Effect of Nitrogen and Sulphur Fertilization on Winter Oilseed Rape Yield

Abstract

Oilseed rape is one of many crops with high nutritional requirements, particularly for nitrogen (N) and sulphur (S). Both macronutrients affect important physiological plant functions and are essential for the proper growth and development of oilseed rape. The objective of the experiment was to investigate the impact of nitrogen and sulphur fertilization on the yield of the winter oilseed rape cultivar LG Absolut. The experiment was conducted during the 2019/2020, 2020/2021, and 2022/2023 growing seasons on Haplic Cambisol soil formed from loess, with medium levels of mineral nitrogen and sulphur. In the experiment, two nitrogen fertilization treatments (150 and 200 kg ha⁻¹) were compared in combination with three additional sulphur fertilization rates (20, 40, and 60 kg ha⁻¹). The results demonstrated that the effectiveness of N and S fertilization varied between individual years. On average, the highest seed yields were obtained with the application of 200 kg N ha⁻¹ combined with sulphur, regardless of the rate. This was attributed to a significant increase in soil–plant analysis development (SPAD) values, the number of pods per plant, and the thousand-seed weight. The increase in seed yield with the higher nitrogen rate without sulphur ranged from 0.36 to 0.57 t ha⁻¹ compared to the lower rate (control 150 N kg ha⁻¹). Supplementary sulphur fertilization increased seed yield ranging from 0.22 to 0.76 t ha⁻¹. The protein content in the seeds increased, while the fat content decreased, following the application of the higher nitrogen rate. The decrease in fat content was mitigated by higher rates of sulphur. The application of 60 kg S ha⁻¹ yielded similar results of the tested parameters to the lower rates. Therefore, for soils with moderate levels of mineral nitrogen and sulphur, it is recommended to fertilize winter oilseed rape with 200 kg N ha⁻¹ combined with 20 or 40 kg S ha⁻¹.

1. Introduction

Winter oilseed rape (Brassica napus L. var. napus) is one of the most important oilseed crops in the world. Like other species in the family Brassicaceae, it has a high demand for nitrogen and sulphur [1,2,3,4]. In some regions, soil sulphur depletion has led to reduced yields and lower seed quality in oilseed rape. Consequently, fertilization with this macronutrient is frequently included in agricultural experiments [5,6,7]. Grant et al. [8] and Filipek-Mazur et al. [9] reported that sulphur deficiency was primarily due to the cultivation of high-yielding varieties, reduced sulphur content in the atmosphere, and decreased mineralisation of sulphur from soil organic matter. Therefore, they believed that sulphur fertilization should be tailored to the requirements of the crop, soil properties, and weather conditions, while also considering the potential environmental risks.

In Poland, sulphur has been regarded as an important macronutrient for plants, but research has primarily focused on its excess in the soil or its adverse environmental effects [10]. Currently, the situation has changed, and sulphur is increasingly recommended for crop fertilization, especially for oilseed rape [11,12]. Podleśna [13] demonstrated that sulphur fertilization was essential for the proper growth and development of winter oilseed rape. The application of S resulted in a significant increase in the seed yield compared to the control. Grant et al. [8] demonstrated that sulphur rates ranging from 15 to 60 kg ha⁻¹ were sufficient for the proper growth of oilseed rape. On the other hand, Stepaniuk and Głowacka [14] reported that seed yield was influenced by the sulphur rate (20–60 kg ha⁻¹), application date (autumn or spring), and method (soil or foliar application). Moreover, they demonstrated that sulphur fertilization increased straw yield, particularly following foliar application. In contrast, in a study by Losak [15], sulphur fertilization had no impact on straw yields of oilseed rape.

