Physical and Chemical Properties of Bottom Ash and Coir Dust Mix Used as Horticultural Substrates

This study evaluated the use of bottom ash as a root medium component when mixed with coir dust in different ratios. Each of two kinds of bottom ash (coarse bottom ash, CBA, with particle size >= 5 mm; and fine bottom ash, FBA, with particle size >= 5 mm) were mixed with coir dust in the ratios 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, and 0:10 (coir dust:bottom ash, v/v). We investigated the physical properties of mixed substrates, i.e., particle size distribution, container capacity, air space, total porosity, bulk density, and the moisture retention curve; and chemical properties, i.e., pH, electrical conductivity, and nutrient content. Large particles (1.4-5.6 mm in diameter) increased as the ratio of CBA increased, while small particles (< 710 mu m in diameter) increased as the ratio of coir dust increased. As the ratio of FBA increased, small particles (< 106 mu m) increased. As CBA increased, the container capacity decreased but air spaces increased, while increasing FBA reduced the air spaces in the substrate mix. In 100% FBA medium, the maximum container capacity was low (32.5%), and had very low easily available water (below 2%); this suggests that, on its own, FBA has poor potential for use as a horticultural medium. In substrate mixes of acidic coir dust (pH 4.99) and CBA, increasing the proportion of bottom ash gradually increased the pH, which is similar to perlite. Electrical conductivity of the substrate increased as the ratio of FBA in the substrate mix increased, largely caused by the high Ca and Na content of FBA. When the proportion of FBA was increased in the substrate mix by more than 30%, Cd contents were higher than the acceptable level for use in horticulture. Other heavy metals (As, Pb, Cr, Cu, Ni, and Zn) were detected at low concentrations in all substrate mixes, but within the acceptable range for use in horticultural media. Based on these results, bottom ash may be used as a component of horticultural media, but to maintain physical and chemical properties within acceptable limits it is not recommended to exceed 20% of the mixture.