Review
Potential fly-ash utilization in agriculture: A global review
Manisha Basu
a,b,
*
, Manish Pande
a
, P.B.S. Bhadoria
b
, S.C. Mahapatra
c
a
Agriculture & Food Services, SGS India Pvt. Ltd., Gurgaon, Haryana 122015, India
b
Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur, West Bengal 721302, India
c
Rural Development Centre, Indian Institute of Technology, Kharagpur, West Bengal 721302, India
Received 1 September 2008; received in revised form 17 November 2008; accepted 8 December 2008
Abstract
Though in last four decades various alternate energy sources have come into the limelight, the hyperbolic use of coal as a prime energy
source cannot be counterbalanced. Disposal of high amount of fly-ash from thermal power plants absorbs huge amount of water, energy
and land area by ash ponds. In order to meet the growing energy demand, various environmental, economic and social problems asso-
ciated with the disposal of fly-ash would continue to increase. Therefore, fly-ash management would remain a great concern of the cen-
tury. Fly-ash has great potentiality in agriculture due to its efficacy in modification of soil health and crop performance. The high
concentration of elements (K, Na, Zn, Ca, Mg and Fe) in fly-ash increases the yield of many agricultural crops. But compared to other
sectors, the use of fly-ash in agriculture is limited. An exhaustive review of numerous studies of last four decades took place in this paper,
which systematically covers the importance, scope and apprehension regarding utilization of fly-ash in agriculture. The authors con-
cluded that though studies have established some solutions to handle the problems of radioactivity and heavy metal content in fly-
ash, long-term confirmatory research and demonstration are necessary. This paper also identified some areas, like proper handling of
dry ash in plants as well as in fields, ash pond management (i.e., faster decantation, recycling of water, vertical expansion rather than
horizontal), monitoring of soil health, crop quality, and fate of fly-ash in time domain, where research thrust is required. Agricultural
lime application contributes to global warming as Intergovernmental Panel on Climate Change (IPCC) assumes that all the carbon in
agricultural lime is finally released as CO
2
to the atmosphere. It is expected that use of fly-ash instead of lime in agriculture can reduce net
CO
2
emission, thus reduce global warming also.
Ó 2009 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in
China Press. All rights reserved.
Keywords: Coal fly-ash; Agriculture; Soil health; Crop yield; Radioactivity; Global warming
1. Introduction
Fly-ash is the end residue from combustion of pulver-
ized bituminous or sub-bituminus coal (lignite) in the fur-
nace of thermal power plants and consists of mineral
constituents of coal which is not fully burnt. Fine minute
particles of ash are carried away with flue gases in electro-
static precipitators or cyclone separators and are collected
by wet (slurry form) or dry scrubbing method, which
requires large volumes of land, water and energy. Use of
high ash containing (30–50%) bituminous or sub-bituminus
coal in thermal power stations, in addition to severa l cap-
tive power plants, contributes to indiscriminate disposa l
of this industrial waste every year [1,2]. The coal ash by-
product has been classified as a Green List waste under
the Organization for Economic Cooperation and Develop-
ment (OECD). It is not considered as a waste under Basel
Convention. However, in many countries this industrial by-
product has not been properly utilized rather it has been
neglected like a waste substance.
In China, about 100 MT (million tons) of coal combus-
tion products are produced each year [3]. In India,
1002-0071/$ - see front matter Ó 2009 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited
and Science in China Press. All rights reserved.
doi:10.1016/j.pnsc.2008.12.006
*
Corresponding author. Tel.: +91 124 2399990; fax: +91 124 2399764.
www.elsevier.com/locate/pnsc
Available online at www.sciencedirect.com
Progress in Natural Science 19 (2009) 1173–1186
presently, the figure is around 112 MT and is likely to
exceed 170 MT by 2012 [4]. During 2005, the utilization
of fly-ash was 100% in Italy, Denmark and Netherlands
with an annual production of 2 MT, 50–85% in USA and
Germany and 45% in China (Table 1) [3]. In India, fly-
ash utilizat ion has increased from 3% in the 1990s [5] to
38% in 2005 [3]. The reason of low fly-ash utilization in
India is the unavailability of appropriate cost-effective
technologies [6]. According to the report of American Coal
Ash Association [7], in agriculture, wasteland reclamation
and civil engineering purposes use 32% of the fly-ash,
30% of the bottom ash, 94% of the boiler slag and 9% of
flue gas desulfurization sludge.
Many exp eriments and studies on the effect and potenti-
ality of fly-ash as an amendment in agricultural applica-
tions have been conducted by various agencies, research
institutes at dispersed locations all over the world. In this
paper, utilization of fly-ash as a value-added product of
agriculture is reviewed with the aim of helping opening
up the usage of fly-ash and reducing the environmental
and economic impacts of disposal.
2. Physical properties of fly-ash
The physical properties of fly-ash vary widely depending
on the coal type, boiler type, ash content in coal, combus-
tion method and collector setup. Fly-ash generally has a silt
loam texture with 65–90% of the particles having a diame-
ter of less than 0.010 mm [8,9]. Ash from bituminous coal is
usually finer as compared with that of lignite one [10]. Fly-
ash particles are empty spheres (cenospheres) filled with
smaller amorphous particles and crystals (plerospheres).
The cenosphere fraction constitutes as much as 1% of the
total mass and gets easily airborne [11]. In general, fly-
ash has low bulk density (1.01–1.43 g cm
3
), hydraulic con-
ductivity and specific gravity (1.6–3.1 g cm
3
) [9,10,12].
Mean particle densities for non-magnetic and magnetic
particles are 2.7 and 3.4 g cm
3
, respect ively, while the
moisture retention ranges from 6.1% at 15 bar to 13.4%
at 1/3 bar [13].
By virtue of its physical characteristics and sheer vol-
umes generat ed, fly-ash is a serious problem. Some of the
aspects of the problem are [14]:
(1) Due to heavy disposal, fly-ash particles both as dry
ash and pond ash occ upy many hectares of land in
the vicinity of power stat ion.
(2) Because of its fineness, it is very difficult to handle fly-
ash in dry state. Flying fine particles of ash corrode
structural surfaces and affect horticulture.
(3) It disturbs the ecology throu gh soil, air and water
pollution.
(4) Long inhalation of fly-ash causes various serious dis-
eases like silicosis, fibrosis of lungs, bronchitis, and
pneumonitis.
Moreover, the oxides of iron and aluminium present on
the surface of the fly-ash particles attract toxic trace ele-
ments, such as Sb, As, Be, Cd, Pb, Hg, Se, and V, and
they are found to be concentrated largely on the surface
of fly-ash [15]. A study was co nducted by Hicks and Yager
[16] with six bituminus, sub-bituminus and lignite coal-
fired thermal power plants to measure the amount of air-
borne respirable crystalline silica in the breathing zone of
workers engaged in fly-ash-related operations. It was
found that in the bituminus, sub-bituminus and lignite
coal-fired plants, the air samples (60%) collected during
maintenance-related work exceeded the threshold limit.