Stanisławska-Glubiak et al. [16] recommended using sulphur-containing fertilizers with additives that reduce their acidifying effect. Interesting research by Dłużniewska and Kulig [17] showed that weather conditions (precipitation) and nitrogen fertilization increased the incidence of fungal diseases in plants, while sulphur exerted a protective effect against downy mildew (Peronospora parasitica) and verticillium wilt (Verticillium dahliae). In this context, Janzen and Bettany [18] noted that applying excessively high nitrogen rates relative to sulphur resulted in inefficient nitrogen utilization. They estimated the optimal ratio of available nitrogen to sulphur in the soil to be 7:1. On the other hand, Zhao et al. [19] demonstrated that increasing the nitrogen rate above 150 kg ha⁻¹ did not affect seed yield in the absence of sulphur fertilization. Meanwhile, Wielebski et al. [20] demonstrated a significant interaction between nitrogen and sulphur, where the recommended sulphur rate at moderate nitrogen fertilization (160 kg ha⁻¹) amounted to 30 kg ha⁻¹. When a high nitrogen rate (220 kg ha⁻¹) was introduced, an application of 60 kg S ha⁻¹ was considered appropriate. Additionally, sulphur fertilization consistently improved nitrogen efficiency (3.5—55%). Hammac et al. [21] found that the effects of nitrogen and sulphur fertilization were inconsistent, influenced by variable precipitation and air temperatures during the study years. Šiaudinis and Butkutė [22] also reported that oilseed rape yields depended mainly on habitat conditions. The highest yields (average 2.98 t ha⁻¹) were obtained in a wet and moderately cool year.

Nemeth et al. [6] demonstrated that with sufficient sulphur availability for oilseed rape, seed yield was mainly determined by nitrogen fertilizer. Therefore, they recommended conducting soil chemical analyses before making fertilization decisions, although this is more challenging for sulphur [8,23]. Additionally, they advised monitoring plant conditions during the growing season, including satellite data [24]. According to other authors [25,26,27,28], additional leaf diagnostics for oilseed rape are necessary to detect a deficiency or an excess of nutrients, e.g., sulphur. However, as noted by Matula and Pechová [29], large fluctuations in sulphur content in the leaves could lead to misinterpretation, depending on the plant’s developmental stage. This also applies to the analysis of young or older leaves [30]. It should be noted that the uptake of nitrogen and sulphur depends on the genotype, and new varieties of oilseed rape typically have higher requirements for these macronutrients [31].

Castellano and Dick [32] demonstrated that the effectiveness of sulphur fertilization depended on the form of the applied fertilizer. Withers and O’Donnell [25] and Santos et al. [33] showed that ammonium sulphate provided the best results for oilseed rape. As a result, they obtained the highest increase in thousand-seed weight and seed yield.

Fertilizing oilseed rape with nitrogen and sulphur increases seed yield and often also improves seed quality. Spasibionek et al. [34] reported that increasing nitrogen rates (100, 160, and 220 kg ha⁻¹) led to a decrease in fat content but an increase in protein content in oilseed rape seeds. In contrast, applying sulphur at a rate of 30 kg ha⁻¹ increased fat content but decreased protein content. Ahmad et al. [35] found that fat content in oilseed rape seeds increased with the application of 20 kg ha⁻¹ of sulphur, whereas higher sulphur rates were unjustified due to an increase in glucosinolate levels. Barczak et al. [36] demonstrated that spring oilseed rape fertilized with nitrogen and sulphur produced a slightly higher fat yield. Moreover, soil application of sulphur was more beneficial than its foliar use. Gugała et al. [37] reported that certain foliar fertilization treatments reduced the quality of oilseed rape seeds. After the application of amino acids or amino acids combined with sulphur and boron, the latter authors observed an increase in glucosinolate levels.

In agricultural practise, winter rapeseed requires macronutrients, primarily nitrogen, potassium, and phosphorus fertilization, but also magnesium and sulphur. Sulphur fertilization can be divided into pre-sowing and spring, but it should be remembered that it should be synchronized with nitrogen fertilization. The study aimed to determine the optimal nitrogen and sulphur fertilization rates for oilseed rape (cultivar LG Absolut) on soil with moderate levels of both macronutrients.

