1.0 Introduction

The Green Revolution has resulted into a surplus production of grains world over

including India (1, 2). This revolution in agriculture, on the contrary has revealed various

other issues to address like the soil deterioration, fertilizers and pest control

management and crop residue management. Crop residue management is of utmost

relevance because of the impact on soil organic matter (SOM), local and regional air

pollution and essential contribution to global Warming phenomenon. The main

contributors to the global crop residue generation are Wheat, rice, maize, barley, millet

and sorghum (3), of these rice and wheat are the main crops grown in Asian countries

and approximately 748 million tons (MT) residue from rice (623 MT from rice straw and

125 MT from rice husks) (4) and approximately 228 MT wheat residues are generated

each year. Generally, most of the wheat crop residue is used as feed for livestock; only

7%-25% of wheat residue is reported to be burnt in the field (5). However, in case of rice

straw because of its higher silica content, reduction in milk yield of cattle (6), massive

residue generation (1.37 tons per ton grain production) (7), early seedbed formation for

next crop and combined harvesting technology lead to no other alternative for the rice

crop residue than to be burnt in the field or at the most left in the field for microbial

degradation (3). Thus, open crop residue burning (CRB) is a very common practice which

leads to the release of various air pollutants including nonmethane hydrocarbon

compounds (NMHCs) and particulate matter (PM2.5 and PM10) consisting primarily of ash,

polycyclic aromatic hydrocarbons (PAH), and soot (organic carbon & black carbon) in the

atmosphere (8), and also lead to loss of soil fertility by changes in soil C\N ratio, nutrient

loss and killing of friendly pests and bacteria (9). Black carbon (BC) and organic carbon

(OC) emissions lead to weakening of the radiative-convective coupling of the atmosphere

and decrease global mean evaporation and rainfall and are also considered as the most

important cause of the formation of Asian Brown Clouds (ABCs) (10).

These detrimental effects of CRB lead to shift towards a crop residue management (CRM)

system for attaining the sustainability in Agriculture. Although, CRM has received less

space in R&D community but its probable contribution to soil fertility, soil organic matter,

soil structure and soil health will lead towards sustainable agriculture with substantial

carbon sequestration (11). Several alternatives have been suggested for CRM

categorized under physical methods, chemical methods, biochemical and

thermochemical conversion processes which include combustion, gasification and

pyrolysis (12, 13). Thermo-chemical technologies have significant role in CRM and are

preferably promoted over other technologies because these technologies also lead to

carbon sequestration and the by-products can be used as a fuel source having higher

calorific value and cleaner fuel in comparison to fossil fuel (14).

Some recent thermo-chemical technological advancement in crop residue management

is the utilization of crop residue for bio-ethanol (15), bio-oil, syn-gas and bio-char

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production (16). Bio-oil production from the crop residue is recently getting more

recognition because of rising prices of fossil fuels and their pollution issues and also

because bio-oil has a potential to be used as a fuel directly (17). Thus, it is considered as

one of the most reliable technology for crop residue management. The bio-oil production

technology is based on the pyrolysis of the lignocellulosic crop residue in absence of

oxygen in a closed pyrolytic chamber. The pyrolysis conditions have a major role in

determining the yield of the bio-oil. In the pyrolysis of crop residue, along with bio-oil,

syn-gas and some solid Carbon-rich charred material are also produced (17). Bio-gas can

be used as an energy source because of its higher calorific value and less polluting

property (18).

Biochar can be defined as a carbon-rich crystalline graphene structured product obtained

by thermally decomposing the biomass such as wood, manure, leaves or crop residue in

closed pyrolysis chamber at a temperature < 700 0C, in absence of air (14). Biochar is

used as a potential adsorbent material for wastewater, for C sequestration, as energy

source and as a potential soil ameliorating agent in agriculture as it has a good capacity

to improve the soil’s physical, chemical and biological properties. The bio-char has

carbon-negative effect and so it can hold a massive amount of CO2 in the form of soil-C

for centennials to millennial (14, 19).