Similarly, in the case of normal production-related activi-
ties, the samples from bituminus (54%) and sub-bituminus
(65%) coal-fired plants surpassed the limit. In the bitumi-
nus/sub-bituminus and lignite coal, the minimum crystal-
line silica contents were observed to be 7.5% and 1.7%,
respectively [16].
3. Chemical properties of fly-ash
The factors influencing the physical properties are also
responsible for wide variation of chemical properties of
fly-ash. In a study of 11 fly-ashes from various U.S. power
plants, Theis and Wirth [17] found that the major compo-
nents were Al, Fe and Si, with smaller concentrations of
Ca, K, Na, Ti, and S. Fly-ash contain s varying amounts
of numerous trace elements, some of which are required
by plant and animals in varying amounts, whereas some
may have toxic effect. Fly-ash contains essential macronu-
trients including P, K, Ca, Mg and S and micronutrients
like Fe, Mn, Zn, Cu, Co, B and Mo. Some are rich in
heavy metals such as Cd and Ni [18]. According to Kumar
et al. [19] , on an average 95–99% of fly-ash consists of oxi-
des of Si, Al, Fe and Ca and about 0.5–3.5% consists of
Na, P, K and S and the remainder of the ash is composed
of trace elements. It is considerably rich in trace elements
like lanthanum, terbium, mercury, cobalt and chromium
[18,20]. According to Page et al. [21], many trace elements
including As, B, Ca, Mo, S, Se and Sr in fly-ash are con-
centrated in the smaller ash particles [18]. In fact, fly-ash
Table 1
Generation and utilization of fly-ash in different countries.
Country Fly-ash production
(million tons per year)
Fly-ash
utilization (%)
India 112 38
China 100 45
USA 75 65
Germany 40 85
UK 15 50
Australia 10 85
Canada 6 75
France 3 85
Denmark 2 100
Italy 2 100
Netherlands 2 100
Source: [3].
1174 M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186
consists of practically a ll the elements present in soil
except organic carbon and nitrogen [19]. On the basis of
silica, alumina and iron oxide content, fly-ash has been
classified into two types: Class F (low lime) and Class C
(high lime) (ASTM C618). The chemical properties of
the fly-ash are largely infl uenced by the chemical content
of the coal burned (i.e., anthracite, bituminous, and lig-
nite). Anthracite is a hard, compact variety of mineral
coals that has a high lustre. It has the highest carbon
count and co ntains the fewest impurities of all coals,
despite its lower calorific content. Lignite, also referred
to as brown coal, is the lowest rank of coal and used
almost exclusively as fuel for steam-el ectric power genera-
tion. The burning of harder, older anthracite and bitumi-
nous coal typically produces Class F fly-ash. Fly-ash
produced from the burning of younger lignite or sub-bitu-
minus coal is of Class C. Alkali and sulfate (SO
4
) contents
are generally higher in Class C than Class F fly-ash [21].
Lignite or brown coal is used almost exclusively as fuel
for steam-electric power generation, resulting in the pro-
duction of huge amount of fly-ash. Therefore, use of
brown fly-ash in agriculture deserves special attention.
Al in fly-ash is mostly bound in insoluble aluminosili-
cate structures, which greatly confines its biological toxic-
ity [21]. Fly-ash also contains minerals such as quartz,
mullite, hematite, magnetite, calci te and borax, and oxida-
tion of C and N during combustion drastically reduces
their quantity in ash [11]. Depending on the sulfur content
of the parent coal, the pH of fly-ash varies from 4.5 to
12.0 [22] and the type of coal used for combustion affects
the S content of fly-ash [21]. Eastern US anthracite con-
tains generally high S and produces acidic ash, while wes-
tern US lignite coals are lower in S and higher in Ca and
thereby produce alkaline ash [21,23–25]. The coal in India
contains low S but high ash (40%) [26].
A large portion of inorganic compounds vaporizes in
the cooler parts of the installation during the combustion
of ground coal at a high temperature of 400–1500 °Cand
condenses on fly-ash particles [12]. Three groups of
elements were recognized on the basis of this volatiliza-
tion–condensation hypothesis which established correla-
tion between mineral concentrations with the particle
size [27]. These groups are: group I with pronounced con-
centration of As, Cd, Ni, Pb, S, Sb, Se, Ti and Zn; group
II with limited concentration of Be, C, Fe, Mg, Mn, Si
andVandgroupIIIwithnoconcentrationofCa,Co,
Bi, Cu, Sn and Ti. Group I elements are classified as
‘lithofiles’(Al,Ca,Fe,K,Mg,Na,Ti)withlittleor
no enrichment in smaller fly-ash particles, group II ele-
ments as ‘Chalco files’ (As, Cd, Mu, Pb, Sb, Se) with
increased concentration with decreasing particle size
and group III elements (Be, Cu, Ni, V, Co) have interme-
diate behavior and are enriched in smaller particles but
to a lesser extent than those of group II. The properties
and contents of major and trace elements of soil and
fly-ash that are available in the literature are presented
in Table 2.
4. Fly-ash for improving soil properties
Soil properties as influenced by fly-ash application have
been studied by several workers [29–35] for utilizing this
industrial waste as an agronomic amendment. Physical
and chemical properties of soil due to fly-ash amendment
vary according to the original properties of soil and fly-
ash but certain generalization could be made in most cases.
4.1. Soil texture
Alteration of the soil texture is possible through the
addition of appropriate quantities of fly-ash (Several exper-
iments have been performed to measure the physical prop-
erties for a variety of soils mixed with up to 50% fly-ash [8],
which revealed that soil fly-ash mixture tend to have lower
bulk density, higher water-holding capacity and lower
hydraulic conductivity than soil alone) due to its textural
manipulation through fly-ash mixing. Application of high
rates of fly-ash can change the surface texture of soils, usu-
ally by increasing the silt content [36,37]. Fly-ash addition
at 70 t ha
1
has been reported to alter the texture of sandy
and clayey soil to loamy [38,39]. Addition of fly-ash at
200 t acre
1
improved the physical and chemical properties
Table 2
Physical characteristics and the major and trace elements in electrostatic
precipitator (ESP) fly-ash and soil.
