IJERD – International Journal of Environmental and Rural Development (2021) 12-1

Research article
erd

Potential Measurement as a Method for Monitoring the Soil
Chemical Environment
NARONG TOUCH*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: nt207118@nodai.ac.jp

TAKAHIKO NAKAMURA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Received 8 January 2021

Accepted 19 May 2021(*Corresponding Author)

Abstract Soil chemical environment (SCE) affects soil degradation and productivity, thus
understanding of the temporal changes of SCE is important for obtaining higher soil
productivity. Widely used indices of SCE included ion concentration, pH, and redox potential(ORP), but these cannot monitor SCE continuously. Previously, a study proved that continued
potential measurement can represent temporal changes in the water quality at the sea floor.
However, no report related to monitoring the SCE with continued potential measurement has
been found. Therefore, this study proposes a method for evaluating soil ORP using continued
potential measurement, and examine the method’s validity in representing changes in SCE due
to soil reduction, bacteria activation, and soil oxidation. Laboratory experiments wereconducted using soils from rice paddy fields. A brush-type carbon electrode was installed in thesoil layer that then connected to a reference electrode for measurement of the soil ORP.
Different soil conditions were created by mixing the paddy soil with cow manure compost orpotassium sulfate. The soil ORP was automatically recorded every 15 min using a voltage
meter. The ORP of paddy soil decreased temporally and stabilized at 50 days after the start ofthe experiment, suggesting that soil reduction occurred over those 50 days. When testing thepotassium sulfate-paddy soil mixture, the soil ORP rapidly decreased during the first day after
the experiment started. When testing cow manure compost-paddy soil mixture, a larger
decrease in soil ORP was observed compared to that in the paddy soil alone. These findings
suggest that soil reduction is promoted by adding potassium sulfate or cow manure compost.
Among the soil samples tested, there was a range of soil ORPs and trends in the potentialdecreases. Based on the results of this study, it was found that continued potential measurement
is effective in evaluating soil ORP, which represents temporal changes in SCE due to redox
reactions after the addition of cow manure compost or potassium sulfate. In addition, the effectsof bacteria activation are revealed during continued potential measurement of soil ORP.
Keywords potential measurement, carbon electrode, redox reaction, metabolism reaction,soil reduction, soil oxidation

INTRODUCTION
Soil is a medium for plant growth and is one of the fundamental bases for agricultural productions, thus
agricultural productivity is strongly influenced by the use of soil fertilizer (Hatfielf, 2006). Over thelast few decades, the relationship between cropping and managing systems and the soil chemical
environment (SCE) has been discussed with regard to obtaining higher productivity by preventing soil
degradation or erosion. The understanding of SCE is useful for ensuring healthy soil that can sustain
crop production and environmentally friendly agriculture (Yan and Hou, 2018).
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It has been suggested that the transition from conventional to organic or low-input farming has
caused changes in the SCE and the processes related to soil fertility (Clark et al., 1998). As a decline insoil fertility results in a decrease in productivity (Buresh et al., 1997), management of soil fertility andsoil health plays an important role in realizing sustainable agriculture (Prasad and Power, 1997). Sincechemical reactions and nutrient circulation affect soil fertility, understanding changes in the SCE areessential in facilitating successful, high-productivity organic farming.
To date, parameters such as pH, organic matter contents, phosphorus availability, and cationexchange capacity have been widely used as indices for evaluating SCE. However, their measurements
cannot be conducted continuously, thus changes in the SCE over time are underdetermined. Therefore,a method that can continuously monitor SCE is required. Potential measurement is a method that canmonitor the redox environment of soil over time. This method may play an important role in organic
farming. For obtaining a higher productivity of organic farming, the measuring SCE over time canprovide useful information for controlling or improving redox and biological conditions of soils. Thismethod was used by Nagama et al. (2018) when they suggested that water potential measurement candescribe changes over time in the water quality at the sea floor. However, no report related tomonitoring SCE with continued potential measurement has been found in the literature.
OBJECTIVE
This study was aimed at proposing a method for measuring soil ORP and at examining the method’s
validity in representing changes in SCE due to soil reduction, bacteria activation, and soil oxidation.
This was done by continued potential measurement to determine changes in the soil ORP of paddy soil.
The paddy soil samples studies were pure, mixed with cow manure compost, or mixed with potassium
sulfate.
METHODOLOGY
The experimental device comprised a cylindrical bottle with an inner diameter of 120 mm and a heightof 150 mm. The bottle was filled with paddy soil to a depth of 20 mm, and a brush-type carbon
electrode was placed on the soil layer. Then, another soil layer at a depth of 30 mm was placed on theelectrode. Finally, tap water was poured over the soil layers (Fig. 1). The paddy soil was collectedduring the agricultural off-season from a rice filed in Ebina, Kanagawa, Japan. Approximately 150 mmof the surface soil was collected and transported to the laboratory.

