Amendment of fly ash to container substrates for ornamental plant production

Ornamental plants are largely produced in containerized soilless substrates that generally consist of sphagnum peat, bark, and other organic matter. Such substrates initially have a low pH (around 4.0) and limited amount of mineral nutrients. In order to raise pH and increase fertility, commercial ground limestone and chemical fertilizers have to be incorporated into the substrates. Fly ash, a coal combustion byproduct possessing alkalinity and containing essential mineral elements in silt-sized particles, could be an alternative to limestone amendments and nutrient sources for container substrates. This study examined chemical properties of four fly ash sampled from power plants in (1) North Carolina, (2) Michigan(a), (3) Florida, and (4) Michigan(b) and incorporated an appropriate amount of each fly ash or a commercial dolomite, respectively, into a soilless basal substrate, resulting in fly ash- or dolomite-amended substrates. After testing chemical properties of the substrates, seedlings of Pink Splash (Hypoestes phyllostachya Bak.), a herbaceous ornamental foliage plant, were planted in the basal, fly ash-, and dolomite-amended substrates. Substrate pH and plant heights were monitored during the course of plant growth. All fly ashes were able to raise the pH of the basal substrate from 3.8 to 7.4 except the one collected from Michigan(b) that was unable to change the pH regardless of the amount of fly ash used. Plants grown in the substrates amended with fly ash from North Carolina, Michigan(a), and Florida had comparable heights and similar fresh and dry weights as well as overall quality to those grown in the dolomite-amended substrate. Whereas, plants grown in the basal substrate and the substrate amended with Michigan(b) fly ash were significantly smaller and had less fresh and dry weights than those grown on the rest of substrates, thus, plants were not marketable. These results show that fly ash, after selection based on their chemical properties, can be used as alternative to commercial dolomite in soilless substrates for ornamental plant production. Utilization of fly ash as a container substrate amendment may represent a new market for the beneficial use of coal combustion byproducts.