**1. Introduction**

The value of pea (*Pisum sativum* L.) as a crop can be assessed in two ways. Firstly, as the seeds contain 20–24% protein, they are a valuable food and feed source [1]. Secondly, the crop residues that remain in the field after cultivation favourably a ffect the physical, chemical and biochemical properties of the soil [2]. Currently, European Union (EU) rules for integrated plant cultivation and the so-called greening [3] are perfectly tailored for this species in respect to the above requirements. According

to the Agriculture Restructuring and Modernization Agency (ARIMR), the area of pea cultivation in Poland was 56,164 ha in 2019.

For many years, the agricultural practice in EU countries, adapting to introduced directives, has been to use environmentally friendly technologies to reduce the use of pesticides and to eliminate their active substances in the environment [4]. Thus, it is increasingly difficult for producers to limit biotic plant stress, such as pests, disease and weed infestation.

In addition, climate change, especially the periods of drought that increasingly occur during the growing season, has created many problems for growers [5], although some solutions are available, e.g., improvement of water retention by increasing the proportion of organic matter in the soil, limiting (unproductive) evaporation from the soil with agrotechnical methods, as well as the use of biostimulators [6,7]. While many definitions exist, a biostimulator is generally defined as "any substance or microorganism applied to plants with the aim to enhance nutrition efficiency, abiotic stress tolerance and/or crop quality traits, regardless of its nutrient content" [8]. However, it should be noted that biostimulators are not fertilizers in the sense that they do not contain nutrients intended to be delivered to the plant. Nevertheless, they may facilitate nutrient acquisition, e.g., by mobilizing elements in the rhizosphere or by developing new routes of nutrient acquisition, such as fixation of atmospheric nitrogen through the recruitment of bacterial endosymbionts [8].

The use of biostimulators in the cultivation of various plant species, including *Fabaceae*, has been shown to have contradictory effects, with some studies reporting a beneficial effect [9,10], while others have shown no effect [8,11]. As such, it can be assumed that the variation in effects come not only from the composition of the individual products but also from the timing of the application and the time between application and the occurrence of biotic or abiotic stress [12,13].

The sustainability of the soil ecosystem can be evaluated with biologically-based indicators, and soil enzymes have been effectively utilized as indicators of soil quality across a range of farming systems [14]. Improved knowledge of how soil enzymes function, and the factors that influence activity is vital to enhance soil managemen<sup>t</sup> and quality, and food production. Soil enzymes catalyze and expedite organic matter decomposition and regulate nutrient cycling, and can, therefore, be used as a biological index for soil quality. In practice, soil enzymes can be simply integrated, are easily quantified, and are much more sensitive to soil managemen<sup>t</sup> changes (than other soil quality indicators). Their activities are influenced by a range of factors, e.g., soil depth, type, temperature, moisture content, pH level, quality and quantity of available substrates, and managemen<sup>t</sup> regimes. However, the activity of an individual enzyme is not reflective of soil quality as single enzyme activities are not representative of the rate of all metabolic processes (except if they catalyze a single specific reaction). Therefore, to accurately determine the level of soil quality, a number of enzyme activities should be evaluated. Catalases and dehydrogenases are found in the soil as essential parts of complete living microbial cells. They can be used as a measure of general microbial activity in the soil and, therefore, can be employed to derive a biological indicator of fertility (BIF). As members of the oxidoreductases class, these enzymes fulfil the most important functions in the environment [15]. Hydrolases are another important group of soil enzymes and include phosphatases, which participate in the phosphorus cycle.

The vast availability of fertilizers and biostimulants leads to an independent assessment of their value in terms of plant-soil interaction. This is the reason of the aim of the study to determine the effect of selected biostimulators and foliar fertilizers on the development and yield of pea, and to evaluate the fertility and biochemical activity (dehydrogenases, phosphatases, catalases) of the soil that they are grown on.
