Measurement of Redox Potential in Soil

In principle the redox potential provides a simple means of gauging a soil s redox status. However in practice it is difficult to make reliable measurements. 

Stumm and Morgan (1996) discnss the problems for simple aqnatic systems and van Breemen (1969), Ponnamperuma (1972), McBride (1994) and Patrick et al. (1996) discnss the additional problems for soil systems. 1 here give the main points. 

Measnrements of Ea are usually made with a platinum electrode placed in the soil solntion together with a reference half cell electrode of known potential. The platinnm electrode transfers electrons to and from the soil solution withont reacting with it. Reducing half reactions in the soil tend to transfer electrons to the platinum electrode and oxidizing half reactions to remove them. At eqnilibrinm no electrons flow and the electric potential difference between the half cell comprising the platinnm electrode and the soil solntion and the half cell comprising the reference electrode is recorded. 

A further problem, particularly in soil systems, is that several redox systems may be present, in which case the apparent equilibrium potential may be the result 

These factors rather constrain the nsefnlness of Uh measnrements in soil solutions. Inferences about the thermodynamics of redox processes in soils that rely heavily on measurements of redox potential shonld be treated with caution. Nonetheless soil h measnrements provide a ready measnre of redox status, for example in experiments in which constant and pH are reqnired (Patrick et al, 1973). 

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The identities of the solid phases that form remain a mystery. Direct identification is difficult because Fe(II) and Mn(II) solid phases are readily oxidized by O2 and it is therefore necessary to maintain scrupulously anoxic conditions to ensure that the material examined actually represents that in anoxic soil. An alternative is to make indirect assessments through measurements of pe, pH and [Fe +] in solution, but these too are difficult (see section on measurement of redox potential in soil). 

Because it is difficult to make a precise measurement of redox potential in soil, especially in well-aerated soil, reliable data on redox potential in the rhizosphere in this study are lacking. The results presented in Fig. 5 may be inaccurate however, a trend of change toward negative Eh in the rhizosphere soil is inferred. Rappart et al. (1987) indicated that greater amounts of exchangeable copper have been detected in soils at low rather than high redox potential. It is assumed that in the maize rhizosphere in this study, the reduced redox potential was favorable to metal mobilization. 

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