2. Materials and Methods

2.1. Experimental Conditions

The field experiment was conducted at the Podkarpackie Agricultural Advisory Centre in Boguchwała, Poland (21°57′ E, 49°59′ N). The research on winter oilseed rape was carried out over three growing seasons: 2020/2021, 2021/2022, and 2022/2023. Differential nitrogen and sulphur fertilization was a factor in the experiment:

A—nitrogen (150 kg ha⁻¹), control;

B—nitrogen (200 kg ha⁻¹);

C—nitrogen + sulphur (150 kg ha⁻¹ + 20 kg ha⁻¹);

D—nitrogen + sulphur (200 kg ha⁻¹ + 20 kg ha⁻¹);

E—nitrogen + sulphur (150 kg ha⁻¹ + 40 kg ha⁻¹);

F—nitrogen + sulphur (200 kg ha⁻¹ + 40 kg ha⁻¹);

G—nitrogen + sulphur (150 kg ha⁻¹ + 60 kg ha⁻¹);

H—nitrogen + sulphur (200 kg ha⁻¹ + 60 kg ha⁻¹).

The cultivar LG Absolut (Limagrain Europe S.A.S.; Saint-Beauzire, France) was selected for the experiment, as it is recommended for cultivation in this study area. This is an F₁ hybrid with high resistance to fungal diseases and a strong yield potential. A one-factor experiment was performed in four replicates in a randomized block design.

2.2. Soil and Weather Conditions

According to the FAO/WRB classification [38], the soil was Haplic Cambisol formed from loess. Soil samples for analysis were collected in the autumn before nitrogen fertilization. The soil had a slightly acidic pH (5.6–5.9 mol/L KCl), a moderate humus content (1.1–1.4%), and a moderate level of mineral nitrogen (67.2–79.3 kg ha⁻¹). The content of available phosphorus (172–188 mg kg⁻¹ soil) and potassium (214–228 mg kg⁻¹ soil) was high, while the levels of magnesium (53–58 mg kg⁻¹ soil) and sulphur (22–25 mg kg⁻¹ soil) were moderate (

Table 1. Chemical soil properties before the field experiment (0-30 cm).

Parameter	Unit	2020	2021	2022
Humus	%	1.4	1.1	1.2
pH in 1 mol/L KCl	-	5.9	5.7	5.6
Nmineral	kg ha−1	67.2	79.3	75.5
P2O5	mg kg−1 soil	188	179	172
K2O		228	224	214
Mg		58	55	53
S-SO4		24	25	22

). The soil was analyzed at the accredited laboratory of the Regional Chemical and Agricultural Station in Rzeszów, according to Polish standards (PN-ISO 10390: 1997, PB 18, 3rd edition of 31.07.2017, PN-R-04023:1996, PN-R-04022: 1996/Azl :2002, PN-R-04020: 1996/AzI :2004, PN-92/R-04017, PN-92/R-04016, PN-92/R-04019, PN-93/R-04021: 1994, PN-93/R-040 18).

The weather conditions were compiled using data from the Meteorological Station of the Podkarpackie Agricultural Advisory Center in Boguchwała, located approximately 0.5 km from the experimental field.

2.3. Experimental Design and Crop Management

The preceding crop was winter wheat, with the straw removed from the field. After harvesting, the field was disked, followed by a cultivator treatment. The area of a single plot was 15.0 m². Winter oilseed rape seeds were sown at a depth of 1.5 cm, with a row spacing of 25 cm.

In soil, sulphur occurs in organic and mineral forms. The average content of all forms (so-called total content) in arable soils in Poland ranges from 60 mg to 790 mg/kg. In some soils, it is therefore too low for the requirements of winter oilseed rape.

Before sowing, mineral NPK fertilization was applied. The nitrogen (ammonium nitrate—34% N), phosphorus (superphosphate—19% P₂O₅), and potassium (potassium chloride—60% K₂O) rates were 30, 80, and 110 kg ha⁻¹, respectively. In the sulphur-fertilized treatments (C–H), the nitrogen provided by the ammonium sulphate (21% nitrogen and 24% sulphur) was accounted for.

The seeds were treated with Scenic Gold (fluopicolide, fluoxastrobin) at a rate of 1 L/100 kg of seeds. Sowing was carried out on 20 August 2020, 19 August 2021, and 25 August 2022. The recommended sowing density for the cultivar LG Absolut is 40 seeds/m². Plant protection products were applied according to the pesticide manufacturer’s recommendations following the prior monitoring of the plantation. All chemical applications were carried out using a tractor-mounted sprayer, with a liquid volume ranging from 200 to 300 L ha⁻¹.

