Alkali, or alkaline, soils are clay soils with high pH (>
8.5), a poor soil structure and a low infiltration capacity. Often they have a
hard calcareous layer at 0.5 to 1 metre depth. Alkali soils owe their
unfavorable physico-chemical properties mainly to the dominating presence of
sodium carbonate which causes the soil to swell and difficult to clarify/settle. They derive
their name from the alkali metal group of elements to which the sodium belongs
and that can induce basicity. Sometimes these soils are also referred to as
(alkaline) sodic soils.
Alkaline soils are basic, but not all basic soils are
alkaline, see: "difference between alkali and base"
Causes
The causes of soil alkalinity are natural or they can be
man-made.
The natural cause
is the presence of soil minerals producing sodium carbonate (Na2CO3) and sodium
bicarbonate (NaHCO3) upon weathering.
Coal fired boilers
/ power plants when using coal or lignite rich in limestone produces ash
containing calcium oxide (CaO). CaO readily dissolves in water to form slaked
lime / Ca(OH)2 and carried by rain water to rivers / irrigation water. Lime
softening process precipitates Ca and Mg ions / removes hardness in the water
and also converts sodium bicarbonates in river water into sodium carbonate.
Sodium carbonates (washing soda) further reacts with the remaining Ca and Mg in
the water to remove / precipitate the total hardness. Also water soluble sodium
salts present in the ash enhance the sodium content in water. The global coal
consumption is 7700 million tons in the year 2011. Thus river water is made
devoid of Ca and Mg ions and enhanced Na by coal fired boilers.
Many sodium salts
are used in industrial and domestic applications such as Sodium carbonate,
Sodium bicarbonate (baking soda), Sodium sulphate, Sodium hydroxide (caustic
soda), Sodium hypochlorite (bleaching powder), etc. in huge quantities. These
salts are mainly produced from Sodium chloride (common salt). All the sodium in
these salts enter into the river / ground water during their production process
or consumption enhancing water sodicity. The total global consumption of sodium
chloride is 270 million tons in the year 2010. This is nearly equal to the salt
load in the mighty Amazon River. Manmade sodium salts contribution is nearly 7%
of total salt load of all the rivers. Sodium salt load problem aggravates in
the downstream of intensively cultivated river basins located in China, India,
Egypt, Pakistan, west Asia, Australia, western USA, etc. due to accumulation of
salts in the remaining water after meeting various transpiration and
evaporation losses.
Another source of
man made sodium salts addition to the agriculture fields / land mass is in the
vicinity of the wet cooling towers using sea water to dissipate waste heat
generated in various industries located near the sea coast. Huge capacity
cooling towers are installed in oil refineries, petrochemical complexes,
fertilizer plants, chemical plants, nuclear & thermal power stations, centralized
HVAC systems, etc. The drift / fine droplets emitted from the cooling towers
contain nearly 6% sodium chloride which would deposit on the vicinity areas.
This problem aggravates where the national pollution control norms are not
imposed or not implemented to minimize the drift emissions to the best
industrial norm for the sea water based wet cooling towers.
The man-made cause
is the application of soft water in irrigation (surface or ground water)
containing relatively high proportion of sodium bicarbonates and less calcium
and magnesium.
Agricultural problems
Alkaline soils are difficult to take into agricultural
production. Due to the low infiltration capacity, rain water stagnates on the
soil easily and, in dry periods, cultivation is hardly possible without copius
irrigated water and good drainage. Agriculture is limited to crops tolerant to
surface waterlogging (e.g. rice, grasses) and the productivity is low.
Chemistry
Soil alkalinity is associated with the presence of sodium carbonate
or washing soda (Na2CO3) in the soil,[6] either as a result of natural
weathering of the soil particles or brought in by irrigation and/or flood
water.
The sodium carbonate, when dissolved in water, dissociates
into 2Na+ (two sodium cations, i.e. ions with a positive electric charge) and
CO32− (a carbonate anion, i.e. an ion with a double negative electric charge).
The sodium carbonate can react with water to produce carbon
dioxide (CO2), escaping as a gas or obsorbed by Algae, and sodium hydroxide
(Na+OH–), which is alkaline (or rather basic) and gives high pH values
(pH>8.5).
Notes:
Water (H2O) is
partly dissociated into H3O+ (hydronium) and OH– (hydroxyl) ions. The ion H3O+
has a positive electric charge (+) and the hydroxyl group OH– has a negative
charge (–). In pure, neutral water, the concentration of H3O+ and OH– ions
equals 10–7 eq/l each (respectively 19x10–7 g/l and 17x10–7 g/l), very small
concentrations.
1 eq = 1
equivalent weight corresponds to as many grams of the substance as its formula
weight divided by its valence. It is also known as gram-molecule or mole per
unit of valence. For the hydronium ion (H3O+) the formula weight equals 19, and
for the hydroxyl group (OH–) it equals 17.
