## 1 Introduction  
Fertilization is one of the basic operations performed during the growing season of cereals [1, 2, 3]. The aim of the fertilization of cereals is to obtain a high yield per unit area with an optimal quality of the obtained grain at relatively low production costs and the use of appropriate cultivation technologies [3]. Wheat can be fertilized with both mineral and organic fertilizers [2]. The fertilization of cereals improves the yield but also affects the quality of the crop by providing nutrients to the plants. The optimization of fertilization, mainly nitrogen fertilization, is very important not only for the plants themselves (applying too high a dose of nitrogen not only does not improve the yield and quality of the wheat grain obtained, but also causes it to decrease) but also for the environment, as unused fertilizer runs off into water bodies, causing their eutrophication. Moreover, it is also economically unjustified, especially at a time of continuous price increases of nitrogen fertilizers [4, 5, 6, 2]. Malt is the dried, partially germinated grain of cereals. It is the most important raw material used in the production of different types of beers. Malting is a process during which the controlled germination of grains takes place under optimal conditions (time, temperature, and humidity [7]). The presence and activity of amyloid, proteolytic, and cytolytic enzymes allows for the more efficient transformation of compounds in malted cereal grains. In addition, malting allows for proper loosening to be achieved, making the chemical constituents in the cereal grain available for mashing [8, 9]. The quality of wheat malt is significantly influenced by protein modification during malting. The degree of proteolytic modification is determined by four parameters: total protein content, soluble protein content, Kolbach index, and free amine nitrogen (FAN) content. The total protein content of malt, as in the case of grain, should be optimal, at 11.0-13.0% d. m. [10]. Both too high and too low total protein contents in wheat malt are undesirable. The shaping of the sensory characteristics of the wheat beer produced (colour, flavour, or saturation), the properties of the beer foam, the degree of haze, and the nutritional value of the product depends on the degree of protein transformation during malting. Wheat gluten proteins (glutenin and gliadin) are modified into water-soluble proteins, which significantly affects the physical properties of the beer foam [10; 11; 12; 13]. The soluble protein content of the wort influences the fermentation process, among other things, to keep the process steady, limit the production of by-products, or achieve stability and haze in the wheat beer produced. The optimal value should be between 650 and 780 mg\(\cdot\)100 g\({}^{-1}\)[10]. The Kolbach number (Kolbach index) is one of the most important quality characteristics of malt. It indicates the degree of protein loosening, which is related to protein transformations during the malting process. It describes the ratio at which the protein substances in the malt were extracted from the wort during congress mashing in relation to the total protein content of the malt (degree of protein degradation [13]). Due to the large variation in the molecular weight of proteins found in wheat grain and, subsequently, wheat malt, the Kolbach number is highly dependent on the variety used and the malting process technology [13]. The optimal value of the Kolbach number for wheat malt is in the range of 37-40%, but KI values in the ranges 35-36.9% and 40.1-45% are also acceptable. Both too low and too high degrees of malt looseness are not desirable [10]. Insufficient protein modification during malting (relatively low KI value) has a positive effect on beer head stability but limits the soluble protein content of the wort and thus the fermentation process, while too high malt loosening (high amino acid content of the malted grain) significantly impairs the stability of the finished product [10].