## 2 Materials and Methods  
The trials were conducted at the Greenhouse Experimental Center of the Warsaw University of Life Sciences in the growing rooms of the Department of Vegetable and Medicinal Plants in 2020 and 2021. For testing, we selected greenhouse cucumber (_Cucumis sativus_ L.) cultivar \"Mewa\" F1 by Rijk Zwaan, which has marketable fruits growing up to 24 cm in length and weighing 200-220 g, with dark, slightly glossy skin. The organic substrate for hydroponic cultivation, carbomat lignite mats from CarboHort, measuring 100 \(\times\) 20 \(\times\) 8 cm, further denoted by the symbol L, were tested. The controls were Grotop Master mineral wool mats from Grodan, measuring 100 \(\times\) 20 \(\times\) 7.5 cm, further denoted by the symbol MW. The microclimate in the growing rooms was controlled with the Hortimax system. Fertilization was controlled using a DGT Volmatic controller with Dosatron dispensers. Before starting the cultivation, new growing mats were flooded with nutrient solution at pH 5.5 and EC 2.6 dS\(\cdot\)m\({}^{-1}\), at a rate of approximately 8 dm\({}^{3}\) per mat. After 48 h, two 5 cm drainage holes were made in the lignite mats on each side of the longer sides of the mat [19], while in the mineral wool mats, four drainage holes were made (two horizontal in each of the shorter sides of the mat and two vertical in each of the longer sides of the mat). Cucumber seedlings, 28 days after sowing (DAS) plants, in the first year of the study were planted into the growing mats on 10 July 2020, and in the second year, on 12 July 2021, at a rate of 3 per mat. The crop was grown until the 35 th week of the year. In 2020, during the growing season, the average daily solar radiation sum was 1474.9 J cm\({}^{-2}\), while the average temperature D/N was 25/23 \({}^{\circ}\)C, and the experimental camera humidity and CO\({}_{2}\) concentration were about 70% and 800 ppm, respectively. In the second year of the experiment, the mean daily solar radiation was 1407.0 J cm\({}^{-2}\), D/N temperature 25/22 \({}^{\circ}\)C, and humidity at the experimental camera and CO\({}_{2}\) concentration were about 70% and 800 ppm, respectively. Plants in the experiment were fed with cucumber standard medium with EC 3.1 dS\(\cdot\)m\({}^{-1}\) and pH 5.5-5.8, designated as control EC. In each year of the 7 days after planting (DAP) experiment, the EC of the nutrient solution was varied for half of the plants in the experiment by dosing capillary medium with an EC of about 7 dS-m\({}^{-1}\) and pH 5.5-5.8, designated as high EC. Four treatments of the experiment were compared: (1) MW/control EC--mineral wool and medium with EC 3.1 dS-m\({}^{-1}\), (2) L/control EC--lignite and medium with EC 3.1 dS-m\({}^{-1}\), (3) MW/high EC--mineral wool and medium with EC 7 dS-m\({}^{-1}\), (4) L/high EC--lignite and medium with EC 7 dS-m\({}^{-1}\). The experiments were established using the randomized block method, in 3 replications, with 9 plants in each repetition. The fertigation medium was prepared from one- and two-component mineral fertilizers designed for hydroponic cultivation. The nutrient solution in the control contained (mg-dm\({}^{-3}\)): N-NO\({}_{3}\) 230, N-NH\({}_{4}\) 10, P-PO\({}_{4}\) 50, K 330, Ca 180, Mg 55, S-SO\({}_{4}\) 80, Fe 2.5, Mn 0.80, Zn 0.33, Cu 0.15 and B 0.33. Dosatron dosing devices (D25 RE2 0.2-2%) were used for diluting the concentrated nutrient solution and for nitric acid to adjust the pH of the working medium. Working medium parameters (pH and EC) were measured daily using a Senmatic portable measuring device. The nutrient solution was dosed into the plants using a capillary system at a rate of 0.5 to 2.5 dm\({}^{3}\) per day per plant. The amount and frequency of the nutrient solution applied during the day depended on the developmental stage of the plant and the current solar radiation and humidity of the substrate. Irrigation was started 30 min after sunrise, and the last cycle was started 2 h before sunset.