Impact of Nitrogen and Sulphur Fertilisation on Phosphorus and Silicon Content and Uptake by Biomass of Spring Wheat

Abstract

Nitrogen and sulphur are among the most important plant nutrients (along with C, H, and O) and the main elements comprising the organic substance of plants. In this study, it is assumed that light soils (Cambisols) do not naturally meet the nitrogen and sulphur needs of spring wheat and, consequently, impact the phosphorus and silicon content in the plant biomass. Therefore, to determine the effect of N and S on the content and uptake of these elements at specific growth stages (BBCH 30–31: in leaves, BBCH 55–59: in whole plants, BBCH 89–90: in grain and straw), a three-year field experiment was conducted using different doses of nitrogen (0, 40, 80, and 120 kg ha⁻¹) and sulphur (0, 50 kg ha⁻¹). The results show that fertilisation with N and S had a significant effect on increasing the content and uptake of P and Si by phytomass in the phenostages studied. In general, as the N fertilisation dose increased, the yields of phytomass and grain increased. A beneficial effect of S on increases in green weight, straw, and spring wheat grain was found. A significant effect of N and S fertilisation on the growth of the Si:P ratio in individual parts of plants in the studied stages was also observed. A significant positive correlation between P and Si content was proven, indicating that the two elements do not act antagonistically towards each other. In contrast, a negative correlation was observed between the P content in plants and their Si uptake. Si is taken up more strongly by plants under conditions of N and S fertilisation, as evidenced by the increase in the Si:P ratio and the fact that plants accumulated on average 3.5 times more Si than P. The highest Si content was found in the green parts of plants in the BBCH 30–31 and BBCH 55–59 stages, while in BBCH 89–92, straw had nearly half that amount and grain contained a thousand times less silicon.

1. Introduction

Common wheat (Triticum aestivum ssp. vulgare) is a genetic hexaploid that originated in Central Asia. It was domesticated in 6000 BC and remains a staple grain in the diet of Homo sapiens to this day. It is has as many as 50,000 cultivars globally [1]. In 2024, global wheat production was 798,482 thousand tonnes. In Poland, which ranks 14th in the world in terms of wheat cultivation area, wheat accounts for 21.6% of the crop structure and is cultivated on 2,386,000 ha of land, including 170,000 ha of spring cultivars. The average grain yield in 2024 was 5.2 t ha⁻¹ [2]. The Descriptive List of Agricultural Plant Cultivars [3] currently includes 60 cultivars of common spring wheat. Wheat grain is used for human consumption and animal feed. The quality of flour depends on the utility value of the grain, which in turn depends on the method and conditions of its production [4].

To date, no published work has presented such a detailed description of the effects of nitrogen and sulphur fertilisation on the phosphorus and silicon content in spring wheat organs. Of particular importance is the fact that these studies examine phosphorus and silicon content in wheat organs collected at several key stages of plant development. In the 1980s, sulphur and sulphur compounds were perceived to have an exclusively adverse impact on ecosystems [5,6]. Now, sulphur deficiency in soils in most countries has led to the use of sulphur fertilisers to increase agricultural production and yield quality [7,8,9,10,11,12]. The second nutrient analysed in this study, nitrogen, is also very important in plant production, as it has the greatest impact on the size and quality of the harvested yield [13,14,15,16].

The inclusion of sulphur in the basic fertilisation of cereals with nitrogen optimises yields by increasing unit productivity and grain quality [17,18]. Sulphur application has been shown to increase the quality, nutrient and protein content of flour [19,20,21,22,23,24]. Properly balanced S and N fertilisation is important due to their joint interactions during uptake and assimilation into the plant [25,26,27,28]. In addition to sulphur, calcium, phosphorus, potassium, and silicon play a particularly important role in building plants’ resistance systems [29,30,31,32,33].