The developmental stages of the plants were determined according to the BBCH scale (Bundesanstalt, Bundessortenamt und Chemische Industrie) used in the European Union [39].

In the spring, nitrogen fertilization (ammonium nitrate—34% N) was applied in two stages: before the start of plant growth, at a rate of 70 kg ha⁻¹ (treatments A, C, E, G) or 90 kg ha⁻¹ (treatments B, D, F, H), and at the budding stage (BBCH 51), at a rate of 50 kg ha⁻¹ (treatments A, C, E, G) or 80 kg ha⁻¹ (treatments B, D, F, H). In the sulphur-fertilized treatments, the nitrogen provided by the ammonium sulphate was accounted for. Sulphur fertilization was applied in the autumn at a rate of 20 kg ha⁻¹ (treatments C and D), and additionally in the spring at a rate of 20 kg ha⁻¹ (treatments E and F) or 40 kg ha⁻¹ (treatments G and H).

Foliar fertilization on all plots was carried out using MIKRO PLUS™ (B, Cu, Fe, Mn, Mo, Zn) at the rates and dates recommended by the manufacturer.

2.4. Field and Laboratory Measurements

Soil–plant analysis development (SPAD) values were measured using a SPAD 502P chlorophyll meter (Konica Minolta, Japan) on 10 plants from each plot. Measurements were taken at the budding stage (BBCH 59) on the upper leaves.

The number of pods per plant, number of seeds per pod, and thousand-seed weight (TSW) were recorded for plants collected from 0.5 m². The seeds were harvested in the second half of July using a field harvester at full seed maturity. The obtained yield was adjusted to 1 ha, accounting for a moisture content of 9%.

2.5. Chemical Analysis of Seeds

Seeds for chemical analysis were collected from each treatment during harvest. The chemical composition of the seeds (total protein and crude fat) was determined using near-infrared spectroscopy with an MPA FT-LSD spectrometer (Bruker, Germany) in the Laboratory of Plant Production at the University of Rzeszów. The yield of both components per hectare was calculated based on the seed yield and the content of crude fat and total protein in the seeds.

2.6. Statistical Calculations

The results were statistically analyzed using TIBCO Statistica 13.3.0 software (TIBCO Software Inc., Palo Alto, CA, USA). The least significant difference was determined using Tukey’s test at p ≤ 0.05.

3. Results

The weather conditions (Figure 1) had a modifying effect on the growth and development of winter oilseed rape. In August 2021 and 2022, low rainfall hindered plant emergence. The weather conditions in the autumn were generally favourable for the growth of winter oilseed rape, although average temperatures were lower than the long-term data. In winter, air temperatures were above the long-term average, and this condition persisted through spring and summer each year. Rainfall was low in March 2021, as well as in May and June 2022. Conversely, rainfall in July exceeded the long-term average, complicating seed harvest, particularly in 2021.

Soil–plant analysis development (SPAD) measurements varied significantly with the nitrogen and sulphur fertilization applied. The SPAD results were inconsistent across seasons. In 2021 and 2023, the application of 200 kg N ha⁻¹ combined with 40 or 60 kg S ha⁻¹ produced the best results. In 2022, the results were confirmed for the same treatments, and additionally for treatment G (150 kg N ha⁻¹ combined with 60 kg S ha⁻¹). The lowest SPAD readings in all analyzed years were recorded for treatment A, with a rate of 150 kg N ha⁻¹. It can therefore be concluded that sulphur fertilization was justified, although its effectiveness depended on the nitrogen rate (Figure 2).

The number of pods per plant significantly increased with the application of treatments D, F, and H, which involved nitrogen application at 200 kg ha⁻¹ in combination with sulphur. Significantly lower results were recorded for the other treatments, particularly for fertilization with nitrogen only at 150 kg ha⁻¹ (Figure 3).

Applying sulphur with a lower nitrogen rate (150 kg ha⁻¹) was also beneficial, though it resulted in less optimal outcomes. The lower number of pods per plant recorded in 2022 was primarily due to a lack of rainfall in May and June.

