3. Soil Eh 10/26/20151Soil Eh

Introduction Electron activity (ae) is an index for expressing the quantity of electrons in a system. Like the definition for the activity of other substances, it denotes the contribution of electrons to the partial molar free energy of the system. It can be expressed as the negative logarithm of the activity of electrons (pe), just as pH for the activity of protons: pe =-logae (1-1) For the oxidation-reduction reaction (oxidant)+e  (reductant) (1-2) at equilibrium K=(reductant)/ (oxidant)(e) (1-3) 10/26/20152Soil Eh

Introduction Under standard conditions Log K=pe 0 (1-4) where K is the equilibrium constant. The equilibrium constant is related to the change in standard free energy (∆G 0 ) of a reaction as follows: ∆G 0 = -RT/nF Ln K (1-5) hence E 0 =RT/nF Ln K (1-6) if n=1, at 25 0 C E 0 =0.059 log K = pe 0 (1-7) Numeral values of pe 0 and E 0 for some redox systems in submerged soils are tabulated in Table 1. 10/26/20153Soil Eh

Table 1. pe 0 and E 0 for some redox systems in submerged soils (Liu, 1985) Systempe 0 (= log K)E 0 (V) ¼ O 2 + H + + e  ½ H 2 O ½ MnO 2 + 2H+ + e  ½ Mn H 2 O Fe(OH) 3 + 3H+ + e  Fe 2 + 3H 2 O ½ NO 3- + H+ + e  ½ NO ½ H 2 O /8 SO /4 H + + e  1/8 H 2 S + ½ H2O /8 CO 2 + H + + e  1/8 CH 4 + ¼ H 2 O H+ + e  ½H /26/20154Soil Eh

Based on the relationship between properties and plant growth, the oxidation-reduction ststus of soils may classifield into four categories, namely oxidizing, weakly reducing, moderately reducing and strongly reducing. The corresponding E h are see Table 2. 10/26/20155Soil Eh

Table 2. Gradiation of redox status of soils (Liu, 1985) Redox statusEh range (mV) ReactionPlant Growth Oxidating> 400O 2 predominant, materials in oxidized form Benefecial to upland crops, not suitable for rice Weakly reducing O 2, NO 3 - and Mn 4 + reducedNormal growth of rice, upland crops affected Moderately reducing (-100)Fe 3 + reduced, organic reducting substances present Upland crops affected Strongly reducing< (-100)CO 2 and H + reducedRice affected by reducing substances 10/26/20156Soil Eh

In Table 3, are tabulated in descending order the theoritical E 0 values and practically measured E h values of some oxidation-reduction systems commonly encountered in paddy soils. Since the pH of most natural systems including paddy soil is ordinarily around 7, all the E h are corrected to this pH. 10/26/20157Soil Eh

Table 3. Sequential reduction of some electron acceptors in paddy soils (in Liu, 1985) Redox reactionE 0 at pH 7 (V)Actually meassured E h (V) O 2 + 4H + + 4e  2 H 2 O NO3- + 2H + + 2e  NO H 2 O0.54o MnO 2 + 4H + + 2e  Mn 2 + 2H 2 O FeOOH + 3H + + e  Fe 2 + 2H 2 O Organic systems-0.03-(-0.2)0-(-0.2) SO H + + 6e  HS - + 4H2O (-0.15) H + + e  ½H (-0.3) Methane fermentation--0.2-(-0.3) 10/26/20158Soil Eh