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

Research article
erd

Temporal and Spatial Distributions of Soil Apparent
Electrical Conductivity in Paddy Rice Fields, Khon Kaen,Thailand
KATSUYUKI SHIMIZU*Tottori University, Tottori, Japan
Email: shimizu@tottori-u.ac.jp

NAHO NOHARA
Hokkaido University, Hokkaido, JapanTADAO YAMAMOTO
Hokkaido University, Hokkaido, Japan

TAKASHI KUME
Ehime University, Ehime, JapanCHULEEMAS BOONTHAI IWAI
Khon Kaen University, Khon Kaen, Thailand
Received 15 January 2019

Accepted 5 May 2020(*Corresponding Author)

Abstract The temporal and spatial distribution of soil apparent electrical conductivity (ECa)was measured to elucidate the process of soil salinization in rainfed paddy rice fields in Khon
Kaen, Thailand. Soil ECa was measured in 2.3 ha of paddy fields using a ground conductivity
meter, utilizing the principle of electromagnetic induction. The average soil ECa wasmeasured at 150 cm and 75 cm depth at 10 m intervals. Observations were conducted every
two weeks for two months, from mid-May 2018 to mid-July 2018. The collected data wereinterpolated using an inverse distance weighted method with 1 m intervals and then analyzed
in terms of temporal and spatial distributions. The results showed that (1) higher soil ECa
was observed in the northern part of the study area near a swamp throughout the monitoring
period; (2) the average soil ECa at a depth of 0–75 cm ranged from 233 to 282 mS/m, while
it ranged from 236 to 268 mS/m at a depth of 0–150 cm; (3) the average soil ECa in the upper
layer was slightly higher than that of the lower layer in May, but this difference became
smaller over time and the situation reversed in mid-July; and (4) a larger spatial distribution
was observed in the upper layer compared with the lower layer based on the difference of acoefficient of variation of soil ECa. A soil salinization process was clearly observed.
Keywords electric conductivity, soil salinization, rainfed paddy fields, wet season, resilience

INTRODUCTION
In Thailand, saline soil covers an area of about 2.3 million ha, especially in inland areas (Pongwichianet al., 2013), and the accumulation of salt occurs in rainfed paddy fields. A major source of the salt
is the Rock Salt Member of the Maha Sarakham Formation, which consists mainly of rock salts,
wherever it is exposed or lies close to the surface. However, other potential sources of salt wereformerly classified as salt-free strata (Wongsomsak, 1986). The management of these soils is very
problematic and strictly depends upon the degree of salinization and the mode of the salt-forming
processes. As general countermeasures, the management of these soils involves leaching, proper
drainage control, land leveling, surface mulching, organic amendments, deep plowing, and the useof salt-tolerant plant varieties (Land Development Department). However, before taking any
countermeasures, it is important to understand to what extent fields are affected by salt. ToparkNgarm et al. (1990) conducted physical and chemical analyses of soil samples collected from a depthⒸ ISERD

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IJERD – International Journal of Environmental and Rural Development (2020) 11-1of 30 cm at 5 cm intervals in Ban Thum, Khon Kaen. Hanchai et al. (2010) monitored changes inbiotic activity in relation to the chemical and physical properties of salt-affected soil before and aftertree planting. However, the high-resolution temporal and spatial distributions of salt accumulation
have not been well studied in the Khon Kaen Province.
OBJECTIVE
This study aimed to elucidate the high-resolution temporal and spatial distribution of soil apparent
electrical conductivity (ECa) in paddy fields and areas adjacent to them in Khon Kaen, Thailand.
When combined with groundwater level fluctuation, rainfall, and microtopography data, themonitoring results should contribute to the understanding of the soil salinization mechanism.
METHODOLOGYMeasurement and analysis of soil ECa
Although there are many saline-damaged farmlands in northeastern Thailand, a rain-fed paddy rice
block of Ban Pasan village in Khon Kaen was selected as a study site because of its clear spatial
distribution of the degree of salinity. The study area, in Ban Pasan, Khon Kaen, encompassed paddy
fields and an adjacent forest and swamp (N16.296° W102.612°; elevation 243 m). The difference inelevation between the south and north ends of the monitoring area is 28 cm and the area is quite flat(< 1/1,000). The groundwater level in the study area ranged from 140 cm below the ground surface
in April to 0 cm in September. Soil ECa was measured using a ground conductivity meter (EM38MK2; Geonics Limited), which simultaneously measured both quad-phase (conductivity) and inphase (magnetic susceptibility) components within two distinct depth ranges, without any
requirement for soil-to-instrument contact (GEONICS). With a maximum effective exploration depthof 150 cm, this meter is used for agricultural, archaeological, and general soil science purposes e.g.
Kume et al. (2003), Zukemura et al. (2016), and Rafael et al. (2011).
N100 mArea of soil EC

Fig. 1monitoring
Study area
The study area is shown in Fig. 1. The yellow dotted line indicates the domain of the ECa survey.
Rice had been planted in a rainfed paddy in the southern part of the domain, with a fallow area in thecenter. A swamp was located to the northeast, and a tree plantation was located to the west. Soil ECa
measurements were conducted at – 10 m horizontal intervals, with about 150–160 points, at twodifferent depths, 75 cm and 150 cm vertically. Measurements were collected five times between 16Ⓒ ISERD

