Soil OM is 50-65% C, so we use 57.5% SOM x = OC and SOM = OC/0.575 e.g., how much SOM do you have with 2% OC? SOM = 2% ÷ = 3.5% or 2% ÷ 0.50 to 0.65 = 4 to 3% OC

pH dependent surface charge: S-OH + H + ↔ S-OH 2 + protonation (gains protons, attracts anions) S-OH ↔ S-O - + H + deprotonation (loses protons, attracts cations) S-OH + OH - ↔S-O - + H 2 Odeprotonation alkaline conditions (loses protons, attracts cations) pKa’s and Henderson-Hasselbalch eqn tell us whether a compound will be mostly charged (usually negatively) or uncharged at a given pH Acidic conditions

Point of Zero Charge PZC  suspension pH at which the particle surface has zero net charge:  p = 0 1. When pH < PZC the particle surface is positively charged 2. When pH > PZC the particle surface is negatively charged 3. At PZC, settling of flocs occurs – important in aggregation and retention of ions during irrigation, leaching, etc. * uncharged particles don’t repel each other

pH 0, positively charged): S-OH + H + ↔ S-OH 2 + pH > PZC (  p < 0, negatively charged): S-OH ↔ H + + S-O - pH = PZC (  p = 0, uncharged): H + + S-O - ↔ S-OH

pH below the pH ZPC

pH at the pH ZPC

pH above the pH ZPC

Soil components vary in PZC 1.Fe and Al oxides (Oxisols, tropical soils) have high PZC (pH 5-10) 2.Soil organic matter has low PZC (pH<5) 3.Silicate clays have low PZC (pH 2-5) Interpretation: low PZC = net negative charge over wider soil pH range  more cation adsorption and more CEC High PZC = net positive charge in acid conditions or in lower range of soil pH  more anion adsorption and less CEC 4.Consider the distribution of soil components in the profile – where would you expect to see more or less anion and cation adsorption? more CEC in Ap or Bt horizons, more AEC in oxide-rich horizons or low OM depths

pH for zero point of charge for minerals Mineral pH ZPC Gibbsite Hematite Goethite 7 – 8 Amorphous Fe(OH) Kaolinite Montmorillonite SiO Note that Al and Fe oxides have a high pH ZPC Kaolinite and montmorillonite have low pH ZPC

Types of PZC PZC,  p = 0 –Apply electric field, PZC reached when particles flocculate or stop moving PZNC (N for net), CEC-AEC =0;  is +  os +  d = 0 –Measure Na + and Cl - sorption with pH; PZNC calculated from intersection point PZNPC (P for proton),  H = 0 (or zero variable charge) PZSE (SE for salt effect), intersection of two potentiometric titration curves –Most commonly measured

Desorption removing an ion or molecule from a surface particle and putting it back into solution. Important for decontamination of soil or sediments and to determine the mobility of contaminants Hysteresis  apparent irreversibility of sorption (forward and backward reactions did not coincide)

Hysteresis causes: Experimental error: failure to attain equilibrium during sorption experiments Chemical or biological transformations not accounted for in sorption study Trapping of ions or molecules in soil micropores resulting in very slow release  short term lab sorption experiments may be inadequate to predict behavior over long time periods under field conditions.

q Ceq Example of hysteresis during desorption in a batch equilibrium sorption experiment

Adsorption (open symbols) and desorption (full symbols) isotherms of water at 25 °C on (a) a TiO2 film deposited at 80 °C for 2 h and (b) the same powder after heating at 450 °C