Editorial
Salinization Status and Salt Removal Techniques
Mitsuhiro Inoue
Professor, Arid Land Research Center, Tottori University
1. Introduction
The rapid growth in human population has caused serious
problems such as food crisis, depletion of water resources,
energy shortage and environmental deterioration. The
problem of land deterioration, especially salinization is
occurring in more than 100 countries in the world1), and lands
are rapidly becoming salinized and sodified. Salinization is
one of the main causes for the decline in global agricultural
production. The excess salt concentration in soil cannot be
reduced over time using normal irrigation and crop
management methods2). Realizable measures to prevent
salinization are urgently needed for sustainable agriculture.
This article discusses the causes of salinization, its current
status, ways to reduce salt accumulation, salinization
prevention measures and the techniques to remove salts from
salt accumulated lands.
2. Causes of salinization
Salinization is regulated by soil water movement and
composition and quantity of salts, and is greatly affected by
the base material of soil, groundwater level, and weather
conditions. Salt-affected soils that are naturally observed can
be classified into two: those formed by natural causes such as
minerals contained in soil and rainfall; and those formed by
artificial causes such as improper irrigation and water
management.
2.1 Natural causes
The base material of soil is formed by the physical and
chemical weathering of rocks, and soil is formed as it is
decomposed by microorganisms and so forth. In chemical
weathering, rainwater contains carbon dioxide dissolved from
atmosphere and becomes weakly acidic carbonic acid and
decomposes rocks. Major exchangeable cations such as
sodium (Na+), calcium (Ca2+), magnesium (Mg2+) and
potassium (K+) adsorbed to the negative charge of soil such as
clay minerals dissolve in water and moves to bind to anions to
form salts, including chlorides (NaCl, CaCl2, MgCl2, etc.),
sulfates (K2SO4, CaSO4, etc.), and carbonates (Na2CO3,
K2CO3, CaCO3, MgCO3, etc.). While the solubility of these
salts depend on the temperature, the it decreases in the
following order when the temperature is 20C; 74.5 for
calcium chloride > 54.6 for magnesium chloride > 35.9 for
sodium chloride > 25.5 magnesium sulfate > 21.5 sodium
carbonate > 19.5 for sodium sulfate > 0.21 for calcium sulfate
(gypsum) > 0.17 for calcium hydroxide > 6.5 x 10-3 (g/100
mL) for calcium carbonate. That is, ions with low solubility
tend to move less in the soil. For instance, a cross-sectional
study on arid lands soil reports that the layer with
accumulation of calcium carbonate, which is low in solubility
and easily identified as white deposit, occurs and the layer
appears in a deeper stratum when precipitation is larger4).
As described above, occurrence of salinization depends on the
processes of soil formation, difference in solubility of salts,
rainfall conditions and so forth. In addition, in the arid lands
salts are deposited from the soil water by evaporation and
accumulate in the top soil since rainfall is low and soil water
movement is predominantly upward. In this case,
salinization advances as the evaporative power, the
groundwater level, the clay content and water content in soil
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are higher. In addition, rock salt may also be produced as the
stratum that used to be an ocean floor in the ancient times is
lifted up and dried. In areas close to coasts, natural disasters
such as entry of seawater splattered by typhoon or strong wind,
entry of seawater in low, flat lands by tsunami, and entry of
seawater due to land subsidence caused by an earthquake may
cause salinization through natural processes.
2.2 Artificial causes
To ensure food supply for the growing population, the
environment surrounding agriculture has been changing
dramatically in the recent years. For instance, the global
population doubled from 1960 to 2000, grain production
increased to 2.5-fold due to breed improvement, introduction
of irrigation and fertilization and increase in cultivated area,
and the irrigation area to 1.8-fold. The demand for
agricultural water which accounts for 70% of the total global
demand for water increased to 1.9-fold, and 40% of all food is
produced in irrigated farmlands, which accounts for
approximately 18% of the total area of farmlands (1.5 billion
ha). As a consequence, groundwater is depleted by excessive
irrigation, the quality of irrigation water is deteriorating, and
accumulation of salts is facilitated by insufficient draining and
evaporation, causing crop yields to drop by salt damage and
farmers to abandon the farmland eventually as salinization
advances.
