J. Jpn. Soc. Soil Phys.
土壌の物理性No. 152 p.9 ∼ 23 (2022)

Effects of inﬂuent water quality on potassium
leaching from paddy soil

Tomijiro KUBOTA1 , Ryoichi MASUDA2 , Tatsuhiro NISHIKIORI3 , Shigeto FUJIMURA3

Abstract: Additional potassium (K) fertilization, a practice undertaken to reduce the absorption of radioactive cesium (Cs) by rice in paddy ﬁelds where radioactive Cs was
deposited, is gradually being discontinued owing to costissues. Understanding the dynamics of K in paddy ﬁelds
and establishing a reliable and efﬁcient K fertilization technique is necessary to continue reducing the absorption ofradioactive Cs by paddy rice. Therefore, to help understand the factors that cause K leaching from paddy soils
and the effect of K supply by irrigation water, we used acompact thumb-size column (diameter 2.1 cm × length 5cm) and examined the K leaching from paddy soil following inﬂuents of different concentrations and cation composition ratios. The test soil (ﬁne-grained ordinary lowland
paddy soil) was collected from a paddy plot that received
additional potassium application. The coefﬁcients of variation during soil ﬁlling of the columns were less than 2 %on average and approximately 6 % for soil exchangeable
K in the column tests, with sufﬁcient reproducibility. The
equilibrium content of soil exchangeable K formed after
water percolation was determined by the concentration ofK+ and the composition ratio of major cations — K, Na,
Ca, and Mg — in inﬂuent water. The potassium adsorption ratio (PAR) of inﬂuent water was a valid indicator of
K leaching. The effects of ion species on K leaching werein the order Mg2+ > Ca2+ > Na+ based on a 1 mmol L−1change in their concentration and Ca2+ > Mg2+ > Na+based on a 10 % change in their concentration, with divalent cations having a greater effect and Na+ having asmaller effect.
Key Words : potassium leaching, thumb-size column test,
PAR (potassium adsorption ratio), irrigation water quality,
paddy soil

1. Introduction
The accident at the Tokyo Electric Power Company
1 Institute for Rural Engineering, National Agriculture and Food Research

Organization (NARO), 2-1-6 Kannondai, Tsukuba, Ibaraki, 305-8609,
JAPAN. Corresponding author: Tomijiro KUBOTA
2 Former NARO.
3 Agricultural Radiation Research Center, Tohoku Agricultural ResearchCenter, NARO, 50 Harajukuminami, Arai, Fukushima, Fukushima, 9602156, Japan.
2022 年 7 月 31 日受稿 2022 年 10 月 31 日受理

Holdings’ Fukushima Daiichi Nuclear Power Station
caused by the Great East Japan Earthquake and subsequent tsunami in March 2011 resulted in the release oflarge amounts of radioactive materials. In farmland whereabsorption of radioactive materials by crops was a concern, an additional application of potassium (K) fertilizer
effectively reduced radioactive Cs concentrations in rice
plants (Fujimura et al., 2013) and supplemental K application was initiated in 2012. As a result, the quantity ofbrown rice exceeding the food standard for radioactive Cs
content has greatly reduced since 2012, and a signiﬁcanteffect of additional K fertilization on the reduction has
been conﬁrmed. However, starting in 2015, trials began to
discontinue additional K applications due to high expenditures, and since 2016, an increasing number of regions
have gradually discontinued additional K applications (Fujimura, 2021). Therefore, establishing a reliable and efﬁcient K fertilization technique is required for rice cultivation, and understanding the dynamics of K in the soil is a
critical issue.
The migration of K in the soil is affected mainly bycation exchange and selective adsorption by some clay
minerals. Cation exchange was identiﬁed in the late 1800s
as a phenomenon in which bases of alkali salts, including K
in soils, were exchanged and transferred primarily throughthe negative charge possessed by clay particles (Johnston
et al., 2016). Thus, K migration is closely related to the dynamics of other major cations. The magnitude of the relative cation selectivity of clays is affected by the ionic radius
and the amount of charge per ion, which is known experimentally to be in the order Cs+ > Rb+ > K+ ∼
= NH+
4 >
+
+
Na > Li for the monovalent cations (Bolt et al., 1978).
Additionally, 2:1 type layered silicate minerals, such as
illite and vermiculite, are highly selective for K+ , alongwith Cs+ , Rb+ , and NH+
4 (Sawhney, 1972; Goulding,1987), which also affects K mobility. These ions withsmall ionization and relatively large ionic radii are easily
dehydrated and adsorbed as naked ions to be sandwiched

10

土壌の物理性between the unit layers of 2:1 type layered silicates (Wada,
1997). However, because soils are mostly mixtures ofmany clay minerals and contain many cations, it is generally difﬁcult to describe the dynamics of ions in a rigorous theoretical or experimental manner (Evangelou andPhillips, 2005). Furthermore, the migration of K in theform of clay colloids and the ionic form has been pointedout (Brouder et al., 2021).
Some studies on the soils of upland ﬁelds have investigated the effect of the quality of irrigation water onsoil K leaching (Kolahchi and Jalali, 2007; Jalali and
Merrikhpour, 2007; Heidari and Jalali, 2016). However,studies on K leaching in paddy soils have been scarce until
recently.
The content of exchangeable K in soil signiﬁcantly
affects the radioactive Cs concentration in brown rice.
Therefore, among studies related to Fukushima, studies on
K balance in paddy ﬁelds, including studies on K supply
by irrigation water and leaching by percolation, have been
conducted in recent years. Nishikiori et al. (2020) measured the K balance, including the K supply from irrigationwater and K leaching to the subsurface, in a paddy ﬁeld located on an alluvial river site with a considerable level of
percolation. They found that K leaching was greater when
there was excessive irrigation water.
In cooperation with ﬁve prefectural agricultural experiment stations in Japan, the National Agriculture andFood Research Organization (NARO) compiled guidelines(NARO, 2021) for applying K in paddy rice based on the
K balance of paddy soils to promote appropriate K fertilization in paddy ﬁelds. According to the report, the presence or absence of return of rice straw has a decisive effecton the K balance. If the rice straw is returned, it almost
compensates for the amount of K carried out by harvest;
therefore, the K balance of the paddy ﬁeld will not become
negative due to fertilization and supply from irrigation water. Here, the amount of K supplied by the irrigation water
was assumed to be approximately 15 kg ha−1 when the K+
concentration in the irrigation water was 2.5 mg L−1 .
As mentioned above, the dynamics of K in agriculturallands, including the quantity of K supplied through water, has been a focus of research in recent years. It can be
expected that differences in the quality of irrigation wateror water inﬁltrating into the soil would affect K leaching.
However, few studies have quantitatively evaluated the effect of water quality on the dynamics of K in paddy soils.
Therefore, in this study, we attempted to investigate therelationships between various inﬂuent water qualities andresponses of K leaching by using compact thumb-size column experiments. We ﬁrst examined the effects of different concentrations of major cations in the inﬂuent wa-