Properties Fly-ash
a
Soil
b
Bulk density (g cc
1
) <1.0 1.33
Water-holding capacity (%) 35–40 <20
Porosity (%) 50–60 <25
Major elements in percentages
N – 0.01–1.0
P 0.004–0.8 0.005–0.2
K 0.15–3.5 0.04–3.0
Ca 0.11–22.2 0.7–50
Mg 0.04–7.6 0.06–0.6
S 0.1–1.5 0.01–2.0
Al 0.1–17.3 4–30
Na 0.01–2.03 0.04–3.0
Fe 36–1333 0.7–55
Trace elements in mg kg
1
Mn 58–3000 100–4000
Zn 10–3500 10–300
Cu 14–2800 2–100
B 10–618 2–100
As 2.3–6300 0.1–40
Cd 0.7–130 0.01–7.0
Co 7–520 1–40
Cr 10–1000 5–3000
Hg 0.02–1.0 –
Mo 7–160 0.2–5.0
Ni 6.3–4300 10–1000
Pb 3.1–5000 2–100
Se 0.2–134 0.1–2.0
–: Data not available.
a
Unweathered ESP fly-ash generated from F-grade coal with 40% coal
ash: [26].
b
Red lateritic soil of order Ultisols. Source: [21,28].
M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186 1175
of soil and shifted the USDA textural class of the refuge
from sandy loam to silt loam [40].
4.2. Bulk density
The particle size range of fly-ash is similar to silt and
changes the bulk density of soil. (Several experiments have
been performed to measure the physical properties for a
variety of soils mixed with up to 50% fly-ash [8,36], which
reveals that soil fly-ash mixture tend to have lower bulk
density, higher water-holding capacity and lower hydraulic
conductivity than soil alone.) Chang et al. [8] observed that
among five soil types, Reyes silty clay showe d an increase
in bulk density from 0.89 to 1.01 g cc
1
and a marked
decrease in soils having bulk density varying between
1.25 and 1.60 g cc
1
when the corresponding rates of fly-
ash amendment increased from 0% to 100%. Application
of fly-ash at 0%, 5%, 10% and 15% by weight in clay soil
significantly reduced the bulk density and improved the soil
structure, which in turn improves porosity, workability,
root penetration and moisture-retention capacity of the soil
[2,41]. According to Prabakar et al. [42], addition of fly-ash
up to 46% reduced the dry density of the soil in the order of
15–20% due to the low specific gravity and unit weight of
soil.
4.3. Water-holding capacity
Fly-ash application to sandy soil could permanently
alter soil texture, increase microporosity and improve the
water-holding capacity [43] as it is mainly comprised of
silt-sized particles. Fly-ash generally decreased the bulk
density of soils leading to improved soil porosity, workabil-
ity and enhanced water-retention capacity [21]. A gradual
increase in fly-ash concentration in the normal field soil
(0, 10, 20 up to 100% v/v) was reported to increase the
porosity, water-holding capacity, conductivity and cation-
exchange capacity [44]. This improvement in water-holding
capacity is beneficial for the growth of plants especially
under rainfed agriculture. Amendment with fly-ash up to
40% also increased soil porosity from 43% to 53% and
water-holding capacity from 39% to 55% [45]. Fly-ash
had been shown to increase the amount of plant available
water in sandy soils [46]. Chang et al. [8] found that fly-ash
amendment increased the water-holding capacity of sandy/
loamy soils by 8%, which in turn caused improvement in
hydraulic conductivity and thereby helped in reducing sur-
face encrustation. Water -holding capacities of fly-ashes
from different thermal power plants in Eastern India were
compared, and the effect of size fractionation on the
water-holding capacity was determined in an investigation
by Sarkar and Rano [47]. Results revealed that the fly-ash
obtained from a thermal power plant working on stoker-
fired combustor produced the highest water-holding capac-
ity, followed by the one working on pulverized fuel com-
bustor. Fly-ash collected from super thermal power plant
had the least water-holding capacity (40.7%). The coarser
size fractions of fly-ashes in general comprised higher
water-holding capacity than the finer ones. According to
Jala and Goyal [26], the Ca in fly-ash readily replaces Na
at clay exchange sites and thereby enhances flocculation
of soil clay particles, keeps the soils friable, enhances water
penetration and allows roots to penetrate compact soil
layers.
4.4. Soil pH
Depending on the source, fly-ash can be acidic or alka-
line, which could be useful to buffer the soil pH [48–50].
The hydroxide and carbonate salts give fly-ash one of its
principal beneficial chemical characteristics, the ability to
neutralize acidity in soils [51–53]. Fly-ash has been shown
to act as a liming material to neutralize soil acidity and pro-
vide plant-available nutrients [46]. Most of the fly-ash pro-
duced in India is alkaline in nature; hence, its application
to agricultural soils could increase the soil pH and thereby
neutralize acidic soils [50]. Researches have shown that the
use of fly-ash as liming agent in acid soils may improve soil
properties and increase crop yield [51]. According to Poyki-
o et al. [54], the concentration of easily soluble Ca
(24.5 g kg
1
(dry weight)) in the fly-ash from a fluidized
bed boiler at the industrial power plant of Laanilan Voima
Oy in Oulu, North ern Finland was 15 times higher than the
typical value of 1.6 g kg
1
(dry weight) in arable land in
Central Finland. It is indicative of the fact that fly-ash is
a potential agent for soil remediation and soil fertility
improvement. The use of excessive quantity of fly-ash to
alter pH can increase the soil salinity especially with
unweathered fly-ash [55]. An appreciable change in the soil
physicochemical properties, an increase in pH and
increased rice crop yield were obtained by mixed applica-
tion of fly-ash, paper factory sludge and farmyard manure
[56,57].
4.5. Biological properties
Information regarding the effect of fly-ash amendment
on soil biological properties is very scanty [58]. The results
of several laboratory experiments revealed that application
of unweathered fly-ash particularly to sandy soil greatly
inhibited the microbial respiration, enzymatic activity an d
soil N cycling processes like nitrification and N mineraliza-
tion [59–63]. These adverse effects were partly due to the
presence of excessive levels of soluble salts and trace ele-
ments in unweathered fly-ash. However, the concentration
of soluble salts and other trace elements was found to
decrease due to weathering of fly-ash during natural leach-
ing, thereby redu cing the detrimental effects over time [64].
