Figure 4. The redox status (rH2) of soils with a good structure (VESS)
was more oxidized than for soils with a poor structure. The colors on
the points describe the soil OM concentration.
Overall, redox can be an important additional measure for soil health
quantification (Table 1). It correlates with soil structure (VESS) and
biological activity (CO2burst). As it does not correlate
with OM, it is a measure of the active organic matter pool, possibly
similar to water soluble organic carbon (Haney et al., 2012). However,
especially the pH-corrected rH2 score correlates with
soil clay content, so the interpretation should be based on soil
texture. Although redox correlates with other soil health indicators, it
is not a replacement for those. For example, soils with a good structure
have a high redox (Figure 4), but also some poor structure soils may
also have similar values. Soils with high biological activity have low
redox potentials (Figure 3), but a low redox potential can also be found
in a soil with moderate biological activity.
As the redox potential changes more rapidly with management than
CO2burst (Figure 2), it is a promising indicator to
track management-induced changes in soil health. In this application, it
could also be used to classify soils based on their Eh and pH (Figure
1). These results supported the earlier findings on the use of soil
redox potential to classify soils and to follow their change due to
management (Husson, 2013; Husson et al., 2016). As the earlier studies
were conducted in a warm temperate climate with lower OM and clay
content, similar findings in high OM and clay environments support
continuing investigations into the role of redox in quantifying soil
health.
A major challenge in interpreting the redox results is the integrative
nature of the redox potential. It is a combined effect of all ongoing
soil oxidation and reduction reactions, which defines the redox
potential. It may be hypothesized that the organic matter decomposition
would drive the potential in a drying-rewetting test, but in this study,
soil texture was found to correlate with redox potential, suggesting
that other reactions than organic matter related were also driving the
potential. Further studies on the interpretation of redox in different
textures and farming systems would be needed to make it an applicable
tool for managing soils.
Table 1. Correlation between soil redox status (Eh and pH corrected rH2)
and commonly used soil health indicators (Structure VESS; CO2burst;
Organic matter; and Clay content) (Weil and Brady, 2016).