Nitrogen and sulphur fertilization did not affect the number of seeds per pod (Figure 4). However, it should be noted that the measurements obtained in 2022 were the lowest. It should therefore be concluded that this yield component is the least modified by the experimental factor tested.

The application of nitrogen and sulphur significantly affected the thousand-seed weight (TSW) in all studied seasons. In 2021 and 2023, the most beneficial effect on TSW was achieved for a rate of 200 kg N ha⁻¹ combined with 40 or 60 kg S ha⁻¹. In 2022, TSW was significantly different in treatment A compared to treatments F and H (Figure 5).

Fertilization with a higher rate of nitrogen without sulphur (treatment B) significantly increased seed yield compared to a lower rate (treatment A). The observed differences in seed yields between the higher nitrogen rate (treatment B) and the lower rate (treatment A) were 0.43, 0.36, and 0.57 t ha⁻¹ in 2021, 2022, and 2023, respectively.

The additional application of sulphur was advantageous for both the lower and higher nitrogen rates. In 2022 and 2023, a higher nitrogen rate combined with 20 kg S ha⁻¹ (treatment D) resulted in effects comparable to those achieved with higher sulphur rates in treatments F and G. However, this trend was not observed in 2021. Fertilization with a lower rate of nitrogen (150 kg ha⁻¹), together with sulphur, had a favourable effect on seed yield, except in 2023, when higher rates of S (40 and 60 kg ha⁻¹) gave better results (Figure 6).

The difference in seed yield between treatments D, F, and H compared to nitrogen-only treatment B was 0.44–0.76 t ha⁻¹ in 2021, 0.30–0.45 t ha⁻¹ in 2022, and 0.45–0.60 t ha⁻¹ in 2023. The difference in seed yield between treatments C, E, and G compared to nitrogen-only treatment A was 0.42–0.61 t ha⁻¹ in 2021, 0.22–0.42 t ha⁻¹ in 2022, and 0.48–0.75 t ha⁻¹ in 2023.

It should be noted that the application of nitrogen at a rate of 200 kg ha⁻¹ without sulphur provided statistically similar results to the rate of 150 kg N ha⁻¹ combined with sulphur fertilization.

Fertilization with nitrogen at 200 kg ha⁻¹ or with nitrogen and sulphur at 200 kg ha⁻¹ and 20 kg ha⁻¹, respectively, reduced seed fat content compared to the treatments with a lower nitrogen rate. After applying 200 kg N ha⁻¹ together with 40 or 60 kg S ha⁻¹, no significant reduction in seed fat content was observed. The most favourable effect on fat content was observed with nitrogen application at a rate of 150 kg ha⁻¹, regardless of the sulphur rates.

The highest fat yield was obtained after the application of 200 kg N ha⁻¹ together with sulphur. However, a statistically similar result was achieved with the application of 150 kg N ha⁻¹ combined with higher sulphur rates (40 or 60 kg ha⁻¹).

The protein content in the seeds was significantly higher with the application of 200 kg N ha⁻¹ compared to the 150 kg N ha⁻¹ rate. Sulphur fertilization did not modify seed protein content in any treatment. However, the highest protein yield was obtained when 200 kg N ha⁻¹ was applied together with sulphur (

Table 2. Fat and protein content (% DM) in seeds and yield of both components (t ha⁻¹).