Moreover, the use of extremely alkaline (pH 11–12) fly-ash
could also be the reason for those adverse effects. The
application of lignite fly-ash reduced the growth of seven
soilborne pathogenic microorganisms as reported by Kar-
pagavalli and Ramabadran [65], whereas the population
of Rhizobium sp. and P-solubilizing bacteria were increased
1176 M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186
under the soil amended with either farmyard manure or
fly-ash individually or in combination [66]. Gaind and
Gaur [67] found that the application of fly-ash at 40 t ha
1
in conjunction with Pseudomonas striata inoculation
improved the bean yield, nutrient uptake by grain and
highest population of the bacteria in the inoculated series,
though both 40 and 60 t ha
1
of fly-ash along with P.
striata resulted in the same amount of available P
2
O
5
in
the soil (Table 3). The so il fly-ash environment was the
most suitable for the proliferation of these bacteria,
thereby contributing towards enhanced availability of soil
phosphorus [68]. Amendment of Class F, bituminous fly-
ash to soil at a rate of 505 Mg ha
1
did not cause any neg-
ative effect on soil microbial communities and improved
the populations of fungi, including arbuscular mycorrhizal
fungi and gram-negative bacteria as revealed from analysis
of community fatty acids [58].
A pot-culture experiment was conducted by Garampalli
et al. [69] using sterile, phosphorus-deficient soil to study
the effect of fly-ash at three different concentrations viz.,
10 g, 20 g and 30 g fly-ash kg
1
soil on the infectivity and
effectiveness of Vescicular-arbusculer mycorrhiza (VAM)
Glomus aggregatum in pigeonpea (Cajanus cajan (L.)
Millsp.) cv. Maruti. All the three different concentrations
of fly-ash amendment in soil were found to significantly
affect the intensity of VAM colonization inside the plant
roots and at higher concentration (30 g fly-ash kg
1
soil);
the formation of VAM fungal structure was suppressed
completely. The dry weight of the pigeonpea plants under
the influence of fly-ash amendment in VAM fungus-
infested soils was found to be considerably less (though
not significant enough) when compared to the plants grown
without fly-ash that otherwise resulted in significant
increase in growth over the plants without G. aggregatum
inoculation. However, fly-ash amendment without VAM
inoculation was also found to enhance the growth of plants
as compared to control plants (without fly-ash and VAM
inoculum). Tiwari et al. [70] isolated 11 bacterial strains
from the rhizospheric zone of Typha latifolia and inocu-
lated separately in the fly-ash with additional source of
carbon to investigate their ability to increase the bioavail-
ability or immobilization of toxic metals like Cu, Zn, Pb,
Cd and M n. It was found that most of the bacterial strains
either enhanced the mobility of Zn, Fe and Mn or immobi-
lized Cu and Cd with the exception s that NBRFT6
enhanced immobility of Zn and Fe and NBRFT2 of Mn.
The study also revealed that NBRFT8 and NBRFT9
enhanced bioavailability of Cu and all the strains immobi-
lized Cd. They explained that it was the specific function of
bacterial strains, which caused the mob ility/immobility of
trace metals from the exchangeable fract ions depending
upon the several edaphic and environmental factors.
Therefore, based on the extractability of metals from fly-
ash, bacterial strains can be utilized to enhance the phy-
toextraction of metals from fly-ash by metal-accumulating
plants or for arresting their leaching to water bodies.
5. Fly-ash as a source of plant nutrients
To solve the soil-shortage problem in subsided land of
coal mines, the principal chemical properties of artificial
soil comprising organic furfural residue and inorganic fly-
ash were examined by Feng et al. [71]. The results indicated
that the artificial soil was suitable for agricultural use after
irrigation and desalination. The available nutrients in the
artificial soil could satisfy the growth demand of plants,
and the pH tended to neutrality. Chemically, fly-ash con-
tains elements like Ca, Fe, Mg, and K, essential to plant
growth, but also other elements such as B, Se, and Mo,
and metals that can be toxic to the plants [34,35,72–74].
Lime in fly-ash readily reacts with acidic components in
soil leading to release of nutrients such as S, B and Mo
in the form and amount favourable to crop plants [26].
Fly-ash contains negligible amount of soluble salt and
organic carbon and adequate quantity of K, CaO, MgO,
Zn and Mo. However, it is potentially toxic to plants due
to high B content (345 mg kg
1
) [75]. After application of
fly-ash, the downward move of nutrients through soil col-
Table 3
Effect of fly-ash on the rhizosphere population of PSB and available P
2
O
5
content of soil under soybean crop.
Treatments Available P
2
O
5
in soil (mg kg
1
) Population ( 10
4
g
1
soil) Grain yield (g plant
1
)
30
a
90
a
30
a
90
a
F
0
(control) 14.3 10.2 – – 4.23
F
0
+ P. striata 19.3 17.7 14 16 7.0
F
20
24.3 19.8 02 02 6.07
F
20
+ P. striata 26.8 20.2 21 18 10.13
F
40
26.8 20.4 02 02 6.93
F
40
+ P. striata 34.7 27.0 28 15 11.67
F
60
26.8 18.6 04 04 27.0
F
60
+ P. striata 34.7 24.8 29 31 8.90
F
80
19.2 15.6 02 02 6.50
F
80
+ P. striata 29.4 25.4 16 12 6.07
S. Em± 2.4 3.1 0.81 0.82 1.8
CD (P = 0.05) 7.2 9.2 2.44 2.46 5.4
F
0
,F
20
,F
40
,F
60
and F
80
, rates of fly-ash application to soil (t ha
1
); PSB, phosphate-solubilizing bacteria.
a
These are sampling interval in days. Source: [67].
M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186 1177
umn and the availability of nutrients for plant growth
became limited to a depth of 80 cm from the soil surface
[76]. According to Khan and Khan [44], a gradual increase
in fly-ash concentration in the normal field soil from 0, 10,
20 up to 100% v/v increased the pH, thereby improving the
availability of sulfate, carbonate, bicarbonate, chloride, P,
K, Ca, Mg, Mn, Cu, Zn and B. They also found that addi-
tion of fly-ash to acidic and alkaline soil decreased the
amounts of Fe, Mn, Ni, Co and Pb released from acid soil.
However, the release of these metals from alkaline soil
remained unchanged. The changes in the selected proper-
ties and heavy metal contents of three soil types in India
were studied by Veeresh et al. [77]. The mixtures of soil
with different proportion of fly-ash and sludge, either alone
or in combination, at a maximum application rate of
52 t ha
1
were incubated for 90 day at near field capacity
moisture level. Sewag e sludge, due to its acidic and saline
nature, high organic matter and heavy metal contents,
had more impact on soil properties than the fly-ash. Elec-
trostatic precipitator (ESP) ash collected directly from
thermal power station in Bathinda, India, was more fine-
textured, lower in pH and richer in nutrients than the ash
of dumping sites [30]. The ashes had both higher saturation
moisture percentage and lower bulk density as compared to
the normal cultivated soils. The dominant cation on the
exchange complex was found to be Ca
2+
followed by
Mg
2+
,Na
+
and K
+
in addition to high S content. In a
study with methi (Trigonella foenum–graecum), Inam [35]
applied different basal doses of fly-ash at 0, 5, 10 and
15 t ha
1
along with two doses of nitrogen (40 and
20 kg ha
1
). Uniform basal dose of 30 kg P and
40 kg K ha
1
was also applied. In general, fly-ash at
10 t ha
1
with 20 kg N ha
1
proved better, while higher
dose of fly-ash proved deleterious. Fly-ash is not recog-
nized as an optimal source of phosphorus as it was found
inferior to monocalcium phosphate [78]. However, it has-
tened Ca
2+
and Mg
2+
uptake by legumes [21].
