8Ion exchangeExchangeable ions on soil surface trading places with ions in solution.
9On soil surfaces, there are: Exchangeable and Nonexchangeable Ions : Exchangeable: weakly held, in contact with soil solution, ready for quick replacement.“outer sphere complex”Nonexchangeable:“inner sphere complex”adsorbed by strong bonds or held in inaccessible places(e.g., the K+ between layers of illite)not part of ion exchange !
10Cation exchange capacity (CEC) Sum total of exchangeable cations that a soil can adsorb. ( prevents nutrients from leaching away from roots)
11CECExpressed in:milliequivalents per 100 g (meq/100g)
12Base saturation% of exchange sites occupied by basic cations Basic cations are cations other than H+ and Al+3 Base saturation + H+ ion saturation should equal 100%
13For midwest US soilsNotice neutral pH (7.0) requires a base sat of 80%. (neutral pH is not 50% because most base cations have a + charge of 2)
14equilibriumStrive for equivalent proportions of solution and exchangeable ions.Upset equilibrium by:removal by plantsleachingfertilizationweatheringInitiate ion exchange
15Ion exchange example: Add H+ ions to soil : Ca+Ca+Ca+Ca+H+Ca+Ca+soilH+Ca+++H+Ca+Ca+H+H+Ca+H+Ca+Ca+Ca+solutionCa+Ca+Ca+exchangeablesolutionexchangeablesolution
16Rules of ion exchange Process is Reversible Charge by charge basis Ratio Law:ratio of exchangeable cations will be same as ratio of solution cations
17Add K fertilizer… K+ Ca+2 Ca+2 K+ Ca+2 + K+ + K+ K+ Ca+2 K+ K+ K+ Same ratio
18Energy of adsorption Strong --------------------------------------Weak Al+3 > Ca+2 > Mg+2 > [K+ = NH4+ ] > Na+ > H+(based on charge and hydrated radius)
19Soil pH importance Determines solubility of nutrients Before plants can get nutrients, they must be dissolved in soil solutionMicrobial activity also depends on pH
20pH negative log of the hydrogen ion concentration (also a measure of OH- concentration)If H+ concentration > OH- : acidicIf OH- > H+ : basicSoil pH is pH of solution, NOT exchange complex
22In soil, both H+ and Al+3 ions produce acidity Al+3 produces H+ ions when it reacts with water.(when pH below 6: Al+3 is the cause of acidity)
23Causes of soil basicity Hydrolysis of basic cationsHydrolysis of carbonates
241. Hydrolysis of basic cations: (especially Ca+2, Mg+2, K+, NH4+, Na+) (also called exchangeable bases)Extent to which exchangeable bases will hydrolyze depends on ability to compete with H+ ions for exchange sites.NaNaNaNa+H2O+ OH-NaHNa+NaNaNaNa
25K+ and Na+ are weakly held compared to Ca+2 and Mg+2. Recall energy of adsorptionSo, K+ and Na+ are hydrolyzed easily and yield higher pHs .
262. Hydrolysis of carbonates (especially CaCO3, MgCO3, Na2CO3) As long as there are carbonates in the soil, carbonate hydrolysis controls pH.Calcareous soils remain alkaline because H+ ions combine with OH- to form H2O.For those soils to become acid, all carbonates must be leached.Basic cations replaced by Al+3 and H+CaCO3 + H2OCa+2 + HCO3- + OH-Na2CO3 + H2ONa + HCO3- + OH- (higher pH because Na more soluble)
27Causes of soil acidity Accumulation of soluble acids Exchangeable acids (Al+3, H+)
28Accumulation of soluble acids at faster rate than they can be neutralized or removed Carbonic acid(respiration and atmospheric CO2)b. Mineralization of organic matter(produces organic, nitric, sulfuric acids)Precipitation increases both a and b
292. Exchangeable acids Exch. H+ or Al+3 dissociate Al+3 ties up OH- from water, releases an equivalent amount of H+ ions.Al H2OAlOH H+
30CEC and pH Only 2:1 silicate clays do not have pH-dependent CECs. Others are pH-dependent:1:1 kaolinite:low pH: low CEChigh pH: high CECOxidic clays