Although the search for alternatives to peat for soilless cultivation began several decades ago, no material has yet been fully substituted for it into the formulation of substrates. Similarly, perlite (PE), an amendment often used to increase air-filled porosity in floricultural substrates, should also be replaced to obtain sustainable substrates. In this study, compost and rice hulls (RHs) are evaluated as peat and PE partial substitutes for bedding plant production. Ten substrates are formulated starting from 40% (v/v) of commercial peat substrate, with the remainder comprising different proportion of compost and either PE or RHs (20-60% compost, 0-40% PE or RHs). A 100% commercial peat blend is also used as a control. The substrates are analyzed for physical and chemical characteristics and used to grow three bedding plant species (Impatiens walleriana ʻWhiteʼ, Petunia ×hybrida ʻCompact Roseʼ and Tagetes patula ʻLemonʼ). The plant growth trial is performed twice. Bulk density and water-holding capacity are found to increase along with the increase of compost in the substrates while air-filled porosity decreases. Electrical conductivity and all macronutrients except ammonium-nitrogen and phosphate, are also found to increase. Analysis of plant growth shows a progressive decline parallel to the increase of compost content, with no consistent effects of the amending material used. Nevertheless, plant growth with substrates containing only 40% of peat appears to be acceptable for all species, although sensitive species (e.g., impatiens) need relatively high amount of PE or RHs and low amount of compost, while for tolerant species (e.g., petunia) the suitable amount of compost can rise to about 40%.
