3. Soil Eh

1. 3. Soil Eh Soil Eh

2. 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) Soil Eh

3. Introduction Under standard conditions Log K=pe0(1-4) whereKis the equilibrium constant. The equilibrium constant is related to the change in standard free energy (∆G0) of a reaction as follows: ∆G0= -RT/nF Ln K (1-5) hence E0=RT/nF Ln K (1-6) if n=1, at 250C E0=0.059 log K = 0.059pe0 (1-7) Numeral values of pe0 and E0 for some redox systems in submerged soils are tabulated in Table 1. Soil Eh

4. Table 1. pe0 and E0 for some redox systems in submerged soils (Liu, 1985) Soil Eh

5. 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 Eh are see Table 2. Soil Eh

6. Table 2. Gradiation of redox status of soils (Liu, 1985) Soil Eh

7. In Table 3, are tabulated in descending order the theoritical E0 values and practically measured Ehvalues 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 Eh are corrected to this pH. Soil Eh

8. Table 3. Sequential reduction of some electron acceptors in paddy soils (in Liu, 1985) Soil Eh