NRES 201 Lecture 26 Outline of Last Lecture I Cation Exchange II Cation exchange capacity CEC III Determination of CEC Outline of Current Lecture IV Cation exchange capacities V Cation exchange and plant nutrient uptake VI Anion exchange VII Anion exchange capacities VIII Soil weathering and ion exchange IX Sorption of organic compounds X Clays and soil physical properties Current Lecture Cation Exchange capacities of soil colloids Kaolinite o Non swelling o No isomorphous substitution Montmorillonite o Swelling o Interlayer cations not fixed o 80 of CEC due to isomorphous substitution Vermiculite o High CEC due to swelling with Ca or Mg as the dominant interlayer cation o Much lower CEC with interlayer K or NH4 which prevents swelling Illite o Low swelling and CEC with K as the dominant interlayer cation Chlorite o Low swelling and CEC due to positively charged interlayer hydroxyl sheet Hydrous Oxides o No isomorphous substitution Organic Matter o Much higher CEC than clay minerals o Entirely pH dependent o Accounts for about 50 of soil CEC These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Cation Exchange capacities of surface soils Generalizations o Importance of clay content and composition for mineral soils o Histosols much higher in CEC Cation exchange and plant nutrient uptake Cation exchange by roots o Plant roots generate H and exhibit CEC Due to intercellular deposits of calcium pectate in the cortex o Cations compete for plant uptake o Availability depends on CEC Cation Saturation Anion versus Cation Exchange Cation exchange o Positive ion o Negative surface Anion exchange o Negative ion o Positive surface Origin of colloidal charge Cation exchange arises from o pH dependent charge o Permanent charge isomorphous substitution Anion exchange is due entirely to pH dependent charge Opposing effects of pH Cation exchange favored by alkalinity Anion exchange favored by acidity Anion exchange capacities of surface soils Generalizations o Illinois soils very limited in AEC o Greatest potential for AEC in acidic andepts and oxisols Soil weathering and ion exchange An increase in weathering intensity o Decreases CEC o Increases AEC
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