6. Use of fly-ash in composting
In sewage sludge composting, lime is used to raise the
pH and thereby to kill pathogens and to reduce the avail-
ability of heavy metals enriched in sludge [79]. Since alka-
line coal fly-ash contain a large amount of CaO, it can
serve the purpose of lime [80], as it reduced the availability
of heavy metals by physical adsorption and precipitation at
high pH [81]. Moreover, it is also cheaper than lime. Co-
composting of fly-ash at 20% level with wheat straw and
2% rock phosphate (w/w) for 90 day recorded lowest
C:N of 16.4:1 and highest available and total phosphorus
[82]. Mixing alkaline fly-ash with highly carbonaceous
acidic material to make compost for soil treatment had also
been suggested [18]. The low nitrogen content of fly-ash is
an important constraint for its agricultural application. In
a study, Bhattacharya and Chattapadhyaya [83] investi-
gated the possibility of improving the N status in mixtures
of fly-ash and organic matter by implementing vermicom-
posting technology. Different combinations of fly-ash and
cow dung viz., fly-ash alone, cow dung alone and fly-
ash + cow dung at 1:1, 1:3 and 3:1 ratios were incubated
with and without epigeic earthworms (Eisenia foetida) for
50 day. Results revealed that different bio-available forms
of N, such as easily mineralizable NH
4
þ
and NO
3
, consid-
erably increased in the series treated with earthworms. It
could be largely attributed to augment ed microbiological
activity in the vermicomposted samples and also to consid-
erable rise in the concentration of N-fixing bacteria in this
series. Among the three combinations, the highest avail-
ability of N was recorded in 1:1 mixture of vermicompost-
ed fly-ash and cow dung. For proper fly-ash/s ludge ratios,
the fly-ash could also act as an outstanding neutralizer in
the acidic waste. Leaching of heavy metals from the aggre-
gate sampl es was below the environmental limits within a
pH range between 3 and 9 [84,85].
7. Fly-ash for improving crop growth and yield
Several reports are available related to the use of fly-ash
as a soil amendment for the benefit of a large number of
field crops. The safe and sustainable use of sewage
sludge/fly-ash combination on agricu ltural soils is sug-
gested to be a highly promising endeavor from environ-
mental point of view [86]. Fly-ash, having both the soil
amending and nutri ent-enriching properties, is helpful in
improving crop growth and yield in low fertility acid later-
itic soils [87]. Many researchers [2,21,32,56,78,88–93] have
demonstrated that fly-ash increased the crop yield of wheat
(Tritiucm aestivum), alfalfa (Medicago sativa), barley
(Hordeum vulgare), bermuda grass (Cynodon dactylon),
Sabai grass (Eulaiopsis binata), mung (Vigna unguiculata)
and white clover (Trifolium repens) and improved the phys-
ical and chemical characteristics of the soil. Furr et al. [94]
demonstrated that alfalfa, sorghum (Sorghum bicolor), field
corn ( Zea mays), millet (Echinochloa crusgalli), carrots
(Daucas carota), onion (Allium cepa), beans (Phaseolus vul-
garis), cabbage (Brassica oleracea), potatoes (Solanum
tuberosum) and tomatoe s ( Lycopersicon esculentum) grew
on a slightly acidic soil (pH 6.0) treated with 125 MT ha
1
of unweathered fly-ash and that these crops showed higher
contents of As, B, Mg and Se. Application of weathered
coal fly-ash at 5% resul ted in higher seed germination rate
and root length of lettuce (Lactuca sativa) [95]. The crop
response to fly-ash application may vary widely from ben-
eficial to toxic depending on the concentration of various
elements present in it [32,96]. Application of fly-ash extract
in the lower concentration range of 0.5–1.0% (w/w) had no
significant effect on germination and seedling growth of
corn and soybean, whereas higher concentration of fly-
ash extract had deleterious effect on germination, viability,
number of roo ts, shoot and root length, fresh weight and
dry matter of seedling of both the crops [97]. Use of swine
manure with fly-ash balanced the ratio between monova-
lent and bivalent cations (Na
+
+K
+
/Ca
2+
+Mg
2+
), which
are detrimental to the soil and thereby increased the avail-
1178 M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186
ability of Ca and Mg [98]. Application of fly-ash at 10 and
20 t ha
1
improved rice yield from 1.02 to 3.83 t ha
1
in
1979 and 4.65 t ha
1
in 1980. Similarly, wheat yield was
improved from 0.57 t ha
1
(control) to 2.53 t ha
1
in
1979 and 2.85 t ha
1
during 1980s [99]. Amendment of
fly-ash up to 40% improved the growth and yield of rice
crop, whereas the gradual decline in plant grow th and yield
parameters was found from 60% to 100% fly-ash-amended
soil. This adverse effect was attributed to salinity caused by
higher levels of sulfate, chloride, carbonate and bicarbon-
ate in fly-ash-amended soil [45]. Possessing alkalinity and
containing some essential mineral elements, coal fly-ash
could be an alternative to lime amendment and a nutrien t
source of container substrates for ornamental plant growth
[100]. Fly-ash amendment also improved the performance
of oilseed crops such as sunflower (Helianthus sp.), sesame
(Sesamum indicum), turnip (Brassica rapa) and groundnut
(Arachis hypogaea) [26,34,87,101,102]. According to
Kuchanwar et al. [5], application of 10 t fly-ash ha
1
and
25:50:0 kg NPK ha
1
resulted in better growth and yield
attributes which led to the highest pod yield of groundnut.
Medicinal plants such as cornmint (Mentha arvensis) and
vetiver (Vetiver zizanoides) were successfully planted in
fly-ash mixed with 20% farmyard manure and mycorrhiza
[103–105]. Amendment of different fly-ash–soil combina-
tions resulted in high yield of aromatic grasses, particularly
palmarosa (Cymbopogon martini) and citronella (Cymbopo-
gon nardus), which was due to increased availability of
major plant nutrients [106,107]. Lee et al. [108] applied
fly-ash at 0, 40, 80, and 120 Mg ha
1
in paddy soil to deter-
mine boron (B) uptake by rice and characteristics of B
accumulation in the soil. Results indicated that in all fly-
ash treatments, B content in rice leaves and available B
in soil at all growing stages were higher than those of con-
trol but all were below toxicity levels. Boro n occluded in
amorphous iron and aluminium oxides was 20–39% of
total B and was not influenced by fly-ash application. Most
of the B accumulated by fly-ash application was residual B
which is of plant-unavailable form and comprised >60% of
the total B in soil . Therefore, it could reasonably be stated
that fly-ash could be a good soil amendment for rice pro-
duction without B toxicity.
