Journal of Applied Botany and Food Quality 88, 68 - 77 (2015), DOI:10.5073/JABFQ.2015.088.010 Humboldt-Universität zu Berlin, Faculty of Life Sciences, Albrecht Daniel Thaer-Institute of Agricultural and Horticultural Sciences, 1 Division Biosystems Engineering and 2 Division Urban Plant Ecophysiology, Berlin, Germany Evaluation of substitutes for rock wool as growing substrate for hydroponic tomato production Dennis Dannehl 1 *, Johanna Suhl 1 , Christian Ulrichs 2 , Uwe Schmidt 1 (Received December 24, 2014) * Corresponding author Summary To reduce the rock wool waste, the present study is focused on the evaluation of sheep wool, cultivated Sphagnum biomass and hemp, which may be used as replacement for rock wool as grow- ing substrate for hydroponic tomato production. As such, physical and chemical properties of substrates, the plant growth, yield, fruit characteristics, as well as primary and secondary metabolites of to- matoes were considered. The marketable fruit yield of plants grown in Sphagnum slabs (12.8 kg plant -1 ) was reduced to only a small extent compared to the yield produced by rock wool slabs (13.8 kg plant -1 ). Sheep wool (12.3 kg plant -1 ) and hemp (10.4 kg plant -1 ), however, showed high- er deviations. The lowest yield of blossom end rot (BER) fruit was produced by Sphagnum. Compared to this result, the BER-yield was nearly 2-fold higher caused by sheep wool. The soluble solid content in fruit ripened by the hemp material was decreased compared to those caused by the remaining substrates. Furthermore, it was found that the volume of easy available water (EAW) was mainly respon- sible for changes in plant development. As such, a high correlation was found between EAW and: leaf area (r = 0.851); lowers (r = 0.785); lycopene (r = -0.918); ß-carotene (r = -0.997); penolics (r = -0.918); L-ascorbic acid (r = -0.848). The indings suggested that cultivated Sphagnum biomass dried and pressed as slabs can be used as replacement for rock wool slabs, whereas the usage of slabs consisting of hemp and sheep wool is not suitable as growing substrate for hydroponic tomato production. Introduction Nowadays, not only the product quality but also the sustainable production plays a major role when foods are purchased by the consumer, where economic, social and ecological aspects are con- sidered in the purchase decision (Vermeir and Verbeke, 2008). Since the ecological part also includes the waste management during the production cycle, a multitude of scientists are concerned with the further development of hydroponic systems in greenhouses in order to realize a more environmentally friendly production of vege- tables. The transition to closed hydroponic systems was a irst step to re- lieve the environment, whereas substrates as growing material can contribute to the main waste low in greenhouse production (PaPa- doPoulos and Gosselin, 2007). This applies in particular to the rock wool substrate, which is the most common substrate used for the cultivation of tomato, cucumber and red pepper in hydroponic systems in a wide range of countries (benoit and Ceustermans, 1995; shinohara et al., 1999; JeonG and hwanG, 2000; bussell and mCkennie, 2004). This growing medium is often disposed after one culture period and cannot always be recycled, whereby up to 150 m 3 of rock wool waste per ha are produced per year resulting in this fact that landills threaten to become scarce (Pieters et al., 1998; Göhler and molitor, 2002). In the Netherlands, for instance, 90 % of the used rock wool slabs are recycled and returned to the mate- rial cycle in form of new rock wool products, whereas the entire rock wool waste of the Canadian greenhouse production is stored exclusively on landills (Van den bosCh, 2004; PaPadoPoulos and Gosselin, 2007). Furthermore, an average primary energy demand of 275 kWh is required to produce one cubic metre of rock wool, where 167 kg CO 2 are released into the environment (brandhorst et al., 2012). In these calculations, however, the energy expenditure for recycling processes is not considered. Despite these circumstances, rock wool slabs are preferably used as a horticultural substrate in soilless systems due to numerous ad- vantages, such as a high total pore space, as well as inert, sterile and homogeneous conditions caused by the production process of this material and the achievement of constant yields during the culti- vation period (olymPios, 1992; bussell and mCkennie, 2004). However, the use of environmentally friendly growing substrates, which are, for example, biodegradable or can be used over several years, may reduce the waste low under protected growing condi- tions (Van os, 1994; Pieters et al., 1998). In this context, rock wool surrogates should be used, which provide equally good results in terms of fruit quantity and quality as achieved using rock wool substrates. Therefore, in recent years, various growth media were tested for their suitability as substrate for the hydroponic cultivation of vegetables. shinohara et al. (1999), for instance, found no dif- ferences in growth, yield and soluble solid content (SSC) of tomato plants, regardless of whether they were grown in rock wool, coco- nut ibre, bark or rice husk. The same applies if exactly the same substrates were used for further experiments. Similar results were found when rock wool was compared with almond shell as grow- ing media, whereas the SSC of tomato plants grown in the organic media was higher than that synthesized on rock wool (urrestarazu et al., 2005). Furthermore, manios et al. (1995), martinez and abad (1992), and allaire et al. (2005) demonstrated that hydro- ponically cultivated tomato plants showed comparable yields of to- matoes when they were produced in substrates consisting of sieved pumice and peat-lite (85% : 15%, v/v), perlite and peat (85% : 15%, v/v), sepiolite and perlite (80% : 20%, v/v), sepiolite and leonardite (97% : 3%, v/v), peat and composted bark (66.6% : 33.4%, v/v) or rock wool. However, tomato yields in substrates mixed from fresh white spruce and ir sawdust (40% : 60%, v/v), as well as from white spruce and ir shavings (40% : 60%, v/v) were lower than those obtained with rock wool (allaire et al., 2005). Based on these results, the present study is focused on the evalua- tion of cultivated Sphagnum biomass, sheep wool and hemp pressed as substrate slabs, which may be used as replacement for rock wool slabs as growing substrate for hydroponic tomato production. The main objectives of this research were to analyse the physical proper- ties of the used renewable organic substrates compared to rock wool and to ind out interactions between the physical properties of these substrates and the plant development, as well as the tomato yield. Furthermore, information about the inluence of the physical proper- ties of different substrates used for hydroponic systems on the accu- mulation of primary and secondary metabolites are scarce, although it is well known that the latter possess health-promoting properties