Impacts of soil slaking and drying on release and retention of
cations during desalinization from saline soil
Abul Hasnat Md. SHAMIM1,2 and Takeo AKAE1

Abstract: The mechanism of soil slaking is the breakingof soil aggregates by entry of water which is accompanied
by salt releasing process when the soil contains salt. The
objective of this paper is to describe the impacts of soil
drying and slaking on cations release to outer solution andshift of cations during desalinization under different initial water contents (IWC). A slaking test on natural andair-dried soils was carried out for evaluating the effect ofslaking in different IWC (60, 50, 40, 30, 20 and 10 % byweight). After 24 hours immersion in water, Na was predominantly released in every case both of the natural andair-dried soils. At the water content of 30 % that is around
optimum slaking water content, maximum Na was released
and at the water content of minimum slaking rate the release of Na was lowest. However, too dry condition (10 %water content) did not contribute to release Na more. On
the other hand, we did not ﬁnd any special relationship between the IWC and release of other cations like Ca, Mg
and K in outer solution. Most of the Ca2+ and Mg2+ stayin exchange site (60 to 80 %) in soil and little amountsof them came out in outer solution. Water-soluble cations
in slaked and unslaked soils followed the order of Na
K> Mg> Ca whereas, the amount of exchangeable statecations followed Na+ > Mg2+ > Ca2+ > K+ . As a whole,
the proportion of Na released in outer solution of naturalsoil was greater than that of air-dried soils. The results indicate that the water content for optimum slaking and theslower slaking rate is more efﬁcient for desalinizing of Na
from the saline soil.
Key Words : soil slaking, soil water content, cations release, desalinization, exchangeable state

1. Introduction
Salinity within irrigated soils clearly limits productivity
in vast areas of Japan and other parts of the world. Soil
degradation caused by salinization and sodiﬁcation is ofgreat concern in the modern world because it reduces potential agricultural lands (Tanji, 1990; Sadiq et al., 2007).
Although salinity has adversely affected agriculture forthousands of years, the recognition that salt-affected land
1 The Graduate School of Environmental Science, Okayama University,
Okayama 700-8530, Japan. Corresponding author: Abul Hasnat Md.
Shamim2 School of Agriculture and Rural Development, Bangladesh Open University, Gazipur-1705, Bangladesh.
2010 年 6 月 29 日受稿 2011 年 2 月 23 日受理
土壌の物理性 118 号, 3–11 (2011)

can be used for agriculture is relatively recent. Interest in
the use of saline land resources has escalated over the last
20 years, with a renewed focus on saline agriculture in arange of countries including Pakistan (Qureshi and BarrettLennard, 1998), Australia (Barrett-Lennard and Malcolm,
1995; Marcar et al., 1995; Bennetta et al., 2009), the Middle East and North Africa (ICBA, 2006). In humid areas
like Japan, residual salt in polder soil and salt in groundwater affected by sea water intrusion can cause salt accumulation and damage crop growth, in particular, on polders
and low land areas along sea coasts.
In reclaiming those saline soils to ﬁnd out the effective(an efﬁcient and low cost) method is required. Soil slaking
is the process of soil aggregates collapse when they are
rewetted after drying. The slaking has long been studied
from the stand point of stability of aggregates. However,it has not been studied from that of salt removal. Drying
followed by rewetting and slaking is commonly found in anatural soil processes. During the process, salt in the soil
moves and accumulates to inner and outer surface of soil
blocks and released to outer solution. But the details of
cation shifting are not well understood. Akae et al. (2003)
had also reported that slaking is the salt releasing process
and salt concentration was decreased from shallow layer tothe deeper layer accompanied with land drying processes.
Note that breakdown of soil aggregates results in collapseof pores, which reduces inﬁltration rate, leads to runoff anderosion, and may, subsequently cause soil degradation.
Our previous research successfully identiﬁed the optimum water content for slaking of a studied soil and salt
release from soil blocks (Shamim and Akae, 2010). The
optimum soil slaking occurred when the gravimetric water content was around 30 % equivalent to pF 4.25 (1800kPa). It is close to the shrinkage limit (31.8 %) which is thecritical water content for air-entry of this soil. The results
revealed that the maximum salt is released at the soil watercontent of maximum slaking. In addition, the amount ofsalt released into equilibrium water after 24 hours immersion was proportional to the fraction of slaked soils (slaking rate). Here, we hypothesize that desalinization may beenhanced by simply putting the soil at the proper soil watercontent by land drying practice or by rewetting.
In this study, the amount of Na in the outer solution