8. Saving of chemical fertil izers
Use of fly-ash along with chemical fertilizers and organic
materials in an integrated way can save chemical fertilizer
as well as increa se the fertilizer use efficiency (FUE).
According to Mittra et al. [109], integrated use of fly-ash,
organic and inorganic fertilizers saved N, P and K fertil iz-
ers to the range of 45.8%, 33.5% and 69.6%, respectively,
and gave higher FUE than chemical fertilizers alone or
combined use of organic and chemical fertilizers in a
rice–groundnut cropping system (Table 4).
9. Fly-ash as pesticide
According to Narayanasamy [110], more than 50 species
of insect pests of various major crop s were suscept ible to
fly-ash treatment. He also stated that fly-ash dusting at
40 kg ha
1
on rice could control both chewing and sucking
pests such as leaf folder, yellow caterpillar, spiny beetle, ear
head bug, brown bug, black bug, grasshoppers, brown
plant hopper and green leafhopper. Serious polyphagous
pests of cotton such as Helicoverpa armigera and Spodop-
tera litura also could be controlled effectively. Scientists
from other research centres have also proved that the fly-
ash could be effectively used to keep away pests from many
vegetables such as brinjal, ladies finger, tomato and cauli-
flower. According to Narayanasamy [111], fly-ash con-
trolled the larvae of crop pests by affecting their
mouthparts and digestive system, and it could also induce
plant resistance against diseases such as the blast fungus
of rice. The addition of 5% fly-ash to soil was also found
to significantly increase the growth of tomato plants and
reduce the amount of galling on the roots caused by
root-knot nematode [112]. The application of 30% fly-ash
reduced the penetration and reproductive potential of
root-knot nematode on tomatoes [113]. Alkaline fly-ash
was added to swine manure at 10% and 20% (w/v), and
production of CO
2
was studied over 12-day period [114].
They observed reduction in CO
2
production and concluded
this as the effect of high pH value, caused by fly-ash addi-
tion, rather than inhibition of microbial activity and
noticeable mobilization of inorganic phosphorus in the
fly-ash-amended manure, probably as a result of microbial
activity. Prospect of use of fly-ash as a dust insecticide,
adjuvant in insecticide formulations and a carrier in pesti-
cide formulation were also reported [115,116]. Bio-efficacy
of fly-ash-based herbal pesticides on certain insect groups
was tested by Arputha Sankari and Narayanasamy [117].
Among the eight fly-ash-based herbal pesticides applied
to rice and vegetables, fly-ash + turmeric 10% dust and
fly-ash + neem seed kernel 10% dust were found to be the
Table 4
Saving of chemical fertilizers and nutrient use efficiency under different modes of fertilization sources in rice–peanut cropping system.
Fertilization sources Saving of chemical fertilizer (%) Nutrient use efficiency (kg grain or kg pod kg
1
nutrient)
NP KN P K
Chemical fertilizer (CF) – – – 34.4 34.4 45.9
Organic
a
+ CF 37.5 22.0 32.0 37.2 86.5 59.8
Organic + Fly-ash + CF 45.8 33.5 69.6 45.4 105.5 72.9
a
Mean of farmyard manure and paper factory sludge at 30 kg N ha
1
for rice and half of these dose for peanut. Source: [109].
M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186 1179
most effective against all the test insects, including Epi-
lachna on brinjal and Spodoptera on okra, followed by
fly-ash + vitex 10% dust and fly-ash + Eucalyptus 10% dust
and fly-ash + Ocimum 10% dust. Thiyagarajan and Naray-
anasamy [118] reported successful use of fly-ash as an insec-
ticide in horticultural crops. Development and use of four
modes of fly-ash as termiticide was tried, and the results
revealed that topical application of the fly-ash on termites
was superior followe d by fly-ash pa ste treatment, fly-ash-
dusted wood and fly-ash + moist soil mixture against
worker termites. Mandibulate type soldiers went through
highest mortality with the topical application of fly-ash fol-
lowed by fly-ash-pasted wood. However, no variation in
the impact of the fly-ash application at different times of
treatment was noticed [117]. Bio-efficacy of fly-ash-based
herbal pesticides against pests rice and vegetable has been
reported [119]. Pesti cide dusting formulation with fly-ash
up to 40% was a suitable disper sant to solve the problem
of agglomeration in the case of pulverized white clay, and
moreover, it also saved the time, electricity, manpower,
natural resource with no adverse effect on paddy (Oryz a
sativa), tomato ( Solanum lycopersicum), brinjal (Solanum
melongena) and jatropha (Jatropha curcas) in the trials with
regard to yield and quality [120].
10. Effect of fly-ash on uptake of nutrients and toxic elements
and quality of crop yield
The high concentration of elements like K, Na, Zn, Ca,
Mg and Fe in fly-ash increases the yield of agricultural
crops. However, application of unweathered fly-ash may
have a tendency of accumulating elem ents such as B,
Mo, Se and Al, which at toxic levels are responsible for
reductions in the crop yields and consequently influence
animal and hum an health [121]. Fly-ash application might
also decrease the uptake of heavy metals including Cd,
Cu, Cr, Fe, Mn and Zn in plant tissues [122,123], which
could be probably due to the increased pH of fly-ash-
amended soil. According to El-Mogazi et al. [124], the
supply of As from fly-ash to plants might be short-term.
Integrated nutrient treatments involving fly-ash at
10 t ha
1
, organic wastes and chemical fertilizers resulted
in higher uptake of N, P, K, Ca, M g, Fe, Mn, Zn and
Cu in rice grain than application of only chemical fertiliz-
ers, which in turn was responsible for higher rice yield
[125–127]. They also observed lower concentration of Cd
and Ni in both grain and straw of rice and the reason
might be the increase in soil pH due to the application
of fly-ash to the rice crop which precipitated the native
Cd and Ni. In rice-based cropping system, uptake of N,
P, K, Ca, Mg, S, Fe, Mn, Zn and Cu by subsequent
mustard crop was higher under the residual fertility of
fly-ash at 10 t ha
1
+ paddy straw at 5 t ha
1
+ chemical
fertilizers or fly-ash at 10 t ha
1
+ farmyard manure
at 5 t ha
1
+ chemical fertilizers or fly-ash at 10 t ha
1
+
green manure at 2.5 t ha
1
+ chemical fertilizers as
compared to chemical fertilizers or fly-ash alone [127].