Reference electrodeTap waterSoil

Voltmeter

20 mm

50 mm

100 mmBrush-type electrode(100*100) mm2
120 mmCase 1: paddy soil, Case 2: Cow manure compost-paddy soil mixture,
Case 3: potassium sulfate-paddy soil mixture

Fig. 1 Experimental device and measurement methodⒸ ISERD67

IJERD – International Journal of Environmental and Rural Development (2021) 12-1

The soil layer was prepared under different conditions to ensure differences in the SCE. For
example, certain paddy soil was mixed with cow manure compost, and another mixture had potassium
sulfate. The mixing ratio (mass basis) of paddy soil to cow manure compost or potassium sulfate was
9:1. The electrode material was carbon cloth (News Company, PL200-E), which was heated at 500°C
for 1 h prior to use to improve its performance, as was suggested by Nagatsu et al. (2014). The heatedcarbon cloth with a width of 100 mm and a length of 100 mm was separated into fibers to form abrush-type electrode (Fig. 1).
The bottles were then submerged in a container that was filled with tap water (Fig. 1). In thecontainer, the electrode in the soil layer was connected to a reference electrode (Toyo Co., RE-7A) for
measuring the electrode potential. This electrode potential was the soil ORP. The potential wasrecorded automatically every 15 min by a voltmeter (T&D Corp., VR-71). A plastic-coated copper
wire was used for all the connections in the measurements.
RESULTS AND DISCUSSION
Accuracy of Potential Measurement

Soil ORP of Case 1-1 (V vs Ag/AgCl)

To examine the accuracy of potential measurement, two paddy soil samples were made under the same
condition, and the soil ORP measurements of the two samples were compared (Fig. 2). The soil ORP
measurements taken during the first day were used because they nearly reflected the electrode potential.
As time increases, the SCE may differ from one sample to another, even though they were made under
the same condition because soil reduction (organic matter decomposition) may differ.
As demonstrated in Fig. 2, the maximum variance in potential was only +0.004 V (1.33%),
indicating this method’s high accuracy. Therefore, it can be said that the proposed method, i.e.,
potential measurement, can accurately represent temporal changes in the SCE owing to redox reactions
and bacteria activation in soils.
0.35

0.34

0.33

0.32

0.31

0.3

0.3

0.31

0.32

0.33

0.34

0.35

Soil ORP of Case 1-2 (V vs Ag/AgCl)

Fig. 2 Comparison of the measured soil ORP from two samples under the same condition
Potential Measurement for Representing Soil Reduction
Fig. 3 illustrates the comparison of temporal changes in soil ORP for each sample. The paddy soil used
was in an oxidized state as it was collected during the agricultural off-season. Thus, the initial potentialⒸ ISERD

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was 0.35 V which then temporally decreased and almost stabilized after 50 days. Approximately twomonths are required for complete soil reduction.
On day 50, the soil ORPs were −200 mV for the paddy soil, −260 mV for the potassium sulfatepaddy soil mixture, and −475 mV for the cow manure compost-paddy soil mixture. Previously, it wasreported that different redox reactions result in a difference in soil ORP (Liesack et al., 2000; Matocha,2005). Specifically, soil ORP indicates redox reactions occurring in soils. The results of this study
indicate that different redox reactions occur because of different soil compositions. Compared withpaddy soil alone, a large decrease in soil ORP was observed when potassium sulfate was added to thepaddy soil. This was due to the reduction reaction of sulfate in the soil. Since methanogens are present
in cow manure, methane production occurred in the paddy soil the manure was mixed with, resulting ina large decrease in soil ORP. In summary, potential measurement is useful for understanding changes
in SCE due to redox reactions in soils.