The examples of artificial causes for salinization include: [1]
excessive irrigation causing the groundwater containing salts
to rise from deep strata even when the quality of irrigation
water is good, [2] supply of irrigation water higher in salt
concentration in downstream area of a river than the upstream
area, [3] application of fertilizers over many years in
greenhouse cultivation, [4] water logging caused by the rise in
groundwater level in neighboring areas due to considerable
leak of water from earthen irrigation canals, [5] rise in
groundwater level within the field due to stagnation of water
seeping into the ground in surface irrigation such as basin
irrigation and furrow irrigation, [6] formation of plow sole
layer by tread pressure directly under the root zone due to
introduction of large tractors, etc., which causes the soil water
content in the root zone to rise and facilitates evaporation, [7]
increase in evaporation on bare lands formed by large-scale
felling in large woodlands as absorption of water by tree roots
stops and groundwater level rises, [8] increase in salt
concentration in soil due to insufficient quantity of water in
planned leaching, [9] irrigation with groundwater with
elevated salt concentration in the neighboring region or
downstream region, although the salt concentration in the root
zone is reduced by leaching, and [10] use of irrigation water
containing salt due to the mixture of industrial effluent or
agricultural drainage. Salinization is becoming more
advanced all over the world. Salt damage in agricultural land
is usually caused by the lack of awareness that irrigation itself
is adding salt to the land. For example, if water with
electrical conductivity of 1 dS/m (equivalent to 0.5 g/L salt) is
used for irrigation in annual quantity of 1,000 mm (daily
irrigation quantity of 3 mm or smaller) for 100 years just like
the irrigation water used in the mid-stream region of Yellow
River in China, assuming the weight density of salt at 2 g/cm3,
it would accumulate 2.5 cm of white salt throughout the land
Geotechnical Engineering Magazine, 60-1 (648)
Editorial
surface. Furthermore, a considerable amount of salt is
supplied to the field when we consider fertilization since
fertilizers also contain salt.
3. Current status of salinization
When we compare salinization to a disease of the land, as a
disease naturally requires diagnosis and treatment, it is
necessary to develop a salinization risk assessment method by
grasping the conditions of salinization for diagnosis5).
3.1 Classification of salt affected soil
Salt affected soil is classified as saline soil (pHe lower than
8.5, ECe 4 dS/m or higher, and ESP lower than 15%), sodic
soil (pHe 8.5 or higher, ECe lower than 4 dS/m, and ESP 15%
or higher), and saline-sodic soil (pHe lower than 8.5, ECe 4
dS/m or higher, and ESP 15% or higher)3). Here, pHe is the
pH value for saturated extraction solution from soil, ECe is the
electrical conductivity of the saturated extraction solution
from soil, and ESP is the exchangeable-sodium percentage
which expresses the rate of absorbed amount of sodium in
cation exchange capacity. It is necessary to determine the
type of salt affected soil, since the measures to prevent
salinization varies by the type, as leaching is good for saline
soil and addition of calcium materials is good to improve
water permeability for sodic soil.
3.2 Measurement of salt concentration
The meaning of salt concentration in soil water varies
depending on how electrical conductivity is measured as listed
below; [1] electrical conductivity (ECw) of solution directly
collected from soil by suction under negative pressure using a
solution collection device which is inserted into the soil, [2]
electrical conductivity of saturated extraction solution (ECe)
which is obtained by collecting soil, adding enough distilled
water to make it into a paste and preprocessing it by filtration
under reduced pressure or centrifugation, [3] electrical
conductivity of the 1:5 extraction solution (EC1:5) which is
obtained by collecting soil and adding distilled water in a
quantity 5 times that of the dry weight of the soil, and [4]
apparent electrical conductivity (ECa) measured by inserting
4-electrode sensor or TDR sensor into unsaturated soil3).
While ECw measured in [1] is directly involved when
considering the cause of absorption by plant roots or salt
damage, the method in [3] is relatively more popularly used in
soil diagnosis in Japan and method in [2] in the U.S.A., and
the classification of salt affected soil is determined based on
ECe. In Australia, airborne electromagnetics for salinity and
groundwater mapping is an established method for grasping
the salinization status over a large area of land in
three-dimensional analysis including the depth direction by
installing an airborne electromagnetic exploratory device on
an airplane or a helicopter6).
3.3 Current status of salinization in overseas
From the beginning, humans have never lived in lands with
high salt concentrations. The Mesopotamian civilization,
which is one of the four grate civilizations, flourished with
ancient agriculture and stock farming in the fertile land
enriched by the Euphrates. However, salinization advanced
due to conflicts among tribes, deforestation and improper
irrigation, and their prosperity diminished. Similarly, the
Indus civilization also declined due to salinization caused by
river flooding and water logging. More recently, significant
salinization is occurring in farmlands of Kazakhstan7) and the
problem of shrinking Aral Sea8) is well-known as the results of
improper irrigation and water management system after the
large-scale farmland development in Central Asia by the
former Soviet Union during the 1960s. Salinization is also a
problem in Mexico5), China and Australia6), indicating that the
measures of salt removal are of urgent need over the world.
