## 1 Introduction  
_Brassica_ vegetables are temperate climate crops that can be direct-seeded or transplanted out in the field. Seedlings can be grown under cover or in an open field. Seedlings grown from starter plugs in heated greenhouses or foil tunnels are best suited for early planting. In turn, seedlings grown directly in the soil bed in unheated tunnels or in the field should be selected for late planting [1]. Seedling quality plays a very important role in agriculture because it affects the growth and yield of transplanted crops. High-quality seedlings have desirable morphological characteristics such as strong stems, an optimal shoot-to-root ratio, and thick, dark green leaves [2]. Starter plugs grown in trays are becoming increasingly popular [3]. This propagation method provides optimal conditions for plant growth. Seedlings grown in trays are easy to maintain, and their growth can be regularly inspected at a relatively low cost and low labor input. Seedling starter trays can be easily transported across long distances, and ready-to-plant plugs can be stored in trays for several days without compromising their quality if the planting date is delayed [4]. Plants grown in less-supportive environments have to be resistant to physiological stress [5]. However, stress affecting seedlings for a long time leads to a marked decrease in their quality. The choice of crop species and cultivar is largely determined by market demand [6]. Consumers are becoming increasingly aware of vegetable quality and global food security, which forcesproducers to enhance the functional properties of food crops. Agricultural practices aiming to improve crop quality should also increase productivity per unit area, while minimizing the environmental impact of food production [7; 8]. This goal can be achieved through the use of organic and profitable raw materials [9]. Horticultural substrates influence plant growth and development, and they improve crop yields and quality. Well-formed seedlings guarantee high yields and high crop quality. According to [PERSON] et al. [10], substrates should supply plants with nutrients to promote effective seed germination and seedling emergence. A substrate has good physical, chemical, biological, and sanitizing properties [11; 12; 13]. The availability of organic substrates has decreased in Poland and other countries in recent years. In Poland, alternative substrates are imported from Latvia and Lithuania, and environmentally friendly peat alternatives are used as potting media to maintain or improve crop yields [14; 15]. The findings of [PERSON] et al. [13] were that the resource supply does not generally indicate a limitation to an extended use of alternative growing media constituents in Europe. In a maximal demand scenario, the amounts considered would also be sufficient to completely replace peat. However, in this study of [PERSON] et al. [13], the current supply for nationally sourced alternative materials could be scarce for some countries like the Netherlands or the Baltic States. Competition for wood resources, e.g., with the energy sector, could limit their use in the growing media sector. Moreover, the conditions set by the EU Fertilising Products Regulation (EU) 2019/1009 [16] might hamper a large use of wood fibers as growing media constituent. For bark, green waste and coir by-products, an increased demand from the growing media sector may support the mobilization of additional resources. For coir by-products, a future rise in the international demand might lead to a strong competition and the exhaustion of the world's potential reserves. Transportation costs play an important role for the access to biomass potentials [13]. The following research hypothesis was formulated: specialty substrates based on Latvian peat improve the morphological and chemical parameters of _Brassica_ crops grown from seedlings. The aim of this study was to evaluate the influence of selected substrates on the biometric parameters and leaf nutrient contents of broccoli, cauliflower, and white cabbage seedlings.