第 152 号（2022）

ter on the leaching of K from paddy soils. Subsequently,
we hypothesized that the K+ concentration of inﬂuent andthe composition ratio of the major cations would affect theleaching. The potassium adsorption ratio (PAR) was used
as an indicator to test this hypothesis.
In addition, leaching tests were conducted using thumbsize columns. Column sizes used in similar studies varied;for example, 5 cm in diameter (D) × 20–50 cm in length(L) (Matsukawa and Nakano, 1992), 4.9 cm (D) × 14 cm(L) (Matsukawa et al., 1996), 14.6 cm (D) × 60 cm (L)(Li et al., 1994), 2 cm (D) × 15 cm (L) (Colombani et al.,2015), and 5 cm (D) × 20 cm (L) (Li et al., 2013). In
recent years, the International Organization for Standardization (ISO) has promoted the international standardization of the “Up-ﬂow percolation test” (ISO, 2019). This
standard is primarily intended to evaluate the leaching ofcontaminants. However, inorganic ions such as Ca2+ andCl− were also evaluated by Yasutaka et al. (2017). In thatstandard, the column size is speciﬁed as D 5–10 cm and
L 30 ± 5 cm. As described above, column diameters ofapproximately 5–10 cm and 15–30 cm lengths are often
used. If the required results can be obtained, the smaller
the column, the more labor-saving the experiment can be.
In this study, we attempted to use a column with a compact
size of 2.1 cm (D) × 5 cm (L) and examined the method’s
reproducibility.

2. Potassium adsorption ratio (PAR) andits characteristics
It is important to avoid sodium (Na) hazard that causes
salt damage and reduces permeability due to soil particle
dispersion in irrigated agriculture in arid and semi-arid regions. Therefore, the sodium adsorption ratio (SAR) has
been deﬁned and widely used as an indicator of the risk ofirrigation water (USDA, 1954).
SAR = √

[Na+ ][Ca2+ ] + [Mg2+ ](1)

where [Na+ ], [Ca2+ ], and [Mg2+ ] are the concentrations ofeach ion (mol L−1 ). Using irrigation water with high SAR
promotes the leaching of Ca2+ , Mg2+ , K+ , and phosphorus from the soil and affects groundwater quality (Jalali
and Merrikhpour, 2007). Sposito and Mattigod (1977)
examined the theoretical chemical basis for the potential
SAR of water in exchange equilibrium with soil and foundthat the Gapon cation exchange equilibrium constant is thechemical basis for the SAR.
Similar to Na+ , K+ , one of the major monovalentcations, can cause harmful effects when presentin excess;論文：Effects of inﬂuent water quality on potassium leaching from paddy soil

however, its effects are weaker than that of Na+ . Therefore, as an indicator, PAR was deﬁned similarly to SAR,
with Na+ replaced by K+ (USDA, 1954).
PAR = √

[K+ ][Ca2+ ] + [Mg2+ ](2)

The characteristics of this equation as an indicator of theeffect of K in irrigation water are that it ignores the effect of
Na+ , does not make a chemical distinction between Ca2+and Mg2+ , and reﬂects the effect of concentration as wellas composition ratio. Assuming a solution with a constantcomposition ratio of cations, PAR is proportional to thesquare root of the [K+ ].
The PAR of the soil solution reﬂects the amount of K
adsorbed on the exchange surface of clay and soil organic
matter, which may be an explanatory factor for aggregation and dispersion within the soil. Shainberg et al. (1987)
studied the effect of PAR, among other indicators, on thedecrease in hydraulic conductivity. The PAR has also been
examined as a factor explaining its effectiveness as a water
quality indicator for assessing the irrigation history of agricultural land and groundwater environments. Smiles andSmith (2004) examined the PAR (called KAR in the paper)of soil solutions in agricultural land irrigated with piggery
efﬂuent and showed that PAR is a good indicator of the effect of irrigation with piggery efﬂuent. Grifﬁoen (2001) investigated the effect of fertilizer application to agriculturalland on groundwater quality and showed that the behaviorof K is explained well using PAR at near neutral pH, where
the major cations in the groundwater are divalent.
Parameters similar to SAR and PAR are found in many
theoretical ion exchange equations (Bresler et al., 1982).
Unlike those parameters, however, SAR and PAR are expressed in terms of ion concentration, not ion activity.
Therefore, they do not consider the reduction of free ionsdue to the formation of complexes. In a case study using the K adsorption ratio of soil solution and considering
the ionic activity, Ramamoorthy and Paliwal (1965) studied the availability of soil K when applied to paddy rice
grown in 40 soils with different properties in India. They
found a correlation between the potassium adsorption ratio
and crop response in many districts, considering the ionicactivity.
A parameter similar to PAR was used in the study of the
Q/I relationship. The Q/I relationship quantiﬁes the relationship between the K intensity factor (factor I) and K
absorption/release by the soil (factor Q), using factor I as
the activity
ratio of ion pairs of K+ and Ca2+ + Mg2+ , i.e.,√
aK+ / (aCa2+ + aMg2+ ) (Beckett, 1964a). Speciﬁcally, theQ/I relationship examines the change in soil potassium absorption relative to the factor I of the solution by reacting

11

a 0.01 M CaCl2 solution with several levels of K concentration and soil at a constant solid-liquid ratio and for aconstant time (Eguchi et al., 2017). If the activity ratio offactor I can be regarded as proportional to the concentration ratio, it is isomorphic to PAR when replaced by the√concentration ratio[K+ ]/ ([Ca2+ ] + [Mg2+ ]) (Evangelou
et al., 1986).
Using the concentration of K+ in the inﬂuent and thecomposition of K in the major cations as multiple explanatory variables may easily explain the K leaching. However,in this study, we would like to reduce the number of explanatory variables and concentrate on PAR as a simpler
indicator to grasp the phenomenon and explain the leaching and accumulation of soil exchangeable K.