Like crop residue management, fly ash management has been drawing attention of the

policy makers for a long time because of its massive production from combustion of coal

in various Thermal Power Plants (TPPs) for electricity generation. The combustion of coal

produces a sufficient amount of energy along with various coal combustion by-products

(CCPs) or coal combustion residues (CCRs) like fly ash, bottom ash and boiler slag,

fluidized bed combustion ash and other solid fine particles (20, 21). The major fly ash

generating countries are U.S., Russia, China and India with a fly ash generation potential

of these countries contributing about 750 MT/yr (22). The fly ash utilization potential of

fly ash is greater in developed countries as compared to developing one. U.S., Europe,

and Japan have fly ash utilization potential of 39%, 47% and 82% [23] respectively,

whereas rest countries fly ash utilization potential averaged around 25% (24).

As estimated by MoEF (2007), fly ash production in India was 112 MT in year 2005-06

and was expected to increase upto 170 MT/yr and 273 MT/yr till the end of 2012 and

2020, respectively (25). Usually fly ash is utilized in cement and concrete industries,

brick formation, road making, landfill and as value added materials like adhesives,

adsorbent, wood substitutes, zeolite and importantly in soil amelioration (21). Fly ash

management through its utilization for ameliorating cultivable as well as non-cultivable

soil is applied for a long time because of the potential of fly ash to provide the major

macro and micro nutrients to the plants and related microbial communities (24, 26). Fly

ash has the capacity to decrease soil aggregation capacity, increase water holding

capacity, moisture content, porosity, electrical conductivity, etc. India has around 175

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Mha non-cultivable land area because of water logging, higher sand proportion, salinity,

acidic nature and alkalinity which have a greater scope for utilization of fly ash (21, 24,

26, 27). Likewise fly ash, biochar also improves soil properties by increasing CEC,

improving pH, increasing porosity and permeability, decreasing soil bulk density,

increasing surface area, water holding capacity (28), and also increasing microbial

activities by providing them a source soil-C, soil-N and soil-P (Earthworm, Mycorrhizal and

bacterial activities) after its incorporation into the soil (29, 30).However, high toxic metal

content of the fly ash are one of the major constrain in its utilization as a soil amendment

agent, but the toxicity varies with soil types (21, 24, 26, 27).

A few studies have reported that the use of zeolites like alumina has increased the bio-oil

production from the crop residue (31, 32). Fly ash has a natural zeolytic properties (33,

34), thus it can be used along with crop residue to produce bio-oil and the solid remains

of this process i.e., biochar mixed fly ash can be used as for soil amendment. Also,

biochar is reported to decrease the leaching properties of various metals in soil (19), thus

it might be beneficial to use this combination of fly ash and biochar which might give an

overall synergistic positive effect to soil.

2.0 Review of Literature

2.1 Biochar from Crop Residues

All carbon-rich residues derived from fossil fuels and biomass by fire or heat are

considered as black carbon which includes solid combustion residues as well as

condensation products like char, charcoal, biochar, soot, graphite black C and graphite

(35). The exact chemical nature of a biochar produce depends upon the type of biomass

used and pyrolysis condition (14). Various studies from different parts of the world

accounts the biochar formation from different crop residues like from olive kernels (36);

straws of canola, corn, soybean and peanut (37); rice straw and rice husk (38), wheat

straw (39), rape and sunflowers residues (40); and also from residues of sugarcane,

sorghum, millet, coconut, oil palm, coffee, cocoa, maize, etc. (41). Also, earlier reports

suggested the formation of biochar from poultry litter (42) wood, municipal biowastes

(43), yard wastes, etc. which are in practice for a long time for soil amendment (14).

Various studies have reported that the yield of biochar production from crop residue is

twice to that of the wood (44).

2.1.1 Physiochemical Characteristics of Crop Residue Biochar

2.1.1.1 General Biochar Properties

The biochar characteristics are very much variable and they depend mainly upon the

biomass source and operating pyrolysis conditions like highest treatment heat (HTT),

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pressure, reaction residence time, vapour residence time, moisture content of biomass

source, reaction vessel, pre-treatment, flow rate of gas /air and post treatment. Biochar

has large surface area, high pore space (micropores, mesopores and macropores) and

permeability, lower bulk density and high water holding capacity (WHC) (28).

