Coal-fired thermal power plants remain the primary electricity source in much of the developing world, generating vast quantities of fly ash (FA) as a solid waste by-product. This ultrafine particulate contains heavy metals and, in some cases, radioactive elements, creating disposal challenges and raising concerns about environmental contamination, bioaccumulation, and human health risks. At the same time, fly ash is rich in plant-essential nutrients such as potassium, calcium, magnesium, and trace elements, and its application as a soil amendment can improve physical structure, nutrient availability, and biological activity. Research indicates that, when applied judiciously, fly ash can enhance plant growth, chlorophyll synthesis, phenolic compound content, and crop yield, including in medicinal species. However, the potential for toxic element transfer through the food chain necessitates rigorous risk assessment and management strategies. This study examines both the hazards and agronomic opportunities of FA, highlighting the importance of balancing environmental safety with the potential for sustainable agricultural use.
agriculturecoalfly ashsolid wastewaste managementsoil healthcrop yieldThe author(s) declare that no financial support was received for the research and/or publication of this article.section-at-acceptanceWaste ManagementIntroduction
Coal is used extensively worldwide as the primary source of energy for power generation. In the thermal power station, electricity is produced by coal combustion, and as a product, FA is produced (Rusănescu and Rusănescu, 2023; Basu et al., 2009; Patil et al., 2013; Kuznia, 2025). In India, thermal power plants account for over 70% of total electricity production and generate substantial quantities of FA. National data for 2022–23 indicate that approximately 140 million tonnes of FA were produced, with only about 60% effectively utilized Figure 1 (Central Electricity Authority, 2023).
Usage of fly ash, year 2022–2023.
Pie chart showing fly ash usage for 2022-2023. Cement accounts for 26%, while 22% remains unused. Bricks and tiles use 10%, low-lying area reclamation 9%, and mine filling 7%. Other uses include roads and flyovers, concrete, agriculture, other sectors, and hydropower.
The Basel Convention has considered FA as a waste of the green list. However, it is worthwhile to note that many countries are still treating this only as waste and are not able to re-utilize it. Only few developed countries are able to reuse ~100% (Italy, Denmark and Netherlands); while other countries (west Germany, France) reuse ~85%; whereas reuse in India is very low, increased during year 1990 (3%) to year 2005 (38%; Kishor et al., 2010; Bhattacharjee and Kanpal, 2002) further enhanced to 78% including significant use of 26% in cement, 21% in roads and flyovers, 10% in bricks and tiles, 9% for reclamation of low lying area and 7% in mine filling (Central Electricity Authority, 2023; Jeyarajp and Sankararajan, 2024). The main factor responsible for this less-utilized proportion of FA is the involvement of more costly and less efficient technology. It can be stated that consumption is lower than the production rate. The quantity of water, energy, and land area required for the efficient dumping of FA is significant. FA from coal-based thermal power plants is managed through a range of disposal and reuse strategies. Historically in India, wet disposal was the predominant method, where FA was mixed with water and stored in lined or unlined ponds, a practice that consumes significant land and water resources and carries risks such as groundwater contamination and embankment failure. In recent years, there has been a shift towards dry disposal methods—such as lined engineered landfills and dry stacking—where FA is handled in dry form to minimize environmental hazards, particularly the risk of leachate infiltration into groundwater. Reuse options are also expanding, with FA being incorporated into products like cement, concrete, bricks, blocks, and materials for road construction and mine filling, helping reduce landfill demand. India’s regulatory framework mandates complete FA utilization, and progress towards this target has been notable. Advanced applications, such as geopolymer binders and alkali-activated materials, are emerging globally, offering higher-value uses. Furthermore, stabilization and encapsulation processes, which involve binding FA with other materials to reduce contaminant mobility or embedding it in dense matrices to limit dust and pollutant release, provide additional pathways for its safe management. Even after this significant volume of FA remains unutilized, and due to the fine nature of particles and content of heavy metals, its throwing is always reported as a liability to the environment, causing soil degradation (Budania and Dahiya, 2018; McBride and West, 2005; Mittra et al., 2003; Montes-Hernandez et al., 2009; Arivazhagan, 2011).
FA contains toxic heavy metals (e.g., As, Pb, Cd, Cr). The adverse impact of FA is associated with the leaching of these lethal elements from FA to soils, ultimately affecting underground water, which leads to a change of elemental composition in plants as well as accelerating the introduction of these toxic elements via the food chain (Carlson and Adriano, 1993). Due to the fineness of particles, even after using emission control devices, power plant units are not able to fully control the leakage of FA in air which is affecting human health. Due to prolong exposure to FA, irritation affected various organs, e.g., eyes, lungs fibrosis, skin, pneumonitis and bronchitis (Carlson and Adriano, 1993). Dry FA is a fine, powder-like material obtained from the flue gases of coal-fired thermal power plants. It forms as a byproduct during coal combustion and is composed mainly of non-combustible mineral constituents such as silica, alumina, and iron oxide, along with small amounts of unburned carbon. FA Considering its fine nature, handling of it is a serious concern and intensify the environmental pollution (air pollution, water pollution and soil pollution), tremendously affecting the ecology and cultivation, disintegrate the structural surfaces (Pujari and Dash, 2006). FA particles size is in the range of silt and during mild breeze (6–10 km/h), dry form can be get lifted and travels for several hundred meters downwind direction. To manage such a huge amount of FA, for stabilization against wind and water erosion, it is recommended to start vegetation on the landfill, promoting and providing shelter and creating habitat for wildlife which ultimately leads to form a more aesthetically delightful landscape (Adriano et al., 1980). FA changes the properties of the soil (physical and chemical), impacting plant existence and its growth, hence progress on revegetation process of these impacted locations proceed very slowly.
