Evaluation of fly ash as a component of potting substrates

Abstract

A series of laboratory and glasshouse experiments were undertaken to assess the potential for incorporation of fly ash in soilless potting substrates.

The physical and chemical properties of a commercially available bark based substrate, the University of California (UC) 1:1 peat:sand mix and a range of test substrates containing fly ash were characterised. In test mixtures, fly ash was substituted for a portion of either 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. However, high pH, increased substrate strength and reduced air-filled porosity 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 substrates was assessed. Two watering regimes, capillary watering and irregular hosing, were used to identify effects of available water capacity on plant growth, but no effect was identified. Test mixtures containing fly ash as 20% of the substrate volume produced growth equal to that in the UC mix, with substrates containing 10% ash producing significantly greater growth of tomato and petunia. At rates of incorporation > 20% reduced plant growth was attributed to both adverse physical and chemical characteristics of the substrate.

As fly ash is available at low cost and can be successfully substituted for a considerable portion of the expensive peat component, its use at low application rates in potting substrates may be desirable from an economic viewpoint.