The selection of a growing medium is a key factor in soilless systems (Samadi, 2011). For successful root penetration and shoot growth, soilless substrate should have high total porosity, low bulk density, optimal WHC, and excellent stability (Deepagoda et al., 2013; Richards et al., 1986). Water-deficit conditions can decrease nutrient uptake, reduce plant growth, and may cause plants to permanently wilt (Deepagoda et al., 2013; Hu and Schmidhalter, 2005), and excess water conditions may reduce microbial activity, nutrient uptake, and root growth (García et al., 2008). Therefore, optimal water and air space management is essential to sustain a healthy rhizosphere, specifically in restricted environments, such as greenhouse containers (Deepagoda et al., 2013).
Physical properties of substrate, such as particle size, are a key factor of growing medium selection (Bohne and Wrede, 2005). Growing bolboschoenus (Bolboschoenus planiculmis) seedlings in homogeneous-sized quartz particles greater than 3 mm resulted in lower biomass, rhizome length, and spacer length (i.e., interramet distance), compared with medium (1.5 mm) and small (0.75 mm) particle substrates (Huang et al., 2013). Samadi (2011) grew cucumber (Cucumis sativus) using soilless system and diverse particle sizes of perlite and organic substrates. The substrate particle sizes were very coarse (2 to 3 mm), coarse (1 to 2 mm), medium (1 mm), fine (0.5 to 1 mm), and very fine (<0.5 mm). Samadi indicated that fine-grade perlite had higher fruit weight (50%), plant height (25%), and leaf area (70%) compared with very-coarse-grade perlite. Richards et al. (1986) found that sieving-off coarse pine bark particles (diameter >2 mm) improved WHC and did not significantly reduce aeration levels in the growing mixture. However, changing particle-size ratio of small (1 to 3 mm) to large (3 to 6 mm) diameter did not affect growth of hybrid bermudagrass [Cynodon dactylon × Cynodon transvaalensis (Sloan et al., 2008)].
Proper physical properties of soilless substrate enhance plant growth and productivity (Cannavo and Michel, 2013; Deepagoda et al., 2013). A substrate with very fine particle size normally has higher WHC and less aeration compared with coarse (well-aerated) substrate (Ingram et al., 1993). Poorly aerated media may concentrate roots in the top portion of the container and increase the susceptibility to root rot pathogens and micronutrient deficiencies (Ingram et al., 1993). Although air-filled porosity was found to be the highest in coarser particles, pore effectiveness coefficient (the degree of linkage between pores) was found to decrease with the increasing coarseness of the substrate (Caron et al., 2005). Murphy et al. (2013) found that coarse substrate (particles >4 mm) inhibited growth possibly by creating excessively large pores. In fine particle substrate (relatively low air-filled porosity), oxygen deficiency in root rhizosphere might occur when plants exhibit high growth rates and intensive root respiration (Allaire et al., 1996). Overall, particle size influences the substrate porosity, WHC, and plant growth parameters (Munoz et al., 1993).
Volcanic tuff is widely used in Jordan and many other Mediterranean countries for growing cut flower plants including lily (Lilium sp.). Tuff substrate derived from pyroclastic volcanic material is characterized by high porosity and surface area. Volcanic tuff is a reliable substrate for soilless systems. It has a good acidity resistance, cation exchange capacity, and aeration (Nawasreh et al., 2015; Polat et al., 2004). The total porosity of volcanic tuff ranges from 60% to 80% depending on its origin and the sieving/grinding process (Raviv and Lieth, 2008). Croton (Codiaeum variegatum) that was grown in perlite and amended with volcanic tuff was able to use 29% less water and fertilizer than when grown in peatmoss–perlite medium (Karam et al., 2004). Growing cherry tomato (Solanum lycopersicum) in volcanic tuff resulted in higher marketable yield than perlite and sand substrate (Al-Ajmi et al., 2009). However, commercial tuff particle size available for soilless agriculture ranges from ≤1 to 10 mm. Given that particle diameter can significantly affect substrate water availability and aeration, it is a crucial prerequisite to determine the appropriate particle size for successful plant growth in soilless system using volcanic tuff. Overall, for efficient use of volcanic tuff as a soilless substrate for lily production, it is essential to identify an optimal particle-size range that potentially improves growth and flower quality. The objective of this study was to assess the influence of tuff substrate size on growth and flower quality of two asiatic hybrid lily cultivars using soilless system in greenhouse conditions.
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