In India, construction and biomass generation are the major areas for FA utilization, year 2022–2023 (Central Electricity Authority, 2023). Utilization in construction area includes various activities, e.g., production of cement, manufacturing of brick and road embankments (Jeyarajp and Sankararajan, 2024). While various areas included in biomass production are agricultural, floriculture, and forestry. However, most FA utilization in forestry is established for growing few economically important trees, e.g., timber wood, biodiesel crops, firewood, plywood trees, paper tree. Therefore, FA application for biomass generation is very important strategies having economic importance as well as to protect environmental degradation (Panday et al., 1985, 2009a,b; Sajwan, 1995, Sao et al., 2007; Schnappinger, et al., 1975; Sharma et al., 2001; Sharma and Uma Singh, 2007; Siddiqui et al., 2022).
Physico-chemical characteristics of FA
Physico-chemical attributes of FA depend up on various factors, e.g., coal origin (anthracite, lignite, bituminous and sub-bituminous) and chemical composition, combustion conditions and heating values, emission control, handling and storage techniques (Chakraborty et al., 2025; Abhishek et al., 2025).
Physical properties
Composition and physical characteristics of FA depends upon various facts, e.g., nature and type of coal, type of boiler and combustion conditions, content of ash in coal, control on emission, conditions associated with storage, handling procedure and collector set up. The empty sphere of FA is filled by small amorphous particles and crystals. Due to the presence of porous particles and other carbonaceous nature units, FA has light texture properties and relatively higher surface area. Few particles of FA are small in size and spherical in shape while some are of large size, as a result different forms of FA present with wide range of properties (Natusch and Wallace, 1974) e.g. mean particle density (2.7 to 3.4 g cm-1), bulk density (1 to 1.8 g cm-1), moisture retention range (6.1 to 13.4%), water holding capacity (49 to 66%) and specific gravity (2.1 to 2.6 g cm-1) (Abhishek et al., 2025; Chakraborty et al., 2025).
Chemical properties
The components which are accountable for the observed differences in the physical characteristics of FA, also affecting the chemical attributes. FA is broadly categorized into Class F and Class C based on its chemical composition and the type of coal from which it originates, as outlined in ASTM C618. Class F FA is typically derived from the combustion of bituminous or anthracite coals and is rich in silica and alumina, with the combined content of silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃) generally exceeding 70%. It contains low calcium oxide (CaO), usually less than 10%, and exhibits pozzolanic properties, requiring the presence of an activator such as lime or cement to form cementitious compounds. In contrast, Class C FA is produced from the combustion of sub-bituminous or lignite coals and contains a higher proportion of calcium oxide, often in the range of 15–30%, with the combined (SiO₂ + Al₂O₃ + Fe₂O₃) typically between 50 and 70%. This higher calcium content imparts both pozzolanic and self-cementing characteristics, enabling it to harden upon contact with water without additional activators. The distinction between these classes is significant, as it governs the material’s reactivity, strength development potential, durability performance, and its suitability for various applications in construction, soil stabilization, and other engineering uses (Abhishek et al., 2024; Chakraborty et al., 2024).
The chemical composition of FA is largely determined by the type of coal burned and the combustion process, with major constituents typically including silicon dioxide (20–60%), aluminium oxide (5–35%), and iron oxide (4–40%), along with variable amounts of calcium oxide (1–40%) and minor oxides such as magnesium, potassium, and sodium, generally below 10%. When in contact with water, FA can produce leachates with pH values ranging from acidic to strongly alkaline (approximately 4–12), depending on the mineral phases present and their buffering capacity. Electrical conductivity (EC) values commonly vary between 1.6 and 24 mS cm−1, reflecting the concentration of dissolved salts. In terms of nutrient content, FA may supply macronutrients such as calcium (5–177 g kg−1), magnesium (5–61 g kg−1), and potassium (2–35 g kg−1), as well as micronutrients including boron, copper, manganese, zinc, and molybdenum. However, it also contains potentially toxic trace elements, with reported ranges (mg kg−1) for arsenic at 0.5–279, lead at 0.4–252, chromium at 3.4–437, cadmium at 0.1–18, mercury at 0.005–4.2, selenium at 0.08–19, boron at 10–1,300, nickel at 1.8–258, and zinc at 4–2,300. The solubility and environmental mobility of these elements are influenced by factors such as pH, redox conditions, and particle surface chemistry, making detailed chemical and leachate analysis essential for evaluating the reuse potential and environmental implications of FA (Adriano et al., 1980; Goetz, 1983). All the elements similar to soil are present in FA, except carbon and nitrogen (Kumar et al., 2000). It is well known that higher content of aluminium is organically toxic while in FA aluminium is present in the form of aluminosilicate structure which has very less toxicity (Page et al., 1979). Due to the presence of these elements, variance is observed in the chemical properties of FA, e.g., colour variation is due to Iron oxide, for the range of pH content with sulphur present in the coal used is responsible (Matti et al., 1990). Which means high Sulphur content in coal will produce acidic ash whereas alkaline ash is produced by low Sulphur content.