3. Materials and methods
3.1 Test soil
The test soil samples were collected from paddy ﬁeld
soil in Matsukawa, Fukushima City, Fukushima Prefecture, and passed through a 2 mm diameter sieve after airdrying. The soil is classiﬁed as ﬁne-grained ordinary lowland paddy soil (NARO, 2019). The plot where the test
soil was taken had been fertilized with additional K to reduce the absorption of radioactive Cs, and the soil was
sampled early in the season after rice planting. In addition, rice straw return was practiced in the plot. The parent
material of the soil is tuffaceous sandstone and siltstone,
called the Matsukawa Formation, a late Miocene or early
Pliocene terrestrial-water stratiﬁed sediment (Yoshida etal., 1983; Sakaguchi, 1995). The distribution area of the
Matsukawa Formation is surrounded by granodiorite zones
formed during the Paleogene, which inﬂuences its characteristics. The physicochemical properties of the test soil
and its mineral composition are shown in Table 1. The
mineral composition of the test soil is rich in kaolinite andcontains some 2:1 type clay minerals such as smectite, vermiculite, and illite.
3.2 Overview of experimental methods
Three different experiments were conducted in thisstudy. This section focuses on matters common to all threeexperiments, and the speciﬁcs of each experiment are described in Section 3.3.
The apparatus used in the experiments is shown in Fig.
1. A small thumb-sized soil column (2.1 cm in diameter,
5.0 cm in length, and 17.3 cm3 in volume) was used forthe soil column tests. The cylindrical acrylic column was
carefully ﬁlled to ensure uniform ﬁlling of the test soil.
The difference in mass between the empty and ﬁlled column was measured during ﬁlling, and the dry bulk density
was calculated. The dry bulk density value was determined

土壌の物理性

12

第 152 号（2022）

Table 1 Physical and chemical properties of the test soil.
Attribute

Description

pH (H2 O)

6.0

Soil-water extracts at 1:5

EC0.09 (dS m−1 )Soil-water extracts at 1:5

CEC

17.6 (cmolc kg−1 )Shaking extraction method (10 % KCl)

Exchangeable Ca (CaO)

2.76 (g kg−1 )Shaking extraction method (1M CH3 COONH4 )Exchangeable Mg (MgO)

0.50 (g kg−1 )Shaking extraction method (1M CH3 COONH4 )Exchangeable K (K2 O)

0.40 (g kg−1 )Simpliﬁed method by using a compact K meter

Phosphate absorption coefﬁcient (P2 O5 )0.54 (g kg−1 )Orthophosphoric acid method

Humus

0.019 (kg kg−1 )Kumada’s simpliﬁed method

Soil texture

CL

Coarse sand: 26 %, ﬁne sand: 18 %, silt: 33 %, clay: 23 %Abundant

Kaolinite

Clay minerals

CommonSmectite, vermiculite, illite, chlorite-vermiculite(By XRD)

Trace

Chlorite

None

Halloysite

Fig. 1 Schematic of apparatus.

as approximately 1.12 g cm−3 using the air-dried soil, thevariation of which is discussed in Section 4.1.
The quality of the inﬂuent water used for the column
tests was determined based on the concentration of major
cations — K, Na, Ca, and Mg — in the irrigation waterof the plot from which the test soils were collected (Table2). The inﬂuent water was prepared by dissolving reagents(KCl, NaCl, CaCl2 , and MgSO4 ) in deionized water as
needed to achieve the required concentrations. The speciﬁc
concentration of each ion in the inﬂuent in each experimentis described in Section 3.3.
A peristaltic pump (MP-2010, Tokyo Rikakikai, Tokyo,
Japan) was used to supply inﬂuent water to the soil column. First, before the start of the test, the inﬂuent water

was slowly fed from the bottom of the column to saturate
the soil, and the water supply was temporarily stopped until the start of the test. This may have caused air to be entrapped, but its effect was not considered. The water ﬂow
Table 2 Cation concentration and PAR of irrigation water intest soil taken ﬁeld during irrigation period in 2020.
Cation concentration (mmol L−1 )

Mean*
Max.
Min.

K+

Na+

Ca2+

Mg2+

0.036
0.052
0.023

0.19
0.31
0.095

0.29
0.37
0.23

0.13
0.21
0.091

∗ Samples were taken 15 times between May and August.

PAR
0.055
0.079
0.037

論文：Effects of inﬂuent water quality on potassium leaching from paddy soil

test was then started at a set ﬂow rate (0.35 mL min−1 ) andtime. In the test, the water was passed through the column
at a constant ﬂow rate from the bottom to the top of the column. The tests were conducted in a laboratory at a room
temperature of 21–25 ◦ C.
The efﬂuent from the column was collected in apolyethylene container (Experiments 1 and 3) or fraction
collector (Experiment 2), and the concentrations of major
cations were determined using an ion chromatography system (ICS-1500, Thermo Fisher Scientiﬁc, Waltham, MA,
USA). However, this paper does not cover the analyticalresults of the samples in Experiments 1 and 3.
In some tests, the water pressure at the bottom of the column was measured with a pressure sensor (HTV-200KP,
HTV-400KP, Sensez, Tokyo, Japan), and the saturated hydraulic conductivity Ks cm s−1 was calculated using thefollowing equation
Ks =

q
L
×
× 0.0981
A ∆H(3)

where q cm3 s−1 is the ﬂow rate at the time of pressure
measurement, A cm2 is the cross-sectional area of the column, L cm is the column length, and ∆H kPa is the pressure difference between the upper and lower ends of thecolumn. In the experiment, the water pressure at the upper end of the column was assumed to be negligibly small
compared to the water pressure at the lower end, and ∆H
was assumed to be equal to the pressure at the bottom of
the column.

