Conclusions

Any general conclusion concerning fly ash and its use as a soil amendment must be drawn with caution due to the variability of physicochemical properties of ashes and amended soils. Thus, summarizing, fly ash is expected to be rich in B because B is volatilized during coal combustion and condenses onto fly ash particles. Boron availability from fly ash is usually high due to the concentration of significant amounts of B on the external surfaces of fly ash particles and to the solubility of B species releasing upon contact of fly ash with water. Boron release from fly ash is strongly and inversely related to pH. Leaching of fly ash in conjunction to pH decrease is expected to reduce total and available B significantly.

Upon fly ash application to soils, available B concentration is expected to increase, but the extent of this increase depends mainly on fly ash B content and rate of application, degree of ash weathering, soil texture, and final soil pH. Due to the enhanced availability of fly ash B, soil constituents are not expected to play a significant role in removing and thus substantially reducing B in soil solution after fly ash addition to soils, even in cases of alkaline pH range where B adsorption maximum occurs.

Leaching of fly ash-amended soil is a more effective way in decreasing excess B, due to fly ash application. Soil texture significantly affects the time needed for effective leaching. Coarsely textured soils are leached more readily than finely textured soils. From a practical approach, in order to avoid the risk of B phytotoxicity, the use of adequately weathered fly ash rather than unweathered and adequate leaching of amended soil in conjunction with the cultivation of plant species tolerant to B could be suggested. Of course, all the preceding points depend significantly on fly ash B content and rate of application. As far as ash application rate is concerned, in general, fly ashes with high B content must be incorporated into the soils at low rates and vice versa.

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