Alongside nitrogen and potassium, phosphorus is a key nutrient that is crucial not only for the proper growth and development of plants but also for the quantity and quality of their yield. It cannot be replaced by other elements [34,35]. Thanks to its presence in phospholipids (and other compounds), it performs structural and storage functions (phytin), and thanks to its ability to influence gene expression, it performs regulatory functions [36]. Phosphorus participates in cellular metabolism directly (e.g., in the form of sugar phosphates) and indirectly (e.g., by regulating enzyme activity during phosphorylation and dephosphorylation). Furthermore, as a component of nucleic acids (DNA and RNA), it participates in the transmission of genetic information, and through its presence in ATP, it participates in energy storage [37]. The form of phosphorus that is directly available to plants is inorganic phosphorus in the form of phosphate ions (H₂PO₄⁻² or HPO₄⁻²) present in the soil solution, referred to as Pi (inorganic P). These ions are mainly absorbed by the cells of the root hair zone. Phosphorus uptake by the plant generally continues throughout the its growth, although it is not uniform throughout the growing season, and there are periods of weaker and stronger uptake of this nutrient. Phosphorus is first accumulated in the vegetative parts of plants (stems and leaves), and in the generative stage, it is transferred to grain, seeds, or fruits thanks to its ability to efficiently remobilise [38].

Silicon (Si) is the second most abundant element in the Earth’s crust after oxygen (28.8%), although it belongs to a group of elements that are neither macro- nor microelements [39]. Silicon occurs in the soil in the form of various minerals, aluminosilicates, sodium, calcium and potassium silicates, as well as silica SiO₂; these forms are very difficult to dissolve and resistant to weathering. Depending on its origin, each kilogram of soil contains between 50 and 400 g (5–40%) of silicon dioxide. The only form of silicon available to plants in soil solutions is orthosilicic acid, H₄SiO₄, also known as monosilicic acid, the content of which is low and often insufficient for plants. Silicon uptake by plants increases under the following conditions: (1) increased soil moisture, (2) increased microbial activity, and (3) improved nitrogen supply to plants [40]. Plants absorb silicon from the soil solution through both passive and active mechanisms [41]. Cereals accumulate the most silicon, reaching a content of about 3%, while dicotyledonous plants accumulate only 0.5% of this element. The total silicon content in a plant depends on the species [42]. The highest amounts of silicon, i.e., above 1% of dry weight, are accumulated by plants called silicon “accumulators”, represented by species that are leaders in global agricultural production, such as rice, sugar cane, wheat, and beet [43,44]. For years, soil and foliar fertilisers containing silicon have been used in countries with temperate and tropical climates [45,46]. Plants accumulate the largest amounts of silicon in their leaves, where it is stored directly under the epidermal layer in the form of amorphous polymers SiO₂ × nH₂O (silica gel), forming the so-called insoluble cuticular silicon layer. Silicon also accumulates in large quantities in roots, root hairs, and cereal husks. The silicon layer under the cuticle strengthens plant tissues and acts as a specific protective mechanical barrier, which protects tissues against the penetration of pathogens and feeding by pests, strengthens the cell walls of the epidermis and increases plant resistance to lodging, increases photosynthetic efficiency by improving leaf exposure to sunlight, and reduces epidermal transpiration, protecting plants against water loss [47,48,49,50,51]. Silicon also reduces the effects of excess manganese, aluminium, and iron, as well as zinc deficiency, especially when phosphorus is in excess [40].

Silicon enhances phosphorus uptake, primarily by increasing soil P availability [33]. Lumsdon and Farmer’s [52] research proves that there is strong competition between phosphate (H₂PO₄) and silicon ions (H₃SiO₄) for sorption sites in soil. Adsorbed phosphorus is displaced by silicon in the soil solution and becomes available to plants. Silicon compounds optimise soil fertility by improving its water, and physical and chemical properties and maintaining phosphorus in a form available to plants. Other studies show that silicon fertilisation induces certain changes in root morphology [53,54]. Consequently, greater root mass distribution, root length, root surface area, and number and length of root hairs and lateral roots have a beneficial effect on the plant, increasing the uptake of several nutrients, including phosphorus [55,56].

According to the literature and our own previous research, excessive nitrogen fertilisation (overfertilization) leads to soil degradation, weakening and damaging plants. Excess nitrogen is harmful to the environment, humans, and animals, as well as to the profitability of agricultural production. Sulphur deficiency in agricultural production can also cause serious economic and ecological problems. Plants lacking sulphur contain higher amounts of harmful, soluble nitrogen compounds (nitrates, nitrites, and nitrosamines). Sulphur deficiency in plant tissues increases their susceptibility to stress factors. The addition of S increases N utilisation from the soil, thereby reducing N volatilisation and leaching into groundwater [5,6,7,8].

