Enhancing the benefits of fly ash as a soil amendment by pre-leaching

Previous studies have shown that the potential benefit (increased water-holding capacity) of fly ash as a soil amendment can be compromised by phytotoxic constituents of the ash, specifically excessive levels of B and soluble salts. Therefore, greenhouse and column studies were conducted to determine if pre-leaching an ash-amended soil would decrease the detrimental components of the ash to levels that would have no effect on crop growth. In the greenhouse study, a Hammonton loamy sand was amended with fly ash at three rates (0, 20, and 40% by weight), thoroughly mixed, and subjected to four leaching regimes (equivalent to 0, 25, 50 or 100 cm of rainfall). After leaching, pots were filled with ash-soil mixtures and sown to corn (Zea mays L.). Subsamples of the potting mixtures were analyzed for pH, electrical conductivity (EC), and extractable elements. After 55 days of growth the plants were harvested and analyzed for dry weight and elemental composition. Soil columns (20 cm in length) were packed with the same ash-soil mixtures used in the greenhouse study and subjected to the same leaching regimes presented above. Leachate and ash-soil mixtures after leaching were analyzed for B and EC. Ash amendments to the soil resulted in levels of B and soluble salts (EC) that could reduce plant growth. Ash amendments delayed emergence and reduced corn growth. Pre-leaching the ash-amended soils improved corn growth and significantly reduced B levels and EC, but had little effect on soil or plant heavy metal content. The column study demonstrated that B and soluble salts should leach rapidly from similar soils inasmuch as soluble (B) and EC values approached background levels within the first 10 to 20 cm of leachate. It was concluded that potential phytotoxic effects of fly ash amendments can be minimized by leaching the amended soil before crop establishment.