Mineralogical properties
FA is predominantly composed of both amorphous (glassy) aluminosilicate phases and various crystalline minerals formed during coal combustion. The amorphous glassy fraction often comprising 70–90% of the material, is created by rapid quenching of molten silicate particles and largely governs FA’s pozzolanic reactivity. Common crystalline constituents identified via XRD include mullite (3Al₂O₃·2SiO₂) and quartz (SiO₂), typically deriving from the thermal transformation of aluminosilicate sources and residual siliceous grains, respectively. Iron oxide phases such as hematite (Fe₂O₃) and magnetite (Fe₃O₄) are frequently observed, sometimes as surface inclusions on glassy spherules. Studies on FA from power plants in Poland reported precisely these phases as mullite, quartz, and iron oxides embedded in glass spheres with hematite and magnetite especially enriched on particle surfaces. Similarly, investigations of FA from Vietnam indicated that magnetic fractions are rich in magnetite and hematite, whereas non-magnetic components primarily consist of quartz and mullite.
In addition, high-calcium (Class C) FA may contain anhydrite (CaSO₄), free lime (CaO), periclase (MgO), calcite (CaCO₃), and cement-like silicate phases such as di-calcium silicate and tri-calcium silicate, reflecting the presence of calcium-bearing minerals in the feed coal. The mineralogical mix and relative abundances are influenced by variables such as coal type, combustion temperature, and cooling rate—factors which determine how much of the original mineral assemblage is retained in crystalline form versus converted to glass. Overall, FA’s reactivity, density, and applicability in cementitious, ceramic, or geopolymeric materials are strongly controlled by this interplay between the reactive amorphous matrix and embedded crystalline phases (Hodgson and Holliday, 1966; Chakraborty et al., 2024; Chakraborty et al., 2025).
Improvement in soil potency
Numerous research has been already accomplished by studying the FA impact on soil. Application of FA to soil can induce significant changes in its physical and chemical environment due to the ash’s mineral composition, alkalinity, and fine particle size. One of the most pronounced effects is on soil pH. Most FA, particularly Class C types with higher calcium oxide content, exhibits an alkaline reaction (pH often > 9), and when applied to acidic soils, it can raise the pH, thereby reducing exchangeable acidity and enhancing base saturation. This liming effect can improve nutrient availability for certain crops but may also increase the solubility of some oxyanion-forming trace elements (e.g., As, Mo, Se) under strongly alkaline conditions. Soil productiveness improved significantly with use of FA as an agronomical adjustor (Rajakumar and Patil, 2019). Cation exchange capacity (CEC) may be altered indirectly. FA itself has limited inherent CEC compared to clay minerals, but its fine texture and amorphous aluminosilicate glass can contribute to additional exchange sites over time as weathering releases secondary minerals (e.g., smectite, allophane). The dissolution of FA also supplies soluble bases (Ca2+, Mg2+, K+, Na+) that occupy exchange sites in the soil, potentially displacing acidic cations (H+, Al3+) and improving nutrient-holding capacity in the short term. In sandy or degraded soils, FA particles can physically fill pore spaces, reduce bulk density, and improve moisture retention, which may indirectly influence nutrient retention and CEC dynamics (Ghodrati et al., 1995; Capp, 1978; Buck et al., 1990; Kene et al., 1991; Khan and Khan, 1996; Singh and Siddiqui, 2003; Jala and Goyal, 2006; Rautaray et al., 2003; Phung et al., 1979; Gupta et al., 1990; Gupta et al., 2007; Lau and Wong, 2006; Abhishek et al., 2024; Abhishek et al., 2025). FA contains higher concentration of CaO and can be used as alternative of lime. Hence, remediation of soil can be achieved by using FA which also improves the soil fertility, improvise the nutrient status which ultimately resulting in the increase of crop yield (Wong and Wong, 1986; Schutter and Fuhrmann, 2001; Sarangi et al., 2001; Christensen, 1987; Jiang et al., 1999). Various research studies identified that crop plants of some families are most tolerant to FA, e.g., Brassicaceae, Fabiaceae and Poaceae. While it has been observed that Acacia sp. and Leucaena leucocephala can tolerate higher content and positive survival ratio in dull, deserted, and areas contaminated with metals (Cheung et al., 2000).
Microbial activity in FA amended soils often shows an initial decline, particularly when high application rates elevate pH beyond the optimal range for native microbial communities or when soluble salts raise electrical conductivity to inhibitory levels. However, as the soil-ash system equilibrates, microbial biomass and enzymatic activity may recover or even surpass baseline levels, aided by improved nutrient availability (e.g., B, Mo, P, Ca) and enhanced moisture conditions. The response varies with FA composition, application rate, soil type, and management practices; in neutral or slightly acidic soils, moderate FA additions can stimulate microbial processes linked to nutrient cycling, whereas in highly alkaline environments, sensitive microbial taxa may be suppressed. In addition, nitrogen content of infertile soils can be improved by the planting legume plants and using bacteria well known symbiotic nitrogen-fixing relationship (Vajpayee et al., 2000). On FA amended soils, leguminous plants grow well and demonstrate absence of any injury symptoms (Singh S. N. et al., 1997; Singh A. K. et al., 1997; Ahmad and Alam, 1997). Overall, FA acts as a soil amendment with both corrective and disruptive potential (Narayanasamy, 2002; Narayanasamy, 2003). Its capacity to modify pH, supply base cations, and influence CEC can benefit certain soils, but the associated shifts in microbial community structure and function require careful management to avoid adverse ecological effects. Long-term monitoring of chemical equilibria and biological indicators is therefore essential for sustainable application (Jones and Lewis, 1960).