Tested Factor	Fat	Fat Yield	Protein	Protein Yield
Fertilization
A	47.5 ± 0.8 a	1.64 ± 0.3 d	19.6 ± 0.8 b	0.68 ± 0.3 c
B	46.2 ± 0.9 b	1.81 ± 0.3 bc	21.3 ± 0.9 a	0.83 ± 0.4 b
C	47.9 ± 0.8 a	1.83 ± 0.4 bc	19.7 ± 0.7 b	0.75 ± 0.3 bc
D	46.5 ± 0.8 b	2.00 ± 0.4 a	21.6 ± 0.7 a	0.93 ± 0.3 a
E	48.4 ± 0.7 a	1.91 ± 0.2 ab	19.9 ± 0.9 b	0.79 ± 0.4 b
F	47.1 ± 0.9 ab	2.10 ± 0.4 a	21.6 ± 0.9 a	0.96 ± 0.4 a
G	48.5 ± 0.7 a	1.96 ± 0.2 ab	19.9 ± 0.6 b	0.81 ± 0.2 b
H	47.0 ± 0.9 ab	2.12 ± 0.3 a	21.8 ± 0.7 a	0.98 ± 0.3 a
Years
2021	47.9 ± 0.8 b	1.97 ± 0.4 b	20.8 ± 0.6 b	0.86 ± 0.4 b
2022	45.5 ± 0.7 c	1.52 ± 0.3 c	21.7 ± 0.9 a	0.72 ± 0.4 c
2023	48.7 ± 0.9 a	2.30 ± 0.6 a	19.5 ± 0.5 c	0.92 ± 0.6 a

Results are expressed as mean values ± standard deviations. Mean values with different letters (a–d) in columns are statistically different (p < 0.05).

). Both the protein and fat content, as well as the yield of these components, varied in the study years.

4. Discussion

Winter oilseed rape has substantial nutrient requirements due to its high-yielding potential and elevated seed fat and protein contents. Nitrogen is most important in the fertilization of this species [1,2,3,40], but sulphur also plays a crucial role [5,7,41,42]. The results of field experiments with the N and S fertilization of oilseed rape are not always conclusive, particularly regarding the latter micronutrient [43]. Donald et al. [44] found no significant effect of sulphur on oilseed rape seed yield. This was likely due to the high availability of sulphur from the atmosphere during the wet year of the study. Zhao et al. [19] reported that when adequate sulphur levels were available in the soil, sulphur fertilization did not significantly affect seed yield, its components, or the protein and fat content in seeds. In contrast, Mahler et al. [41] demonstrated that the varieties ‘Bridger’ and ‘Cascade’ responded to sulphur fertilization; however, no reaction was observed in the variety ‘Dwarf Essex’.

The present study has demonstrated that winter oilseed rape requires both nitrogen and sulphur fertilization. The application of sulphur was particularly beneficial with higher nitrogen rates (200 kg ha⁻¹). However, the impact of nitrogen and sulphur fertilization was influenced by the weather conditions during the study years. Overall, seed yield was most favourably affected by nitrogen at 200 kg ha⁻¹ combined with sulphur at 20 or 40 kg ha⁻¹. A higher sulphur rate of 60 kg ha⁻¹ did not yield the expected results. This has been confirmed by other authors [45,46], who showed that the application of sulphur at a rate of 40 kg ha⁻¹ was particularly recommended at higher nitrogen rates. Zhao et al. [19] reported that applying a rate of 300 kg N ha⁻¹ and 50 kg S ha⁻¹ increased the yield of oilseed rape by 10.7% compared to nitrogen fertilization without sulphur. McGrath and Zhao [45] also demonstrated that the combined application of nitrogen (180 and 230 kg ha⁻¹) and sulphur (40 kg ha⁻¹) had the most beneficial effect on oilseed rape yield. As a result, these authors achieved an increase in seed yield ranging from 0.7 to 1.6 t ha⁻¹ compared to the control. However, seed yield did not increase in response to sulphur application when nitrogen was not applied or added at low rates (50 or 100 kg ha⁻¹). Hřivna et al. [47] demonstrated in a pot experiment that under low sulphur concentrations in plants, seed yield increased only up to a rate of 0.9 g N per pot, and then gradually decreased. Varényiová et al. [48] obtained the highest average seed yield (3.96 t ha⁻¹) with the application of 40 kg S ha⁻¹. However, increasing the sulphur rate to 65 kg ha⁻¹ led to an 11.4% reduction in yield compared to the lower rate.

