( prevents nutrients from leaching away from roots)leaching away from roots)
CECCEC
Expressed in:Expressed in:
milliequivalents per 100 g (meq/100g)milliequivalents per 100 g (meq/100g)
Base saturationBase saturation
% of exchange sites occupied by % of exchange sites occupied by basic basic cationscations
Basic cations are cations Basic cations are cations other than Hother than H++ and and AlAl+3+3
Base saturation Base saturation
+ H+ ion saturation+ H+ ion saturation
should equal 100%should equal 100%
For midwest US soilsFor midwest US soils
Notice neutral pH (7.0) Notice neutral pH (7.0) requires a base sat requires a base sat of 80%.of 80%.
(neutral pH is not 50% (neutral pH is not 50% because most base because most base cations have a + charge cations have a + charge of 2)of 2)
equilibriumequilibrium
Strive for equivalent proportions of Strive for equivalent proportions of solutionsolution and and exchangeableexchangeable ions. ions.
Upset equilibrium by:Upset equilibrium by:
removal by plantsremoval by plants
leachingleaching
fertilizationfertilization
weatheringweathering
Initiate ion exchange
Ion exchange example:Ion exchange example:Add HAdd H++ ions to soil : ions to soil :
soil
Ca+
Ca+
Ca+Ca+
Ca+
Ca+Ca+
Ca+
Ca+Ca+
Ca+Ca+
Ca+
Ca+
+
H+
H+
H+H+
H+H+
solution
exchangeable solution
+
H+
H+Ca+
Ca+
exchangeable solution
Rules of ion exchangeRules of ion exchange
Process is Process is ReversibleReversible
Charge by chargeCharge by charge basis basis
Ratio LawRatio Law: : ratio of exchangeable cations will be ratio of exchangeable cations will be
same as ratio of solution cations same as ratio of solution cations
Add K fertilizer…Add K fertilizer…
Ca+2
K+
Ca+2
+Ca+2 Ca+2K+
K+
K+ K+K+
K+
K++
1 Ca : 2 K 1 Ca : 2 K
Same ratio
Energy of adsorptionEnergy of adsorption
Strong --------------------------------------WeakStrong --------------------------------------Weak
AlAl+3 +3 > Ca> Ca+2+2 > Mg > Mg+2+2 > [K > [K++ = NH = NH44+ + ] > Na] > Na+ + > H> H++
(based on charge and hydrated radius)(based on charge and hydrated radius)
Soil pH importanceSoil pH importance
Determines solubility Determines solubility of nutrientsof nutrients Before plants can get Before plants can get
nutrients, they must be nutrients, they must be dissolved in soil dissolved in soil solutionsolution
Microbial activity also Microbial activity also depends on pHdepends on pH
pHpH
negative log of the hydrogen ion concentrationnegative log of the hydrogen ion concentration
(also a measure of OH(also a measure of OH-- concentration) concentration)
If If HH++ concentration > OHconcentration > OH-- : : acidicacidic
If If OHOH-- > H > H++ : : basicbasic
Soil pH is pH of solution, NOT exchange complex
General soil pH conditions:General soil pH conditions:
““Slightly acid” Slightly acid”
6.0 – 6.66.0 – 6.6
““Moderately acid”Moderately acid”
5.0 – 6.05.0 – 6.0
““Strongly acid”Strongly acid”
< 5.0< 5.0
““Slightly basic”Slightly basic”
7.4 – 8.07.4 – 8.0
““Moderately basic”Moderately basic”
8.0 – 9.08.0 – 9.0
““Strongly basic”Strongly basic”
> 9.0> 9.0
In soil, both In soil, both HH++ and and AlAl+3+3 ions produce acidity ions produce acidity
AlAl+3+3 produces produces HH++ ions when it reacts with ions when it reacts with water.water.
((when pH below 6: when pH below 6: AlAl+3+3 is the cause of acidity is the cause of acidity))
Causes of soil basicityCauses of soil basicity
1.1. Hydrolysis of basic cationsHydrolysis of basic cations
2.2. Hydrolysis of carbonatesHydrolysis of carbonates
1. Hydrolysis of basic cations:1. Hydrolysis of basic cations:(especially (especially CaCa+2+2, Mg, Mg+2+2, K, K++, NH, NH44
++, Na, Na++))
((also called also called exchangeable basesexchangeable bases))
Extent to which exchangeable bases will hydrolyze Extent to which exchangeable bases will hydrolyze depends on ability to compete with H+ ions for depends on ability to compete with H+ ions for exchange sites.exchange sites.
NaNa
Na NaNa
Na
Na Na
NaNa
+H2O H
+ + OH-
KK++ and Na and Na++ are weakly held compared to are weakly held compared to CaCa+2+2 and Mg and Mg+2+2..
Recall energy of adsorptionRecall energy of adsorption
So, KSo, K++ and Na and Na++ are hydrolyzed easily and are hydrolyzed easily and yield higher pHs .yield higher pHs .
2. Hydrolysis of carbonates2. Hydrolysis of carbonates(especially (especially CaCOCaCO33, MgCO, MgCO33, Na, Na22COCO33))
• As long as there are carbonates in the soil, As long as there are carbonates in the soil, carbonate hydrolysis controls pHcarbonate hydrolysis controls pH..
• Calcareous soils remain alkaline because H+ ions Calcareous soils remain alkaline because H+ ions combine with OHcombine with OH-- to form H to form H22O.O.
• For those soils to become acid, all carbonates For those soils to become acid, all carbonates must be leached.must be leached.
• Basic cations replaced by AlBasic cations replaced by Al+3+3 and H and H++
CaCO3 + H2O Ca+2 + HCO3- + OH-
Na2CO3 + H2O Na + HCO3- + OH- (higher pH because Na more soluble)
Causes of soil acidityCauses of soil acidity
1.1. Accumulation of soluble acidsAccumulation of soluble acids
2.2. Exchangeable acids (AlExchangeable acids (Al+3+3, H, H++))
1.1. Accumulation of soluble acidsAccumulation of soluble acidsat faster rate than they can be neutralized or at faster rate than they can be neutralized or
removedremoved
a.a. Carbonic acidCarbonic acid(respiration and atmospheric CO(respiration and atmospheric CO22))
b. b. Mineralization of organic matterMineralization of organic matter
(produces organic, nitric, sulfuric acids)(produces organic, nitric, sulfuric acids)
Precipitation increases both a and bPrecipitation increases both a and b
2. Exchangeable acids2. Exchangeable acids
Exch. HExch. H++ or Al or Al+3+3 dissociate dissociate
AlAl+3 +3 ties up OHties up OH- - from water, releases an from water, releases an equivalent amount of H+ ions.equivalent amount of H+ ions.
Al+3 + H2O AlOH+2 + H+
CEC and pHCEC and pH
Only 2:1 silicate clays do not have pH-dependent CECs.Only 2:1 silicate clays do not have pH-dependent CECs.
Others are pH-dependent:Others are pH-dependent:
1:1 kaolinite:1:1 kaolinite:low pH: low CEClow pH: low CEChigh pH: high CEChigh pH: high CEC
Oxidic claysOxidic clays