Soil Chemistry - University of Minnesota Duluth：德卢斯明尼苏达大学土壤化学.pptVIP

Class evaluations Soil Chemistry Ion Exchange Ions adsorbed to soil surfaces can be exchanged with ions in soil solution. Cations and anions Ion exchange Organic colloids and inorganic micelles (clays) are sites of ion exchange Where do ions in soil come from? Release from organic matter Rain Weathering of parent material Ion exchange Exchangeable cations (on soil surfaces) cannot be removed by leaching. Soluble cations (in solution) can be removed by leaching. When soil is dried… …exchangeable cations hold to adsorption sites on soil surfaces. …soluble cations (and anions) precipitate or crystallize as salts. Examples of soluble cations precipitating Ion exchange Exchangeable ions on soil surface trading places with ions in solution. On 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 ! Cation exchange capacity (CEC) Sum total of exchangeable cations that a soil can adsorb. ( prevents nutrients from leaching away from roots) CEC Expressed in: milliequivalents per 100 g (meq/100g) Base 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% For midwest US soils Notice neutral pH (7.0) requires a base sat of 80%. (neutral pH is not 50% because most base cations have a + charge of 2) equilibrium Strive for equivalent proportions of solution and exchangeable ions. Upset equilibrium by: removal by plants leaching fertilization weathering Initiate ion exchange Ion exchange example:Add H+ 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+ exchang