The present research was conducted on soil with average levels of mineral nitrogen and sulphur. As a result, the increase in oilseed rape seed yield obtained after applying a higher nitrogen rate without sulphur compared to a lower rate ranged from 0.36 to 0.57 t ha⁻¹, depending on the year of the study. In contrast, varying sulphur fertilization led to an increase in seed yield ranging from 0.22 to 0.76 t ha⁻¹, depending on the nitrogen rate and year of the study. Withers and O’Donnell [25] demonstrated that the average increase in oilseed rape seed yield following sulphur fertilization ranged from 10% to 17% on soil with low sulphur levels. On calcareous soil without sulphur deficiency, the seed yield increased by only 8%. On the other hand, Balik et al. [49] and Malhi and Gill [50] demonstrated that oilseed rape fertilization with sulphur is justified even on fertile soils. The control treatment yielded 3.7 t ha⁻¹, whereas the treatment with nitrogen resulted in a 49% higher yield, and the treatment with both nitrogen and sulphur yielded a 60% increase. Ahmad et al. [51] concluded that sulphur fertilization was more effective when applied in two rates rather than one, e.g., before sowing and during the growing season. Hřivna et al. [47] and Groth et al. [52] showed that the results of oilseed rape fertilization experiments were dependent on location and weather conditions. Rathke et al. [53] have concluded that the optimal fertilization of oilseed rape depends on various factors, such as the preceding crop, agronomic practises, variety, site conditions, and the form of fertilizer used.

In our study, it was confirmed that weather conditions modified the effects of N and S fertilization in oilseed rape, particularly influencing yield components and seed yield in 2022 with low rainfall in May and June. Ma et al. [54] achieved the highest seed yield with a rate of 150 kg N ha⁻¹ combined with 20–40 kg S ha⁻¹, which is partly similar to the results of our research. Ahmad et al. [55] reported that the number of pods per plant and seed yield significantly increased following nitrogen and sulphur fertilization. However, thousand-seed weight decreased with rising nitrogen rates. In our study, no statistical evidence was found for a reduction in the thousand-seed weight under the influence of N and S fertilization. Jankowski et al. [46] demonstrated that nitrogen fertilization improved seed yield by increasing the number of pods per plant and thousand-seed weight. Sulphur fertilization also increased seed yield (0.85–0.90 t ha⁻¹), particularly in years with high autumn rainfall. Zhao et al. [19] demonstrated that nitrogen fertilization increased seed yield by enhancing the number of pods, while sulphur fertilization improved yield by reducing pod drop. In the experiment by Hřivna et al. [47], sulphur fertilization primarily increased the seed weight per pod, from 61.9 mg to 79.8 mg. Farooq et al. [56] concluded that the application of nitrogen (120 kg ha⁻¹) combined with sulphur at a rate of 40 kg ha⁻¹ increased the number of pods per plant (204), seeds per pod (23), thousand-seed weight (4.26 g), and seed yield (2289 kg ha⁻¹). Sienkiewicz-Cholewa and Kieloch [57] reported that a sulphur rate of 20 kg ha⁻¹ did not significantly affect the yield of oilseed rape. The results regarding the advisability of fertilization with nitrogen and sulphur are therefore ambiguous and require further research.

In our study, the application of a higher nitrogen rate (200 kg ha⁻¹) combined with sulphur positively influenced the number of pods per plant and thousand-seed weight but did not affect the number of seeds per pod compared to nitrogen fertilization alone. Ma et al. [58] indicated that yield components and seed yield of oilseed rape were mainly dependent on nitrogen fertilization, but in some years, sulphur fertilization also produced good results, leading to an increase in seed yield of 3–31%. Balint et al. [31] demonstrated that two oilseed rape varieties (Surpass 600 and 46C74) responded positively to nitrogen and sulphur fertilization. Therefore, these authors argued that further cultivar development in oilseed rape was necessary to improve nutrient utilization.

The soil–plant analysis development (SPAD) measurements performed in our study indicated a beneficial effect of N and S fertilization on the plants; however, the results were not reproducible across years. In 2021 and 2023, the application of 200 kg N ha⁻¹ combined with 40 or 60 kg S ha⁻¹ yielded the best results, and the plants were well nourished. In 2022, these results were confirmed for the same treatments, but additionally for treatment G (150 kg N ha⁻¹ combined with 60 kg S ha⁻¹). It should therefore be concluded that sulphur fertilization was justified, but its effectiveness depended on the nitrogen rate. Pużyńska et al. [59] found that sulphur and boron fertilization had a beneficial effect on the SPAD index. Higher values of this index were obtained in spring compared to autumn. This result was influenced by the rate of N applied in spring, which increased the chlorophyll content in plants. Jarecki [60] proved that the multi-component foliar fertilization of oilseed rape increased the SPAD measurement and other parameters compared to the control, while Ostrowska et al. [61] did not show a significant effect of nitrogen fertilization on the SPAD index.