According to Mittra et al. [109], the uptake of Zn, Cu,
Cd and As by rice and pe anut from fly-ash-amended soil
were within the safe limits as given by Prevention of Food
Adulteration Act (1997). Application of fly-ash-stabilized
sludge to an acid loamy soil significantly increased the
corn yield as well as reduced the uptake of heavy metals
including Cu, Zn, Ni and Cd present in sludge [128].
Topac et al. [129] used lignite fly-ash as an additive in
three different alkaline stabilization processes and tested
the effects on some chemi cal properties of wastewater
sludge. The results revealed that sludges added with 40%
fly-ash (dry weight) caused no significant differences in
the sludge properties; however, application of 80% and
120% fly-ash reduced the concentrations of NH
4
–N,
No
3
–N, available P and soluble B in sludges. A significant
decrease in available forms of N and P and a significant
increase in pH were found in the processes of alkaline sta-
bilization (10–15% quicklime + 40–120% fly-ash) and
alkaline pasteurisation (10–15% quicklime + 40–120%
fly-ash + heating at 70 °C for 30 min).
Alkaline fly-ash was also reported to act as binding
agent for fixation of heavy metals and nutrien ts in waste
and organic matters [114,130,131]. Some scientific workers
used modified fly-ash for removal of pollut ants from water
and wastewater [132–135] and as adsorbent for water and
wastewater reclamation [131]. Boron toxicity is the major
problem in agricultural use of fly-ash. Co-application of a
readily oxidizable organic substrate could prohibit B-
induced inhibition of microbial respiration [21]. Increased
selenium accumulation in plant tissues with increased fly-
ash application calls for close examination and use of
appropriate quantity of weathered fly-ash depending upon
the end use of the produced biomass [25,136]. Rios et al.
[137] observed that coal fly-ash can be used for removal
of heavy metals like Fe, As, Pb, Zn, Cu, Ni, Cr from acid
mine drainage. Goodarzi and Hower [138] tested fly-ashes
produced from Canadian power plants using pulverized
coal and fluidized bed co mbustors for their carbon content
to determine their ability to capture Hg and reported that
the quantity of carbon in the fly-ash alone does not deter-
mine the amount of Hg captured. The types of carbon like
isotropic and anisotropic vitrinitic, isotropic inertinitic and
anisotropic Petcoke, the halogen content, the types of fly-
ash control devices, and the temperatures of the fly-ash
control devices also play major roles in the capture of
Hg. Wang et al. [139] tested single and co-adsorption of
heavy metals and humic acid (HA) on fly-ash and observed
that for single-pollutant system, adsorption of Pb
2+
was at
18 mg g
1
,Cu
2+
was at 7 mg g
1
and HA was at
36 mg g
1
, while in the case of co-adsor ption, the presence
of HA in water woul d provide additional binding sites for
heavy metals, resulting in increased adsorption of Pb
2+
and
Cu
2+
to 37 and 28 mg g
1
, respectively, for Pb–HA and
Cu–HA systems, respectively, at pH 5.0 and 30 °C. They
reported that the heavy metal ions present in the system
competed with the adsorption of HA on fly-ash, thus
resulting in a decrease in HA adsorption.
1180 M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186
11. Radionuclides in fly-ash
Compared to common soils or rocks, majority of fly-
ash are not considerably enriched in radioactive elements
or in a ssociated radioactivity [140]. While less than 10%
thorium is contained in phosphate minerals such as mo n-
azite or apatite, the concentration of natural uranium
may vary from 14 to 100 mg kg
1
, and in exceptional
cases, it may be as high as 1500 mg kg
1
and it is concen-
trated more in finer-sized particles of fly-ash. Fly-ash con-
tains the radiochemical pollution of uranium and thorium
series [141,142] along with other radioactive contaminants
like
222
Ru and
220
Ru [55]. Though several reports regard-
ing the presence of radionuclides in fly-ash are available,
studies on their impact are lacking [143]. The results of
radioactivity analyses reveal ed that the activity levels of
gamma emitting radionuclides
40
K,
226
Ra,
228
and Ac were
within the permissible limits and that mixing of fly-ash
with soil at 24% (v/v) was of no consequence [144].
Mathur et al. [145] investigated the radon exhalation rates
in coal and fly-ash samples from the thermal power plants
at Kolaghat (WB, India) and Kasimpur (UP, India) using
sealed-can technique having LR-115 type II detectors.
They observed that the radon exhalation rate from fly-
ash samples from Kolaghat was higher than from coal
samples and that the activity concentration of radionuc-
lides in fly-ash was enhanced after the combustion of coal,
while fly-ash samples from Kasimpur showed no appre-
ciable change in radon exhalation. Papastefanou [146]
examined the radioactivity of coals and fly-ashes in
Greece and found that the activity concentrations of the
coals ranged from 117 to 435 Bq kg
1
for
238
U, from 44
to 255 Bq kg
1
for
226
Ra, from 59 to 205 Bq kg
1
for
210
Pb, from 9 to 41 Bq kg
1
for
228
Ra and from 59 to
227 Bq kg
1
for
40
K. He reported that these levels are
comparable to those present in coals of different countries
worldwide. The activity concentrations of the fly-ashes
produced in coal-fired power plants ranged from 263 to
950 Bq kg
1
for
238
U, from 142 to 605 Bq kg
1
for
226
Ra, from 133 to 428 Bq kg
1
for
210
Pb, from 27 to
68 Bq kg
1
for
228
Ra and from 204 to 382 Bq kg
1
for
40
K. These results indicated that there is an increment
of the radionuclides in fly-ash as compared to the coal
during combustion and the enrichment factors ranged
from 0.60 to 0.76 for
238
U, from 0.69 to 1.07 for
226
Ra,
from 0.57 to 0.75 for
210
Pb, from 0.86 to 1.11 for
228
Ra
and from 0.95 to 1.10 for
40
K. Mittra et al. [109] found
that fly-ash-amended soil at 40 t ha
1
showed higher
radioactivity (Bq kg
1
)of
226
Ra,
228
Ac and
40
K than fly -
ash and for
137
Cs the trend was reverse. They also
reported that the radioactivity due to the addition of
fly-ash was due to dilution effect of soil, though these
marginal variations were within the safe limits. Kumar
et al. [147] observed that the radon exhalation rate from
fly-ash was less than that from soil and coal, although
fly-ash contains a higher concentration of uranium than
typical soil.
