Fly Ash Use in Agriculture: a Perspective

Md. Wasim Aktar*

Pesticide Residue Laboratory, Department of Agricultural Chemicals, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur-741252, Nadia, West Bengal, India

INTRODUCTION

Fly ash has a potential in agriculture and related applications. Physically Fly Ash occurs as very fine particles, having an average diameter of < 10 m m, low to medium bulk density, high surface area and very light texture. Chemically the composition of Fly Ash varies depending on the quality of coal used and the operating conditions of the Thermal Power Stations. Approximately on an average 95 to 99% of Fly Ash consists of oxides of Si, Al, Fe & Ca and about 0.5 to 3.5% consists of Na, P, K and S and the remainder of the ash is composed of trace elements. In fact, Fly Ash consists of practically all the elements present in soil except organic carbon and nitrogen (Table 1). Thus it was found that this material could be used as an additive / amendment material in agriculture applications. In view of the above, some agencies/ individuals/ institutes at dispersed locations conducted some preliminary studies on the effect and feasibility of fly ash as an input material in agricultural applications. Some amount of experience was been gained in the in the country and abroad regarding the effect of fly ash utilisation in agriculture & related applications.

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Table 1:

Physical and Chemical Characteristics of Indian Fly Ash and Soil

Properties Fly Ash Soil

BD (g cm-1) <1.0 1.33

W.H.C. (%) 35-40 <20

Porosity (%) 50-60 100 tons per acre) are generally required to significantly influence soil physical properties such as water holding capacity and aggregation. In most instances, fly ash is added to soils primarily to affect chemical properties such as pH and fertility, and loading rates are limited by chemical effects in the treated soils. Plant growth on fly ash-amended soils is most often limited by nutrient deficiencies, excess soluble salts and phytotoxic B levels (Page et al., 1979; Adriano et al., 1980). Fly ash usually contains virtually no N and has little plant-available P. However, newer power plants may be adding ammonia as a flue gas conditioner to limit NOX emissions which may lead to some plant-available N. Application of fly ash to soil may cause P deficiency, even when the ash contains adequate amounts of P, because soil P forms insoluble complexes with the Fe and Al in more acidic ashes (Adriano et al., 1980) and similarly insoluble Ca-P complexes with Class C ashes. Amendment of K-deficient soil with fly ash increases plant K uptake, but the K in fly ash is apparently not as available as fertilizer K, possibly because the Ca and Mg in the fly ash inhibit K absorption by plants (Martens et al., 1970).

In some cases, soils have been amended with fly ash in order to correct micronutrient deficiencies. Acidic-to-neutral fly ash has been found to correct soil Zn deficiencies, although alkaline fly ash amendment can induce Zn deficiency because Zn becomes less available with increasing pH (Schnappinger et al., 1975). Fly ash application has also been shown to correct B deficiencies in alfalfa (Plank and Martens, 1974). In some cases, plant yields after fly ash application have been reduced because of B toxicity (Martens et al., 1970; Adriano et al., 1978). Soil amendment with fly ash to alleviate B deficiencies should be carefully monitored in order to avoid B toxicity. Fly ash often contains high concentrations of potentially toxic trace elements. Plants growing on soils amended with fly ash have been shown to be enriched in elements such as As, Ba, B, Mo, Se, Sr, and V (Furr et al., 1977; Adriano et al., 1980). Although trace amounts of some of these elements are required for plant and animal nutrition, higher levels can be toxic. Highly phytotoxic elements often kill plants before the plants are able to accumulate large quantities of the element; which limits their transfer to grazing animals. Elements such as Se and Mo, however, are not particularly toxic to plants and may be concentrated in plant tissue at levels that cause toxicities in grazing animals. Soils amended with high rates of fly ash may accumulate enough Mo to potentially cause molybdenosis in cattle (Doran and Martens, 1972; Elseewi and Page, 1984).

Finally, amendment of soil with fresh fly ash may increase soil salinity (reported as soluble salts or electrical conductance-EC) and associated levels of soluble Ca, Mg, Na, and B. Incorporation of 80 T/A unweathered fly ash from a Nevada power plant increased soil salinity 500 to 600% and also caused a significant increase in soluble B, Ca, and Mg (Page et al., 1979). Fly ash that has been allowed to weather and be leached by rainfall for several years generally has much lower soluble salt and soluble B concentrations and is more suitable for use as a soil amendment (Adriano et al., 1982). In general, ashes which have been wet-handled in the plant and stored in ponds will be much lower in soluble salts and B than dry-collected ashes.

Use of Fly Ash in Acidic Spoil and Coal Refuse Revegetation

Alkaline fly ash can aid in the reclamation of acidic spoils and refuse piles, although one-time ash applications do not appear to be effective in maintaining increased pH if pyrite oxidation is not completely stopped and neutralized. The pH of an extremely acidic surface mine soil and a coal refuse bank in West Virginia was initially raised to near neutral by application of high rates of alkaline (pH 11.9) fly ash. Soil pH dropped 1 to 2 units over the next two growing seasons, however, presumably because of continued pyrite oxidation in the spoils and leaching of Ca and Mg oxides from the fly ash (Adams et al., 1972). Jastrow et al. (1981) used fly ash as an alternative to lime in a greenhouse experiment involving acidic coal refuse. The initial pH of the refuse was 3.5.