4

土壌の物理性

Table 1 Fundamental properties of Kojima Bay Polder (paddy
ﬁeld; depth: 50 – 70 cm).
Properties
Unit
TextureSilty clay
Clay
38.2
%
Silt
50.4
%
Sand
11.4
%
Soil pH
7.5(Soil : Water = 1 : 2.5)
Hardness
3.83
mm
Particle density
2.51
g cm−3
Plastic limit
39.5%
Liquid limit
72.9%
Electrical Conductivity
2.92
dS m−1(Soil : Water = 1 : 2.5)
Exchangeable ions (Bolt et al., 1976)
Sodium19.6
c molc kg−1
Potassium
3.26
c molc kg−1
Calcium
7.72
c molc kg−1
Magnesium
10.7
c molc kg−1
Water-soluble ions (Soil : Water= 1 : 5 by weight)
Sodium8.45
c molc kg−1
Potassium
0.74
c molc kg−1
Calcium
0.28
c molc kg−1
Magnesium
1.26
c molc kg−1

surrounding the slaked soil blocks was mainly discussed,
which is an exact measure of Na exclusion from the soil
block. In addition, sites and states (exchangeable sites andsoluble salts) of cations in soil were examined.

2. Material and methods
2.1 Sample collection and analysis method
The soil was collected from the depth of 50 to 70
cm (salinized layer) of Kojima Bay Polder (paddy ﬁeld:
34◦ 32’15.1” N and 133◦ 55’37.0” E), located in Okayama
prefecture in western Japan. The soil is classiﬁed to OchricGley Sols and the land use is rice cultivation. The clay
minerals of the soil are composed of mainly halloysiteillite intermediate, nontronite and montmorillonite accompanied with quarz, feldspars, chlorite and free sesquioxides(Kawaguchi et al., 1957). The disturbed soil samples were
air-dried and mixed with distilled water at the ratio of 1 : 5
by weight. Then the supernatant water was taken and EC1:5
were measured using a portable electrical conductivity meter (B-173; Horiba Ltd.). The soil pH was measured in a
1 : 2.5 (Soil : Water by weight) suspension using a portable
pH meter (B-212; Horiba Ltd.).
Soluble cation concentrations were measured using an
atomic absorption spectrophotometer (AAS; Z 5300; Hitachi Ltd.) in the supernatant water after centrifuging
the natural soil. Insoluble salts and exchangeable cations
were extracted using ammonium acetate. The 1.0 g of the

第 118 号

(2011)