In neutral
water, the pH, being the negative log value of the H3O+ concentration in eq/l,
is 7. Similarly, the pOH is also 7. Each unit decrease in pH indicates a
tenfold increase of the H3O+ concentration. Similarly, each unit increase in pH
indicates a tenfold increase of the OH– concentration.
In water with
dissolved salts, the concentrations of the H3O+ y OH– ions may change, but
their sum remains constant, namely 7 + 7 = 14. A pH of 7 therefore corresponds
to a pOH of 7, and a pH of 9 with a pOH of 5.
Formally it
deserves preference to express the ion concentrations in terms of chemical
activity, but this does hardly affect the value of pH.
Water with
excess H3O+ ions is called acid (pH < 7), and water with excess OH– ions is
called alkaline or rather basic (pH > 7). Soil moisture with pH < 4 is
called very acid and with pH > 10 very alkaline (basic).
The reaction between Na2CO3 and H2O can be represented as
follows:
2Na+ + CO32− + 2H+
+ 2OH– => 2Na+ + 2OH– + H2CO3
The acid H2CO3 is unstable and produces H2O (water) and CO2
(carbon dioxide gas, escaping into the atmosphere). This explains the remaining
alkalinity (or rather basicity) in the form of soluble sodium hydroxide and the
high pH or low pOH.
Not all sodium carbonate follows the above chemical
reaction. The remaining sodium carbonate, and hence the presence of CO32− ions,
causes CaCO3 (which is only slightly soluble) to precipitate as solid calcium
carbonate (limestone). Hence, the calcium ions Ca++ are immobilized.
Sodium exchange process between ions adsorbed at the surface
of clay particles and those in the soil moisture
The presence of abundant Na+ ions in the soil solution and
the precipitation of Ca++ ions as a solid mineral causes the clay particles, which
have negative electric charges along their surfaces, to adsorb more Na+ in the
diffuse adsorption zone (DAZ, see figure, officially called diffuse double
layer[7]) and, in exchange, release Ca++, by which their exchangeable sodium
percentage (ESP) is increased as illustrated in the figure.
Na+ is more mobile and has a smaller electric charge than
Ca++ so that the thickness of the DAZ increases as more sodium is present. The
thickness is also influenced by the total concentration of ions in the soil moisture
in the sense that higher concentrations cause the DAZ zone to shrink.
Clay particles with considerable ESP (> 16), in contact
with non-saline soil moisture have an expanded DAZ zone and the soil swells (dispersion).
The phenomenon results in deterioration of the soil structure, and especially
crust formation and compaction of the top layer. Hence the infiltration
capacity of the soil and the water availability in the soil is reduced, whereas
the surface-water-logging or runoff is increased. Seedling emergence and crop
production are badly affected.
Note:
Under saline
conditions, the many ions in the soil solution counteract the swelling of the
soil, so that saline soils usually do not have unfavorable physical properties.
Alkaline soils, in principle, are not saline since the alkalinity problem is
worse as the salinity is less.
Alkalinity problems are more pronounced in clay soils than
in loamy, silty or sandy soils. The clay soils containing montmorillonite or
smectite (swelling clays) are more subject to alkalinity problems than illite
or kaolinite clay soils. The reason is that the former types of clay have
larger specific surface areas (i.e. the surface area of the soil particles
divided by their volume) and higher cation exchange capacity (CEC).
Note:
Certain clay
minerals with almost 100% ESP (i.e. almost fully sodium saturated) are called
bentonite, which is used in civil engineering to place impermeable curtains in
the soil, e.g. below dams, to prevent seepage of water.
The quality of the irrigation water in relation to the
alkalinity hazard is expressed by the following two indexes:
1) The sodium adsorption ratio (SAR,[6] )
The formula for calculating sodium adsorption ratio is:
[Na+]
{Na+/23}
SAR = ───────────── = ──────────────
√[Ca++/2 +
Mg++/2] √{Ca++/40 + Mg++/24}
where: [ ] stands for concentration in
milliequivalents/liter (briefly meq/l), and { } stands for concentration in
mg/l.
It is seen that Mg (Magnesium) is thought to play a similar
role as Ca (Calcium).
The SAR should not be much higher than 20 and preferably
less than 10;
When the soil has been exposed to water with a certain SAR
value for some time, the ESP value tends to become about equal to the SAR
value.
2) The residual sodium carbonate (RSC, meq/l,[6]):
The formula for calculating residual sodium carbonate is:
RSC = [HCO3– +
CO3=] − [Ca+++ Mg++]
= {HCO3–/61 +
CO3=/30} − {Ca++/20 + Mg++/12}
which must not be much higher than 1 and preferably less
than 0.5.
