Review Paper Fly ash as a soil ameliorant for improving crop production—a review Sudha Jala, Dinesh Goyal * Department of Biotechnology and Environmental Sciences, Thapar Institute of Engineering and Technology, Deemed University, Patiala 147 004, Punjab, India Received 4 September 2003; received in revised form 17 September 2004; accepted 17 September 2004 Available online 11 November 2004 Abstract Fly ash, a resultant of combustion of coal at high temperature, has been regarded as a problematic solid waste all over the world. Many possible beneficial applications of fly ash are being evaluated to minimize waste, decrease cost of disposal and provide value- added products. The conventional disposal methods for fly ash lead to degradation of arable land and contamination of the ground water. However fly ash is a useful ameliorant that may improve the physical, chemical and biological properties of problem soils and is a source of readily available plant macro and micronutrients. In conjunction with organic manure and microbial inoculants, fly ash can enhance plant biomass production from degraded soils. Detailed studies on the nature and composition of fly ash, con- ducted during the latter half of the 20th century have helped in repeatedly confirming the various useful applications of this hitherto neglected industrial waste. The purpose of this paper is to review the available information on various attributes of fly ash and explore the possibility of exploiting them for agronomic advantage. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Fly ash; Soil amendment; Plant productivity 1. Introduction Environmental pollution due to release of smoke, gases, effluents and solid wastes from industries is one of the major issues of global concern. Today 52% of the capacity for generating electricity in the United States alone is fueled by coal and the consumption of coal worldwide is projected to increase by 36% by the year 2020 (USDE, 2000). Combustion of bituminous and sub-bituminous coal and lignite for generation of electricity in thermal power plants produces solid wastes such as fly ash, bottom ash, boiler slag and flue gas desul- phurization (FGD) materials, which are commonly known as coal combustion by-products (CCPs) (Vom Berg, 1998). Fly ash is the mineral residue consisting of small particles that are carried up and out of the boiler in the flow of exhaust gases and are collected from the stack gases using electrostatic precipitators (ESP), flue gas desulphurization (FGD) systems and bag houses (Mattigod et al., 1990). Approximately 70% of the by- product is ESP fly ash, which is the most difficult to han- dle. Nearly 10–12% of the by-product is bottom ash, which is a coarse and solid mineral residue. The individ- ual particles are much larger than fly ash particles and fall down through the airflow to the bottom of the boiler. The material is mechanically removed from the bottom of the boilers. Boiler slag constitutes 4–6% of bottom ash produced by some older boilers, where the material is actually melted in the boiler, quenched in the bottom hoppers and becomes hard and glassy. FGD material (10–12%) is the solid material resulting from the removal of SO 2 gas from the utility boiler stack gases in the flue gas desulphurization process. The material is produced in the flue gas scrubbers by reacting slurried limestone or lime with the gaseous SO 2 to produce CaSO 3 . In US alone over 118 mt (million tons) of coal com- bustion products are generated each year (ACAA, 0960-8524/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2004.09.004 * Corresponding author. Fax: +91 0175 2393738. E-mail address: d_goyal_2000@yahoo.com (D. Goyal). Bioresource Technology 97 (2006) 1136–1147