12. Effect of fly-ash on ground water
Physical and chemical characteristics of fly-ash and
hydrogeologic and climatic conditions of the disposal site
are the main factors, which determine the influence of
ash on ground water [148]. Weathered fly-ash contains
higher level of soluble salt; therefore, deposition of this
ash causes more ground–water contamination. In case of
unweathered ash, there is generally a higher release of sol-
uble salts initially, but it de clines rapidly with time
[12,149,150]. When water saturated, weathered ash from
a settling pond is deposited in a landfill, there is a rapid
release of leachat e containing much lower concentration
of soluble salts, while it may take a year or longer for
dry unweathered ash to absorb sufficient moisture to
release leachates [150].
Fly-ash contains trace and heavy metals, which readily
percolate down from conventionally used earth-lined
lagoons. The solubility of trace and heavy metals present
in fly-ash is <10% [151]. Laboratory experiments by Natu-
sch [152] revealed that 5–30% of toxic elements especially
Cd, Cu and Pb are leachable. Moreover, the concentration
of these elements in fly-ash is very low; hence, the chance
for leaching of these elements to ground water is negligible.
However, close monitoring of this aspect may be advisa ble.
Experiments conducted at Central Fuel Research Institute
(CFRI), Dhanbad, India showe d that there was no nega-
tive influence of fly-ash application on the quality of
ground water and that the trace and toxic metal contents
were within the permissible limits. The potential use of
fly-ash from coal-fired power plant for the removal of
Zn(II) and Ni(II) from aqueous solutions has been
reported by Cetin and Pehlivan [53]. A study conducted
on soils from Italian mine site contaminated severely with
heavy metals showed decreased levels of heavy metal con-
tent in percolating water when mixed with fly-ash, which
was indicative of the fact that fly-ash in such soils can lead
to immobilization of heavy metal ions [153] .
13. Fly-ash utilization and global warming
Agriculture plays a major role in the global fluxes of the
greenhouse gases like carbon dioxide, nitrous oxide, and
methane. Many studies suggested that additional opportu-
nities have arisen for lessening the GWP (global warming
potential) by altering the agronomic practices [154]. With
the assumption by the Intergovernmental Panel on Climate
Change (IPCC) that all the carbon in agricultural lime
(aglime) is eventually released as CO
2
to the atmosphere,
the US EPA estimated that 9 Tg (Teragram = 10
12
g=
10
6
metric tonne) CO
2
was emitted from an approximate
20 Tg of applied aglime in 2001 [155]. As per another esti-
mate, in US agriculture only, aglime is applied to the tune
of 20–30 Tg year
1
and the same study estimated that 4.4–
6.6 Tg CO
2
was emitted in 2001 from that lime [156].
Bernoux [157] estimated the net CO
2
fluxes from liming
of agricultural soils in Brazil for the period 1990–2000.
M. Basu et al. / Progress in Natural Science 19 (2009) 1173–1186 1181
The calculation was based on the methodology proposed by
the IPCC, but separately conducted for the five administra-
tive Brazilian regions. The summarized annual CO
2
emis-
sion for Brazil varied from 4.9 to 9.4 Tg CO
2
year
1
with
a mean CO
2
emission of about 7.2 Tg CO
2
year
1
. But agri-
cultural lime can be a source or a sink for CO
2
, depending
on whether reaction occurs with strong acids or carbonic
acids. A study showed that infiltrating waters tended to
indicate net CO
2
uptake, as did tile drainage waters and
streams draining agricultural watersheds. As nitrate con-
centrations increased in infiltrating waters, lime switched
from a net CO
2
sink to a source, implying nitrification as
a major acidifying process [158]. One experimental study
demonstrated that 1 ton of fly-ash could sequester up to
26 kg of CO
2
, i.e., 38.18 ton of fly-ash per ton of CO
2
sequestered. This confirmed the possibility to use this alka-
line residue for CO
2
mitigation [159]. Use of fly-ash as soil
ameliorant in place of lime could lead to reduction in CO
2
emissions, thus contributing to minimize global warming
[160].
14. Summary
To meet the growing energy demand and thereby increase
power generating capacity, the dependency on coal for
power generation and disposal of fly-ash will continue to
increase along with various unavoidable problems. More-
over, keeping in view of developmental problems like bur-
geoning population, growing food demand, shrinking
natural resources, it is necessary to sustain the production
of crop yield as well as soil health in an eco-friendly way.
Hence, it is required to involve fly-ash more effectively in
agriculture sector to exploit its various physical and chemical
properties fully, which are beneficial for soil and crop health.
In view of the above discussions, the salient points from
this extensive review could be summarized in the following
sections:
(1) Advantages of fly-ash use in agriculture:
(i) It could be stated that the potentiality of fly-ash
for its use in agriculture is popularizing day by
day due to the fact that it contains almost all
the essent ial plant nutrients i.e., macronutrients
including P, K, Ca, Mg and S and micronutrients
like Fe, Mn, Zn, Cu, Co, B and Mo, except
organic carbon and nitrogen.
(ii) It is now well proved that though it can substitute
lime, a costly amendment for acid soils, it cannot
be a substitute for chemical fertilizers or organic
manures. However, integrated application of all
these can foreshorten the plant uptake of different
heavy metals from fly-ash-amended soils as well
as can reduce the use of chemical fertilizers and
thereby reduces environment pollution.
(iii) Fly-ash is also useful for stabilizing erosion-prone
soils. Phytoremediation can prevent cycling of
toxicants from fly-ash and growing of multipur-
pose tree species on problem soils.
(iv) According to IPCC, agricultural lime application
contributes to global warming through emission
of CO
2
to the atmosphere. Use of fly-ash instead
of lime as soil ameliorant can reduce net CO
2
emission and thereby lessen global warming.
(2) Disadvantages/app rehensions that need further
research:
(i) In spite of several advantages of using fly-ash in
agricultural field, many are afraid of its natural
radioactivity and heavy metal content.
Researches have proved that the effect of this
radioactivity and heavy metal content is in the
safe limit if fly-ash is being applied in optimum
quantity. However, long-term confirmatory
research and demonst ration are necessary for
convincement at the grass root level, as is the case
for any new agricultural input materials like fer-
tilizers, amendments or pesticides.
(ii) As in the case with fertilizers and any other agri-
cultural inputs, the amount, time and method of
fly-ash application would vary with the type of
soil, the crop to be grown, the prevailing agro-cli-
matic condition and also the type of fly-ash avail-
able. Research on these aspects need s attention
for utilization of fly-ash in a better way.
(iii) Simultaneously, in future, attention should be
given on some important areas related to fly-ash
utilization, like proper handling of dry ash in
plants as well as in fields, ash pond management
(i.e., faster decantation, recycling of water, verti-
cal expansion rather than horizontal, etc.), long-
term studies of impact of fly-ash on soil health,
crop quality, and continuous monitoring on the
characteristics of soil as well as fly-ash.
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