Temperature has a very important role in determining the characteristics and application

of biochar (Table 1) as biochar prepared at low temperature can be used for controlling

release of nutrients from fertilizers (45) and high temperature leads to the formation of

activated carbon like material (46). Chemically, biochar is a highly aromatic compound

that contains random stacks of graphitic layers (35) i.e., has high carbon content

followed by oxygen, hydrogen and nitrogen and has higher C\N, C\H, C\O ratios and

lesser volatile matter content than the parent material (47). Biochar is highly stable,

resistant to various erosion, having high cation exchange capacity (CEC) and a variable

range of pH (depends upon biomass source and heating temperature) (14). High cation

exchange capacity (CEC) of biochar is due to the presence of various functional groups

like pyranone, phenolic, carboxylic, lactone and amine (48). Several workers regard

biochar as a rich source of nitrogen alongwith carbon, which can be used to further

improve soil nutrient status (49). Because of its higher adsorption capacity in comparison

to soil, biochar is regarded as a better soil phosphorus retaining material (35). Further,

biochar favors microbial soil communities like N2-fixing bacteria (50), arbuscular

mycorrhizal (AM) fungi (51) and several other organisms like earthworms (29), by

providing them space and nutrients and protect them from predation and desiccation

(30, 52, 53).

2.1.2 Crop Residue Biochar Properties – A Comparative Statement

Various physicochemical properties of biochar produce from various crop residue sources

are summarized in Table-1. The effect of pyrolysis temperature can easily be generalized

from the data given in Table-1, as the rise in pyrolysis temperature leads to decrease in

nitrogen and oxygen content, increase in carbon, phosphorus, ash content and pH (37,

54) is reported. Surface area of biochar also increases with increase in pyrolysis

temperature; however it is 100 times substantially less than the surface area of activated

charcoal. Porosity of the biochar increases with increase in pyrolysis temperature

because of the loss of volatile matter, thus, in turn leading to decrease in bulk density.

2.1.3 Life cycle analysis of biochar

Biochar has been found as a very stable and resistive element in soil and so it has very

long life (thousand to millions of years) in soil, because the recalcitrance of biochar

depends upon biomass source, pyrolysis condition, soil properties and climate (55). Terra

Preta soil or Amazonian Dark Earths (ADE), regarded as a type of biochar, is considered

as an example for describing the longevity of biochar in soil (56). Various lab-scale

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studies has predicted that biochar has a mean residence time of 1300-4000 years in soil

(57). Its degradation and mineralization is very slow and because of this property, it is

considered as a good method for mitigating Climate change by locking a huge amount of

atmospheric CO2 (15). Also, various studies suggested that a fraction of C (present as

mineral carbonates and organic molecules) from biochar, called labile carbon content, is

mineralized abiotically and biotically to CO2 within a short period of time (14). Rumpel et.

al., (2006) (58) reported that biochar amended soil in a steeply sloped area was more

prone to water erosion than the other soil organic matter because of its low density, less

mineral interactions and lesser biodegradability.

2.1.4 Biochar as a soil amendment agent: for Sustainable Agriculture

Enhanced mineralization of soil organic matter and depletion of soil nutrients are

currently considered as the two important limitations for the sustainable agriculture.

Biochar having high adsorption capacity and nutrient retention capacity as well more

stable nutrient source is considered as an effective soil amendment than compost and

organic manure (59). The amount of biochar incorporation in soil requires the

understanding of soil characteristics and climatic conditions. Although, after performing

various experiments of biochar application to soil, especially for crop production,

Lehmann concludes that: “crops respond positively to biochar addition up to 50 MgC/ha and may

show growth reduction only at very high applications”. Crop residue biochar as a soil amendment

agent for sustainable agriculture, can be described in following sub headings:

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Table 1. Chemical characterization of biochar from different sources

