IJERD – International Journal of Environmental and Rural Development (2020) 11-1

Research article
erd

Evaluating Changes in the State of Organic Matter Presentin Agricultural Soils based on Loss on Ignition Method
TAKAHIKO NAKAMURA
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan

NARONG TOUCH*
Faculty of Regional Environment Science, Tokyo University of Agriculture, Tokyo, Japan
Email: nt207118@nodai.ac.jp
Received 5 December 2019

Accepted 25 March 2020(*Corresponding Author)

Abstract Loss on ignition (LOI) method is widely regarded as a quick, inexpensive method
for evaluating organic matter characteristics in terrestrial soils and littoral sediments.
Previous studies examined the LOI method alone to evaluate the state of organic matter,
finding that changes in the state of organic matter in littoral sediments could be represented
by LOI method-based indices related to combustion temperature: (LOI300−LOI200)/LOI600
and LOI300/LOI600. However, these new indices have not been applied to agricultural soils.
In this study, laboratory experiments were conducted to examine the validity of our proposed
indices for evaluating the state of organic matter in agricultural soils. Paddy soil, Andosol,and cow manure compost were allowed to decompose over three months. Measurements ofpH, redox potential (ORP), and weight LOI at 200, 300, and 600ºC were measured at one andthree months after the experiments started. Temporal decreases in ORP were observed,
highlighting the decomposition of organic matter present in each material. The(LOI300−LOI200)/LOI600 metric changed over time for all materials, whereas theLOI300/LOI600 metric was almost constant. We concluded that (LOI300−LOI200)/LOI600 canrepresent changes in the state of organic matter in agricultural soils. Furthermore, we
discovered that LOI200/LOI600 can represent changes in the state of organic matter withreasonable accuracy, rather than the more complicated (LOI300−LOI200)/LOI600. This finding
should make LOI measurements easier and more affordable.
Keywords loss on ignition, organic matter state, decomposition, combustion,
agricultural soils

INTRODUCTION
Soil organic matter (SOM) is any plant or animal material, including leaf litter, microbial biomass,water-soluble organics, and humus at different stages of decomposition (Stevenson, 1994; Brady andWell, 1999). SOM has a major impact on the physicochemical properties of soils (Tisdall and Oades,1982). For example, SOM has been shown to affect the cation exchange capacity, structure, water
infiltration rate, water-holding capacity, erodibility and conversion, and pesticide adsorption of soil(Schulate, 1995; Ding et al., 2002).
Several methods have been proposed to measure the organic matter of terrestrial soils and littoral
sediments. For instance, the Walkley-Black method is a wet oxidation method used to determine theamount of organic matter from the organic carbon content of the soil (Swift, 1996; Kerven et al.,2000). Another method, loss on ignition (LOI), uses dry combustion method to directly determine
the amount of organic matter (Cambardella et al., 2001; Konen et al., 2002). Automated CHNS
analysis conducted on gas chromatographs has been used to quickly determine the elemental contents(i.e., C, H, N, and S) with high precision (Telek and Marshall, 1974). With advancements intechnology, it is now possible to accurately analyze the organic matter characteristics of a sample
based on molecular weight measurements, thermogravimetric analysis, and spectrometric analysis(Cuypers et al., 2002; Hong et al., 2010). To the best of our knowledge, the LOI method is the mostⒸ ISERD51

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economical method that is suitable for rapidly analyzing large numbers of samples with theadvantage of time and cost savings, it does not require a measurement expert, and it is an accessible
method for developing countries.
To date, some authors have evaluated the amount and state of SOM on the basis of the LOI
method, alone. For instance, the ratio of weight losses on ignition at 300°C (LOI300) and at 600°C(LOI600) was found to describe the state of organic matter present in littoral sediment (Hibino et al.,2014). Subsequent refinement revealed that (LOI300−LOI200)/LOI600 metric more accurately
represents the state of organic matter in littoral sediments than LOI300/LOI600 metric, especially,changes in the state of organic matter due to decomposition (Touch et al., 2015; Touch et al., 2017).
However, these metrics have not been used to evaluate the state of SOM in agricultural soils, so theirvalidity should be tested in that setting.
OBJECTIVESThis study aims to examine the validities of LOI300/LOI600 and (LOI300−LOI200)/LOI600 metrics inevaluating changes in the state of organic matter present in paddy soil, andosol, and cow manurecompost. These materials are allowed to decompose over three months, and changes in their pH andredox potential (ORP) are measured for understanding the decomposition of organic matter present
in each material. Then, the association between temporal changes in LOI300/LOI600 and(LOI300−LOI200)/LOI600 metrics and the decomposition of organic matter is investigated for
examining the validities of these metrics. Furthermore, this study also aims to propose a more
economical metric for examining the state of organic matter present in organic agriculture materials.
METHODOLOGY
Materials and Experimental Procedures
The paddy soil used in experiments was collected from a rice field (Ebina, Kanagawa, Japan) during
the agricultural off-season. Approximately 150 mm of the surface soil was collected and transported
to the laboratory. Approximately 200 mm of the surface soil of an andosol was collected from afarmland (Nakai, Kanagawa, Japan) during the agricultural off-season. Finally, commercial cow
manure compost was purchased and used in this study. All of the materials were used in theexperiment without any pre-treatment. Each material was placed in the experimental device as shownin Fig. 1.

