Fly ash effect on improving soil properties and rice productivity in Korean paddy soils

Abstract

Paddy soils in Korea generally require the addition of Si to enhance rice productivity. Coal combustion fly ash, which has a high available Si content and alkaline pH, was selected as a potential source of Si in this study. Two field experiments were carried out to evaluate rice (Oryza sativa) productivity in silt loam and loamy sand soils to which 0, 40, 80, and 120 Mg ha⁻¹ of fly ash were added with 2 Mg ha⁻¹ Si as a control. Fly ash increased the soil pH and available Si and P contents of both soils. The amount of available B increased to a maximum of 2.57 mg kg⁻¹, and the B content of the rice plants increased to a maximum of 52–53 mg kg⁻¹ following the addition of 120 Mg ha⁻¹ fly ash. The rice plants did not show toxicity effects. The highest rice yields were achieved following the addition of around 90 Mg ha⁻¹ fly ash. The application of fly ash increased Si, P and K uptake by the rice plants, but did not result in an excessive uptake of heavy metals in the submerged paddy soil. In conclusion, fly ash could be a good supplement to other inorganic soil amendments to improve the nutrient balance in paddy soils.

Introduction

Rice is the main crop in Korea, where paddy soils are generally characterized by low pH values and low levels of available Si. In the 1990s, average values for pH and available SiO₂ were 5.6 and 72 mg kg⁻¹, respectively, which were lower than the optimum ranges of 6.0–6.5 and 130–180 mg kg⁻¹ defined by the Rural Development Administration (RDA, 1999).

The rice plant requires high levels of silica, which increases resistance to pathogens and lodging (Mengel and Kirkby, 1987, Deren et al., 1994) and stimulates the absorption of elements such as N, P, and K (Hu and Wang, 1995). Korea has historically experienced a lack of available Si in paddy soils. Silicate fertilizer in the form of by product slag from iron manufacturing is now supplied at 4-year intervals by the Korean government, which is not adequate. Therefore, an alternative source of cheap Si is currently required.

Fly ash from coal combustion is a glassy material with a very high available Si content (Adriano et al., 1980). In 1997, approximately 3.7 million tons of waste fly ash were produced in Korea, and production could reach 6 million tons by the year 2010 (Cha et al., 1999). Most electrical power stations in Korea import bituminous coal from various areas of world. The addition of alkaline fly ash, which has a pH over 9.0 (Cha et al., 1999), can reduce soil acidity to a level suitable for agriculture (Moliner and Street, 1982) and can increase the availability of Si, Na, K, Ca, Mg, B, S and other trace nutrients (Elseewi et al., 1981, Druzina et al., 1983, Wong and Wong, 1989, Ko, 2000). The commercial use of fly ash as a fertilizer in crop production is uncommon in most countries, because coal ashes may also contain non-essential elements that adversely affect crop, soil and groundwater quality (e.g., As, B, Cd, Se) (Adriano et al., 1978, Adriano et al., 1980, Page et al., 1979). Despite potential negative effects on environmental quality, coal continues to be the prime source of energy in Korea. Hence, the disposal of fly ash is likely to remain a serious issue. The use of fly ash in the manufacturing of cement and concrete is beginning in Korea, but even if this industry becomes prosperous, a large portion of the ash will require other environmentally friendly uses. The present investigation was carried out to determine the effect of fly ash on improving soil properties and rice productivity in Korean paddy soils.