2.1. Experimental Set Up
Α preliminary cascade cultivation system was built in an experimental polyethylene covered greenhouse (ground area of 160 m2), located at the University of Thessaly near Volos (latitude 39°44′, longitude 22°79′, altitude 85 m) on the coastal area of eastern Greece. The greenhouse was equipped with a single continuous roof vent and a fan heater. Air temperature and relative humidity were automatically controlled using a climate control computer (Argos Electronics, Athens, Greece). To test the cascade system and the effect of the efficiency of reusing the drainage solution (DS) for a secondary cultivation, a series of different crop-combinations and system-layouts were carried out from September to December (1st Period) and from March to April (2nd Period). In both periods, the primary crops were commercial and high-value vegetables such as tomato and cucumber while the secondary crops plants had shorter production cycles and a broader concentration range of nutrient demands were applied.
During the 1st Period, cucumber (Cucumis sativus cv. Long Krateros) was chosen as a primary crop and rosemary (Rosmarinus officinalis), basil (Ocimum basilicum) and peppermint (Mentha x piperita) were chosen as secondary crops. Two crop rows comprised of 13 containers (plastic bags with dimensions of 1 m in length and a volume of about 40 L, called herein slabs) filled with perlite, were used to cultivate the primary crop with 2 plants per slab. The primary crop was fertigated with fresh nutrient solution with set-points for electrical conductivity (EC) at 2.1 dS m−1 and pH 5.7. The nutrient solution was supplied via a drip irrigation system. The irrigation dose for each treatment was set to cover the 30% of leaching fraction. The nutrient solution supplied to the primary crop was a standard nutrient solution for cucumber grown in open hydroponic systems adapted to Mediterranean climatic conditions, with the following composition: 3.0 mM K+, 6.0 mM Ca2+, 2.0 mM Mg2+, 1.0 mM NH4+, 11.5 mM NO3−, 1.5 mM H2PO4−, 3.5 mM SO42−.
For the secondary crop cultivation, 6 rows with 9 substrate slabs each were used. To achieve a randomized block design in the secondary cultivation, in each of the 6 lines, 9 plants of each species were cultivated (3 plants per slab; 27 plants per line). In total, three fertigation treatments were applied in two replications, in which the secondary crop plants where supplied with: (i) standard fresh nutrient solution (FS, the same one applied to the primary crop), comprising the control treatment (0%D+FS), (ii) nutrient solution obtained by mixing the drainage solution of the primary crop and FS at a ratio of 25/75 (25%D+FS) and (iii) nutrient solution obtained by diluting the drainage solution of the primary crop with water (W) at a ratio of 40/60 (40%D+W). The pH of the nutrient solution was adjusted using nitric acid. The fertigation treatments were applied one day after transplanting (DAT) of the secondary crops and the experimental period lasted 81 days.
During the 2nd Period, tomato plants (Solanum lycopersicum cv. Elpida) were chosen as the primary crop and spearmint (Mentha spicata), celery (Apium graveolens), dill (Anethum graveolens) and parsley (Petroselinum crispum) as secondary crops. The set-points for EC and pH were 2.0 dS m−1 and 5.7, respectively, while the leaching fraction for the irrigation dose was setting up in 30% as well.
The layout used during the 2nd Period for the secondary crops was the same as for the 1st Period. Three fertigation treatments were applied: (i) fresh nutrient solution (FS) comprising the control treatment (0%D+FS), (ii) nutrient solution obtained by mixing the drainage solution of the primary crop and FS at a ratio of 10/90 (10%D+FS) and (iii) nutrient solution obtained by diluting the drainage solution of the primary crop with water at a ratio of 15/85 (15%D+W). The fertigation treatments were applied 1 day after transplanting of the secondary crops and the experimental period lasted 57 days.
Both primary and secondary crops were grown under natural light conditions. The treatments started when the primary crop plants had about 10 leaves each, were about 1 m in height and had a leaf area index of about 0.8, in both periods.
Air temperature (T, in °C) and relative humidity (RH, in %) were measured using a temperature-humidity sensor (model HD9008TR, Delta Ohm, Italy) calibrated before the experimental period, placed 1.8 m above ground level. Irradiance (Rg, i, in W m−2) inside the greenhouse was recorded using a solar pyranometer (model SKS 1110, Skye instruments, Powys, UK) located 1.8 m above ground.
The electrical conductivity (dS m−1
) and pH of the irrigation solution (IS) of both primary and secondary crops was measured automatically using EC (DK 5689, Greisinger, Regenstauf, Germany) and pH (GHM-Greisinger, Regenstauf, Germany) sensors. The EC and pH values of the DS were recorded manually twice a week using a portable sensor (Combo, Hanna Instruments, Woonsocket, RI, USA). The volume of the DS drained from the primary and secondary crops was also measured. Samples of the IS and DS were collected and nitrate, K, Ca, Na, Mg and Mn contents were measured. Extraction was performed using the Kjeldahl Nitrogen method (TKN) based on Kjeldahl (1883) protocol [13
]. Nutrient elements were determined by ICP (ICP-OES, SPECTRO Analytical Instruments GmbH, Kleve, Germany).
Plant height and the number of stems of the secondary crops were recorded from 18 plants per crop and treatment. The plant leaf chlorophyll content was measured twice during the growing season of the 2nd Period, using non-destructive sensing by means of an Opti-Science sensor performing measurements in contact with the leaf (CCM 200, Opti-Science, Hudson, NH, USA). In total, 20 measurements were taken at young and fully developed leaves per plant of secondary crop and treatment at DAT 28 and 56. The CCM 200 m records relative measurements of chlorophyll content index (CCI).
Additionally, in order to estimate the fresh (FM) and dry matter (DM) of the crops, two destructive-samplings were performed in the middle and at the end of each cultivation period (i.e., during the 1st Period, the destructive-sampling was performed at DAT 43 and 80, while during the 2nd Period, the destructive-sampling was performed at DAT 28 and 56). On each sampling date, three plants per secondary crop, replicate and treatment [number of samples (n) = 6 per treatment] were dried in a forced-air oven for 72 h at 70 °C.
The total amount of nutrient solution irrigated to and drained from the crops during the entire cultivation period was estimated for each crop and treatment studied. In addition, the needs for additional (not drained) water of each secondary crop were also estimated.
The absorption concentration of several elements was calculated based on the model described in Katsoulas et al. (2015) [14
The water use efficiency (WUE) was estimated by dividing the biomass produced by the volume of the water applied. If we consider the cascade cropping system (primary and secondary crop) as one system and that the drainage solution collected from the primary crop is not a new resource input in the secondary crop, then, in order to estimate the WUE of the secondary crops, only the fresh water needs could be considered. Thus, the WUE of the secondary crops was calculated taking into account the FM or DM produced and the additional (fresh) water needs for each species.