13

ﬂow volume. The qualities of the inﬂuent water (Table3) were set as standard water (hereinafter referred to as“1×”), low-concentration water (hereinafter referred to as“0.3×”), high-concentration water (hereinafter referred toas “3×”), and deionized water (hereinafter referred to as“DI”). The water quality of the standard water (1×) was
determined with reference to the average of actual irrigation water quality. However, the range of variation of values (0.3× − 3×) was set wider than the actual range ofvariation in order to examine the response of K leaching tochanges in inﬂuent water quality. The ﬂow volumes were
set in eight steps ranging from 120 to 7500 mm, with a
ﬂow rate of 0.35 mL min−1 and a ﬂow time of approximately 2–120 h. Since the sampling plots had relatively
low inﬁltration rates and assuming that the water ﬂow volume for one irrigation season of paddy rice is 1200 mm,
the ﬂow volumes set in the experiment are roughly equivalent to 0.1–6 years of the ﬂow volume. After each test,
the soil was air-dried and analyzed for soil exchangeable
K. Furthermore, in Experiment 1, there was basically norepetition of the same experimental conditions.
3.3.2 Experiment 2: Characteristics of major
cations leaching from soil columns
In Experiment 2, we investigated changes in the efﬂuent of major cations from the soil columns over time bycollecting efﬂuent at predetermined time intervals and an-

After the test, the column was dismantled, the mass of
the wet soil was measured, and the volumetric water content was calculated from the wet mass of the soil and themass of the sample when the column was ﬁlled. Since themass of air-dried soil was used to calculate volumetric water content, there may have been a slight underestimation.
After the test, the soil was air-dried and then measured forsoil exchangeable K. The exchangeable K was analyzed
in triplicate using a simple analytical method (Nakayama,
2013; Nakayama et al., 2018) for the exchangeable K content of soil using a compact K ion meter (LAQUA twin
K-11, Horiba, Kyoto, Japan) as shown in Fig. 2.
Furthermore, to examine the reproducibility of this test
using small columns, the coefﬁcients of variation of dry
bulk density and volumetric water content in Experiments
1 and 3 and water ﬂow volume and soil exchangeable K in
Experiment 3 were examined.
3.3 Speciﬁc experimental conditions
3.3.1 Experiment 1: Effect of water quality andﬂow volume on K leaching
Experiment 1 was designed to investigate the responseof K leaching to changes in inﬂuent water quality andFig. 2 Procedures for measuring soil exchangeable K using
a compact ion meter (Nakayama, 2013; Nakayama et al., 2018with some modiﬁcation).

土壌の物理性

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第 152 号（2022）

Table 3 Cation concentration of inﬂuent water in Experiments 1 and 2.
Item
1×
0.3×
3×
DI

Cation concentration (mmol L−1 )Description
Standard water
Varying 4 cations simultaneously
Deionized water

K+

Na+

Ca2+

Mg2+

0.031
0.0094
0.094
0.00

0.17
0.051
0.51
0.0

0.35
0.11
1.1
0.0

0.15
0.045
0.45
0.0

PAR
0.044
0.024
0.077
–

Table 4 Cation concentration and PAR of inﬂuent water and number of test repetitions in Experiment 3.
Description

Cation concentration (mmol L−1 )Concentration factor

K+

Na+

Ca2+

Mg2+

PAR

Repetition

Standard water

1×

0.031

0.17

0.35

0.15

0.044

4

Varying 4 cations
simultaneously

0.3×
3×
10×

0.0094
0.094
0.31

0.051
0.51
1.7

0.11
1.1
3.5

0.045
0.45
1.5

0.024
0.077
0.14

3
3
3

Varying only K+ conc.

0.3×
3×
10×

0.0094
0.094
0.31

0.17
0.17
0.17

0.35
0.35
0.35

0.15
0.15
0.15

0.013
0.13
0.44

3
3
3

Varying only Na+ conc.

0.3×
3×
10×

0.031
0.031
0.031

0.051
0.51
1.7

0.35
0.35
0.35

0.15
0.15
0.15

0.044
0.044
0.044

3
3
3

Varying only Ca2+ conc.

0.3×
3×
10×

0.031
0.031
0.031

0.17
0.17
0.17

0.11
1.1
3.5

0.15
0.15
0.15

0.061
0.029
0.016

3
3
3

Varying only Mg2+ conc.

0.3×
3×
10×

0.031
0.031
0.031

0.17
0.17
0.17

0.35
0.35
0.35

0.045
0.45
1.5

0.050
0.035
0.023

3
3
3

Varying Ca2+ andMg2+ conc.

0.3×
3×
10×

0.031
0.031
0.031

0.17
0.17
0.17

0.11
1.1
3.5

0.045
0.45
1.5

0.081
0.026
0.014

2
1
2

alyzing the water quality. The inﬂuent and ﬂow rates were
the same as in Experiment 1. The ﬂow time was set to 72
h (the set ﬂow volume was approximately 4600 mm), andthe water was collected 22 times between 0.2 h and 71.9 husing a fraction collector (Model 2110, Bio-Rad, Hercules,
CA, USA). The collected water was measured for pH (pH22, Horiba, Kyoto, Japan) and analyzed for major cations.
There were no repetitions in Experiment 2.

and 10×) for each of the four major cations, and a combination of two cations (Ca2+ , Mg2+ ).

