Efficient production of horticultural seedlings in nurseries requires plants that can develop rapidly and uniformly until they are well established. To that end, the role played by substrates is critical (Sterrett, 2001). Many materials, alone or mixed, can be used to produce substrates with the appropriate physical, chemical, and biological characteristics to grow different crops under diverse cultural conditions (Burés, 1997). Currently, sphagnum peat provides the base material for the majority of commercial substrates used in nurseries; its excellent physical, chemical, and biological properties make it ideal for growing horticultural seedlings (Abad et al., 2001; Burés, 1997). Peat is collected from bogs, marshes, and wetlands, which are often fragile ecosystems of great ecological and archaeological value (Bustamante et al., 2008). Unfortunately, the high demand for peat in horticulture has led to constant exploitation of peatlands, the consequent depletion of this resource, and the ecosystem degradation (Raviv, 1998; Sterrett, 2001). Since the late 1970s, alternatives to peat have been sought worldwide (Raviv et al., 1986).
Many authors have indicated the viability of vegetable waste composts (VW), solid urban wastes (UW), and the residues of the wine industry [vine pomace (VP)] as seedbed substrates for the production of horticultural seedlings in nurseries (Castillo et al., 2004; García-Gómez et al., 2002; Herrera et al., 2008; Kostow et al., 1996; Pérez-Murcia et al., 2006). The use of these composts also helps solving a major environmental problem: their disposal. Every year, Europe alone produces more than 1.8 billion megagrams of total waste—3.5 Mg of waste per person (Bernal and Gondar, 2008).
Immaturity and instability are common problems of composts when used as seedbed substrates for horticultural seedlings (Carmona and Abad, 2008). Even well-stabilized compost can have properties considered to be limiting factors for horticultural use, such as the presence of heavy metals, low water-holding capacity, aeration problems, or excessive content of salt and nutrients (Carmona and Abad, 2008; García-Gómez et al., 2002). The use of very saline substrates to grow tomato seedlings can lead to physiological, morphological, and biochemical abnormalities. Effects of salinity on tomato seed germination are closely linked to the substrate's concentration, the species, and the cultivar used (Cuartero and Fernández-Muñoz, 1999; Foolad and Lin, 1997; Srinivas, 2001), as well as plant height, leaf number, stomatal density, leaf area (Al-Karaki, 2000; Romero-Aranda et al., 2001), and root development. When the damage caused is severe, chlorosis and necrosis appear, especially at the leaf edges (Al-Karaki, 2000; Romero-Aranda et al., 2001). Combining peat with compost can attenuate some of the physical and chemical problems that composts present when used alone (Bustamante et al., 2008; Raviv et al., 1986).
Several authors have shown the viability of UW, VW, and VP as components of substrate mixes for tomato seedlings (Castillo et al., 2004; Herrera et al., 2008; Kostow et al., 1996). Herrera et al. (2008) reported that a mixture of 30% UW, 65% peat, and 5% perlite (v/v) allowed the production of tomato plants with development properties similar to those produced using the common nursery substrate mix of blond peat, black peat, and coconut fiber. In addition, these authors also noted that to reduce the negative effects of the high electrical conductivity (EC; 11.4–19.8 dS·m−1) and pH (8.1–8.8) of their compost, the ratio of the compost component should not exceed 30% (v/v). Ribeiro et al. (2007) evaluated a compost obtained from forestry waste and solid swine manure (EC = 0.27 mS·cm−1, pH 6.88) as a substrate for the production of tomato seedlings. The greatest growth was achieved with 100% composted substrate, while the least growth was achieved with peat alone. Castillo et al. (2004) reported that pH, salinity, and organic matter content of UW have great influence on the quality of tomato seedlings. In addition, they reported that EC values of more than 9 dS·m−1 were associated with plants of poorer quality.
The above-mentioned studies suggest that the partial substitution of peat by composts for tomato seedlings production is feasible. However, they failed to define the optimal ratios with respect to the physical and chemical properties of such composts. The aim of the present work was to examine the effect of seedbed substrates with several EC values on the growth of tomato seedlings, based on different peat, UW, VW, and VP compost ratios.
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