## 1 Introduction
[25] found that lignite is difficult to recycle, and it has been treated as problematic waste. From an agricultural point of view, it is organic matter that contains cellulose, lignin, humins or humic acids and can be used as organic fertilizer in conventional crops. The compounds contained in brown coal (huminous acids, humic acids) play an important role in the absorption of nutrients by plants and act as chelates, preventing leaching and degradation of nutrients. This also contributes to reducing the use of artificial mineral fertilizers. Humic acids have also been found to contain substances such as auxins and gibberellins, which are essential for proper plant growth and development. It has also been proven that soil fertility is improved by the use of humic acids, whose sources include lignite [26; 27]. With this in mind, lignite after several seasons of hydroponic cultivation can be intended as an organic fertilizer containing humic acids that improve soil properties. This is a type of cascade proposal for the use of lignite, which was proposed using a substrate of _Miscantlus_[7], but without combustion, which reduces greenhouse gas emissions. Despite strictly controlled conditions in hydroponic cultivation, both mineral and organic substrates result in excessive accumulation of salts and ions in the substrate [28]. The accumulation of ions in the substrate and the increase in EC electrolytic conductivity affect plant morphology, may lead to a reduction in leaf area and root and shoot length,and affect the amount of fresh and dry matter of the plant [3; 29]. Salinity stress, where there is also a high EC of the substrate, affects physiological activity through changes in the transport of primary and secondary metabolites and disturbances of photosynthesis, including disturbances of the proper functioning of photosynthetic pigments [2; 29; 30]. The environmental pollution that results from hydroponic cultivation using mineral medium is one of the problems to be solved. Another challenge is to increase the quality of fruits and vegetables by increasing the content of biologically active substances. One way to obtain an increased amount of secondary metabolites and biologically active substances is to apply eustressors in the form of salt stress and high EC of the substrate [20; 31; 32; 33]. Salinity stress can affect the color of cucumber fruit [34], increase the content of vitamin C [35], carotenoids, phenols [36], or acids and antioxidant compounds [37]. On the other hand, salt stress is considered to strongly reduce plant yield [2; 33]. By applying salt stress to obtain an increase in biologically active compounds, a strong reduction in yield can be induced [2; 36; 38] and lead to adverse physical changes, such as the ability to retain water and minerals in the soil [9; 39]. Increased nutrient solution ion concentration (high EC) in hydroponic cultivation using lignite substrate increases cucumber fruit quality parameters without significantly reducing yield [20]. There have been no reports to date on the effects of the physical properties of lignite substrate on morphological parameters and plant nutritional status, as well as cucumber fruit yield and quality. The results obtained will help direct research into improving the universal lignite substrate and the use of meters to measure changing physical parameters of the substrate during cultivation. Such solutions will allow future control and management of plant growth and yield in hydroponic crops with lignite substrate. The aim of this study was to investigate the correlation between bulk density and water holding capacity of the substrate and morphological parameters, macro- and micronutrient content of leaves, and cucumber fruit yield and quality in hydroponic cultivation.