EVALUATION OF FLY-ASH AS A COMPONENT OF POTTING SUBSTRATES

A series of laboratory and glasshouse experiments were undertaken to a ssess the potential for incorporation of fly ash in soilless potting s ubstrates. The physical and chemical properties of a commercially avai lable bark based substrate, the University of California (UC) 1:1 peat :sand mix and a range uf test substrates containing fly ash were chara cterised. In test mixtures, fly ash was substituted for a portion of e ither the peat or sand component of the UC mix, at rates of 10, 20, 30 and 50% of the mix volume, Incorporation of fly ash greatly increased the plant available water capacity (10-1500 kPa) of the substrate. Ho wever, high pH, increased substrate strength and reduced air-filled po rosity were considered adverse effects, particularly at ash rates > 20 %. The growth of tomato (Lycopersicon esculentum), petunia (Petunia x hybrida grandiflora) and Boston fern (Nephrolepis exaltata) in the sub strates was assessed. Two watering regimes, capillary watering and irr egular hosing, were used to identify effects of available water capaci ty on plant growth, but no effect was identified. Test mixtures contai ning fly ash as 20% of the substrate volume produced growth equal to t hat in the UC mix, with substrates containing 10% ash producing signif icantly greater growth of tomato and petunia. At rates of incorporatio n > 20% reduced plant growth was attributed to both adverse physical a nd chemical characteristics of the substrate. As fly ash is available at low cost and can be successfully substituted for a considerable por tion of the expensive peat component, its use at low application rates in potting substrates may be desirable from an economic viewpoint.