The existing literature on this subject shows a lack of studies on the content of phosphorus and silicon and their interaction in the individual development stages of spring wheat under conditions of nitrogen and sulphur fertilisation. To bridge this research gap, a three-year field experiment was conducted using different doses of nitrogen (0, 40, 80, 120 kg ha⁻¹) and sulphur (0, 50 kg ha⁻¹). In this study, the effect of fertiliser nitrogen and sulphur on the content and uptake of phosphorus and silicon by spring wheat biomass and the Si:P ratio in individual growth stages (BBCH 30–31 in leaves, BBCH 55–59 in whole plants, and BBCH 89-9 in grain and straw) was analysed.

2. Materials and Methods

2.1. Site Description

The field experiment was conducted in the village of Malice, which is situated in southeastern Poland. Its precise location is defined by the following coordinates: east longitude 23°45′ E and north latitude from 50°42′ N. The following macroregions are distinguished in the Zamość region: the Lublin Upland, Roztocze, the Sandomierz Basin, Volhynian Polesie, the Zachodnio-Wołyńska Upland, and Pobuże [57].

Data on meteorological conditions are described in

Table 1. Weather conditions during the vegetative period in the research years 2009–2011, and the long-term averages from 1971–2011 (Meteorological Station in Zamość).

Season	Sum–Mean (April–August)
	April	May	June	July	August	k *	p	t
2009	0.5	2.4	2.1	0.4	0.8	1.3	349.1	2652
2010	1.1	2.0	1.1	2.1	1.3	1.6	443.4	2715
2011	1.1	0.7	1.0	2.7	2.3	1.6	414.6	2581
1971–2011	1.7	1.5	1.5	1.6	1.1	1.5	367.3	2336

* k—Selyaninov’s hydrothermal coefficient [k = (p × 10)∑t)], according to Skowera et al. [58]. Ranges of values (k) were classified as follows: k ≤ 0.4—extremely dry; 0.4 < k ≤ 0.7—very dry; 0.7 < k ≤ 1.0—dry; 1.0 < k ≤ 1.3—relatively dry; 1.3 < k ≤ 1.6—optimal; 1.6 < k ≤ 2.0—relatively humid; 2.0 < k ≤ 2.5—humid; 2.5 < k ≤ 3.0—very humid; k > 3.0 extremely humid. p—precipitation (mm); t—temperature (°C).

. Using data on rainfall and air temperature during the vegetation period (April–August), Selyaninov’s hydrothermal coefficient was calculated (Table 1). According to the calculations, the weather conditions in the 2009 growing season were defined as dry verging on optimal (1.3), while the weather conditions during the other two vegetation seasons (2010, 2011) were determined to be optimal to somewhat wet (1.6). According to long-term averages, the weather conditions for the growing seasons from 1971 to 2011 were determined to be optimal (1.5).

2.2. Description of the Soil Site

The subject of this study was the Tybalt cultivar of spring wheat (Triticum aestivum L.), which was fertilised with varying amounts of nitrogen (factor I) and sulphur (factor II). This article presents the results of a three-year field experiment conducted in 2009–2011.

The experiment was established on Cambisols [59] consisting of light silty sand (sand: 68%; loam: 31%; clay: 1%). The experiment was initiated in the last 12 days of September 2008. The soil in the experimental field was characterised as slightly acidic (pH_KCl = 5.6; determined by potentiometry with using a Methrohm—605 pH-meter). The following were determined: the content of N-total (Nt) (0.9 g kg⁻¹; measured using Kjeldahl’s method; PN-EN ISO 20483:2014-02) [60], the content of Nmin (72.8 kg ha⁻¹, from the sum of N-NO₃ + N-NH₄ × 1.38 (soil bulk density, mg m⁻³; PN-R-04038:1997) [61], the content of C-total (Ct) (9.2 g kg⁻¹; measured via combustion by CNS-2000 LECO), a high content of available phosphorus (P₂O₅) (54.5 mg kg⁻¹; determined via double lactate extraction at pH 3.6 (1:50 m/v) and measured by colorimetric assay—Egner Riehm DL method; PN-R-04023:1996) [62], a medium potassium content (K₂O) (88.6 mg kg⁻¹; extracted as P₂O_5, measurement by photometric method, PN-R-04022:1996/Az1:2002 [63]), and the magnesium (Mg) content (34.8 mg kg⁻¹; extracted with 0.0125 mol L⁻¹ CaCl₂ (1:10 m/v ratio) and measured by AAS, PN-R-04020-1994/Az1:2004 [64]). The content of sulphate sulphur (S-SO₄) was low (14.2 mg kg⁻¹; extracted with 0.025 m L⁻¹ KCl and measured by ion chromatography).