Improving crop growth and yield
Due to presence of hydroxide and carbonate, FA has unique chemical characteristics to neutralize acidity in soils (Matsi and Keramidas, 1999). Soil amendment using FA is well known for changing the pH and improvement in texture of soil, also to provide essential plant nutrients which results in the increase of crop production (Pandey, 2008). Several field studies as well as greenhouse experiments confirm about the positive impact of FA on plant growth and ability to enhance the agronomic capacity of soil due to chemical constituents of FA (Wong and Wong, 1989; Singh and Singh, 1986; Inam, 2007).
FA contains substantial concentration of Calcium, Phosphorous, Magnesium, Potassium and Sulphur. Therefore, supplemental supply of FA leads to enhancement of nutrient uptake and causes significant progress in plant growth (Kalra et al., 1997), ultimately results in increase of crop yield (Basu et al., 2007). Increase in the accessibility of Sulphur content from FA is the main factor behind this increase of crop yield. In agronomic crops, e.g., barley and B. grass (Page et al., 1979), FA addition up to 8% w/w concentration results in the considerable increment. FA application enhances the growth, improves the metabolic rate and leads to higher photosynthetic pigmentation as observed in maize and soybean (Mishra and Shukla, 1986). By using the FA in a concentration up to 40%, yield of tomatoes increased by 81% (Khan and Khan, 1996), while using FA in concentration of 25% w/w, various vegetables obtained in higher yield, e.g., brinjal, tomato, cabbage (Goyal et al., 2002). Also, by applying low concentration of FA, i.e., 5%, rate of seed germination increases as well as lettuce root length amplified (Lau and Wong, 2006; Ou et al., 2020).
To establish the effect of FA on the productivity and growth of Brahmi (Bacopa monnieri L.; Panda et al., 2020), FA used for soil amendments and studied in a pot culture experiment. B. monnieri seedlings cultivated using different concentration of FA for 90 days. It has been observed that using amended soil (FA 25%), in comparison with garden soil (control) and extra FA treatments, significantly improvise the tolerance index (>100%), growth in plant biomass and essential oil content. In comparison with seedlings which are grown on garden soil, no change observed in the content of chlorophyll in leaf and photosynthetic parameters when 25% of FA uses as medium. However, observed that higher concentration of FA adversely impacts these parameters and content declines significantly. Uses of higher concentration of FA results in the oxidative damage and also induce some anti-oxidative enzymes activities in B. monnieri indicating about the capability of plant to withstand oxidative stress and its tolerance. Research study demonstrated that for cultivation of B. monnieri L, soil can be amended with FA (25%) which results in enhancement of plant biomass along with production of essential oil (Panda et al., 2020).
Similarly, FA effects studied on the growth of medicinal plant Withania somnifera L. (Dunal), also known as ashwagandha (Kumar et al., 2017). Various parameters, e.g., effect on seed germination, plant height, leaf area per leaf, total leaf area per plant, plant fresh weight, chlorophyll a & b, carotenoids. Due to increasing concentration of FA, most of the studied parameter shows positive impact and enhancement up to the use of FA concentration (10–15%). Further using high concentration of FA, observed a decline in the growth parameters. Thus, for growth of medicinal plant Withania somnifera, 10–15% FA incorporation in soil is suitable (Kumar et al., 2017).
Using pot culture conditions, impact of FA application studied on Calendula officinalis (Varshney et al., 2021). Under different composition of FA in soil (% ration as Control, 10, 20, 40, 60, 80 and 100% of FA), impact studied on biochemical parameters (nitrate and nitrate reductase, soluble protein and reducing sugar), photosynthetic pigments including carotenoids, chlorophyll (both a and b) and total chlorophyll, metal accumulation (Cadmium, Cobalt, Chromium, Copper, Iron, Manganese, and Zinc) and antioxidant defense activity. The results of this study indicated that physico-chemical properties of soil significant improvised with the addition of FA (40%) in soil. Beneficial impact also observed in various other biochemical parameters of plants along with enhancement in photosynthetic pigment. However, observed that under high FA applications these parameters declined. While contrary to this, with increasing FA application, antioxidant enzyme activities (SOD & CAT and Peroxidase) increase to prevent the heavy metal stress due to uptake from FA. Also, levels of antioxidant enzyme increased in leaves at high FA applications indicating heavy metal stress as well as mitigation of reactive oxygen species (Varshney et al., 2021).