% dry weight basis

SourcePT (0C) pH

SA (m2/gm) C N O H P C/N H2O Ash References

Corn stover 500 8.90 4.20 25.00 0.60 5.00 1.10 n/a 41.67 9.1 69.00 54

Corn stover 515 9.50 4.40 45.00 0.50 1.00 1.70 n/a 90.00 11.5 55.00 54

Coconut shell 550 8.90 15.10 80.10 0.50 2.50 n/a n/a 160.20 12.4 n/a 54

Peanut hulls 481 8.00 1.00 59.00 2.70 12.00 2.30 n/a 21.85 7.2 18.00 54

Corn cob 400 9.00 < 0.1 80.10 0.60 8.80 3.70 n/a 133.50 3.1 3.70 54

Sugarcane bagasse 350 5.00 n/a 75.20 0.66 15.80 4.60 n/a 113.94 3.42 3.60 54

Poultry litter 400 10.30 n/a 42.30 4.20 n/a n/a n/a 10.07 n/a n/a 54

Cottonseed hull 500 8.50 <0.1 78.70 2.50 6.90 2.50 n/a 31.48 6.5 7.90 54

Cottonseed hull 800 7.70 322.00 84.30 0.60 6.60 0.60 n/a 140.50 5.9 9.20 54

Rape residue 550 n/a n/a 72.20 1.30 25.60 0.90 n/a 55.54 3.2 21.80 40

Sunflower residue 550 n/a n/a 63.40 1.60 34.30 0.70 n/a 39.63 4.73 28.90 40

Wheat straw n/a n/a n/a 43.20 0.61 39.40 5.00 n/a 70.82 n/a n/a 47

Rice hulls n/a n/a n/a 38.30 0.83 35.45 4.36 n/a 46.14 n/a n/a 47

Olive kernel 800 n/a n/a 75.68 1.35 12.18 0.79 n/a 56.06 n/a n/a 36

Canola straw 500 9.39 n/a 63.40 0.04 n/a n/a 0.301585.00 n/a 18.40 37

Canola straw 700 10.76 n/a 54.90 0.04 n/a n/a 0.501372.50 n/a 28.55 37

Soyabean straw 500 10.92 n/a 62.60 0.40 n/a n/a 0.40 156.50 n/a 17.85 37

Soyabean straw 700 11.10 n/a 57.90 0.10 n/a n/a 0.60 579.00 n/a 23.70 37

Corn straw 500 10.77 n/a 41.90 0.90 n/a n/a 0.40 46.56 n/a 50.70 37

Corn straw 700 11.32 n/a 24.50 0.80 n/a n/a 0.70 30.63 n/a 73.30 37

Peanut straw 500 10.86 n/a 48.50 1.50 n/a n/a 0.10 32.33 n/a 32.50 37

Peanut straw 700 11.15 n/a 47.00 1.50 n/a n/a 0.12 31.33 n/a 38.50 37(Here, PT= pyrolysis temperature; SA= surface area; H2O= moisture content; and n/a= data not available)

6

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2.1.4.1 Direct use of Biochar

As biochar is a highly porous structure, so after its addition to soil it leads to increase in

the soil aeration, soil water holding capacity and decrease in soil aggregation, soil

strength and soil bulk density. The pH of biochar varies from slightly acidic to alkaline

range (mostly in alkaline range from pH = 8.0-10.0), so alkaline pH leads to better

functioning of soil microbial communities and resurrecting buffering capacity to soil after

its application (37). Also, biochar application can be beneficial in acidic soil reclamation

and the soil that has been degraded by long term continuous cultivation (60). Various

anionic functional groups of biochar (48) on its surface behaves as an cation exchange

resin leading to the retention of essential cations for exchange, thus increase the soil

CEC, leading to increased crop productivity (59, 61). Biochar application to soil increases

the soil organic carbon pool and soil-N. During the initial periods of its application,

biochar has less resistance and is more prone to degradation because a small fraction of

carbon is present in the labile form (62), so the microbial activities is enhanced during

this time period. Excellent nutrient retention property of biochar leads to longer retention

of nutrients in topsoil after fertilizer application. High porosity and larger surface area of

soil after biochar amendment would lead to the growth of microorganism thus leading to

better symbiosis of the crop with bacteria and fungi, which results in the dissolution of

nutrients and bioavailability of nutrients for the crop (30).

2.1.4.2 Indirect use of biochar

The nutrient retention capacity of biochar leads to the reduction of fertilizer use, so it

indirectly results in reduction production, energy and environmental cost. Also, biochar

application to soil leads to subdued release of N2O and CH4 like potent Green house

gases (43, 63). According to an estimation of Woolf (2008) (64), all the crop residues of

the world if converted into biochar, would sequester about 1 gigatonne of carbon to soil

and is assumed to be a better carbon capture and storage (CCS) alternative for

mitigating Climate Change. Biochar has a carbon negative effect on the atmosphere (15,

55). By-products of biochar production (syn-gas and bio-oil) from crop residue are cleaner

fuel with high calorific value, thus can be used as an alternative source of energy (38-

40).

2.1.5 Crop residue Biochar as an adsorbent material

Lou et al., (2011) (65) reported that the rice-straw biochar amended in soil sediments has

a significant sorption capacity for pentachlorophenol (PCP), a genotoxic material for seed

germination. Xu et al., (2011) (66) was reported significant adsorption of methyl violet

from aqueous solutions by various crop residue derived biochars due to electrostatic

attraction and specific interaction between dye and various negative charges on biochar

8

surface. Similarly, a comparative adsorption study of rice-straw biochar and fly ash by

Lou et al., (2012) (67) advocated biochar as a significant sorption material for FCP than

aged fly ash. However, fresh fly ash has greater sorption capacity for FCP.