2.3. Experimental Design

The forecrop for spring wheat was potato, which was fertilised with 30 t ha⁻¹ of cattle manure. After harvesting the potatoes, medium ploughing was carried out at a depth of 20 cm.

The field experiment included two factors:

1. Nitrogen application at doses of 0, 40, 80, and 120 kg ha⁻¹;

2. Sulphur application at doses of 0 and 50 kg ha⁻¹.

The experiment was set up in a split-plot design with four replicates. Eight fertiliser combinations (kg ha⁻¹) were used across four blocks (replicates): 1. N-0, S-0; 2. N-0, S-50; 3. N-40, S-0; 4. N-40, S-50; 5. N-80, S-0; 6. N-80, S-50; 7. N-120, S-0; 8. N-120, S-50. The arrangement was as follows: 4 levels N (A) × 2 levels S (B) × 4 replicates (k) gave 32 basic treatments (plots). Plots measured 30 m² (5 m × 6 m) and for harvest, 20 m² (4 m × 5 m). The main experimental units A (where N fertilisation was applied at 4 rates) were randomly distributed and located in half of the field, adjacent to each other. No sulphur fertilisation was applied to half of the A units (0, 40, 80, and 120 kg N ha⁻¹) (B). To the other half of the A units with nitrogen fertilisation, sulphur was added at a rate of 50 kg ha⁻¹ (B). This plot arrangement allowed for the implementation of appropriate agricultural techniques throughout the entire experimental field.

Nitrogen was applied in the form of ammonium nitrate (34%), and 50 kg ha⁻¹ of sulphur was applied in two portions, i.e., 40 kg S ha⁻¹ before sowing in the form of kieserite (MgSO₄ × H₂O) and 10 kg S ha⁻¹) via foliar application and between the middle and the end of heading (BBCH 55–59) in the form of magnesium sulphate heptahydrate (MgSO₄ × 7H₂O) (5% solution of SO₃ per 300 L H₂O ha⁻¹). The distribution of nitrogen and sulphur doses is recorded in

Table 2. The doses and phases of nitrogen (N) and sulphur (S) fertiliser application and chemicals used in the protection of spring wheat against pests.

Fertiliser	Dose		Time of Treatments
		Spring, Before Sowing	BBCH 28	BBCH 30–31	BBCH 55–59
Sulphur (S)	50 kg ha−1	40	-	-	10
Nitrogen (N)	0 kg ha−1	-	-	-	-
	40 kg ha−1	40	-	-	-
	80 kg ha−1	40	-	40	-
	120 kg ha−1	40	-	40	40
Chemical protection *
Granstar 75 WG 1	g ha−1	-	20
Puma Super 069 2	L ha−1	-	1	-	-
Alert 375 SC 3	L ha−1	-	-	1	-
Tilt CB 39.5 4	L ha−1	-	-	-	1
Decis 2.5 5	L ha−1	-	-	-	0.25

* explanatory notes: weeds—¹ tribenuron-methyl, ² fenoxaprop-P-ethyl; fungi—³ flusilazole + carbendazim, ⁴ propiconazole + carbendazim; insects—⁵ deltamethrin.

. Before sowing the wheat grain, phosphorus (39.6 kg P ha⁻¹ in the form of granular triple superphosphate, 46%,) and potassium (83 kg K ha⁻¹ in the form of potassium chloride, 60%) fertilisers were applied to all experimental plots. Magnesium lime and calcium carbonate were applied to the plots with the various fertiliser combinations to balance the soil pH.

During the growing season, the spring wheat plants were protected from pests. The selection of preparations and their doses are provided in Table 2.

The area of the plots used for sowing and observation was 30 m², while the harvested area was 20 m² (4 m × 5 m). Seeds of the spring wheat cultivar Tybalt were sown in the first ten days of April, depending on climate and soil conditions, and the grain was harvested by hand in the first ten days of August.

Table 3. Dates of phenological stages (BBCH) of spring wheat growth in the experiment.