30 mm

120 mmDeionized water
1, 3 months

120 mm40 mm

Material➢ Paddy soil from rice field➢ Andosol from farmland➢ Cow manure compostSurface soil➢ pH➢ Redox potential➢ Loss on ignitionFig. 1 Diagram of the experimental device and some procedural details used in this study
The experimental device comprised a cylindrical bottle with an inner diameter of 120 mm and aheight of 150 mm. The bottle was filled with each material to a depth of 120 mm. The bottle wasthen filled the rest of the way with deionized water (Fig. 1), and placed under ambient conditions forthree months. After one and three months, the top 40 mm of the surface soil was collected from thebottle for measurements of pH, ORP, and weight LOI.
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Soil Analysis
Each collected sample (paddy soil, andosol, or cow manure compost) was mixed before pH and ORP
were measured. A D-50 pH/ORP meter (Horiba, Japan) was inserted directly into the sample. Afterthe pH and ORP measurements were completed, samples were prepared for the measurement ofweight LOI. Each sample was oven-dried at 50 ± 5°C for more than three days, and the dried weight
was then measured. A temperature of 50°C is suitable for drying samples before determination oforganic matter content (Takata et al., 2017). An electronic muffle furnace (Advantec, KL-420) wasused to burn each of the dried samples for 4 h at each of three temperatures: 200, 300, and 600°C
according to Saito et al. (1977). The sample was weight after burning at each temperature. LOI for
each stage was determined by comparing these values with the 50 °C oven-dried weight. The
measurements of LOI were performed in triplicate and had an error of ± 6 mg.g-1.
RESULTS AND DISCUSSION
Temporal Decomposition of Organic MatterFigure 2 shows the temporal changes in pH and ORP for each material while under ambientconditions. The ORP values started at 304±4 mV, 318±1 mV, and 96±9 mV for the andosol, paddysoil, and cow manure compost, respectively. The OPR of the paddy soil decreased to 93 mV overone month, and to −241 mV over three months. Decreases in the ORP of andosols and cow manurecompost were also found. After three months, the ORP was −226 mV for the andosol, and −437 mVfor cow manure compost.
400

Blue: 1 month, Red: 3 months
Andosols
Paddy soil
Cow manure compost

ORP (mV vs. Ag/AgCl)

300
200

1000R= 0.86

R= -0.99-100-200

R= -0.99-300-400-500

5678910pH (-)

Fig. 2 Temporal changes in pH and redox potential (ORP)
Variations in pH and ORP can represent the decomposition of organic matter present in littoral
sediments (Touch et al. 2015). A decrease in ORP indicates the decomposition of organic matterbecause oxidants are used in the decomposition processes. Thus, our results showed clear signs ofdecomposition of organic matter over time, as was found by Touch et al. (2015).
Validities of LOI300/LOI600 and (LOI300−LOI200)/LOI600
As revealed by Fig. 2, the decomposition of organic matter present in each material decomposed overtime. This means that a correlation between the proposed indices and operation time should beobserved. Figure 3 depicts the temporal patterns of LOI300/LOI600 and (LOI300−LOI200)/LOI600
metrics. Low correlation between LOI300/LOI600 metric and operation time was confirmed, with thismatric remaining nearly constant for each material over the span of experiment (Fig. 3a). This finding
suggests that LOI300/LOI600 metric is unable to reflect the decomposition of organic matter present inandosol, paddy soil, and cow manure compost.
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0.8

0.8

LOI300/LOI600 (-)

0.6

R= -0.55

0.5

R= 0.52

0.4
0.3
0.2
0.1

(LOI300-LOI200)/LOI600 (-)