January, 2012
4. Reduction in salt accumulation and preventive measures
To treat, we need measures to prevent accumulation of salts or
keep salinization from advancing.
Salinization can occur through two mechanisms; [1]
accumulation of salts caused when the salt concentration near
the ground surface rises and they precipitate as the
groundwater level rises and the soil water converts from liquid
phase to gas phase in evaporation process, and [2]
accumulation caused by salts that enter from outside the
system over a long period by irrigation, fertilization and so
forth or a short period by a disaster and so forth. Salt moves
upward in the case of [1], and downward in [2].
4.1 Upward movement of salts
When the salts move upward, the basic measures to reduce salt
accumulation and prevent salinization are to keep the
groundwater level from rising and to reduce evaporation by
suppressing capillary rise. Specific measures include: [1]
drainage through civil engineering measures using open
ditches (drain ditches) or conduits, mechanical draining by lift
pumps installed in wells, and biological draining by absorption
from plant roots such as poplar as measures to lower the
groundwater level, [2] mulching to reduce the level of
evaporation by laying straws, fallen leaves, gravel, sand and
so forth as a measure to reduce evaporation and suppress the
upward movement of soil water9), [3] traditional deep
cultivation method to form a dry layer above the ground
surface by plowing deep into the ground to cut off the
capillary rise in the root zone, and [4] civil engineering
measure of burying capillary barriers to cut off the capillary
rise by laying a gravel layer and so forth between the root
zone and the groundwater level10).
4.2 Downward movement of salts
When the salts move downward, the typical cases are rainfall
and irrigation. To prevent the salt concentration in the root
zone from rising, amount of applied irrigation (Ir) shall be set
up by adding the amount of water draining (D) below the root
zone into the ground to the crop evapotranspiration (ETc) as
the leaching requirement. That is, the amount of leaching to
wash the salts in the root zone down shall be added to ETc as
an extra volume. Here, a simple one-dimensional salt
balance is considered by measuring the salt concentration of
irrigation water entering the root zone (ECi) and the salt
concentration for drainage going downward from the root
zone (ECd), disregarding the condensation or dissolution of
salt in irrigation water or absorption by plant roots. Since the
above electrical conductivity is proportional to the
concentration of electrolytes (mol/L) in soil water, the amount
of salt entering the root zone is proportional to the product of
Ir and ECi, and the amount of salt leaving the root zone is
proportional to the product of D and ECd. If these amounts
entering and leaving the root zone are equal, the salt
concentration in the root zone can be controlled at a low level.
Based on this concept, salt accumulation can be prevented by
defining the leaching requirements (LR = D/Ir) as the ratio of
amount of irrigation against the amount of water draining and
determining the amount of irrigation for leaching as follows in
leaching plan3).
Ir = ETc / (1 - LR) …………………. (1)
What is important in this case is to facilitate draining from the
ground sufficiently so that the groundwater does not rise in
concurrence with leaching. Especially in the case of clay soil,
draining through conduits is effective. It is also important to
save water as possible even if irrigation water contains salts in
any concentration in order to address sustainable agriculture.
Furthermore, as case examples of measures to prevent entry of
salts from outside the system, civil engineering measures such
as [1] establishment of tide embankment or windbreak forest
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Editorial
to prevent entry of seawater due to typhoons or strong winds,
[2] lining of irrigation canals to prevent rise in groundwater in
the area due to leak from the canals, and [3] separation of
irrigation canals and drainage canals to prevent entry of
industrial effluent or agricultural drainage with high salt
concentration are considered effective.
5. Techniques to remove salt from lands with salt
accumulation
To treat, we also need measures to remove salt from salt
accumulated lands to restore farmlands. In arid regions,
there are many lands where salt crystals have accumulated on
the surface of farmlands that really look like “salt accumulated
lands.” This section introduces the hydraulic, chemical,
biological and civil engineering salt removal techniques.