Season	Growth Stage BBCH
	0	1	2	3	5	8	9
	Code BBCH
	00 *	10	23	31	51	83	99
2009	10 April	19 April	6 May	22 May	9 June	28 July	12 August
2010	5 April	14 April	2 May	20 May	5 June	25 July	7 August
2011	2 April	13 April	3 May	19 May	7 June	19 July	5 August

* explanatory notes: BBCH 0—germination, 00—dry seed (caryopsis); BBCH 1—leaf development, 10—first leaf through coleoptiles; BBCH 2—tillering, 23—three tillers detectable; BBCH 3—stem elongation, 31—first node at least 1 cm above tillering node; BBCH 5—inflorescence emergence, heading, 51—beginning of heading: tip of inflorescence emerged from sheath, first spikelet just visible; BBCH 8—ripening, 83—early dough; BBCH 9—senescence, 99—harvested product.

shows the sowing and harvesting dates and the phenological stages of the spring wheat plants [65]. Wheat grain was sown at a density of 500 plants per m².

2.4. Chemical Analysis

Plant material was sampled during three development stages, i.e., from stem elongation to the first node stage (BBCH 30–31), the stage between the middle and end of heading (BBCH 55–59), and the stages of fully ripe grain and straw (BBCH 89–92). In development stages BBCH 30–31 and 55–59, plants were collected from an area of 1 m² to determine fresh and air-dried weight. At BBCH 89–92, samples of mature plants were collected to assess the effect of nitrogen and sulphur fertilisers on grain and straw yield. Plant material used for the determination of dry matter and the measurement of phosphorus and silicon content was collected from an area measuring 1.0 m². Plant samples (30 plants) were taken from the same sowing rows across a specific experimental block. The fresh samples of plant material were weighed and then crushed and dried at 60 °C. The content of dry matter was determined following oven-drying at 105 °C. Chemical analyses were performed on the grain (in 2009–2011) to determine the content of P and Si at the Central Research Laboratory in University of Life Sciences in Lublin. For P, the Accreditation Certificate of Testing Laboratory Nr CLB/ESA/28/2019 version 3 from 4 March 2019, was obtained, confirming that the experiment satisfied the requirements of the PN-EN-ISO/IEC 17025–2005 standard. After mineralisation in concentrated sulphuric acid, we assayed the total phosphorus (P) content by reacting ammonium heptamolybdate and potassium antimony (III) oxide tartrate in an acidic medium with dialysed and diluted solutions of phosphate, which formed an antimony–phospho–molybdate complex. This complex was reduced to an intensely blue-coloured complex by L(+) ascorbic acid and measured at 880 nm (Skalar SANplus flow analyser; HQ-Skalar Analytical B.V., 4823 AA Breda, The Netherlands).

For Si analysis, the Accreditation Certificate of Testing Laboratory Nr CLB/ESA/5/2019 version 3 from 10 December 2019 was obtained, confirming that the experiment met the requirements of PN-EN-ISO 11885:2009 [66]. Samples were ground in an analytical mill, and from the homogeneous mass, approximately 0.5 g of each sample was weighed on an analytical balance with an accuracy of 0.0001 g. The samples were transferred to Teflon thimbles and poured into 5 cm³ of HNO₃ (Suprapur-Merck, Darmstadt, Germany). After sealing, the thimbles were transferred to a digester rotor. Digestion was carried out in a CEM Mars Xpress microwave oven at 210 degrees Celsius and a pressure of approximately 7 atmospheres. The resulting clear digests were quantitatively transferred to 50 cm³ volumetric flasks and diluted to the mark with deionized water (conductivity: 0.055 µS/cm. The obtained solutions were analysed on an inductively coupled plasma mass spectrometer (ICP Mass Spectrometer Varian MS-820, SpectraLab Scientific Inc., Markham, ON, Canada). The gas used to generate the plasma was argon from Air Liquide (99.999% pure). A reaction chamber (CRI) was not used in the analysis. The following instrument settings were used: plasma flow—16 dm³/min; nebulizer flow—0.98 dm³/min; RF power—1.38 kW; sampling depth—6.5 mm. Determination was performed using the standard curve method. Ultra Scientific standards with 99.999% purity were used for the analysis. The obtained results are expressed in mg kg⁻¹ of fresh weight. The analysis was conducted to confirm the quality of the results by measuring a blank sample, duplicate samples, and the certified reference material “NIST-1577c Bovine Liver”.

2.5. Statistical Analyses

Analysis of variance (ANOVA) and the Snedecor F test were used for statistical analysis of the results. The significance of differences was calculated using the Tukey test (p = 0.05). The mean results were then compared with post hoc analysis, and the Pearson correlation coefficients were calculated. The calculations were carried out using the statistical programmes Statistica 10 (StatSoft Poland) and Excel 7.0.