R= 0.25

0.7

Andosols
Paddy soil
Cow manure compost

0.7
0.6
0.5

R= 0.990.4

R= 0.990.3
0.2

R= -0.99

0.1000123Operation time (month)40(a) LOI300/LOI600

123
Operation time (month)4(b) (LOI300-LOI200)/LOI600

Fig. 3 Temporal variations of LOI300/LOI600 and (LOI300-LOI200)/LOI600
On the other hand, the (LOI300−LOI200)/LOI600 metric for each material had a strong correlation(R = 0.99 for all samples) with operation time (Fig. 3b). This demonstrates that
(LOI300−LOI200)/LOI600 metric can indicate the decomposition of organic matter present in thesematerials. When combined with the findings of previous research (Touch et al., 2017), we concluded
that the (LOI300−LOI200)/LOI600 metric can represent the decomposition of organic matter either inlittoral sediments or in agricultural soils, including compost materials.
Interestingly, different tendencies in the variation of (LOI300−LOI200)/LOI600 metric wereobserved among the materials used. Specifically, (LOI300−LOI200)/LOI600 metric increased in theandosol and cow manure compost, while it decreased in the case of paddy soil (Fig. 3b). This reflects
differences in organic matter decomposition characteristics among these materials due to a differencein the initial pH-ORP (Fig. 2) and organic matter content (e.g., Fig. 3a) of each material.
New Index for Evaluating the Decomposition of Organic MatterTo explore the source of the difference in the performance between (LOI300−LOI200)/LOI600 metricand LOI200/LOI600 metric, Figure 4 shows the behavior of LOI200/LOI600 metric. This metric had astrong correlation (R > 0.87) with operation time for each material (Fig. 4a), but was not as strong as(LOI300−LOI200)/LOI600 metric (R = 0.99). Furthermore, it was observed that LOI200/LOI600 metric
had a strong correlation (R > 0.87) with (LOI300−LOI200)/LOI600 metric (Fig. 4b), suggesting thatLOI200/LOI600 metric can be used instead of (LOI300−LOI200)/LOI600 metric. This means that
LOI200/LOI600 metric can be viewed as a new index for evaluating the decomposition of organic
matter in these materials. This may provide an advantage in terms of cost and time savings, because
only the weight losses on ignition at 200 and 600°C must be measured for this simplified metric.
As noted earlier, we observed different tendencies in the variation of (LOI300−LOI200)/LOI600
metric (Fig. 3b). According to Cuypers et al. (2002), the burning temperature of organic matter
increases as the organic matter becomes more humified. For the cow manure compost,(LOI300−LOI200)/LOI600 metric increased (Fig. 3b), while LOI200/LOI600 metric decreased (Fig. 4a).
This suggests a release of the organic matter burned at 200−300°C (OM200−300) and the loss of theorganic matter burned at 50−200°C (OM50−200). The paddy soils, however registered a formation ofOM50−200 and a loss of OM200−300.
Generally, cow manure compost contains the organic matter that readily decomposes, while theorganic matter in paddy soils and andosols tends to be humified. Hence, these results reflect
decomposition of OM200−300, leading to the release of OM50−200 in paddy soils, while the humificationof OM50−200 leads to the formation of OM200−300 in cow manure compost. It can be said that the tendencyof the temporal change in each metric reflects the decomposition characteristic of organic matter.

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0.8

0.6
0.5

0.7
LOI200/LOI600 (-)

LOI200/LOI600 (-)

0.8Andosols
Paddy soil
Cow manure compost

0.7

R= 0.960.40.3

R= -0.87

0.2

R= -0.91

0.1

0.6
0.50.4

R= -0.87

0.3
0.2

0.10

R= -0.97

R= -0.8900123Operation time (month)4(a) Operation time and LOI200/LOI600

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8(LOI300-LOI200)/LOI600 (-)(b) (LOI300-LOI200)/LOI600 and LOI200/LOI600

Fig. 4 Relations between operation time and (LOI300-LOI200)/LOI600 with LOI200/LOI600
CONCLUSION
Our laboratory experiments examined the validities of LOI300/LOI600 and (LOI300−LOI200)/LOI600
metric in evaluating changes in the state of organic matter present in paddy soil, andosol, and cow
manure compost. Temporal changes in LOI300/LOI600 metric were not observed, while(LOI300−LOI200)/LOI600 metric had a strong correlation with operation time. This reinforced the ideathat (LOI300−LOI200)/LOI600 metric can represent the decomposition of organic matter, either inlittoral sediments or agricultural soils, including compost materials. Furthermore, we found out that
LOI200/LOI600 metric had a strong correlation with (LOI300−LOI200)/LOI600, suggesting that thismatric can be used as a simpler proxy for (LOI300−LOI200)/LOI600 metric. Finally, the proposedmetrics, i.e., (LOI300−LOI200)/LOI600 and LOI200/LOI600, were used to identify the decomposition andhumification of organic matter. For example, the humification of organic matter caused a decreasein LOI200/LOI600 metric and an increase in (LOI300−LOI200)/LOI600 metric. The tendency of thetemporal change in each metric reflected the decomposition characteristic of organic matter.
ACKNOWLEDGEMENTS
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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