The above expression recognizes the presence of bicarbonates
(HCO3–), the form in which most carbonates are dissolved.
While calculating SAR and RSC, the water quality present at the
root zone of the crop should be considered which would take into account the leaching
factor in the field.The partial pressure of dissolved CO2 at the plants root
zone also decides the Calcium present in dissolved form in the field water.
USDA follows the adjusted SAR for calculating water sodicity.
Solutions
Alkaline soils with solid CaCO3 can be reclaimed with grass
cultures, organic compost, waste hair / feathers, organic garbage, waste paper,
etc. ensuring the incorporation of much acidifying material (inorganic or
organic material) into the soil, and enhancing dissolved Ca in the field water
by releasing CO2 gas. Deep plowing and incorporating the calcareous subsoil
into the top soil also helps.
Many times salts' migration to the top soil takes place from
the underground water sources rather than surface sources. Where the
underground water table is high and the land is subjected to high solar
radiation, ground water oozes to the land surface due to capillary action and
gets evaporated leaving the dissolved salts in the top layer of the soil. Where
the underground water contains high salts, it leads to acute salinity problem.
This problem can be reduced by applying mulch to the land. Using poly-houses
during summer for cultivating vegetables/crops is also advised to mitigate soil
salinity and conserve water / soil moisture. Poly-houses filter the intense
summer solar radiation in tropical countries to save the plants from water
stress and leaf burns.
Where the ground water quality is not alkaline / saline and
ground water table is high, salts build up in the soil can be averted by using
the land throughout the year for growing plantation trees / permanent crops
with the help of lift irrigation. When the ground water is used at required
leaching factor, the salts in the soil would not build up.
Plowing the field soon after cutting the crop is also
advised to prevent salt migration to the top soil and conserve the soil
moisture during the intense summer months. This is done to break the capillary
pores in the soil to prevent water reaching the surface of the soil.
Clay soils in high annual rain fall (more than 100 cm) areas
do not generally suffer from high alkalinity as the rain water runoff is able
to reduce/leach the soil salts to comfortable levels if proper rain water
harvesting methods are followed. In some agricultural areas, the use of
subsurface "tile lines" are used to facilitate drainage and leach
salts. Continuous Drip irrigation would lead to alkali soils formation in the
absence of leaching / drainage water from the field.
It is also possible to reclaim alkaline soils by adding
acidifying minerals like pyrite or cheaper alum or Aluminium sulfate.
Alternatively, gypsum (calcium sulfate, CaSO4. 2H2O) can
also be applied as a source of Ca++ ions to replace the sodium at the exchange
complex. Gypsum also reacts with sodium carbonate to convert into sodium
sulphate which is a neutral salt and does not contribute to high pH. There must
be enough natural drainage to the underground, or else an artificial subsurface
drainage system must be present, to permit leaching of the excess sodium by
percolation of rain and/or irrigation water through the soil profile.
Calcium Chloride is also used to reclaim alkali soils. CaCl2
converts Na2CO3 into NaCl precipitating CaCO3. NaCl is drained off by leaching
water. Spent acids (HCl, H2SO4, etc.) can also be used to reduce the excess
Na2CO3 in the soil.
Where urea is made available cheaply to farmers, it is also
used to reduce the soil alkalinity / salinity primarily. The NH4 (Ammonium)
present in urea which is a weak cation releases the strong cation Na from the
soil structure into water. Thus alkali soils absorb / consume more urea
compared to other soils.
To reclaim the soils completely one needs prohibitively high
doses of amendments. Most efforts are therefore directed to improving the top
layer only (say the first 10 cm of the soils), as the top layer is most
sensitive to deterioration of the soil structure.The treatments, however, need
to be repeated in a few (say 5) years time.Trees / plants follow gravitropism.
It is difficult to survive in alkali soils for the trees with deeper rooting
system which can be more than 60 meters deep in good non-alkali soils.
It will be important to refrain from irrigation (ground
water or surface water) with poor quality water.
One way of reducing sodium carbonate is to cultivate
glasswort or saltwort or barilla plants. These plants sequester the sodium
carbonate they absorb from alkali soil into their tissues. The ash of these
plants contains good quantity of sodium carbonate which can be commercially
extracted and used in place of sodium carbonate derived from common salt which
is highly energy intensive process. Thus alkali lands deterioration can be
checked by cultivating barilla plants which can serve as food source, biomass
fuel and raw material for soda ash and potash, etc.
Leaching saline sodic soils
Saline soils are mostly also sodic (the predominant salt is
sodium chloride), but they do not have a very high pH nor a poor infiltration
rate. Upon leaching they are usually not converted into a (sodic) alkali soil
as the Na+ ions are easily removed. Therefore, saline (sodic) soils mostly do
not need gypsum applications for their reclamation.
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