disturbed air-dried soil was mixed with 200 ml solution(0.0114 M SnCl2 .6H2 O and 1 N CH3 COONH4 ) then the
mixture was shaken for 1.5 hours. After settling and centrifuging, the cations in the supernatant were analyzed by
the AAS. Finally, we subtracted the amount of free ionsfrom that of the total ions to obtain true amount of exchangeable cations (Bolt et al., 1976). Fundamental physical properties of soils; particle size distribution, soil density, hardness, Atterberg Limits were measured by the standard methods of the Japanese Geotechnical Society (TheJapanese Geotechnical Society, 2000).
2.2 Fundamental properties of the studied soil
Fundamental properties of the studied soils are given in
Table 1. The studied soil showed a silty clay texture (clay38.2, silt 50.4 and sand 11.4 %); the pH and EC1:5 were
7.5 and 2.92 dS m−1 , respectively. The soil has relatively
higher liquid limit of 72.9 %. Both the water-soluble (8.45c molc kg−1 ) and exchangeable ion Na (19.6 c molc kg−1 )of the natural soil were higher than that of other ions like
K, Ca and Mg (Table 1). The mass basis natural watercontent of the studied soil was 94.9 %.
2.3 Preparation of soil sample for slaking test
The ﬂow chart for slaking test is given in Fig. 1. The
slaking test was conducted at the Agricultural Land Research Laboratory, Graduate School of Environmental Science, Okayama University, Japan during February, 2009
following basically to Experimental Guide of PhysicalMechanical Properties of Soil (The Editing Committee ofExperimental Guide of Physical-Mechanical Properties ofSoil, JSIDRE, 1983). The natural soil was separated into
two batches, one batch was air-dried and another was notdried (natural). Each batch of soil was mixed thoroughly
and the air-dried batch was re-saturated. The soils were
packed into stainless steel cylinders (2.5 cm height and 5
cm diameter) by pushing them into the soil. The specimens were kept in a desiccator at 30 ◦ C for drying treatment to attain different initial soil water contents of 60,
50, 40, 30, 20 and 10 % by weight. The weight of eachspecimen was monitored carefully until the desired watercontents. The each water content treatment was replicatedthrice. After completing water content adjustment the soil
specimens were sealed and kept in a refrigerator.
Lastly, the each soil specimen was placed on the 4.75
mm sieve and immersed into tap water (EC: 0.112 dS m−1 ,volume: 9080 cm3 ) for slaking test. After 24 hours immersion in the water the slaked (fell down through the sieve)and unslaked (remained on the sieve) soils were collected
and weighted separately. The cations in the extract solution (Soil : Water = 1 : 5) from the slaked and the unslaked
soils were measured by AAS. The exchangeable cations of
the slaked and unslaked soils were also measured by thesame method described previously.

論文：Impacts of soil slaking and drying on release and retention of cations

5

Fig. 1 Flow chart for slaking test.

2.4 Calculation of slaking rate of soil
The slaking rate, SR (%) was calculated by using thefollowing equation;
SR(%) = s/(s + u) × 100(1)

Where s represents the weight of the slaked soil and u
indicates the weight of the unslaked soil.
2.5 Calculation of cations in solution and soil
The salt released into outer solution (Mi ) was calculatedby the following equation;
Mi = Ci ×Vw(2)

Where Ci is concentration of the cations; Vw refers to thevolume of the outer solution of the slaking test.
As the cation concentrations were measured on 1 : 5 solution (C1:5 ), the amount of a cation in the slaked soil, Ms
is given by equation (3),Ms = (Cs1:5 × 5 ×Ws )/ρw

(3)

Where Ws represents the weight of soil; ρw is the densityof the water. Similarly, cation in the unslaked soil, Mu isgiven by equation (4);Mu = (Cu1:5 × 5 ×Ws )/ρw

(4)

The total cation remains in the soil, Mt , is given by sumof Ms and Mu .
Mt = Ms + Mu

(5)

Letting the amount of released Na in the outer solution
as RNa , then desalinization rate of Na ion on soluble salt,
DR, is deﬁned by equation (6).
DR(%) = RNa /(RNa + Mt,Na ) × 100

(6)

6

土壌の物理性第 118 号

(2011)

dicates that after experiencing air-drying, soil changed toslake much more easily. The slaking rate was very high
(99 %) under 30 and 20 % in comparison with any otherwater contents. The slaking rate at water content of 10 %was comparatively higher than that of 60 and 50 and 40 %but lower than 30 and 20 % water content in both of natural and air-dried soils (Fig. 2). This is attributed to that thestrong linkage developed between soil particles by the intensive drying, which might enhance resistance to slaking.
3.1.3 Difference between natural and air-dried
soil
Fig. 2 Dependence of slaking rate against initial water contents
after 24 hours immersion in water. Vertical bar indicates ± SE ofthree replicates.