3.3.3 Experiment 3: Effects of different cation
composition ratios in the inﬂuent on K leaching

4. Results and discussion

In Experiment 3, the effects of different cation composition ratios in the inﬂuent on K leaching and accumulation were investigated. There were 19 patterns of inﬂuent
quality settings (Table 4), and the tests were conducted intriplicate with some exceptions. The quality of the inﬂuent
water consisted of standard water (1×) and its variations(0.3×, 3×, and 10×), variable concentrations (0.3×, 3×,

4.1 Reproducibility of compact column tests

These tests were conducted assuming a water ﬂow volume of 1200 mm and a ﬂow time of 20 h. After the tests,
the soil samples were air-dried and analyzed for soil exchangeable K. In some of the tests, settings and results
were shared with Experiment 1.

The advantage of using a smaller column is that asmaller amount of test soil to be prepared. Therefore, evenwhen designing tests that include repetitions under various conditions, as in this study, the total amount of sample
required is small. Correspondingly, the labor required totreat the sample before and after the test and the amount論文：Effects of inﬂuent water quality on potassium leaching from paddy soil
Table 5 Dry bulk density, volumetric water content, and their
coefﬁcients of variation (CV) of soil columns in Experiments 1
and 3.
Dry bulk
density*(g cm−3 )Volumetricwater content**(m3 m−3 )Mean CV ( %) Mean
Experiment 1
Experiment 3

1.12
1.13

1.6
1.8

0.562
0.595

n***

CV ( %)
6.8
1.7

45
64

∗ Dry bulk density might be slightly overestimated because the massof air-dried soil was used in the calculation.
∗∗ Volumetric water content was measured after dismantling thecolumn.
∗∗ Volumetric water content might be slightly underestimated because

the mass of air-dried soil was used in the calculation.
∗∗∗ Experiments 1 and 3 share some tests.

Table 6 Fluctuation of ﬂow volume and soil exchangeable K
content in column tests in Experiment 3.
Flow volume (n = 64)
(mm)
Mean

Min.

Max.

1241

1145

1339

CV ( %)

3.2

Soil exchangeable K*
CV ( %)
Mean

Min.

Max.

5.8

0.9

11.8

∗ 17 tests of three repetitions each.

of water needed are also small. In contrast, because thecolumn is small, lower reproducibility of the test, which
may be reduced by, for example, non-uniform sample ﬁlling and its effects, is a concern. Therefore, in this section,
we discuss the reproducibility of the sampling method.
In total, 96 tests were conducted in the three different
experiments. Using some of these data, Table 5 shows theproperties and the coefﬁcients of variation (CV) of ﬁllingthe columns. The dry densities of Experiments 1 and 3
were similar, and their CVs were small, indicating goodsoil ﬁlling reproducibility. In contrast, there were differences in volumetric water content, and the CV was slightly
larger in Experiment 1.
Table 6 shows the range and CV of measured ﬂow volume and the CVs of the experimentally obtained soil exchangeable K values. In this study, the ﬂow rate was controlled by a peristaltic pump, and the ﬂow time was determined. Therefore, the ﬂow volume varied even under
the same settings owing to the ﬂuctuation of the actual
ﬂow rates. In Experiment 3, a ﬂow volume of 1200 mm
was assumed, but the actual ﬂow volume was 1145–1339
mm. The variation in values of soil exchangeable K content reﬂects not only the direct variation of factors due tothe analytical method, such as the variation caused by theprocedure of the simpliﬁed analytical method but also theeffect of differences in the ﬂow volume. The values of ex-

15changeable K obtained in this experiment had an average
CV of about 6 % and were sufﬁciently reproducible when
the range of values was considered.
4.2 Effect of inﬂuent water quality and ﬂow volume on K leaching (Experiment 1)
The results of Experiment 1 are shown in Fig. 3, indicating the effect of inﬂuent water quality and ﬂow volume
on exchangeable K of test soil. The difference between
the initial value of soil exchangeable K and the value of
exchangeable K at any given ﬂow volume indicates theamount of K leaching by the ﬂow volume. K leaching increased as the ﬂow volume increased, and those differences
in inﬂuent water quality signiﬁcantly affected the K leaching. The order of leaching rates was 3× > 1× > 0.3× >DI until the ﬂow volume was about 500 mm; i.e., the leaching proceeded in the order of increasing concentration, after which the K leaching rate gradually decreased with increasing ﬂow volume. If DI is excluded, the soil exchangeable K contents almost reached equilibrium in the order ofincreasing concentration of inﬂuent. The equilibrium exchangeable K contents were 0.17 and 0.07 g K2 O kg−1 for
3× and 0.3× of inﬂuent water quality, respectively, a difference of approximately 0.1 g K2 O kg−1 .
With 0.3×, 1×, and 3× inﬂuent water quality, soil exchangeable K increased slightly with increasing ﬂow volume after the K leaching stopped. In the case of DI, theexchangeable K did not reach equilibrium until approximately 7400 mm, and the content of exchangeable K decreased gradually due to leaching.
The average pH values of the inﬂuent were 5.8, 5.6, 5.6,
and 5.6 for DI, 0.3×, 1×, and 3×, respectively. The pH
values of the efﬂuent increased to 6.9, 6.6, 6.5, and 6.3, respectively, after more than 100 h of water ﬂow (a ﬂow volume of approximately 6000 mm or more). This increase

Fig. 3 Effect of inﬂuent water quality and ﬂow volume on soil
exchangeable K (Experiment 1).

