**1. Introduction**

Sugar beet is one of the most important crops in the EU, as it is the only raw material for sugar extraction. Sugar beet acts as a good breaker of cereal crop rotations in the field and is also a good pre-crop for cereals (except for spring barley [1]), which are the most abundant arable crops in the EU. During most of the 20th century, sugar beet was a strategic crop in the Czech Republic. With the change from a centrally planned economy to a market economy in 1989, followed by the application of EU quotas restricting beetroot yields, sugar beet has undergone significant changes both in regards to the size of sown areas and in yields per hectare. Today, sugar beet is grown on an average area of 61,000 ha in the Czech Republic.

Beetroot and top yield and the quality of sugar beet are affected by a wide range of factors. Some of these factors are controllable by the farmers, such as crop rotation [2], tillage practices [3–6], or fertilization, however, some of them are not, such as weather conditions [7]. Fertilization represents a crucial factor influencing the final yield and quality, especially fertilization with nitrogen (N). The under application of N leads to a lower yield of beetroots and lower sucrose yield, while an over–application of N leads to imbalanced partitioning of assimilates, decreased sucrose content, and increased concentrations of impurities, resulting in reduced sucrose extraction [3,8–12] due to higher water retention by the beetroots and a lower amount of dry matter. An over–application of N also increases the concentration of soluble N compounds in the beetroots and this prevents subsequent extraction of sugar.

The determination of the optimal nitrogen dose varies from site to site, and therefore is site-specific dose. According to Chatterjee et al. [12], a single dose of 146 kg ha−<sup>1</sup> of N was recommended in North Dakota and Minnesota for sugar beet, irrespective of soil type and soil organic matter content, but this recommendation should be lowered to 112 kg ha−<sup>1</sup> of N, based on their two years of research. According toDeBruyn et al. [13], a dose of 157 kg ha−<sup>1</sup> of N was associated with the highest beetroot yield, while 136 kg ha−<sup>1</sup> of N was associated with the highest profit, in their three year experiment in Canada. In Europe, much attention is being paid to sugar beet nutrition experiments. Islamgulov et al. [14] experimented with the hybrid Hercules and found that 160 kg ha−<sup>1</sup> of N provided the highest economic efficiency under the conditions of the middle Cis-Ural region. According to Malnou et al. [15], who analysed the response of sugar beet to N fertilization at five sites within the UK, a dose of 100 kg of N per ha, in the absence of organic manure, should be applied for maximum yield. Similar results (100–110 kg ha−<sup>1</sup> of N) were published by Jaggard et al. [16], who analysed 161 experiments from England in their meta-analysis. The optimal dose can be determined by modelling. There are several models applicable depending on the crop evaluated, the data obtained, and the answer to the question being asked [17]. The quadratic model offers an answer to the maximum yield depending on the dose of nutrients. This model is very suitable for winter wheat because, with an increasing dose of nitrogen, wheat yields initially increase and begin to decline after reaching a critical value [18]. However, determining the dose of nutrients, in this way, may not be statistically significantly different from the lower dose of applied nutrients. Not every crop follows a parabolic course for the dependence of yields and doses of applied nutrients. For example, the reaction of sugar beet yields on doses of nitrogen may be linear [19], even the differences between the analysed fertilizer treatments are not significant. In that case, a linear-plateau model can provide useable answers [12,17,19].

Previously, the sugar beet crop, in the Czech Republic, was commonly fertilized with organic manures. We deliberately state "previously", because today's situation is completely different. There is a shortage of organic manure due to a reduction in animal production and there has been a significant split between animal and plant production, manifested by an insufficient amount of organic matter incorporated into the soil. The common doses of farmyard manure applied directly to potatoes and sugar beet range from 20 to 40 tons per hectare in the Czech Republic. As compared with mineral fertilizers, the content of nutrients in organic manures is non-standardized. Thus, the nutrient content may vary, depending on the animals from which it came, their diet, and other aspects. The mineralization process is also strongly dependent on weather conditions [20], and therefore farmers may not know exactly how much nutrients they applied to the soil, which may explain the recommendation to not use farmyard manure for sugar beet fertilization [21,22]. However, rising prices for mineral fertilizers [21] and the practice of growing sugar beet for the organic market [23] have increased the interest in the application of organic manures to sugar beet, especially in the USA, because the application of manures directly to sugar beet has a long tradition in Europe. Organic manures work in two ways. The first way represents direct releasing of nutrients into the soil environment through the process of mineralization. The second way represents the beneficial influence on the soil's physical, chemical, and biological properties [24–29], especially maintaining and increasing soil organic carbon (SOC) content. This indirect positive effect of livestock manure on crop yields was evidenced by Hlisnikovský et al. [18].

Concerning the issues discussed above, we analysed a three-year sequence in a longterm field experiment, and focused on how mineral fertilizers (different doses, NPK1–4), farmyard manure (FYM), and combinations of FYM and NPK (FYM + NPK1–4) affected the yield and quality of sugar beet beetroots and tops. In this paper, we also recommend the dose of fertilizers according to the linear-plateau regression model. The evaluation included three years (2016, 2017, and 2018). All three years were characterized by different weather conditions. The year 2016 was very warm and very dry, but with relatively good conditions for sugar beet. The year 2017 was warm, with normal precipitation. The year 2018 was extraordinary warm and very dry, significantly affecting sugar beet beetroot and top yields, therefore, in our experiment, we covered the unfavourable conditions

that occurred more frequently and were connected with global weather change. Finally, an analysis of soil properties affected by the fertilizer treatments is also provided.