5.1 Hydraulic salt removal technique (leaching)
Leaching requires more water than it is necessary for
consumption by the crops in order to remove the salts, and this
leads to the problem of depletion in water resources in arid
regions. Here, let us consider how much time it would take
to move the dissolved salts downward from a salt accumulated
land by leaching. Supposing that the groundwater level is
sufficiently deep and salinization doesn’t occur during
non-irrigation period and that there is regular downward,
one-dimensional flow with no salt adsorption in soil and little
salt movement due to diffusion, then advection will dominate
the downward flow of salts and the time of salt travel (t) can
be calculated with a simple equation. That is, salt movement
depends on the average soil water velocity (v), and the
distance traveled (L) is the product of the time of salt arrival
(t) and the average velocity (v). Since the flux (q) defined as
the quantity passing through the unit area in unit period is the
product of average volumetric water content (θ) and the
average velocity (v), the time of arrival is calculated as
follows:
t = L×θ / q ……………………….. (2)
This equation helps in simple calculation of salt advection
velocity. For example, when the annual precipitation (added
with the amount of irrigation including the value for leaching)
is 1,500 mm, annual amount of evapotranspiration 1,250 mm,
the groundwater level 20 m in depth, and the average
volumetric water content in the vadose zone (unsaturated zone
from the ground surface to the groundwater level) 0.25 m3/m3,
the time of salt arrival when it reaches the groundwater is
calculated as 20 years11). This result indicates that it would
take 20 years for the salts to travel the distance of 20 m. This
value, of course, depends on the annual leaching, irrigation
and precipitation. If the annual precipitation is 3,000 mm,
the time of arrival will be 2.86 years, and salts can be removed
deeper into the ground if the groundwater level is deep and
irrigation water is available in abundance.
The prerequisites for leaching include that there is sufficient
supply of water from outside the system, that drainage is
ensured with open ditches or conduits not to allow the
groundwater level to rise, and that the permeability is
maintained. However, in actual cases, there are many issues
of secondary salinization of farmlands due to depletion in
water resources, insufficient drainage from low/flat lands, rise
in groundwater level and increased salt concentration in
downstream farmlands.
5.2 Chemical salt removal technique (improvement of
sodic soil)
The measures of salt removal vary depending on whether the
soil in the salt accumulated land is saline or sodic. Advance
in salinization strongly depends on factors including [1]
condensation by evaporation associated with intense solar
insolation energy, [2] selective precipitation of inorganic salts
associated with the solubility of soluble salts, [3] intensity and
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cumulative quantities of rainfall and irrigation, and [4] soil
properties and structure of soil. In addition, due to the
matters of solubility as described earlier, calcium carbonate
precipitates first during the phenomenon of salt accumulation,
and chlorides dissolve first in dilution by rainfall or irrigation.
Sodification advances even during the leaching process as the
salts containing sodium runs downward and helps the
formation of sodic soil.
Sodic soil which contains high levels of sodium is strongly
alkaline and is distributed mainly in low/flat lands. Leaching
effect cannot be expected as water permeability and air
conductivity deteriorate by dispersion of colloid substances
and swelling of clay soil, forming salt crusts on the ground
surface. Furthermore, it poses a severe ecological
environment in which even salt-tolerant plants cannot grow as
it is strong alkaline. To improve this sodic soil, it is
recommended that water-soluble calcium materials such as
gypsum (CaSO4•2H2O) and calcium chloride (CaCl2•2H2O)
should be added to the soil to improve the water permeability,
followed by leaching with irrigation water containing some
salt instead of fresh water so that the absorbed sodium is
replaced by calcium5). Physical improvements such as deep
plowing and plow sole layer crushing are effective in
improving the permeability of clay soil layers with poor
drainage.
5.3 Biological salt removal technique (phytoremediation)
While it also depends on the type of soil including the clay
content, leaching does not remove the salts sufficiently from
the root zone as most water pass through macropores.
Inexpensive phytoremediation (environmental restoration
using plants) has been gathering attention of many developing
countries in the recent years. Its characteristics include [1]
mitigation of sodification by alkali-resistant plants, [2]
improvement in permeability as the plant roots penetrate the
root zone, [3] wide range of land improvement down to the
deep parts where the plant roots reach if plants such as
sorghum and Sudangrass are used, although application of
calcium materials such as gypsum improves only the applied
parts of the root zone (only about 20 cm of surface layer), [4]
salt removal effect for lowering the electrical conductivity and
ion concentration in the soil layer to the depth of 1.2 m
revealed by soil layer analysis after transplanting salt-tolerant
shrubs12), and [5] continuous prevention of salt damage by
economically useful salt-tolerant plants.