3. Results

3.1. Air-Dried Mass of Spring Wheat

The statistical analysis carried out in this field experiment showed that sulphur fertilisation (SF) and nitrogen fertilisation (NF) had a significant effect on air-dried mass in each phenological phase of spring wheat. No significant interaction between sulphur fertilisation and nitrogen fertilisation was found for the examined plant biomass traits, while the weather conditions during the growing seasons analysed, in 2009–2011, had a significant effect on the magnitude of the studied traits (

Table 4. The influence of nitrogen and sulphur application on the air-dried mass of spring wheat (t ha⁻¹) (2009–2011).

Fertilisation		Air-Dried Mass (t ha−1)
		BBCH
S	N	30–31	55–59	89–92
		Stems + Leaves	Whole Plants	Straw	Grain	Straw + Grain
0S (N × S)	0	0.316 a *	0.567 a	4.494 a	3.570 a	8.064 a
	40	0.364 a	0.657 a	4.851 a	3.670 a	8.521 a
	80	0.367 a	0.797 a	6.391 a	4.750 a	11.141 a
	120	0.368 a	0.813 a	7.010 a	4.837 a	11.847 a
50S (N × S)	0	0.342 a	0.612 a	4.839 a	3.655 a	8.494 a
	40	0.388 a	0.698 a	5.182 a	3.875 a	9.057 a
	80	0.394 a	0.883 a	6.966 a	4.802 a	11.768 a
	120	0.387 a	0.887 a	7.204 a	5.057 a	12.261 a
F-value, p		0.3 n.s.	0.4 n.s.	0.6 n.s.	0.7 n.s.	0.1 n.s.
Mean (S)	0S	0.354 b	0.708 b	5.686 b	4.207 b	9.893 b
	50S	0.378 a	0.770 a	6.048 a	4.347 a	10.395 a
F-value, p		58.7 ***	14.2 *	13.1 *	7.2 *	12.9 *
Mean (N)	0N	0.329 b	0.590 c	4.666 d	3.612 b	8.278 d
	40	0.376 a	0.678 b	5.016 c	3.773 b	8.789 c
	80N	0.380 a	0.840 a	6.678 b	4.776 a	11.454 b
	120N	0.378 a	0.850 a	7.107 a	4.947 a	12.054 a
F-value, p		62.9 ***	60.3 ***	145.5 ***	170.3 ***	182.4 ***
Mean (Y)	2009	0.350 b	0.724 a	5.730 b	4.207 b	9.937 b
	2010	0.354 b	0.729 a	5.725 b	4.131 c	9.856 b
	2011	0.366 a	0.764 a	6.146 a	4.493 a	10.640 a
F-value, p		80.2 ***	2.4 n.s.	7.8 *	17.8 **	12.7 **

Different letters in the same column represent significant differences at p ≤ 0.05; ***, **, and * indicate significance at p < 0.001, <0.01, and <0.05, respectively; n.s.—not significant.

).

Sulphur fertilisation (SF) significantly increased the biomass yield in each phenological phase of spring wheat by an average of 4.87% compared to the control. The yield of leaves and stems in the BBCH 30–31 stage increased significantly after the application of 40 kg N ha⁻¹ and remained stable after increasing subsequent doses of nitrogen (NF). Biomass yield increased by 12.96% on average after nitrogen fertilisation (NF) compared to the control (without nitrogen). The dry mass yield of spring wheat at the BBCH 55–59 phenological stage (the inflorescence emergence phonological stages, from the end of the third season in May to the end of the first season of June) increased significantly after the application of 40 and 80 kg N ha⁻¹ by an average of 28.6% compared to the control (without nitrogen). The straw yield (BBCH 89–92) increased significantly in proportion to each nitrogen dose applied. The grain yield increased significantly after the application of 80 and 120 kg N ha⁻¹ compared to the dose of 40 kg N ha⁻¹ dose and the control variant (without nitrogen fertilisation). Despite unproven statistical differences, it can be observed that in the treatments without sulphur and with sulphur fertilisation, each dose of nitrogen (SF × NF) increased the biomass yield of spring wheat.