Other experiments using FA has been executed which shows significant and strong positive relationships with suitability of various concentration of FA (Kumar and Patra, 2012). To evaluate the impact of increasing composition of FA (from control soil, i.e., 0 to 100%) along with Garden soil (GS), study related with Mentha piperita has been conducted on different parameters of plant. During the study, parameters related with plant growth responses and elemental accumulation, composition of volatile oil and overall yield studied. It is observed that in experiments having concentration of FA at level of ≤50% results in better growth of plant, composition of oil and its yield; whereas with application of FA at composition more than 50%, results in the diminished growth of plant, composition of oil and its yield. Content of essential oil, i.e., Menthol, methyl acetate and menthofuran are changing with the compositional change of FA in soil. In this study, concentrations of Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn in Mentha piperita roots and shoots exhibited significant positive correlations with fly ash (FA) application. Correlation coefficients indicated greater accumulation of Mn and Zn in roots (r = 0.997 and 0.993, respectively) compared with shoots (r = 0.989 and 0.914). Similarly, higher regression coefficients for Fe and Ni in roots (0.982 and 0.989, respectively) confirmed preferential root accumulation, whereas Cu and Cr were more strongly associated with shoots (0.999 and 0.995, respectively) than roots (0.954 and 0.989). Comparable patterns were reported by Singh S. N. et al. (1997) and Singh A. K. et al. (1997) in Vicia faba, where Cr accumulated primarily in shoots, and by Singh and Agarawal (2010), who suggested that detoxification mechanisms at the root level restrict metal translocation. Collectively, these results indicate that most metals are preferentially sequestered in roots, with only partial transfer to above ground tissues (Kumar and Patra, 2012; Singh S. N. et al., 1997; Singh A. K. et al., 1997; Singh and Agarawal, 2010).
Study related with Coleus forskohlii found that value of chlorophyll a and b content is directly related with enhancement of FA concentration in soil (Sinha et al., 2013). In normal soil, C. forskohlii has 2.7 mg/g chlorophyll a, whereas for FA in soil (5 and 10%) chlorophyll a level improved to 2.9 and 3.0 mg/mL, respectively. Similarly, chlorophyll b content was 0.5 mg/g in normal soil while using 5 and 10% FA this content increased to 0.6 & 0.45 mg/g, respectively. Study also shows that phenolic component significantly increased in FA containing soil. Phenolic component in C. forskohlii in normal soil is 1.2 mg/100 mg whereas in 5 and 10% FA soil this increased to 1.6 mg/100 mg and 2.2 mg/100 mg, respectively.
In a study related with Jatropha curcas, pot culture experiments have been performed to study the impact of varying composition of FA with soil on biodiesel plant growth (Raj and Mohan, 2014). Mixture with soil prepared by adding FA in the range from control, i.e., 0 to 100%. Various growth and morphological parameters have been studied including total nitrogen and protein content, plant height, pigment content and number of leaves. During the study of Jatropha curcus it has been observed that all the growth parameters are best in pot having FA concentration of 25% whereas after 1.5 years of seed sowing in 100% FA growth parameters deteriorate significantly. For photosynthesis, primary part of plant is Leaves and to all parts of plant leave food perform the function of metabolic fuel. Therefore, the overall status of plant growth performance and metabolic activities are represented by the foliar response. Average plant height achieved is 92.7 cm, while healthy leaves are in average number of 63, average content of pigment is 0.95 mg/g, total nitrogen content in average is 5.8 mg/g and average content of protein is 38.4 mg/g. Therefore, it is concluded that low dose of FA in soil is suitable for all the growth parameters which represents a step forward in the most suited and environmentally responsive utilization of waste for growth of biodiesel plant.
At thermal power plants, nitrogen is oxidized into gaseous compounds during combustion, which explains its absence in FA. Consequently, the use of FA in landfills can inhibit plant growth due to nitrogen deficiency. Compared with soils, FA contains a higher concentration of phosphorus; however, interactions with aluminum, iron, and calcium in the ash render this phosphorus unavailable to plants (Adriano et al., 1980). To address nitrogen deficiency in FA, leguminous plants capable of utilizing atmospheric nitrogen should be cultivated, as they host symbiotic nitrogen-fixing bacteria (Bhattacharya and Chattapadhyaya, 2004). The use of blue-green algae further enhances soil fertility and improves physicochemical properties such as cation exchange capacity, pH, and electrical conductivity while increasing organic matter, total nitrogen, and phosphorus content. Additionally, the concentrations of metals such as copper, manganese, zinc, nickel, and iron in FA can be reduced through bioaccumulation in the tissue of the alga, Anabaena doliolum (Rai et al., 2000). FA, both individually and in combination with cyanobacteria, has been studied as a green compost for the cultivation of B. juncea, resulting in the production of adequate oil yields (Banerjee et al., 1999). Researchers have also recommended various FA-tolerant species—such as Blumea lacera, Cassia tora, and Sida cardifolia—for effective revegetation of designated landfills and lagoons (Gupta and Sinha, 2008), as listed in Table 1.
Fly ash usage in the growth of medicinal plants.
Plant species name
Mixture (%) of FA-soil
Main observations
References
Bacopa monnieri L.
FA from 25 to 100%
most pronounced improvements in plant growth, oil yield, and sustained photosynthetic activity observed when soils were amended with FA at a 25% application rate.
Panda et al. (2020)
Withania somnifera (L.)
FA from 5 to 100%
Most of the studied parameter shows positive impact and enhancement up to the use of FA concentration (10–15%).
Kumar et al. (2017)
Calendula officinalis
FA from 10 to 100%
Incorporation of FA at a 40% application rate into soil enhanced its physico-chemical properties, while also promoting higher levels of photosynthetic pigments and improving various biochemical parameters in plants.
