Experimental setup
Two nutrient flow technique (NFT) systems were placed in a plastic tunnel greenhouse in Bleiswijk, The Netherlands, during a warm August–October season. The gullies of the NFT systems, each system servicing 16 gullies of 7.7 m long, were mounted in two blocks of eight rows and distributed in alternation. The system of cross-over NFT was applied, leading to a drip irrigation nozzle for every plant, while drain water was directly collected and did not affect neighbouring crops. These individual water nozzles gave a water flow of 2 L per hour. No additional CO2 was applied. The recirculation container of each NFT system contained 250 L of water.
Gullies were planted with 38 lettuces each, leading to a planting density of 12 heads per square meter. Hydrologically speaking, this approach, however, cannot be considered as a repetition. The scheme as well as a picture of the experimental setup can be seen in Figs. 1 and 2.
The hydroponic treatment tank has been filled up with rain water continuously and the RAS treatment tank with 30% RAS water and 70% rain water. To both tanks, hydroponic nutrient solutions (General Hydroponics, FloraMicro, FloraGrow and FloraBloom, 3:2:1 mixing ratio) were added daily. The RAS water was taken from a RAS system cultivating carps just hours before it was added to the hydroponic system. The climate was monitored (temperature, relative humidity (RH), irradiation). Water loss due to evapotranspiration and leakage was replaced continuously in the basin, while the electrical conductivity (EC) and acidity (pH) were measured daily, and kept constant on 1800 μS cm−2 and pH 5.0–6.0 respectively. Every 2 weeks, 40 L RAS water was added to the hydroponic sump.
Water analysis
Once every 2 weeks, water of both systems, as well as the RAS water from an experimental carp RAS system in Wageningen, has been sent to the lab for analysis of the plant-relevant ion composition. The water samples have been measured by the commercial lab Groen Agro Control, Delft, The Netherlands, using HPLC equipment according to the ISO 17025 norm.
Lettuce
Butterhead lettuce (variety Cosmopolia, RZ) were sown in 4 × 4 peat blocks and put directly into the system on the 23rd of August 2016. Seven weeks after planting (11th of October), 20 lettuce shoots were randomly selected, harvested and weighed individually. Prior to sending in the milled lettuce shoots for lead analysis, the lettuce heads of each system were cut into small pieces, weighed and merged in brown paper bags (40 × 20 × 20 cm) and dried (for 24 h at 103 °C) to determine their dry weight. The leaf analysis of the nutrients was performed with an ICP-OES by Groen Agro Control according to their certified analysis protocol.
Statistical analysis
Data are presented as mean ± standard deviation (SD) and ranges respectively of n samples. Analysis of statistical significance and ANOVA were conducted in R (R Core Team 2013). Furthermore, the nonparametric two-sample Kolmogorov–Smirnov test was used to test whether the two (i.e. in the RAS and HP system) Na concentration probability distributions differ. Genstat software was used to conduct a principal component analysis with respect to the lettuce’s nutrient composition.