Where DR is the fraction of Na released into the outersolution divided by the sum of Na in outer solution, watersoluble Na in the slaked and the unslaked soils. It is notedthat this DR does not include the exchangeable Na in thesoil, nor the conﬁned salt.

3. Results and discussion
For discussing the displacement of cations from a site tosite, we speciﬁed the sites into four categories (phases): (1)free cations in outer solution, (2) water-soluble cations insoils (slaked and unslaked) and (3) exchangeable cations.
In addition, some salts may remain in conﬁned or stagnant
pores, although the amount of that cannot be measured.
3.1 Effect of initial water content on slaking rate
Dependence of slaking rate on water content is shown in
Fig. 2. The bars in the ﬁgure show the standard error ofthree replicates.
3.1.1 Natural soil
The slaking rates of the specimen at 60 and 50 % watercontent were only 0.15 and 0.16 %, respectively, after 24hours immersion. It may be due to the sample was initiallysaturated and contained no entrapped air, resulting the low
force of escaping air. Under the water content of 40 %,the slaking rate, 2.54 %, was higher than that of 50 %.
This indicates that the natural soil started to slake below
this water content. A signiﬁcant change was observed atwater content of 30 %. The slaking rate (27.3 %) was muchhigher in comparison with other water contents. It seems
due to that the initial water content of 30 % was close toshrinkage limit and contained more air than that of 40 to 60%. It resulted higher force of escaping air pressure, which
is favourable condition for slaking. The slaking rate kept
the almost same value (26.0 %) at water content of 20%.
3.1.2 Air-dried soil
On the other hand, 88 – 89 % of the air-dried specimens
were slaked at water contents of 60 and 50 % which in-

The slaking rate of air-dried soil was higher than that of
the natural soil at all water contents. It is due to that airdrying increased entrapped air than the natural soils at thesame water content, resulting the higher force of escaping
air pressure. Much more bubbles were observed duringslaking of the air dried soil than the natural soil.
3.2 Effect of initial water content on cations in
the three phases
Table 2 and Table 3 show the variance of measurement
on amount of shifted cations among three categories. They
also express the precision of each cation measurement
in different water contents. These tables reveal that the
cations measurement of Na, Ca and Mg is statistically reliable enough because the range is below 25 % against themean values. Note that the variance of K measurement inwater-soluble phases was so large in comparison with othercations in both the natural and the air-dried soils. This isdue to critically small values of K against precision of measurement, but contribution of K is not important in desalinization from saline soil.
3.3 Cations released into outer solution after 24hours immersion
Fig. 3 shows the cations released into outer solution after 24 hours immersion of different pre-drying treatment(natural and air-dried) of soils. The bars in the ﬁgureshow range of the measured values, indicating the varianceof measurement is small enough statistically. The ﬁgureshows that Na exists predominantly in the outer solution
and the release of Na is maximum at water content of 30%.
3.3.1 Natural soil
At the lower water content of 20 and 10 %, the amountof Na released to outer solution (RNa = 0.25 mmol g−1 and0.25 mmol g−1 , respectively) were high, and it was highest at 30 % (0.35 mmol g−1 ). The RNa at 60, 50 and 40% water content were 0.185, 0.18 and 0.19 mmol g−1 , respectively. The difference in the total Na under different
water content is mainly due to the amount of Na in theouter solution. The fractions of Na in outer solution were
74.6, 73.2, 76.7, 71.5, 70.1 and 65.9% at water content of
10, 20, 30, 40, 50, 60%, respectively. At water content of論文：Impacts of soil slaking and drying on release and retention of cations

7

Table 2 Mean of range/average ratios on amount of cations in three phases in natural soil.
PhasesOuter solution
Exchangeable
Water-soluble

Water content
60∼10%
60∼10%
60∼10%Na∗ IV (%)

17.5
15.3
22.8

K
IV (%)
14.3
25.6
75.7

Ca
IV (%)
23.3
16.8
0.18

Mg
IV (%)
16.1
13.4
15.5

∗ IV (Index of Variance) = mean {range (maximum value − minimum value)/average} on cation concentrationof different water content. Measurements were replicated thrice on each water content, i.e., n = 18.