Varshney et al. (2021)
Mentha piperita
FA from 0 to 100%
Application of FA at levels up to 50% has been shown to enhance plant growth, improve oil composition, and increase overall oil yield.
Kumar and Patra (2012)
Coleus forskohlii
FA from 0 to 10%
FA in soil (10%) increased the chlorophyll a level (from 2.7 of control soil to 3.0 mg/ml), while Phenolic component increased from 1.2 mg/100 mg of control soil to 2.2 mg/100 mg
Sinha et al. (2013)
Jatropha curcas
FA from 0 to 100%.
Soil amendment with 25% FA resulted in notable improvements in seed production, oil content, and subsequent biodiesel yield
Raj and Mohan (2014)
Brassica juncea L.
FA from 5 to 20%
Incorporation of FA (20%) into soil was observed to delay seed germination. Less impact on summer season crops
Kalra et al. (1997)
Cichorium intybus
FA conc of 3%
The alkalizing effect of FA can be harnessed to limit the uptake of potentially toxic elements by plants, when soil pH maintained at level of 5.7.
Scotti et al. (1999)
Lycopersicon esculentum Mill. Cv.
FA from 10 to 100%
Incorporation of FA at 40% was identified as the most economically viable application rate, resulting in an 81% increase in tomato fruit yield and a 30% enhancement in mean fruit weight, thereby improving overall market value.
Khan and Khan (1996)
Brassica juncea L.
FA from 10 to 100%
Plants cultivated in soil amended with 25% FA exhibited a significant increase in biomass, shoot length, and overall plant height compared to the control.
Gupta and Sinha (2006)
Sesbania cannabina L.
FA from 10 to 100%
Increasing trend of Fe and Ni accumulation as well as, malondialdehyde content showed maximum increases of 136% in roots and 120% in leaves at 100% FA application compared to plants grown in ground soil
Sinha and Gupta (2005)
Brassica chinensis
FA from 3 to 30%
A low FA amendment of 3% enhanced the growth of young seedlings, whereas higher application rates (12 and 30%) negatively affected plant growth
Wong and Wong (1989)
Glycine max (soybean)Cicer arietinum (chickpea)
FA from different g/kg
Pulses treated with FA at a rate of 1 g per 5 kg, when stored under ambient conditions, showed no infestation by Callosobruchus chinensis even after 12 months of storage.
Mendki et al. (2001)
Vicia faba
FA from 20 to 80% w/w
Cytogenetic evaluation of vermicomposted mixtures of FA, cow dung, and soil indicated a 15–45% reduction in chromosomal aberration frequencies, while chemical analysis revealed a 10–50% decrease in concentrations of metals such as Cr, Cu, Pb, Zn, and Ni.
Jain et al. (2004)
Zea mays L.
FA from conc 10 t/hac
Maize grain yield increased with FA application up to 10 t/ha, while wheat yield improved with additions up to 20 t/ha before declining at higher rates. Mustard exhibited enhanced seed yield at a FA application rate of 10 t/ha.
Kalra et al. (1998)
Lolium perenne L.
FA from different g/kg of soil
FA with low boron and salt content can serve as an effective liming agent for acid soils when applied at rates not exceeding 40 Mg/ha.
Matsi and Keramidas (1999)
Brassica juncea L.
FA from 25 to 100% FA
All tested concentrations of FA and FA leachate were found to exert detrimental effects on cytology and genetic material.
Karpate and Choudhary (1997)
Echinochloa crusgalli
FA up to 5%
After using FA (45%) amended soil, content of Selenium, Rubidium, Arsenic, Boron, Magnesium elevated.
Furr et al. (1978)
Oryza sativa L.
FA from 40 to 120 mg/ha
The addition of approximately 90 Mg ha−1 of FA resulted in the highest recorded rice yields.
Lee et al. (2006)
Beta vulgaris L.
FA from 2 to 8%
Application of FA at 2% (w/w) promoted plant growth and enhanced yields, whereas higher application rates (4 and 8% w/w) elevated soil pH and electrical conductivity to levels considered undesirable.
Singh et al. (1994)
Blumea Lacera
FA up to 100%
All surveyed plant species demonstrated suitability for revegetation of FA dykes, with S. cardifolia and C. album showing potential for use in decontamination applications.
Gupta and Sinha (2008)
Oryza sativa
FA from 10 to 20 t ha
Application of FA at 20 t/ha− significantly (p < 0.05) increased total plant biomass and aboveground biomass by 17 and 25%, respectively, while underground biomass exhibited reduced growth. At a rate of 17.5 t/ha−, FA amendment enhanced grain and straw yields by 21 and 18%, respectively, compared with the control.
Sarangi et al. (2001)
Acacia auriculiformis
FA up to 70%
Amendments consisting of 30% (w/w) vermiculite or sewage sludge compost were applied to improve the chemical and physical constraints of lagoon ash; however, the amended lagoon ash showed significant reductions in biomass and plant nutrient content compared with agricultural soil.
Cheung et al. (2000)
Helianthus annuus L.
FA from 0.5 to 1.5 kg/m square
FA application enhanced leaf number, leaf area, flower weight, and overall plant biomass, with the most pronounced improvements observed at a rate of 0.5 kg/m2.