The present study has shown that fertilization with nitrogen and sulphur modified the chemical composition of the seeds. The fat content in the seeds significantly decreased following nitrogen fertilization at a rate of 200 kg ha⁻¹, as well as with the combination of nitrogen and sulphur at rates of 200 kg N ha⁻¹ and 20 kg S ha⁻¹; however, higher rates of sulphur (40 or 60 kg ha⁻¹) mitigated the decrease in seed fat content. Protein concentration in seeds was significantly higher after nitrogen fertilization at a rate of 200 kg N ha⁻¹ compared to a rate of 150 kg N ha⁻¹, while sulphur fertilization did not affect seed protein content. Several studies [19,35,52,62] have shown that higher nitrogen rates decrease fat content while increasing protein levels in oilseed rape seeds. Losak [15] indicated that sulphur fertilization had a positive effect on the fat content in oilseed rape seeds, but the increases were not significant compared to the control. Kumar et al. [63] demonstrated that sulphur fertilization did not affect the protein and fat content; however, increasing nitrogen rates significantly elevated protein content while reducing fat proportion in oilseed rape seeds. Jan et al. [64] achieved a significant increase in fat and protein content in oilseed rape seeds by applying a rate of 40 kg S ha⁻¹, whereas a higher rate (60 kg S ha⁻¹) did not further increase the levels of these components. Varényiová et al. [48] reported that the average fat content in oilseed rape was 45.1, 45.5, and 44.0% after the application of sulphur at rates of 15, 40, and 65 kg ha⁻¹, respectively. In contrast, Jankowski et al. [42] showed that sulphur fertilization did not cause significant changes in the seed fat content, but they observed a trend towards increased protein content. McGrath and Zhao [45] concluded that a greater effect of sulphur on seed fat content could be expected in soils with a significant deficiency of this macronutrient.

Spasibionek et al. [34] and Sikorska et al. [65] indicated that the chemical composition of oilseed rape seeds was significantly dependent on weather conditions, i.e., temperature and rainfall. Šiaudinis [66] recorded the highest protein content in seeds during a warm and dry year. This was confirmed by our research, which demonstrated that both fat and protein content in oilseed rape seeds varied across the study years. A higher protein content was obtained in the dry and warm year (2022), while a higher fat proportion was observed in the wetter and cooler year (2023). Poisson et al. [67] concluded that nitrogen and sulphur fertilization should be applied in appropriate N:S ratios. Excessive application of one macronutrient relative to the other can exert an antagonistic effect.

In the present study, the highest fat and protein yields were obtained with the application of 200 kg N ha⁻¹ combined with sulphur. The fat yield was also positively affected by a lower nitrogen rate (150 kg ha⁻¹) when combined with higher sulphur rates (40 or 60 kg ha⁻¹). On average, during the study years, the fat yield was 1.93 t ha⁻¹, and the protein yield was 0.83 t ha⁻¹. Varényiová et al. [48] reported that the average fat yield was 1809, 1802, and 1595 kg ha⁻¹ with sulphur fertilization at rates of 15, 40, and 65 kg ha⁻¹, respectively. Fismes et al. [68] concluded that despite the decrease in seed fat content resulting from high nitrogen rates, the fat yield did not decline because the total seed yield increased.

It must therefore be concluded that fertilization with sulphur at a rate of 60 kg ha⁻¹ produced lower results than expected, which may have been due to the moderate levels of this macronutrient in the soil. Therefore, for soils with moderate levels of mineral nitrogen and sulphur, it is recommended to fertilize winter oilseed rape with 200 kg N ha⁻¹ combined with 20 or 40 kg S ha⁻¹.