16土壌の物理性第 152 号（2022）

Fig. 4 Relationship between PAR in inﬂuent and soil exchangeable K with different concentrations in the same ion composition ratio (Experiment 1).

in pH during the percolation process suggests that H+ contributes to cation exchange. In fact, the gradual progression of leaching in tests using DI is expected to include theeffect of cation exchange by H+ .
The water pressures at the bottom of the column during the tests ranged from approximately 26–70 kPa, andthe saturated hydraulic conductivities were generally inthe range of 1.2–3.3 × 10−5 cm s−1 . However, in somecases, the hydraulic conductivity gradually decreased, andin tests in which large amounts of DI or 0.3× were passed
through the column, it eventually dropped by about oneorder of magnitude. The pH of the efﬂuent was higher inthose cases, and the increase in pH and decrease in soil
exchangeable cations might have affected the decrease in
hydraulic conductivity, i.e., the dispersion of clay colloid
particles and their migration, resulting in soil pore clogging. Shainberg et al. (1987) passed several dilute solutions, including distilled water, through a smectite-sandmixture and evaluated clay swelling and dispersion basedon changes in permeability and the concentration of clayin the efﬂuent. The results showed that the permeability
decreased with lower concentrations of experimental water in tests with Polkville smectite (Mississippi) and SAR
less than 10, with distilled water having the lowest permeability. In our study, in some of the tests in which thedecrease in hydraulic conductivity was signiﬁcant, the turbidity of the efﬂuent was large, suggesting the dispersion
and migration of clay colloidal particles. These details will
be reported at another occasion.
Fig. 4 shows a rearrangement of the data of Fig. 3,
excluding DI, in relation to the PAR of inﬂuent and soil
exchangeable K. The PAR increased in response to increases in the concentration of the inﬂuent. The exchange-

able K contents at equilibrium with ﬂow volumes greaterthan approximately 4500 mm were correlated with PAR(r = 0.995; p ≤ 0.01; n = 6). This means that the soil exchangeable K will reach equilibrium after approximately 4
years of inﬁltration, assuming 1200 mm of inﬁltration perirrigation season.
This result is consistent with the relationship between
factor I and factor Q observed in the Q/I relationship.
While the Q/I relationship indicates the relationship between factors I and Q in the equilibrium state of soil solution and soil in batch tests (Beckett 1964b; Okajima andMatsunaka 1972), the present study looks at the relationship between PAR (contrasted with factor I) and the Kleaching (contrasted with factor Q) at equilibrium, which
emerges from continuous percolation of inﬂuent.
4.3 Characteristics of leaching of major cations
from the soil column (Experiment 2)
Fig. 5 shows the relationship between ﬂow volume andconcentrations of major cations in the efﬂuent from soilcolumns for different inﬂuent water qualities. The ﬁnal
ﬂow volume was approximately 4600 mm for 0.3×, 1×,
and 3×, and approximately 3900 mm for DI. In the DI
test, the permeability of the soil column decreased during
the test, and the ﬂow rate decreased, resulting in approximately 15 % less ﬂow volume than in the other tests.
The concentrations of the major cations in the efﬂuent
gradually decreased immediately after the start of the experiment. For many of the cations, the decrease in concentration stopped, and equilibrium was reached. A comparison of the concentrations for each ion species regarding
the rapidity of stabilization of concentrations based on thewater ﬂow volume from the start of the experiment until

論文：Effects of inﬂuent water quality on potassium leaching from paddy soil

17

Fig. 5 Leaching of major cations from soil columns by inﬂuent water with different water quality, (a) Na+ , (b) K+ , (c) Mg2+ ,and (d) Ca2+ . The vertical axis shows the concentration of each ion in efﬂuent from soil columns (Experiment 2).

equilibrium was reached showed that the solutions stabilized faster in the order 3× > 1× > 0.3× > DI. Also, whencomparing ion species, the order of stability was faster for
Na+ > Mg2+ = Ca2+ > K+ when 3×, 1×, and 0.3× inﬂuents were used. For three inﬂuent types (excluding DI),the concentrations of Mg2+ and Ca2+ had decreased once
to a level below those of the inﬂuent water and then gradually approached those of the inﬂuent water. When DI wasused, the concentration of Na+ showed a tendency to decrease further when the ﬂow volume exceeded 1000 mm.
Similarly, the concentration of K+ in the efﬂuent tended
to decrease further at a ﬂow volume of around 1000 mm
for all water quality inﬂuents. On the other hand, no such
two-step decrease in concentration was observed for theremaining Ca2+ and Mg2+ samples.
Fig. 5 shows that the K+ concentration in the efﬂuent
gradually approached the K+ concentration in the inﬂuent
as the ﬂow volume increased. However, equilibrium was
not reached even after approximately 3900 mm of ﬂow volume in DI and 4600 mm in 0.3×. The concentration in
the efﬂuent remained slightly higher than that in the inﬂuent at 1× and 3×, indicating that K+ had the slowest
leaching rate of the four cations. The concentrations of theother three cations in the efﬂuent approached those in theinﬂuent at a ﬂow volume of a few hundred millimeters at

the earliest and a thousand millimeters at the latest. Conversely, in the case of K+ , the concentration in the efﬂuent
showed a slight deviation from the K+ concentration of theinﬂuent, even after about 4600 mm of ﬂow volume. Thismay be explained by the effect of selective adsorption of
K+ by 2:1 type layered silicate minerals such as illite, vermiculite, and smectite in the test soil.
Many previous studies have investigated the mobility of
exchangeable cations in soil. For example, Kozanecka etal. (2009) investigated the long-term mobility of exchangeable Ca2+ , Mg2+ , K+ , and Na+ cations in soil cross sections to a depth of 1 m in an 85-year upland ﬁeld with ﬁxed
mono-fertilization in Skiernievice, Poland. The mobilityof exchangeable cations was Ca2+ > K+ > Mg2+ > Na+ ,indicating that the mobility of exchangeable cations differs
greatly depending on the soil and other conditions.
4.4 Effect of different cation composition ratios
on K leaching (Experiment 3)
Experiment 1 showed that when the concentration of inﬂuent with the same cation ratio was increased, the soil
exchangeable K content equilibrated at a higher value.
Therefore, the leaching of K was investigated in Experiment 3 by ﬁxing the ﬂow volume at approximately 1200
mm and varying the composition ratio of the major cationsin the inﬂuent.

18

土壌の物理性第 152 号（2022）

Fig. 6 Soil exchangeable K and PAR when only total major cations or K concentration in the inﬂuent is varied, (a) relationship between exchangeable K of test soil and concentration factor with three repetitions, (b) relationship between PAR andconcentration factor (Experiment 3).

