Johanna Suhl

Evaluation of substitutes for rock wool as growing substrate for hydroponic tomato production

To reduce the rock wool waste, the present study is focused on the evaluation of sheep wool, Sphagnum and hemp slabs, which may be used as replacement for rock wool as growing 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 tomatoes 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 higher 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 responsible for changes in plant development. As such, a high correlation was found between EAW and: leaf area (r = 0.851); flowers (r = 0.785); lycopene (r = -0.918); s-carotene (r = -0.997); penolics (r = -0.918); LAA (r = -0.848). The findings suggested that Sphagnum 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.

Reuse of Organomineral Substrate Waste from Hydroponic Systems as Fertilizer in Open-Field Production Increases Yields, Flavonoid Glycosides, and Caffeic Acid Derivatives of Red Oak Leaf Lettuce (Lactuca sativa L.) Much More than Synthetic Fertilizer

Effects of organic waste from a hydroponic system added with minerals (organomineral fertilizer) and synthetic fertilizer on major polyphenols of red oak leaf lettuce using HPLC-DAD-ESI-MS(3) were investigated. Interestingly, contents of the main flavonoid glycosides and caffeic acid derivatives of lettuce treated with organomineral fertilizer were equal to those synthesized without soil additives. This was found although soil nutrient concentrations, including that of nitrogen, were much lower without additives. However, lettuce treated with synthetic fertilizer showed a significant decrease in contents of caffeic acid derivatives and flavonoid glycosides up to 78.3 and 54.2%, respectively. It is assumed that a negative effect of a high yield on polyphenols as described in the growth-differentiation balance hypothesis can be counteracted by (i) a higher concentration of Mg or (ii) optimal physical properties of the soil structure. Finally, the organomineral substrate waste reused as fertilizer and soil improver resulted in the highest yield (+78.7%), a total fertilizer saving of 322 kg ha(-1) and waste reduction in greenhouses.