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May 2018 and 11 July 2018, every two weeks. The collected soil ECa data were interpolated using
an inverse distance weighted method with a 1 m mesh.
RESULTS AND DISCUSSION
Temporal Distribution of Soil ECa
The average soil ECa in the upper soil layer (0–75 cm) and the deeper layer (0–150 cm) aresummarized in Tables 1 and 2, respectively. The average soil ECa between 0 and 75 cm ranged from
233–282 mS/m, while it ranged from 236–268 mS/m between 0 and 150 cm. The average soil ECa
in the upper layer was slightly higher than that of the lower layer in May, but this difference reduced
over time, with the situation being reversed in mid-July. The average ECa peaked in late May, withan average ECa of 282.3 mS/m at 0–75 cm and 268.0 mS/m at 0–150 cm. The highest median ECa
and the highest coefficient of variation (CV) occurred in late May at both depths.
Table 1 Summary of soil ECa in the shallow layer (0–75 cm)
Date
2018/5/16
2018/5/30
2018/6/14
2018/6/27
2018/7/11

Average
247.8
282.3
243.0
242.0
233.1

Max
928.6
989.1
982.9
971.4
895.9

Min
32.6
35.2
26.1
24.1
27.2

Median
214.5
217.2
203.8
217.4
214.5

mS/m
STD
141.9
198.0
149.7
150.5
133.0

CV
0.57
0.70
0.62
0.62
0.57

mS/m
STD
103.3
155.7
120.8
120.2
112.2

CV
0.42
0.58
0.51
0.50
0.47

Table 2 Summary of soil ECa in the deeper layer (0–150 cm)
Date
2018/5/16
2018/5/30
2018/6/14
2018/6/27
2018/7/11

Average243.3
268.0236.1
241.2
237.6

Max
769.7
987.3
928.4
773.0
802.2

Min
52.7
39.2
35.1
38.2
41.2

Median214.0
225.2
211.0
223.4
216.5

Fig. 2 Average soil apparent electrical conductivity (ECa) in the upper (0–75 cm)
and lowest layers (75–150 cm) and daily rainfallⒸ ISERD

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The average soil ECa in the lowest layer (75–150 cm) was then calculated, as shown in Fig. 2.
The difference between the ECa in the upper soil layer (0–75 cm) and the lowest soil layer (75–150cm) was greatest during May, but the difference reduced, and the ECa of the lowest layer was higher
than that of the upper layer from late June. It is generally understood that saline water moves upwards
by capillary action, and the increasing difference in ECa in the upper and lowest soil layers can beexplained by capillary action. The reversal of the difference between the upper and the lowest layers
might have been due to rainfall during late June and there was actually about 100 mm of rainfall (atthe Khon Kaen airpoirt which is the nearest whether station from the study site) during the three days
from June 25-27, 2018.
Spatial Distribution of Soil ECa
Figure 3 shows the spatial distribution of soil ECa at different depths. Higher soil ECa at both depths
was observed in the northern part of the study area near the swamp throughout the monitoring period.
There were some spot high ECa values in the southern paddy fields in May; however, these werelower in June and July. At a glance, the ECa values were highest during May, then decreased in Juneand July. The ECa in and around the rainfed paddy field was about 200 mS/m. The FAO states thatrice is a salt-sensitive crop, with an ECa threshold of 300 mS/m (FAO, 2002). As shown in Figure 3,
the ECa in the central and northern areas exceeded this threshold; therefore, these areas were left
fallow due to the high salinity. The reason of high salinity of the central and northern part of thestudy area is to be discussed by considering spatial and temporal distribution of groundwater level
fluctuation, soil type, and inundation of ground surface.
N

16-May30-May14-Jun

27-Jun11-JulHorizontal (average 0–75 cm)

16-May30-May14-Jun27-Jun11-JulVertical (average 0–150 cm)

Fig. 3 Temporal and spatial distributions of soil apparent electrical conductivity (ECa)Ⓒ ISERD

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CONCLUSION
A high-resolution investigation of soil ECa over a large area was conducted using the electromagneticinduction method. The ECa of two soil layers (0–75 cm and 0–150 cm depth) was measured and asoil salinization mechanism discussed. The results can be summarized as follows: (1) higher soil ECa
was observed in the northern part of the study area near the swamp throughout the monitoring period,(2) the average soil ECa at a depth of 0–75 cm ranged from 233 to 282 mS/m, while it ranged from
236 to 268 mS/m between 0 and 150 cm, (3) the average soil ECa in upper layer was slightly higher
than that of the lower layer in May, but the difference between them was reduced over time and thesituation reversed in mid-July, and (4) a larger spatial distribution was observed for the upper layercompared with the lower layer, judged by the difference of the CV of the soil ECa. Soil salinization
during the wet season was clearly observed. A comprehensive analysis of the soil salinization process,considering the fluctuation of the groundwater level, rainfall, and microtopography is planned for thefuture.
ACKNOWLEDGEMENTS
This work was supported by JSPS KAKENHI Grant Numbers JP17H04630 and International
Platform for Dryland Research and Education, Tottori University.
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