Fig. 6 shows a plot of soil exchangeable K content(Fig. 6 (a)), and PAR of the inﬂuent (Fig. 6 (b)) when
only the K+ concentration in the inﬂuent was varied andother cation concentrations were ﬁxed, and when the overall concentration of major cations was varied (as in Experiment 1) as a control. When only the K+ concentrationin the inﬂuent was increased, the exchangeable K content
was higher than in the control, demonstrating that the difference in cation composition ratio affected the K leaching.
When only the K+ concentration of inﬂuent was increased
by a factor of 10, K accumulation occurred beyond the initial soil exchangeable K content.

of changes in each cation in the inﬂuent on the leaching
and accumulation of K, using some of the results of Experiment 3. Therefore, we ﬁrst calculated the sensitivity
S1ix (where x is the concentration multiplier) of the change
in soil exchangeable K when the concentration of cation ichanges by 1 mmol L−1 using the following formulas:

The shapes of the graphs in Fig. 6 (a) and Fig. 6 (b)are alike. Comparing both graphs, soil exchangeable K
was higher at 0.3× than at 1×. As can be seen from Fig.
3, this may be because the leaching rate was still high andfar from equilibrium at the stage when approximately 1200
mm of the 0.3× inﬂuent had passed through the column.

where exKxi is soil exchangeable K mg K2 O kg−1 and Cxi
is the inﬂuent concentration mmol L−1 . The results showthat the increase in K+ concentration in the inﬂuent contributes signiﬁcantly to the maintenance and accumulationof soil exchangeable K (Table 7). The magnitude of theeffect of a 1 mmol L−1 change in the concentration of eachcation in the inﬂuent was found to be in the order of S1i3X
for Mg2+ >Ca2+ >Na+ . Furthermore, the sensitivity S2ix
of the change in soil exchangeable K for a 10 % change
in the concentration of cation i based on the actual cation
composition of the irrigation water (Table 2) was calculated using the following equations:

Similarly, Fig. 7 (a) shows the soil exchangeable K content when the concentrations of the major cations, Na+ ,Mg2+ , and Ca2+ , were increased or decreased individuallyor when the concentrations of Ca2+ and Mg2+ were increased or decreased simultaneously. Fig. 7 (b) shows aplot of the PAR of the inﬂuent. Interestingly, the ﬁgure in
Fig. 7 (a) is quite similar to that in Fig. 7 (b). Since Na+
is not considered in equation 2, there is no change in thevalue of PAR with changes in Na+ concentration in Fig. 7(b). The K leaching was almost the same in the inﬂuent
with 3× and 1× of Na+ but was higher in the inﬂuent with
10× of Na+ . This indicates that higher concentrations of
Na+ enhanced the K leaching.
Next, we attempted to quantitatively compare the effects

S1i3X =

i − exK i
exK3X
1X
i −CK i
C3X
1X

(4)

S1i10X =ii
exK10X− exK1X
i
i
C10X −CK1X(5)

S2i3X =

S1i3X
i0.1 ·C1X(6)

S2i10X =S1i10Xi
0.1 ·C1X(7)

The magnitude of the effect of a 10 % change in the論文：Effects of inﬂuent water quality on potassium leaching from paddy soil

19

Fig. 7 Soil exchangeable K and PAR for different proportions of cations in the inﬂuent water, (a) relationship between
exchangeable K of test soil and concentration factor (three repetitions with some exceptions), (b) relationship between PAR andconcentration factor (Experiment 3).
Table 7 Effect of ionic species of inﬂuent water on K accumulation/leaching.
Change in soil exchangeable K with
1 mmol L−1 change in concentrationof each cation

Change in soil exchangeable K
with 10 % change inconcentration of each cation

i

S1i3X

S1i10X

S2i3X

S2i10X

K+
Na+
Mg2+
Ca2+
Unit

1250
1.2
−77
−69

1253
−23
−45
−27

3.9
0.020
−1.2
−2.4

3.9
−0.40
−0.68
−0.96

mg K2 O kg−1

mg K2 O kg−1

Note: Positive and negative values indicate accumulation and leaching, respectively.

concentration of each ion, assuming the actual cation composition of irrigation water, was Ca2+ > Mg2+ > Na+ in
S2i3X . The effect on soil exchangeable K of a 10 % increasein K+ concentration is estimated to be equivalent to about a+2+
16 % decrease in Ca2+ concentration (from S2K3X /S2Ca
3X ).
In all cases, the contribution of Na+ to K leaching was
small. This indicates that the exchange selectivity of Na+
for cation exchange in the test soils is relatively low compared to Mg2+ and Ca2+ , supporting the absence of Na+in equation 2.
Based on the above, since there is a close relationship between K leaching and PAR of the inﬂuent, the relationshipbetween PAR and the rate of change in soil exchangeable
K and the relationship with K+ for comparison, are shownin Fig. 8. The rate of change in soil exchangeable K (Rc )was calculated by the following equation:

Rc =(Exchangeable K a f ter test)
× 100 ( %)(Exchangeable K be f ore test)(8)

The relationship between K+ concentration and Rc (Fig.
8 (a)) showed a pattern, but K+ concentration is not highly
representative of K leaching. Conversely, except for a few
data points, the relationship between PAR and Rc (Fig. 8(b)) was almost plotted on a single curve, conﬁrming thatPAR has favorable indicator properties for Rc . The excluded points are the results of two experimental parameters, one with 0.3× of standard water and the other with
0.3× of K+ . All three repetitions for each parameter had
the same trend, and the CVs were small. As seen in Fig. 3,
the initial leaching was slow under low-concentration conditions, such as the excluded points. We presumed that thiswas affected by the fact that K was still in the mid-leachingstage at about 1200 mm of ﬂow volume (Fig. 4).
The obtained regression curve indicated that soil exchangeable K accumulates when using inﬂuent with PAR
greater than 0.3. The exchangeable K contents were estimated to be 0.161, 0.195, and 0.128 g K2 O kg−1 , respectively, for the mean, maximum, and minimum values ofirrigation water quality at the sampling site (Table 2). This