The mechanisms for causing salt damage in plants4) include
[1] suppression of growth as absorption of water by plants is
inhibited by the osmotic pressure of salts contained in soil
water (osmotic stress), [2] physiological disorder caused by
excessive sodium ions and magnesium ions (ion stress), and
[3] exchange of ions adsorbed to the clay particles in soil with
sodium ions, which deteriorates the drainage considerably and
results in root rotting (physiological disorder). Plants are
sensitive to salt during the germination stage, and the growth
is slow even if they are salt-tolerant. Salt-tolerant plants
prevent salt damage through various mechanisms including
reproduction as viviparous seeds which have already
germinated as in mangrove (ecological mechanism), salt
removal by accumulating the salt in hairs that grow on the
surface of leaves which fall off as in chenopodiaceae plants
(morphological mechanism), suppression of sodium
absorption from the root as in common reed, elimination of
absorbed salt from specific tissues (salt glands) as in tamarix,
and storage of sodium in gigantic vacuoles of mesophyll cells
(physiological mechanisms)3). Useful plants from which
harvesting can be expected include cotton, sugar beet, ice
plant, common glasswort(Salicornia spp.), barley, table beet,
bermuda grass, Kochia, and saltgrass, with ECe about 8 – 12
dS/m and sorghum, sunflower, Sudangrass and Swiss chard
with ECe about 3 – 8 dS/m, although yield will be down by 10
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Editorial
3)
– 15% . Here, useful plants indicate plants that can be used
by people and be sources of income, such as production of
fiber or essential oil, food including vegetables and livestock
feed (to be blended in herbage). Ice plant is used in salads
and common glasswort is used in salads or pickled. Such use
of halophilic plants in salt removal measures is economically
feasible. The purpose of phytoremediation is to cultivate
regular crops by restoring the farmland by salt removal and
not to produce halophilic plants. Thus it is important to
ensure soil management in the future.
5.4 Civil engineering salt removal techniques
When white crystallized salt is run downward by leaching, it
will increase the salt concentration in groundwater and
advance secondary salinization. There are several
engineering methods to remove the salt to prevent this.
(1) Enough water resources
If ample water is available from rivers, salts can be removed
by [1] washing off the salt around the ground surface
horizontally by pumping and so forth into the downstream end
to remove the salt (surface runoff removal method, flushing
method), and [2] flooding the field for 2 to 3 days to let the
salt dissolve sufficiently into the irrigation water and draining
into open ditch (drainage ditch) to remove the salt (surface
elution drainage method) - methods effective for clayey soil
with poor draining, and [3] conducting surface irrigation in the
field and facilitating downward permeation and drainage with
subsoil breaking, mole drain and so forth as necessary to
remove the salt (leaching method) as the method effective for
soil with good drainage. Fundamentally, it is important to
take into consideration the separation of irrigation water and
drainage not to cause secondary salinization by letting the
removed salt enter an adjacent field. The salt-containing
drainage should be eventually collected into a salt-water lake
to be used in salt production and so forth.
(2) Limited water resources
When salt concentration is measured in the vertical direction
in salt accumulated land, the concentration is considerably
high around the surface with white deposition of salt caused
by evaporation appearing partially. The methods to remove
salts when the water resources are limited include [1] removal
by civil engineering measures of salt crust and soil with high
salt concentration (surface delamination method, scraping
method), [2] laying the collection sheets soaked in water13) on
the ground surface to absorb and collect salt sufficiently to
remove it from the system (salt absorption and capture
removal method, dehydration method), [3] salt removal
through absorption while saving water by pinpointing on the
white salt accumulation14) (surface suction leaching method),
and [4] applying capillary barriers to cut off the capillary rise
of groundwater and borrowing soil that does not contain salts
as the root zone (capillary cutoff and soil layer improvement
method).
6. Conclusion
Methods of salt removal to restore large areas of lands with
salt accumulation as farmlands that can be carried out both
economically and sustainably need to be discovered by
combining various techniques. For example, it is possible to
[1] improve the soil layer by removing the salt deposited on
the ground surface with surface delamination method using
civil engineering measures, followed by leaching and then
phytoremediation. Here, the problems of the removed salt
and secondary salinization need to be solved. It is also
possible to [2] apply the capillary barrier materials such as
gravel and recycled materials over the land with salt
accumulation to put admixtured new soil without salt content
onto the barrier as the root zone. Here, how the capillary
barrier materials and the admixtured new soil for root zone
can be obtained must be solved. In either case, it is
January, 2012
important to diagnose the soil by monitoring the salinization
status with nondestructive sensors, establish a risk assessment
method for salinization and adopt the appropriate salt removal
technique for the conditions of each land by considering the
salt balance in the farmland over a long period.
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