Amendment with fly ash raised the pH to 4.8, but it dropped to 4.2 by the end of one growing season. In another greenhouse experiment, the application of fly ash to extremely acidic coal refuse resulted in a higher pH and significantly increased barley yields (Taylor and Schumann, 1988). Boron toxicity has been observed in plants grown on fly ash-amended mine spoils, although in some cases toxicity symptoms were apparent but yields were not reduced (Adams et al., 1972; Keefer et al., 1979; Taylor and Schumann, 1988). Jastrow et al. (1981) implicated Mn, Zn, and V toxicity as possible factors in reduction of tall fescue yields on fly ash-amended coal refuse. Coal refuse often contains high levels of trace elements and fly ash application can raise the concentrations of these elements to toxic levels, especially if pH is not controlled.

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Studies on possible negative effects of Fly Ash application



Ground Water

Fly Ashes contain a small amount of trace and heavy metals which may percolate down and pollute ground water. The solubility of these elements is <10% (Rohriman, 1971). Natusch (1975) observed in a laboratory experiments on leaching potential that 5 to 30% of toxic elements especially Cd, Cu and Pb are leachable. Gralloway et.al.(1976) observed that atleast 10% of total Cd would be solubilized in the acidic pH range of 3 to 5. It is unlikely that these will have any major effect on the quality of ground water. However, monitoring of this aspect may be advisable.

At Central Fuel Research Institute (CFRI), Dhanbad it was observed that the quality of ground water did not change with the application of flyash and all the parameters including the trace and toxic metal contents were within the permissible limits. Some other research organisations also observed that Fly Ash has no significant polluting effect on ground water.



Uptake of heavy metals and toxic elements by plants

Fly Ash has ppm level concentration of heavy metals, when applied to soil these elements may get absorbed by plants grown on it which may ultimately enter into food chain. However, the absolute quantities of these elements in flyashes are low which may not result into negative effect. The data on trace element uptake and accumulation by plant are limiting. Despite fairly intensive research over the last 25 years, the data on trace element accumulation are rather sketchy and inconsistent. Boron in FLy Ash is readily available to plants and investigators consider B to be limiting factor in unweathered Fly Ash utilisation (Townsend and Gillham (1975); Elseewi et.al. 1978; Ciravolo and Adriano, 1979). RRL, Bhopal conducted a study regarding the uptake of heavy and trace metals by some vegetable crops and it was observed that the uptake is quite low and remains within the normal range.

Central Fuel Research Institute, Dhanbad observed that there is no significant differences in uptake of trace & heavy metal between control and Fly Ash treated plots. Although Fly Ash contain a moderate amount of trace and heavy metals, the uptake and accumulation of these by plants in very negligible.



Radionuclides

There have been several reports in the literature on the presence of radionuclides in Fly Ash but studies on their impact have been few (Coles et.al. 1978; Gowiak and Pacynas, 1980). The radiochemical pollution of Uranium and Thorium series is always present in Fly Ash (Eisenbud and Petrow 1964). The concentration of natural Uranium varies from 14 to 100 ppm although in exceptional cases it may be as high as 1500 ppm whereas that of Thorium is less than 10 ppm. The Fly Ash concentrates besides other gaseous and trace metal oxides, several radioactive contaminants like 222Ru & 220Ru (Sharma et.al. 1989). Bhaba Atomic Research Centre, Bombay is of the opinion that most of the Indian coals has very low levels of radioactivity which is well below the hazardous limit. Hence radioactivity of Fly Ash may not be a limiting factor for its application for agriculture purposes. Central Fuel Research Institute, Dhanbad observed that there is no significant uptake of radioactive elements by plants and also that there was negligible cumulative build up of these contaminants in soil when Fly Ash applied for agriculture purposes.

Conclusions:

The potential of fly ash as a resource material in agriculture and related areas is now a well-established fact and more and more researchers and `users' are getting convinced with its utility potential in this field. The major attribute, which makes Fly ash suitable for agriculture, is its texture and the fact that it contains almost all the essential plant nutrients except organic carbon and nitrogen. Although fly ash cannot substitute the need of chemical fertilizers or organic manure it can be used in combination with these (or in some cases may part substitute their requirement) to the to get additional benefits in terms of improvement in soil physical characteristics, increased yields etc. As in the case with fertilizers and any other agriculture input , the amount and method of fly ash application would vary with the type of soil, the crop to the grown, the prevailing agroclimatic condition and also the type of fly ash available.

Although, fly ash has many benefits as an input material for agriculture applications, in view of the fear in the minds of many (regarding the levels of natural radioactivity in Fly Ash and/ the characteristic presence of some amounts of heavy and toxic elements in it) there may be some cautions which have to be taken for the time being while using Fly Ash in agriculture. From the information available till now, there appears to be not much ground for concern on these accounts (heavy metals, radioactivity etc) however further confirmatory studies at the ICAR centers would be helpful in bringing out recommendations in this field. Meanwhile there appears to be sufficient ground now for the cautious and judicious use of this useful material, which is otherwise being wasted/ underutilized.

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