Results and discussion
Redox potential can be used to classify soils and monitor
management effects
The redox potential Eh of soils ranged from reduced to slightly oxidized
+340 to +560 mV and from acidic pH 5.8 to slightly alkaline pH 7.6
(Figure 1). Most of the sampled soils were in a region of the Eh/pH
space, which is considered favorable for plant growth (Husson, 2013).
Compared with the favorable region, one soil was too reduced and 10
samples were too oxidized. The reduced sample was a seasonally
waterlogged silty clay loam soil, the oxidized alkaline sample was a low
organic matter sandy loam soil. The acidic and oxidized soils were a
more complex set of soils with either a very high OM level
(>12%) or a very high clay level (>60%). The
two soils in the ”optimum” range were silty clays with an OM 6% and a
history of grass cultivation and horse manure application. Compared to
the four soils evaluated in Husson et al. (2016), the soils in this
study were more reduced (530 vs. 470 mV), which could be expected on the
basis of differences in climate (warm vs. cool temperate) and organic
matter level (2% vs. 8%).
The addition of carbon compounds to the soil through carbon farming
decreased Eh (Figure 1 and Figure 2). This supports the findings of
Husson et al. (2016) where conservation agriculture (residue retention
and minimum tillage) reduced Eh by 10-25 mV. In this study, the
reduction was on average -20 mV (p < 0.05; 95% range -94-+26
mV). The largest reductions (-90 mV) were on sites that had compost soil
amendments applied. This would suggest that soil redox potential reacts
to the addition of readily decomposable organic matter. Overall, the
redox potential test on dried and rewetted soils could differentiate
soils based on their OM and clay status, but it also reacted on
short-term changes to soil management. Poor soil condition
(waterlogging, OM or tillage) was reflected by the placement in the
Eh/pH chart. This reactivity to soil conditions and management makes
redox a promising indicator of soil health.