Fly ash, a by-product of coal combustion, is produced in large quantities in many countries and is partially disposed of in soils. Physically, fly ash consists mainly of silt-sized particles, is characterized by moderate to extremely high water-holding capacity, and possesses cementing properties. Chemically, fly ash is a ferro-alumino-silicate material and contains substantial amounts of macro-and micronutrients for plants (except C and N). Fly ash can be a strongly acidic or a strongly alkaline material, depending on its composition. Consequently, its application to soils can enhance soil fertility, improve soil's physical properties, and raise the pH of acid soils, if it is alkaline in reaction.

However, fly ash may cause undesirable environmental problems, i.e., unacceptable soil pH values, salinization, and B toxicity [1,2]. In fact, certain trace elements found in different fly ashes are considered to be potentially toxic in plants and animals [2], although, according to Page et al., [3] concentrations of trace elements in fly ash and soils are generally comparable, except those of B, Mo, and Se. Concentrations of these three elements in fly ash greatly exceed those generally found in soils.

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