Table 3 Mean of range/average ratios on amount of cations in three phases in air-dried soil.
PhasesOuter solution
Exchangeable
Water-soluble

Water content
60∼10%
60∼10%
60∼10%Na∗ IV (%)

17.7
8.5
17.3

K
IV (%)
14.1
12.8
75.7

Ca
IV (%)
23.4
12.6
2.30

Mg
IV (%)
18.1
17.5
1.30

∗ IV (Index of Variance) = mean {range (maximum value − minimum value)/average} on cation concentrationof different moisture content. Measurements were replicated thrice on each water content, i.e., n = 18.

Fig. 3 Cations released into outer solution after 24 hours immersion in different pre-drying treatments. Vertical bar indicates ± SE ofthree replicates.

30%, maximum Na was released from conﬁned or stagnant
pores to outer solution because the maximum slaking occurred and collapsed aggregates at this water content (Fig.
2). At water content of 10 %, samples slaked in short time
and the RNa was low as 0.25 mmol g−1 . It was shown thatvery dry condition and quick slaking did not contribute themore Na release from the soil.
3.3.2 Air-dried soil
At water content of 30 and 20 %, the RNa was 0.27 and0.26 mmol g−1 , respectively. On the other hand, when
the soil was in the driest condition of 10 % water content
slaking made limited contribution to RNa (0.25 mmol g−1 )which was equal to that of the natural soil.
3.3.3 Difference between natural and air-dried
soil

Both the natural and air-dried soil followed the same
trend for releasing Na in relation to the initial water contents. The water content above 40 % showed less RNa , andbelow water content of 30 % RNa was high. But the effects
were more pronounced in the natural soil. The maximum
release of Na occurred around at the coincident water content as maximum slaking. The results indicate that higher
water content and too rapid slaking at low water contents
were not efﬁcient for removing Na from the soil block.
In terms of other cations release like Ca, Mg and K, all
the soils show almost the similar trends. Note that very
little amount of Ca and Mg comes out to outer solution.
The release of cations to outer solution in soils followed theorder of Na  K > Mg > Ca, and drying did not affect thecomposition of K, Mg and Ca except for a slight increasein Mg and slight decrease in K in a few cases (Fig. 3).

8土壌の物理性第 118 号

(2011)

Fig. 4 Amount of Na in outer solution and left in soils after 24 hours immersion under different initial water contents.

Fig. 5 Amount of K in outer solution and left in soils after 24 hours immersion under different initial water contents.

3.4 Impacts of slaking and drying on watersoluble cations in soils
Fig. 4, 5, 6 and 7 shows the sites and amounts ofeach cation (Na, K, Ca and Mg) after 24 hours immersion, respectively. The site where the cations exist is divided into four categories; outer solution, exchangeablestate and water-soluble state in the soil and salts in conﬁned pores which cannot be measured. It is seen that thefraction of the water soluble state is relatively very small.
Among them Na showed much higher values than the othercations, showing slight increase with drying. We did notﬁnd any signiﬁcant differences in amount of other watersoluble cations like Ca, Mg and K in the slaked and unslaked soils in almost all the cases.
3.4.1 Natural soil
In terms of water-soluble Na (after 24 hours immersion)in slaked and unslaked soils, more Na (8 to 10 %) existed
in the soil at higher water content (60 %). This is becausesome of Na stayed in stagnant part inside the soil in higher
water contents. On the other hand, when soil slakes atlower water content the Na staying in soils was lowest proportion (5 %) which might be due to the Na stayed mainly

close to the surface. It was found higher water content was
not effective from the stand point of Na release from soil.
The water-soluble K, Ca and Mg in soils were in small
amounts (＜ 0.02 mmol g−1 ) in both the higher and lower
water contents.
3.4.2 Air-dried soil
The higher water content of 60 and 50 % held less Na (2to 3 %) as water soluble than the natural soil which means
most of the Na released into outer solution. This is because, the air-dried soil at that water content slaked morethan the natural soil. It is due to decrease in the Na staying in the stagnant part inside the aggregates. The watersoluble Ca and Mg in air-dried soil followed the same trend
as the natural soil.
3.4.3 Difference between natural and air-driedsoil
At lower water content the air-dried soil held slightly
more water-soluble Na than that of the natural soil. But
the amounts of water soluble cations were affected very
little by the drying. The amount of water-soluble cations
followed the order of Na  K > Mg > Ca in all these soils
after 24 hours immersion.

