## 1 Introduction  
In agriculture, various types of fertilizers are applied to enrich soil for crops. The most commonly used kinds are natural fertilizers such as slurry and liquid manure, mineral and organic fertilizers made of a single organic substance, or a mixture such as composts, vermiculopsis and municipal sewage sludge [1]. The limiting factor in the use of both organic and mineral fertilizers is the content of N and P; therefore, their doses should always be calculated on the basis of the content of these elements in soil and fertilizers [2]. It cannot be ignored that the crucial moment is their application method, which should be carried out very efficiently, and fertilizers should be quickly covered with soil in order to minimize nitrogen losses in the form of ammonia, which contributes to environmental contamination. The sludge as a by-product from wastewater treatment plants is characterized by typical fertilizing and cariogenic properties, such as the presence of many nutrients necessary for plants (macro- and micronutrients, including nitrogen and phosphorus) and a high content of organic matter [3]. Currently, the most popular method of sludge disposal is in-cination [4]. However, in the case of sewage sludge from areas with low industrialization and low content of heavy metals, it is encouraged to use them as a substrate to improve the quality of soils by enriching them with organic matter and minerals necessary for plants. Unfortunately, the use of sludge is still associated with the disapproval of society due to the place of its formation and the possibility of occurrence of various pollutants. Moreover, the risk associated with the application of sludge to soil as a fertilizer is the possibility of leaching contaminants (e.g., heavy metals) and other components from soil into the groundwater, which can lead to water contamination. The leaching process depends mainly on the mineral composition of the waste product and soil chemical composition, pH and texture [5]. Therefore, in order to use sludge for agricultural purposes in a safe way, its dose and method of application should be adjusted to the type and properties of soil [6]. Obviously, the content of fertilizers in the sewage sludge may be varied and it mainly depends on the quality of the incoming sewage and the treatment technology used [6; 7]. The mean content of macroelements in the sewage sludge for nitrogen is approx. 2.5%, with fluctuations from 0.9 to 7.6% DS, phosphorus (P\({}_{2}\)O\({}_{5}\)) approx. 3.2% in weight, while potassium (K\({}_{2}\)O) is approx. 0.4% on a dry-weight basis [8]. Moreover, it is estimated that approx. 35% of municipal waste is an organic fraction, which, when applied to soil, improves properties such as: bulk density, porosity, and stability of soil aggregates or water retention capacity [6; 9]. [PERSON] et al. [10] indicate that all these factors contribute to the growth and development of various groups of soil microorganisms, which, also by transforming organic and mineral compounds, enrich soil with nitrogen and biologically active, antibiotic and growth substances [11]. Many studies confirm the positive effect of sewage sludge on the growth of oligo- and macrootrophic bacteria and soil fungi, such as _Geotrichum_, _Fusarium_, _Mucor_, _Penicillium_, _Mordierlla_, _Verlicillium_ and _Trichoderma_ after the direct application of sludge. However, there are also reports that the microorganisms present in sludge are not able to fully inhabit soil environment and in a short time most of them die, becoming an additional source of organic matter for native bacteria [11; 12]. According to [PERSON] et al. [3], the growth of microorganisms in soil treated with sludge contributes to the stimulation of biochemical activity, which guarantees plant vegetation and the final yield at a satisfactory level [12]. However, during the growing season, various agrophages appear in crops, which significantly affects their size and quality.