2.1.6 Risk associated with biochar as a soil amendment agent

The following are the major limitations and risks associated with biochar application to

soil which limit its use as soil ameliorator (68):

1. Application rates: a standardized application rate is still needed.

2. Effect on agrochemicals: application of biochar increases the binding of various

agrochemicals on its surface, thus reduces killing of pests and enhances the

longivity of chemical in soil by avoiding them from microbila decay.

3. PAHs production from pyrolysis: various studies reported PAHs production during

slow pyrolysis of biochar which remain attached with the anioinc surfaces of

biochar and might be causing negative impact to soil and microbila diveristy.

4. Soil albedo: biochar decreases soil albedo by providing black surface to soil and

increases absorption of sunlight, thus indirectly leads to Global warming

phenomenon.

5. Soil residence time: the residence time of biochar in soil is estimated as centinnial

to millenial with an average residence time of 600 years.

6. Soil Organic Carbon (SOM): in the literature not any conclusive report is available

which signify that biochar either increases or decreases the SOM content.

7. Heterogeneous nature of biochar: the properties of biochar are not stable; they

vary with type of biomass, pyrolysis condition and its application.

8. Cost of production: it is also comparatively higher.

2.2 Fly Ash from coal combustion

An inorganic finely divided particulate material generated from the burning of pulverised

coal and collected from the flue gas or electrostatic precipitaors in thermal power plants

is considered as fly ash (34). It is mainly an alluminosilicate compound (analogous to clay

particles) with a potential amount of oxides of Mg, Ca, Fe, and Na alonwith various toxic

trace metals. On the basis of amount of CaO, SiO2, Al2O3 and Fe oxides, ASTM C618

procedure catagorised fly ash into two classes, viz., Class C and Class F. Combustion of

lignite and sub-bituminous coal leads to the formation of Class C fly ash which has a high

content of CaO (33%), Na2O (0.7%) and comparatively lower contents of SiO2, Al2O3 and

Fe oxides (Table 2). Higher content of CaO provides self cementing property of Class C fly

ash. The burning of harder and older bituminous and anthracite coal leads to the

production of Class F fly ash which is more finer than the Class C fly ash because of the

presence of higher contents of SiO2 (53%), Al2O3, and Fe2O3, a lesser CaO (9%) content

(Table 2), thus requires an activator e.g., lime liming property (25, 26).

9

Table 2. Basic Characteristics of Class C and Class F fly ash

Characteristics

Source CoalClass

CClass F Lignite and Sub-bituminous Bituminous and Anthracite

Major Producers

U.S., South African

countries Australia, Canada, China, India,

South Africa, U.S.

Basic Nature Cementious Pozzolonic

Typical Composition

(%)

SiO2 40 55

Al2O3 17 26

Fe2O3 6 7

CaO 24 9

SO3 3.3 0.6

Available alkalies(Na2O) 0.7 0.5

Fineness 8 14

(retained on 325 mess)

(Source: Yunusha et al. 2012 (26))

2.2.1 Morphology and Physical Properties of fly ash

Various SEM and XRD studies revealed that fly ash has a regular shape and size,

consisting of about 2% spherical, hollow and solid structures known as cenospheres and

pleurospheres. Collectively these structures are known as microspheres having very high

thermal and magnetic properties, spherical design and chemical inertness (20, 33, 34).

The bulky microspheres are mainly present as (a). crystalline monolith, (b). porous, (c).

Cenospheres: hollow spheres formed by alluminosilicate in which the particle diameter to

wall thickness ratio can reach more than 50, and (d). Plerospheres: in which a hollow

large sphere is filled with various small spherical particles (21, 34). Fly ash has a great

adsorption capacity for various gases and other chemicals like pesticides due to presence

of these hollow spheres (20). These hollow spheres are responsible for the natural

geolytic behaviour of fly ash (34), also for comparatively lesser bulk density, higher

porosity, water holding capacity (WHC), surface area and electroconductivity (EC) than

soil. Particle diameter of fly ash usually ranges between 1-150 µm, however 60% of fly

ash is constituted by particles having diameter < 3 µm (comprise only 10 wt.% of fly ash).