論文：Impacts of soil slaking and drying on release and retention of cations

9

Fig. 6 Amount of Ca in outer solution and left in soils after 24 hours immersion under different initial water contents.

Fig. 7 Amount of Mg in outer solution and left in soils after 24 hours immersion under different initial water contents.

3.5 Impacts of slaking and drying on exchangeable cations in soils
3.5.1 Natural soil
In terms of exchangeable cations, Na was slightly affected by slaking and drying after 24 hours immersion(Fig. 4). The lower water content held slightly higher exchangeable Na than higher water contents, especially at 30%. It means a part of Na on exchange sites was replaced by
the other exchangeable cations during drying and slaking
under higher water content. Exchangeable K+ below watercontent of 30 % was higher than that of above 40 % watercontents. Most of the Ca2+ and Mg2+ stay in exchange site
(60 to 80 %) and showed much higher proportion at lower
water contents in the natural soil (Fig. 6 and Fig. 7).
3.5.2 Air-dried soil
The air-dried soil showed the different trends in remaining exchangeable Na from the natural soil. Note that the
cations exist in exchangeable state at highest amount at water content of 40 % (Fig. 4). This result is very special, thesoils at this water content did not slake well in comparison
with other water contents (Fig. 2). Whereas, the air-driedsoils show exchangeable K+ is low in lower water contents different from that of Na (Fig. 5). In terms of Ca2+and Mg2+ , the air-dried soil showed similar trends to K+ ,the exchangeable Ca2+ and Mg2+ was in lower proportion
at lower water contents (Fig. 6 and Fig. 7).
3.5.3 Difference between natural and air-driedsoil
The exchangeable Na+ was mainly replaced by Ca2+and Mg2+ at lower water contents. The results also indicate that the exchangeable cations were increased by theeffect of drying (ion concentration in equilibrium solution
was condensed by drying). This is because the Ca2+ andMg2+ ions are more strongly bonded with exchange sitesof soil particles due to their higher valence; on the other
hand, K+ has strong binding capacity with clay particles
in comparison with Na+ ions.
The holding of exchangeable cations followed the orderof Na+ > Mg2+ > Ca2+ > K+ except for a minor increasein Mg2+ and slight increase in Ca2+ than Na+ in few cases
under the natural soil. It is due to the presence of insoluble
Ca and Mg salts in the original polder soil.
3.6 Desalinization rate to outer solution
Fig. 8 shows the desalinization rate (DR) to outer solution in different pre-drying treatments of soils. In thiscase, we considered the Na released into outer solution and

10土壌の物理性第 118 号

(2011)

of salt released into water were 86.4% after 24 hours immersion. It increased to 96.6 % at 27 % slaking rate in thenatural soil. For the air-dried soil, it is highest (93.8 %) at
slaking rate close to 100 %. It means that under the optimum water contents, soil slaking can enhance releasingof soluble salt from the natural soil by 10.2 %. The results suggested that the lower water content and the slower
slaking is more efﬁcient for desalinizing of saline soil.

4. Conclusions

Fig. 8 Desalinization rate or released water-soluble Na fromsoils after 24 hours immersion under different initial water contents. Vertical bar indicates ± SE of three replicates.

In this study, the Na in the outer solution surrounding
the slaked soil blocks were measured, which is a substantial measure of the amount of Na exclusion from the soil
block. In addition, sites and states (exchange sites and soluble salts) of cations in soil were examined. The following
facts were revealed by this study:

water-soluble Na remaining in slaked and unslaked soils
after 24 hours immersion. It is a measure of water soluble
salt released to outer solution from all of water soluble salt
in the soil block.