Pandey et al. (1994)
Eucalyptus globules
FA from 25 to 100%
Among the treatments evaluated for plant growth parameters, soil alone served as the most effective growth medium, followed by a 1:3 FA-to-soil mixture.
Malewar et al. (1998)
Lactuca sativa L.
FA from 10 to 30%
A 25% FA amendment resulted in significantly greater improvements compared to 5 and 10% applications, likely due to the provision of an optimal micronutrient supply supporting various physiological processes
Srivastava et al. (1995)
Cucumis sativus
FA from 10 to 50%
A 50% FA amendment enhanced the growth and yield of cucumber (Cucumis sativus), while the highest levels of photosynthetic pigments, including total chlorophyll and carotenoids, were observed at a 25% amendment rate.
Ajaz and Tiyagi (2003)
Prosopis juliflora L.
FA from 10 to 100%
Plants grown on ameliorated FA exhibited significant increases in biomass, photosynthetic pigment concentrations, protein content, and in vivo nitrate reductase activity compared to those cultivated in unamended FA or garden soil.
Rai et al. (2004)
Oryza sativa
FA from 0 to 50%
FA application rates exceeding 25% generally resulted in increased water-holding capacity (with the exception of Domino loam soil) and reduced bulk density, while lower application rates proved beneficial for enhancing certain agronomic properties of soils.
Chang et al. (1977)
Glycine Max L.
FA from 20 to 80 ton/hac
Application of FA at 40 t/ha, combined with P. striata inoculation, enhanced bean yield and phosphorus uptake by the grain, without adversely affecting the population of the inoculated bacteria.
Gaind and Gaur (2002)
Agropyron elongatum
FA at 0 to 50% (w/w)
The observed yield increase, even at a high amendment rate of 35% ash–sludge mixture (1:1 v/v soil ratio), highlights the potential of this mixture as an artificial soil medium for agricultural applications.
Wong and Su (1997)
Brinjal
FA up to 25%
FA can enhance soil health and boost crop yields; however, its effectiveness is influenced by factors such as soil characteristics, FA composition, and application rate.
RRL (1999)
Groundnut
FA up to 20 ton/ha
The optimal amount and method of FA application depend on soil type, crop species, prevailing agroclimatic conditions, and the specific characteristics of the FA used.
CAS (1997)
Rice, groundnut
FA up to 10 ton/ha
FA contains nearly all the elements found in soil, with the exception of organic carbon and nitrogen, making it a viable additive or amendment for agricultural applications.
IIT (1999)
Oryza sativa L.
FA from 0 to 120 mg/hac
Even at an application rate of 120 Mg ha−1, FA can serve as an effective soil amendment for rice cultivation without inducing boron toxicity.
Lee et al. (2008)
Brassica napus
FA application enhanced early growth vigour and increased seed yield in canola by 20%.
Yunusa et al. (2006)
Helianthus annuus L.
FA from 20 to 100%
In seeds, heavy metals were undetectable at FA application rates up to 40%.
Siddiqui et al. (2004)
Phaseolus vulgaris
FA from 10 to 25%
At a 25% FA amendment, higher accumulations of Fe, Mn, Ni, Cu, and Co were recorded in the roots, whereas Zn, Pb, and Cd were predominantly concentrated in the aerial plant parts.
Gupta et al. (2007)
Vicia faba L.
FA from 25 to 100%
low concentrations of FA amendment enhanced early-stage plant growth, whereas higher application levels exerted inhibitory effects, leading to reduced seedling growth compared to control plants.
Rai et al. (2003)
Triticum aestivum L.
FA from 10 to 100%
FA application promoted the growth of plant and enhanced yields,
Iqbal and Khan (1995)
Cicer arietinum L.
FA up to 100%
Cicer arietinum L. has been identified as a suitable crop for revegetating nitrogen-deficient, amended FA landfill sites.
Gupta et al. (2004)
Trigonella foenum-graecum L.
FA from 25 to 100%
The application of FA influenced fenugreek by altering leaf morphology and physiology, including changes in leaf area, thickness, color, and nutrient composition.
Srivastava et al. (2003)
Brassica campestris L.
FA from 25 to 100%
Low concentrations of coal ash constituents may promote plant growth by serving as a supplementary nutrient source and enhancing soil properties.
Saleem and Ansari (2003)
Brassica juncea
FA from 10 to 20%
With increasing the FA conceentartion from 10% to 20, no significant difference observed on plant growth
Using FA amendments, elemental uptake observed in comparison with control sample
Barman et al. (1999)
Oryza sativa L.
FA from 20 to 100%
Soil amendment with 20 and 40% FA significantly enhanced plant growth and yield across all three cultivars, with the 40% application rate producing greater improvements than the 20% rate.
Singh and Siddiqui (2003)
Mustard, tomato, cabbage, mangold
FA from 0 to 100%
Barley and spinach cultivated on FA accumulated elevated levels of aluminium and manganese in their leaves, leading to toxicity symptoms. Under reduced nitrogen conditions, these crops exhibited lower yields compared to the normal control.
Rees and Sidrak (1956)
Grass and herbs species
In both laboratory batch experiments and a two-year field study, soil amendment with coal FA and peat markedly reduced copper and lead leaching from contaminated soils, with average reductions of 96 and 99.9% in the laboratory and 96 and 97% in the field, respectively.