20

土壌の物理性第 152 号（2022）

Fig. 8 K+ and PAR as indicators of the rate of change in soil exchangeable K (Rc), (a) relationship between K+ and Rc,(b) relationship between PAR and Rc (Experiment 3).

assumed an inﬁltration rate of 1200 mm and negligible effects of rainfall and fertilization.
Thus, the PAR of the inﬂuent was found to be a valid
indicator that reﬂected the leaching and accumulation ofsoil exchangeable K.
4.5 Limitations of this study and future issues
This study has potential limitations. Firstly, differencesin the initial values of the exchangeable K content of thetest soil could signiﬁcantly affect the results; the rate ofchange in exchangeable K used in Fig. 8 is based on theinitial exchangeable K content of the test soil, and thus will
have direct effects. Differences in experimental conditions,such as the ﬂow rate or pH of the inﬂuent, might also affectthe results.
Secondly, this study only examined one of the paddy soil
types. It will be interesting to observe the patterns withdifferent soil types, such as volcanic ash and peat soils.
However, the exchangeable K content at equilibrium may
be predicted from previous studies of the Q/I relationship.
Eguchi et al. (2017) used the Q/I relationship to study theeffect of clay minerals on the effect of K application on the
reduction of radioactive Cs uptake by crops. They foundthat each soil containing characteristic clay minerals suchas smectite, allophane, and vermiculite had unique Q/I relationships. This suggests that, for many soil types as well
as for the test soil, a unique relationship between the PAR
and the equilibrium exchangeable K content is to be expected. However, dynamic experiments such as column
tests are needed to determine the rate of K leaching.
Four cations, K+ , Na+ , Ca2+ , and Mg2+ , were considered in this study. However, for greater precision, it may benecessary to consider the effect of NH+
4 , whose concentra-tion may be temporarily increased by fertilizer application.
Furthermore, several points need to be addressed as an extension of this study. For example, how do the results of
the leaching test performed under pressurized conditions
with a pump differ from those obtained under natural conditions, and how can the results of an accelerated test obtained with a compact column be scaled and utilized?
Based on the ﬁndings obtained in this study, ways to reduce the K leaching from the soil should be considered.
If artiﬁcial inputs, such as fertilizers and soil amendments,
contain unwanted Mg2+ or Ca2+ , one approach is to reduce their amounts. In the future, we would like to combine the compact column tests and ﬁeld experiments toquantitatively evaluate the improvement of K retention capacity when applying soil amendment materials such aszeolite and bentonite.

5. Conclusions
In this study, as part of our efforts to understand thecauses of K leaching from paddy soils and the effects ofK supply by irrigation water, we investigated the characteristics of K leaching from ﬁne-grained lowland paddy
soil by using a compact thumb-size column test. The
leaching/accumulation of soil exchangeable K and outﬂow
of major cations were examined using inﬂuent with different concentrations and ion compositions. The relationshipof soil exchangeable K with the PAR of the inﬂuent wasalso studied. The following results were obtained:(1) The compact column test had sufﬁcient reproducibility for ﬁne-grained paddy soil, with an average CV of論文：Effects of inﬂuent water quality on potassium leaching from paddy soil

less than 2 % for dry bulk density and approximately
6 % for soil exchangeable K obtained after the test.
(2) When inﬁltrating inﬂuent with the same cation composition, the lower the concentration, the slower the
initial leaching; but as the ﬂow volume increases, theleaching progresses. Conversely, the initial leachingrate was larger at higher concentrations, but equilibrium is reached faster and with a higher equilibrium
exchangeable K content.
(3) The soil exchangeable K content at equilibrium ishighly correlated with the PAR of the inﬂuent.
(4) When DI water was used, K leaching also occurred,albeit at a slower rate, possibly because of exchanges
with protons.
(5) The major cation concentrations of eluate tended tostabilize faster in the order Na+ > Ca2+ = Mg2+ >K+ , with K+ being the slowest. This suggests that
selective adsorption of K+ by 2:1 type clay minerals
may have been affected.
(6) The equilibrium content of soil exchangeable K
formed after water percolation was determined by theK+ concentration and the composition ratio of coexisting exchangeable cations in the inﬂuent.
(7) The PAR of the inﬂuent is a promising indicator to
explain the leaching and accumulation of K.
(8) The PAR represents the ionic composition ratio andreﬂects the effect of K+ concentration.
(9) The effects of ion species on K leaching were in theorder Mg2+ > Ca2+ > Na+ based on a 1 mmol L−1change in their concentration and Ca2+ > Mg2+ >Na+ based on a 10 % change in their concentration,with divalent cations having a greater effect and Na+
having a smaller effect.
(10) The effect of a 10 % increase in K+ concentration on
soil exchangeable K is estimated to correspond to an
approximately 16 % decrease in Ca2+ concentration.

21

the quality of this paper.

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要

旨

水田における放射性 Cs の吸収を抑制するためには，カリウム（K）の動態を把握し，信頼性の高い効率的な K の施肥技術を確立する必要がある．そこで，本研究では，水田土壌から交換性 K が溶脱する要因として浸透水の影響を調べるために，親指サイズの小型カラムを用いて，異なる陽イオン濃度や組成比を持つ流入水に対する水田土壌からの K の溶脱を調べた．カラム試験では，通水試験後の土壌の交換
性 K の変動係数は約 6 % であり，十分な再現性を有していた．異なる水質を持つ流入水の浸透により生じる交換性 K の平衡含量は，流入水の K+ 濃度と陽イオン構成比によって決定され，PAR（K 吸着比）が，K 溶脱の有効な指標となった．流入水中の各陽イオン濃度が K の溶脱に及ぼす影響は，二価のイオンで大きく，Na の影響は小さいことがわかった．
キーワード：カリウム溶脱，親指サイズカラム試験，PAR（カリウム吸着比），潅漑水質，水田土壌

1,