Substrate selection is of major importance in ornamental potted plant production. The substrate should possess the correct physical and chemical characteristics and should drain freely while retaining the correct amount of moisture. Peat has been used extensively for indoor and outdoor pot production since the early 1950s either on its own or combined with perlite. However, in recent years, worldwide reduction of peat depots, in conjunction with an effort to support the sustainability of peat forests by minimizing peat mining, has led to a search for alternative materials to be used as soil amendments.
For several decades, this quest involved partial or total substitution of peat with composted agricultural and municipal byproducts, which served to reduce the agricultural use of peat while minimizing the organic load of agricultural and municipal waste toward landfills. Agricultural byproducts have included composted manure, cotton gin trash, and rice hulls (Papafotiou et al., 2001), olive mill wastes (Ntoulas et al., 2011; Papafotiou et al., 2004), and tree barks (Fain et al., 2008a, 2008b; Owen and Altland, 2008).
As an alternative to composted products, industrial byproducts can be recycled and used as soil amendments. Urea formaldehyde resin foam (UFRF) is produced by inflation of a resin with the addition of urea and formaldehyde. This foam has a porous structure and has the capacity to absorb water (up to 60% v/v) depending on the bulk density of the final product (Nektarios et al., 2003a, 2003b, 2005). It is environmentally friendly because it biodegrades over 15 to 20 years while slowly releasing nitrogen to plants that is estimated to be 5% per annum. Like many other substrates used for potted plants, it has an acidic pH (2.8 to 4.5) and may be used to lower the pH of alkaline soil. It is readily available in most countries.
UFRF could be considered a potential substitute for peat because it possesses similar physical characteristics and pH values. UFRF has been found to reduce bulk density and increase air-filled porosity in substrates for turfgrass growth (Nektarios et al., 2003a), in potted plants (Chan and Joyce, 2007; Nguyen et al., 2009), and in green roof substrates (Nektarios et al., 2003b). In addition, UFRF has been found to reduce soil bulk density in turfgrass that was subjected to moderate and severe soil compaction (Nikolopoulou and Nektarios, 2004). UFRF has also been reported to increase the water-holding capacity of soils and substrates, although the amount of easily available water to plants was not improved (Nektarios et al., 2003a, 2003b; Nguyen et al., 2009).
Despite its contribution to the improvement of substrate physical properties, UFRF has a moderate to negligible impact on plant growth. Nektarios et al. (2003a) did not observe any significant improvement on clipping yield, root growth, or tensile strength of turfgrasses grown in a non-amended and UFRF-amended sandy loam soil. However, turfgrass growth was improved by UFRF under moderate compaction but not under severe compaction (Nikolopoulou and Nektarios, 2004). In an intensive green roof substrate, UFRF amendment did not improve the growth of Lantana camara with respect to shoot number and length (Nektarios et al., 2003b) as well as shoot, leaf, and root dry weight (Tsiotsiopoulou et al., 2003). Chan and Joyce (2007) found increased plant height and stem diameter but minimal improvement on leaflet numbers of Flindersia schottiana saplings after incorporating UFRF in composted pine bark medium. Similarly, Nguyen et al. (2009) did not detect any differences in Orthosiphon aristatus shoot length or number by the incorporation of UFRF in either composted bark or sand media.
Therefore, the initial aims of the present study included the following: 1) to evaluate the potential of UFRF to substitute peat or perlite for pot plant production of Lantana; and 2) to determine the best substrate for production of potted Lantana. However, after winter dormancy and on failure of plants growing in UFRF-supplemented substrates to sprout, an additional aim was included, namely, to investigate the causes for UFRF to fail to support plant growth, because in previous studies, UFRF did not exhibit such detrimental effects (Nektarios et al., 2003b; Tsiotsiopoulou et al., 2003).
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