Redox potential (soil science)

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The redox potential of the soil (Eh value) is an important measured variable that, roughly speaking, gives an indication of the availability of oxygen in the soil. This depends on the ratio of oxidized and reduced substances in the soil. The greater the proportion of reductive forms, the more reductive the redox potential .

Redox systems in the soil

Substances can be oxidized or reduced depending on their redox potential. Important redox systems in the soil concern the elements iron, manganese, carbon, nitrogen and sulfur. The redox potential influences many central processes in the soil such as nutrient balance (solubility), biological activity (decomposition rate, aerobic / anaerobic ) or weathering .

Iron plays a key role in the color of the soil. In its oxidized form Fe (III) it is - depending on the climatic conditions - brown ( goethite ) or reddish ( hematite ) and hardly soluble. The reduced form Fe (II) is gray and easily soluble.

Manganese is less clearly visible because its concentration in the soil is lower. The reduced form is easily soluble and colorless. The oxidized form, however, is difficult to dissolve and black.

Soils with reductive / oxidative conditions

Soils show reductive conditions if there is long-term oxygen-poor water, or if there is an overabundant supply of nutrients, through which microorganisms use up all easily usable electron acceptors such as oxygen. Strongly reductive horizons are also called reduction horizons , which are easily recognizable by their typical gray-black color and their sulfur compound odor. These are characteristic of groundwater soils such as gleye and marshes , but also in artificial, extremely highly enriched soils such as rubbish dumps .

Well-ventilated, not over-supplied soils are oxidative. If there are transitions between reductive and oxidative areas, iron and manganese ions are shifted. This leads to the effects of hydromorphism , i.e. concretions and rust stains in the soil, a distinction being made between gleying (influence of groundwater) and pseudo-gleying (influence of backwater).

Field measurement

The relationship between electron uptake and removal is determined with a measuring chain using the potential difference in millivolts mV . In field soil science, this is usually done in at least two different soil depths such as B. in 30 cm and 90 cm. For this purpose, a hole is made with a drill stick to the desired depth, the (platinum) electrode is inserted and the cavity is carefully covered with soil again. If possible, this should be done a few days (at least several hours) before the measurement date in order to determine the balance of the soil and not the air supply through the borehole. The potential is measured with a reference electrode (AgCl) which is inserted into the topsoil.

Sources of error : The measurement usually only gives a rough indication of the redox properties of the soil, as this can be permanently disturbed by the borehole and the silting up. In addition, the soil itself is often inhomogeneous ( aggregates , pore system ) and the redox potential can change within very small distances. Particularly strong fluctuations occur in marbled horizons such as the Go or Sw horizons, in which oxidative and reductive areas occur. Furthermore, seepage water with high precipitation or the groundwater level influence the measurement. Differences of 100 mV over the course of a year are not uncommon. In the case of strong groundwater fluctuations, these can be in the range of up to 800 mV.

In addition, errors can be falsified by insufficient contact between the soil and the electrode, sluggish measuring chains, redox pairs that have not been recorded, or deposits on the electrode. The electrode is "poisoned" by hydrogen sulfide or inactivated by carbonate deposits.

Despite these errors, the measured redox potential is a valuable guide to the conditions in the soil, especially the oxygen supply.

Interpretation of results

In general: the higher the measured value (in mV), the more oxidative the system. A range occurs in the soil that covers approximately −300 mV (strongly reductive) to +800 mV (strongly oxidative).

Important subdivisions are:

Influence of pH

The pH value also influences the redox potential: alkaline pH values ​​tend to lower it, while acidic values ​​make it rise. The measurement is usually carried out at pH 7. The pH value must be recorded if the measuring device does not automatically take it into account. The following applies: 50–100 mV must be added for each pH level upwards and 50–100 mV must be subtracted for each pH level downwards. For average soil pH values, the rule of thumb 59 mV is often used.

If the result is rewritten to hydrogen, this gives the rH value, which is often used in the literature.

rH = Eh / 28.75 + 2 * pH value

The newly determined values ​​run from rH 0 (reductive) to rH 41 (oxidative). Falling below 15 initiates reduction horizons. At rH> 30 there is almost complete oxidation.

Conversely, oxidation and reduction also affect the pH.

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