Soil ORP (V vs. Ag/AgCl)

0.4
Paddy soil (PS)
PS+Cow manure compost
PS+Potassium sulfate

0.3
0.2

0.1
0-0.1
-0.2
-0.3

-0.4
-0.5

05

1015 20 25 30 35
M easurement time (day)404550Fig. 3 Comparison of temporal changes in soil ORP

Soil ORP (V vs. Ag/AgCl)

0.5
Paddy soil (PS)
PS+Cow manure compost
PS+Potassium sulfate

0.4
0.3
0.2
0.1
00

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.91

M easurement time (day)Fig. 4 Comparison of decreasing trends of soil ORP during the first day of the experiment
Potential Measurement for Representing Microbial Activation in Soils
Fig. 4 depicts a comparison of decreasing trends of soil ORP during the first day of the experiment.
Interestingly, large differences in decreasing soil ORP were observed during this time. Compared withpaddy soil, a large decrease in soil ORP was recorded when the soil was mixed with cow compostⒸ ISERD

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manure, and an even larger decrease in soil ORP was recorded when it was mixed with potassium
sulfate. These trends depend on the redox reactions occurring at the electrode and possible potential
losses due to electron transfer at the electrode surface.
Following the Nernst equation (Eq. 1), the electrode potential (E) depends on the standard
potential of redox couple (E0) and the concentration of ions (logarithm term).
E = Eo + (RT/nF) ln (Ox/ Re)(1)

R is the universal gas constant, T is the temperature in Kelvins, n is the number of electronstransferred in the reaction or half-reaction, F is the Faraday constant, Ox is the activity of the oxidized
form, and Re is the activity of the reduced form.
E0 value are different between redox couples; for example, E0 =1.057 V for the Fe(OH)3/Fe2+redox couple and E0 =0.303 V for SO42-/H2S. Therefore, large changes of E will be observed when theredox reaction varies from one to another redox couple. Compared with a biological reaction, achemical reaction occurs rapidly, thus the observed rapid decrease of soil ORP in this study is due tothe change of the redox couple from iron reduction in the paddy soil alone to sulfate reduction in thepotassium sulfate-paddy soil mixture.
In addition, E depends on microbial activation at the electrode. According to Yamasaki et al.
(2018), potential loss (activation loss) occurs during the flow of electrons into an electrode. In apotential measurement, activation loss should be minimized to obtain the equilibrium potential in ashort time. Microbial activation at the electrode is one of the methods for minimizing activation loss.
Wang et al. (2009) succeeded in improving electrode performance by lowering the potential losses byactivating bacteria at the electrode. As seen in Fig. 4, the decrease of soil ORP when cow manurecompost was used was larger than that when paddy soil alone was used. This may be explained in thatmicrobial activation occurs in the cow manure compost. Therefore, changes in the redox couple and thebenefits of microbial activation in soil can be understood through potential measurement.
Potential Measurement for Representing Soil Oxidation

Soil ORP (V vs Ag/AgCl)

Measurement did continue after day 50 for the case of using paddy soil alone. Germination wasconfirmed on day 50. From the measurement taken (Fig. 5), soil ORP started to increase on day 50,
indicating the generation of soil oxidation. On day 170, the soil potential reached 0.32 V which is close
to the initial potential measurement taken. Additionally. the plant growth had occurred (Fig. 5).
0.6
0.5
0.4
0.3
0.2
0.1
0-0.1
-0.2
-0.3

Germination
170 days after theexperiment started

010 20 30 40 50 60 70 80 90 100
Measurement time (d)Fig. 5 Changes in soil ORP of paddy soil due to soil oxidationⒸ ISERD

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The increase in soil ORP was caused by soil oxidation owing to the oxygen supply from theoverlying water layer. Plant growth may have also facilitated the oxygen supply of the soil layer. As
the electrode was placed at a depth of 30 mm from the soil surface, the oxygen supply may haveaffected these long-term measurement results. In other words, the proposed method of potential
measurement may also predict soil oxygen levels.
CONCLUSIONSIn this study, a laboratory experiment was conducted to examine a method for monitoring SCE and to
examine its validity in representing changes in SCE due to soil reduction, bacteria activation, and soiloxidation. It was found out that the proposed method, i.e., potential measurement, had high accuracy(with a variance of 1.33%) for representing SCE. A difference in equilibrium soil ORP was observed
when mixing different fertilizers with paddy soil, indicating that redox reactions in soils can bepredicted through potential measurement. In addition, different trends in decreasing soil ORP wereobserved, suggesting that chemical and biological reactions in soil can be understood from potential
measurement. Finally, soil ORP started to increase on day 50 because of soil oxidation, indicating that
potential measurement can predict soil oxygen supply that causes soil oxidation.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the students of the Rural Environmental Engineering Laboratory,
Tokyo University of Agriculture, for their efforts in collecting data.
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