10

A vast range of surface area and density is revealed by the fly ash ranging from 0.2

m2/gm to 0.8 m2/gm and 1.9 x 106 gm/m3 to 2.9 x 106 gm/m3 [34]. Colour of fly ash

depends upon the source material; however it is usually from greyish to black in colour

(21, 34).

2.2.2 Physico-chemical characacteristics of fly ash

Physico-chemical properties of fly ash depend upon the coal source and combustion

condition. In general, fly ash has an alkaline pH (8.0-11.48) due to presence of alkali and

alkaline earth metals compounds [33]; greater electrical conductivity (EC), mostly

composed of alumina, silica, and iron oxides (~87%); various macro and micro elements

(e.g., Fe, Mg, P, K, Si, Na, S, Ca, Mn, Al, etc.) and trace elements (e.g., As, Ba, B, Cd, Cr,

Cu, Co, Pb, Ni, Hg, Mo, Sc, Se, V, Zn, etc.). General chemical composition of fly ash is SiO2

> Al2O3 > Fe2O3 > CaO > MgO > K2O > Na2O > TiO2 (24). Trace metals (mainly heavy

metals) leaching like Zn, Cd, Pb, As, Se, and B are the major limitation for the use of fly

ash in the soil system which pollute the groundwater (69), but most of these metals are

reported in very less amount in Indian fly ash (70).

2.2.3 Use of fly ash

Fly ash is potentially used in cement industries, brick kilns, road formation, as adsorbent,

zeolyte formation which is further used for removal of heavy metals from wastewaters,

adhesives, wall board, paint, wood substitute, and as a potential soil amendment agent

(21,71-73). Singh et al. (2012) (74) reported a considerable amount of sorption of

metribuzin herbicide in fly ash amended soil, thus reduction of the runoff and leaching

losses from the soil. Use of fly ash in cement and soil amelioration also helps in carbon

sequestration (21). According to an study, 1 tonne fly ash can sequester 26 kg of CO2, i.e.

38.18 tonne fly ash will sequester 1 tonne of CO2, thus making the way for the utilization

of alkaline fly ash residue for CO2 mitigation (75). Fly ash has the capacity to adsorb some

volatile compounds like polynuclear aromatic hydrocarbons (PAHs) (76). The porous

nature of silicates and the surface embedded with activated carbon particles provides the

adsorbing capacity to the fly ash. The pore spaces formed by silicates show hydrophobic

nature and adsorb organic solvents from wastewater; and pore spaces formed by

aluminosilicates show hydrophilic nature and suitable for adsorption of water from organic

solvents [20]. Vitekari et al.,(2012) (77)viewed fly ash as a carrier for bio-pesticides and

bio-fertilizer formulations

2.2.4 Fly ash as a soil amendment agent

Class C fly ash is used for neutralizing the acidic soils. Calcium carbonate equivalent

(CCE) of fly ash is responsible for the neutralization of soil acidity mainly (26). Generally

all the soil macro and micro nutrients are present in fly ash except nitrogen (26). Thus, fly

ash amendment resulted into improvement of soil acidity, soil sodicity, nutrient supply,

concentration and loss, and adverse soil physical properties (26). Fly ash also has almost

similar soil improving properties as of biochar. Low rate fly ash application to soil is

11

reported to increase the VAM (vesicular arbuscular mycorrhiza) colonization (Glomus

aggregatum) in plant roots (Cajanus cajan) and at high rate of application, growth is

totally suppressed [78]. Singh et al., (2011) (79) performed various studies on

amendment of fly ash in three different cultivars of paddy in Indian soil and reported that

fly ash amendment at lower concentration (10%) enhanced the yield of rice.

2.2.5 Negative effects of fly ash

The following are some major negative effects of fly ash (80, 81):

1. High pH (from 8-12) resulting into reduction in bioavailability of a few nutrients.

2. High salinity.

3. High content of phytotoxic elements e.g., B, As, Mo, Se, etc. However, according to

a study by Love et al., (2009) (82) on Cassia occidentalis plant growing on

weathered fly ash suggests that fly ash prompts genotoxicity in plant. Wong and

Selvam (2009) (83) performed co-composting of fly ash with sludge and reported

that the heavy metal content of fly ash decreased with the increase in compost

ash content, however, B content was increased with the increase in ash content.

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