Kumpiene et al. (2007)
Albizia procera
FA from 10 to 30%
FA can be effectively utilized in land reclamation and reforestation projects, particularly in regions degraded by industrial activities, to restore soil functionality and promote vegetation establishment. Growth promoted at lower concentration.
Tripathi and Tripathi (1998)
Cajanus cajan L.
FA from 0 to 100% (w/w)
With adequate technical knowledge and motivation, FA can be effectively utilized to reduce environmental pollution. In India, large-scale initiatives are essential to promote its bulk application, particularly in construction, wasteland reclamation, and biomass production. Positive results observed at lower concentrate
Pandey et al. (2009a, 2009b)
Beta vulgaris L.
FA from 0 to 20%
Application of FA resulted in marked reductions in growth, biomass, and yield of Beta vulgaris at various growth stages. Heavy metal concentrations in plant tissues increased significantly with higher FA application rates. The findings indicate that B. vulgaris is highly sensitive to FA, and therefore, leafy vegetables of this type are unsuitable for cultivation in areas where FA is applied as a soil amendment.
Singh et al. (2008)
Zea mays L.
FA from 0 to 20%
In calcareous soils, FA application at 2 and 5% resulted in improved plant growth compared to the control, whereas rates exceeding 5% led to a significant decline in crop performance. In contrast, acidic soils exhibited positive growth responses with FA additions up to 20%, although yields remained lower than those in unamended soils. The incorporation of FA in acidic soils effectively increased soil pH, thereby reducing metal solubility and decreasing their bioavailability to plants.
Shende et al. (1994)
Oryza sativa L.
FA from 0 to 120 mg/hac
Even at an application rate of 120 Mg ha−1, FA can serve as an effective soil amendment for rice cultivation without inducing boron toxicity.
Lee et al. (2008)
Reduction in chemical fertilizer usage through FA amendment
To establish the effect of using FA on the biochemical and morphological parameters of J. curcas, similar experiments performed. Study carried out using increasing FA composition in soil from 0 to 50%, observed that at 25% of FA plants grown showed much better growth in term of height, leaves area and corresponding number. Leaves of plant grown on 25% FA has higher content of nitrogen 11.73 mg/g, Protein 62.08%, carotenoids 0.27 mg/g, chlorophyll-A (0.52 mg/g), chlorophyll-b (0.41 mg/g) and total chlorophyll content 0.93 mg/g, sulphur 0.15%, sugar 1230.59 μg/g. At two stages, absorption of metals by J. curcas plants (Calcium, Cadmium, Copper, Magnesium, Lead, Iron, Manganese, Nickel and Zinc) were considered. By evaluating the metal concentration, relationship between metal uptake and plant growth evaluated, which confirmed the significant impact of metal uptake on growth of plant (Raj and Mohan, 2014).
It is observed that with the increase of composition of FA in soil, bulk density declines from 1.22 gm/ml to 0.91gm/ml, while decrease in the pH value also from 9.2 to 8.0, respectively. Further using in-vitro study, ethanolic extracts of Jatropha curcas used for antibacterial activity on Staphylococcus aureus and Klebsiella pneumonia. Study results indicate broad antimicrobial activity in samples where extracts obtained from leaf samples of plants which are grown on 25 and 50% FA.
Numerous studies have documented improvements in plant dry matter accumulation, photosynthetic pigment levels, protein content, and overall growth performance at low to moderate FA concentrations (10 to 40%; Mishra et al., 2007; Panda et al., 2020; Kumar et al., 2017; Varshney et al., 2021; Kumar and Patra, 2012; Sinha et al., 2013; Raj and Mohan, 2014; Khan and Khan, 1996; Malewar et al., 1998; Wong and Su, 1997; RRL, 1999; Srivastava et al., 2003; Iqbal and Khan, 1995; Tripathi and Tripathi, 1998; Pandey et al., 2009a, 2009b; Dwivedi et al., 2007; Singh et al., 2021; Thetwar et al., 2006; Tiwari et al., 1982; Tripathi et al., 2004; Khan et al., 1997).
Conclusion
The synthesis of research findings demonstrates that soil amendment with FA can substantially enhance crop growth and yield when applied at appropriate rates. However, elevated application rates (≥50%) have been consistently associated with reductions in growth parameters, likely due to changes in soil chemistry and potential accumulation of toxic elements. These findings emphasize the necessity of optimizing amendment rates to leverage the agronomic benefits of FA while minimizing adverse impacts on plant performance.
With the expansion of coal-based thermal power generation to meet rising industrial energy demands, the production of FA is projected to increase proportionally, intensifying concerns over its disposal. Strategic utilization of FA in agriculture, particularly for the reclamation of degraded or marginal soils, offers a viable pathway to improve soil physico-chemical properties and fertility while enhancing crop yields. This approach, however, must be guided by site-specific application thresholds to avoid excessive accumulation of heavy metals in plants and soils, ensuring that concentrations remain within safe limits for human health.
Future research should focus on establishing region-specific guidelines for FA application in agriculture, coupled with long-term monitoring of soil micronutrient and trace metal status. Such measures will enable the sustainable integration of FA into agricultural systems, balancing waste management imperatives with the need to maintain soil health, protect environmental quality, and support food security.
The authors acknowledge the support received from the leadership and management of K.R. Mangalam University, Gurugram, Haryana, India.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Reviewed by: Shubham Abhishek, Central Institute of Mining and Fuel Research, India