(1) Na was predominantly affected by slaking (and drying) in terms of releasing cations. However, there were not
any special differences between water content and released
water-soluble cations like Ca, Mg and K.

3.6.1 Natural soil
It was shown that desalinization rate was higher at lower
water content after 24 hours emersion. At higher watercontents (60, 50 and 40 %) the DR was low (DR = 86, 92
and 91 % respectively) than that of lower water contents(30, 20 and 10 %; DR = 96, 95 and 96 % respectively).
3.6.2 Air-dried soil
Both the higher and lower water contents showed highdesalinization rate except for 40 % (DR = 90 %). However,if the soil was in excessively dry condition (10 % watercontent), slaking did not release more Na, resulting the low
DR (93 %), which might be due to the strongly trapped Na
between the soil particles during drying.
3.6.3 Difference between natural and air-dried
soil
The desalinization rate of the air-dried soil showed similar trend as the natural soil under lower water content, butshowed the opposite trend at higher water contents. In thenatural soil, at higher water content, soil did not slake well,on the other hand air-dried soil slaked very quickly at thesame water content. The trend was similar to that of slaking (Fig. 2).
The whole result indicates that the desalinization rate issomewhat affected by the initial water content and by theintensity of pre-drying. The soluble salt release to outersolution is slightly enhanced by soil slaking. Shamim andAkae (2010) had also reported that the salt release is linearly proportional to the slaking rate in both of the naturaland the air-dry soils. The higher salt release was caused byhigher slaking rate. At slaking rate is almost zero (60 and
50 % moisture contents) in the natural soil, the proportion(2) Most of the Ca2+ and Mg2+ stay in exchange sites (60to 80 %) in soil and very few come out in outer solution.
(3) The exchangeable cations of the natural soil were increased by the effect of drying. This revealed that shift ofcations were caused to exchange phase by drying.
(4) The lower water content (30 %) released more Na, andat the minimum slaking rate the release of Na was lowest
in both the natural and air-dried soils.
(5) However, when the soil was too dry condition (10 %water content) and slaking was very rapid, Na release was
not enhanced more.
But the further study and research in this challenging
ﬁeld is encouraged in order to discover the most effective
and applicable desalinization method for the soil.

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Japanese Geotechnical Society (In Japanese), Tokyo.

旨

スレーキングの機構は，水の流入による土壌団粒の破壊であり，土壌が塩分を含む時，塩分を解放する過程を伴う．本論文の目的は、異なる初期含水状態での除塩において，スレーキングと乾燥が，陽イオンの外液中への排出と状態の移動に及ぼす影響を述べることである．初期含水比を変えた（60，50，40，

30，20，10 % 重量含水比）自然土と風乾土へのスレーキングの効果を評価するため，スレーキング試験を行った．24 時間の水浸後，自然土と風乾土の全てのケースで， Na が卓越して排出された．最適ス
レーキング含水比にほぼ等しい比較的低い含水比（30 %）で最大の Na が解放され，スレーキング率が最低の含水比で，Na の排出は最も低かった．しかしながら，低すぎる含水比（10 %）は，それ以上のNa の解放に寄与しなかった．一方，外液中の他の陽イオン Ca， Mg，K の解放と初期含水比の間には特別な関係は認められなかった．ほとんど（60 ∼ 80 %） の Ca と Mg は，交換性サイトに停まり，外液中には僅かしか出て来ない．スレーキング後の土に残存する水溶性陽イオンは，Na  K > Mg > Caの順であり，これに対し，交換態の陽イオンの量は Na > Mg > Ca > K の順であった．全体として，外液中に解放された Na 量は自然土の方が風乾土よりも多かった．最適スレーキング含水比と，よりゆっ
くりとしたスレーキングが土壌からの Na の除塩により効率的であることが示された．
キーワード：土壌のスレーキング， 土壌水分状態， 陽イオンの解放，除塩，交換態