**4. Discussion**

As compared with the control, statistically higher beetroot yields in the FYM treatment were recorded only in 2016 (+17.6 t ha−1). In the following years, the application of FYM resulted in comparable yields. If we compare the entire evaluated period (2016–2018), the application of FYM increased the average sugar beet beetroot yield by about 8 t ha−1. In the case of the tops, no differences between the control and the FYM treatments were recorded in individual years. A comparison of the entire analysed period showed that the average top yield was significantly higher in the FYM treatment (+2.0 t ha−1) (Table 2). As mentioned above, the mineralization of manure in the soil strongly depends on the weather and other soil parameters [20]. The years 2016 and 2017 represent seasons with relatively good (2016) and good (2017) conditions, resulting in very high yields in the control (especially in 2017, these yields are very high for the unfertilized control treatment and we assume that they are the result of exceptionally good climatic conditions during the season), and visible effect of the FYM (especially in 2016). The extremely unfavourable weather conditions in 2018 reduced beetroot yields by 43% and top yields by 69%. The explanation for the higher yields in the FYM treatment lies both in the direct supply of nutrients through mineralization and the course of mineralization. According to Barłóg et al. [8], three main periods of beetroot yield formation can be distinguished, i.e., early, midseason, and final period, with N requirements dominating in the first two stages. The FYM is a fertilizer with a high C/N ratio (in comparison with slurries), and contains a high amount of organic N that is not directly available to plants [20], therefore, FYM releases its nutrients slowly and over a longer period, covering critical periods of beet formation (if weather conditions allow it). A similar effect of FYM on sugar beet yield was published in [21]. According to their results, the yield response to different manure ratios across two years plateaued at 23 t ha−1, with a corresponding beet yield of 62.2 t ha−1. The sugar content (SC, %) was not

affected by the FYM application. Both treatments (control and FYM) varied from 19.9% to 20.1%. The same situation happened in the case of other chemical elements in the beetroots and tops (Tables 5 and 6), therefore, the application of the FYM provided higher yields and, consequently, a higher amount of sugar harvested from the field, without significant changes in sugar beet chemical composition. This also applies to the comparison of all other fertilization treatments (Tables 5 and 6), except P concentration in sugar beet tops (Table 6), where P concentration slightly increases in NPK4 and all FYM + NPK treatments.

Application of mineral fertilizers significantly increased beetroot and top yield (Table 3). This result is expected and is in line with other published results [6,9,14,21], as N is the most important element for sugar beet and mineral fertilizers provide readily available N in precisely definable amounts. In our research, it was rather crucial to recommend an average dose of nitrogen that provided the best results during the years including both, the standard and the dry weather conditions. In the case of mineral fertilizers applied without organic manures, it is relatively simple and our results are comparable with other recommendations [12,15,16], while lower than recommendations of [13,14], but every experiment provides site-specific recommendations concerning soil and climate conditions of the site. In our case, a dose of 122 kg ha−<sup>1</sup> N represents a breakpoint between the linear and plateau functions of the developed model, with the corresponding beetroot yield of 66 t ha−1. Application of N above this value does not increase the beetroot yield significantly.

The combined application of FYM and mineral fertilizers (FYM + NPK treatments) had a different course each year. In years with good climatic conditions, the beetroot yield fertilized with the FYM was comparable (2016) or slightly lower (2017, only FYM + NPK4 treatment provided significantly higher yields as compared with the FYM treatment) than in the FYM + NPK treatments (Table 4). A significant difference only became apparent with the advent of drought in 2018, when treatments fertilized with FYM + NPK provided higher yields than treatments fertilized only with FYM. In years with a normal course, manure could cover the demands of beets during the season and provided very good yields. However, in the event of a drought, the efficiency of manure decreased as it responded more sensitively to unsuitable climatic conditions. The positive benefit of mineral fertilizers was also manifested in the case of the tops. The combined application of FYM and mineral fertilizers provided, on average, higher yields than the application of FYM alone. These results were predictable. For this article, it was more important to analyse the response of beetroots and tops to the dose of nutrients and to determine the recommended dose. From this point of view, it is interesting that the breaking point of the linear-plateau model occurred at the same value as in the NPK treatments, i.e., 66 t ha−1, but the amount of nitrogen increased to 165 kg ha−<sup>1</sup> (+53 kg ha−<sup>1</sup> N as compared with the NPK treatments). The same situation occurred in the case of the beet tops, where the break occurred at a yield of 24 t ha−1, and at a dose of 181 kg ha−<sup>1</sup> N (+59 kg ha−<sup>1</sup> N as compared with the NPK treatments). According to the data, the combined application of FYM and NPK did not bring any massive improvement in yields as compared with NPK or FYM applied alone, showing that maximum yielding potential of the sugar beet was reached under local soil-climate conditions. According to [8], the maximum yield potential of sugar beet in Europe is between 110 and 150 t ha−<sup>1</sup> (calculations based on [7]), and around 80 t ha−<sup>1</sup> in Poland, but the farmers' share of the actual yields is only 50 or 60% of that value.

According to the MANOVA results, both, beetroot and top yields were mainly affected by the weather conditions, while the effect of the fertilizer treatment was minor. This was mainly due to the extraordinary dry year in 2018. There is an increasing number of dry years and their frequent occurrence and weather instability, generally, are associated with the current global change in climate conditions. These extreme years should not be surprising in the coming period. The farmers in Europe are already adapting their approaches to this fact by selecting other crop varieties and species and adjusting the timing of cultivation [37].

A slightly different situation is found in the case of soil parameters. Application of NPK without organic manures resulted in generally lower pH values as compared with the control and FYM treatments (Table 7). The applications of FYM + NPK treatments resulted in between these two groups, which mean that FYM reduces the negative impact of NPK on soil pH. The same results were published by [24,38]. By affecting the value of the soil pH, organic manures also modify the environment for and activity of the microbial community in the soil [24], and the availability of nutrients. The concentration of soil N was not affected significantly by the fertilizer treatment, only high doses of applied mineral N (FYM + NPK4) resulted in significantly higher N concentration as compared with the control. In the case of P and K, the highest concentrations of both elements were recorded in FYM + NPK treatments, The combination of FYM with NPK significantly increased the soil C content. This result is in agreemen<sup>t</sup> with the results published by [24,39,40]. On the one hand, application of mineral fertilizers without manures can decrease soil carbon content when the C inputs to the soil from arable crops (including straw, roots, and post-harvest residues) are lower than the C decomposed by the soil microbial community. On the other hand, organic manures contain organic matter that directly affects the physiological, chemical, and biological properties of the soil. From this point of view, the combined application of FYM and mineral fertilizers results in maintaining soil fertility and is a sustainable approach to soil care [24,26,39].