The most favourable yield of leaves and stems at the BBCH 30–31 stage (second season in May) was obtained in the 2011 season, where the Sielianinow coefficient was 0.7, meaning that warm and dry conditions prevailed at that time and were conducive to plant growth. The weather conditions during the analysed growing seasons, 2009–2011, had a significant effect on the yield of whole plants in BBCH 55–59. The most favourable straw and grain yield occurred in the 2011 season, where the Sielianinow coefficient throughout the entire vegetation period was k = 1.6, and the weather conditions were determined to be optimal bordering on humid (Table 4).

3.2. Phosphorus

Sulphur fertilisation (SF) did not significantly affect the phosphorus content or uptake by dry biomass in the BBCH 30–31 stage, but it had a significant effect on the phosphorus content and uptake by dry biomass in BBCH 55–59 and for straw and grain in BBCH 89–92 (

Table 5. The influence of nitrogen and sulphur application on the phosphorus content (g kg⁻¹) and uptake by (kg ha⁻¹) spring wheat (2009–2011).

Fertilisation		Phosphorus
		BBCH
S	N	30–31		55–59		89–92
		Stems + Leaves		Whole Plants		Straw		Grain		Straw + Grain
		Content	Uptake	Content	Uptake	Content	Uptake	Content	Uptake	Uptake
		g kg−1	kg ha−1	g kg−1	kg ha−1	g kg−1	kg ha−1	g kg−1	kg ha−1	kg ha−1
0S (N × S)	0	4.117 a *	1.313 a	5.967 a	3.391 bc	2.000 b	8.99 a	4.267 a	15.22 a	24.21 a
	40	3.950 a	1.445 a	5.720 a	3.572 bc	1.810 c	8.78 a	4.167 a	15.30 a	24.08 a
	80	3.753 a	1.380 a	5.520 bc	4.410 a	1.710 e	10.92 a	4.100 a	19.49 a	30.41 a
	120	3.543 a	1.305 a	5.303 bc	4.290 a	1.643 g	11.52 a	4.033 a	19.51 a	31.03 a
50S (N × S)	0	3.903 a	1.343 a	5.663 b	3.477 a	2.177 a	10.53 a	4.417 a	16.02 a	26.55 a
	40	3.837 a	1.493 a	5.870 b	4.107 a	1.793 d	9.29 a	4.300 a	16.66 a	25.95 a
	80	3.777 a	1.493 a	6.020 a	5.325 a	1.710 e	11.92 a	4.267 a	20.48 a	32.40 a
	120	3.563 a	1.386 a	6.083 a	5.415 a	1.677 f	12.08 a	4.100 a	20.72 a	32.80 a
F-value, p		0.4 n.s.	0.2 n.s.	7.0 *	3.0 n.s.	22.7 **	1.6 n.s.	0.21 n.s.	0.1 n.s.	0.1 n.s.
Mean (S)	0S	3.841 a	1.361 a	5.628 b	3.961 b	1.791 b	10.05 b	4.143 b	17.38 b	27.43 b
	50S	3.770 a	1.429 a	5.909 a	4.581 a	1.893 a	10.95 a	4.271 a	18.47 a	29.42 a
F-value, p		0.7 n.s.	3.3 n.s.	10.1 *	19.6 **	27.4 **	22.0 **	7.19 *	8.1 *	13.6 *
Mean (N)	0N	4.010 a	1.328 b	5.815 a	3.434 bc	2.088 a	9.76 b	4.342 a	15.62 b	25.38 b
	40	3.893 a	1.469 a	5.795 a	3.929 b	1.802 b	9.03 c	4.233 a	15.98 b	25.01 b
	80N	3.765 a	1.436 a	5.770 a	4.867 a	1.710 c	11.42 ab	4.183 ab	19.99 a	31.41 a
	120N	3.553 ab	1.345 a	5.693 a	4.853 a	1.660 d	11.80 a	4.067 b	20.12 a	31.92 a
F-value, p		5.2 *	3.4 n.s.	0.4 n.s.	25.6 **	430.2 ***	47.4 ***	5.61 *	41.1 ***	47.8 ***
Mean (Y)	2009	3.738 b	1.305 b	5.083 c	3.664 b	1.768 c	9.95 b	4.225 a	17.66 b	27.61 b
	2010	3.444 b	1.217 b	6.406 a	4.695 a	1.860 a	10.49 ab	4.206 a	17.34 b	27.83 b
	2011	4.235 a	1.662 a	5.816 b	4.454 a	1.818 b	11.06 a	4.188 a	18.77 a	29.83 a
F-value, p		28.9 **	53.3 ***	74.8 ***	19.7 **	33.6 **	11.2 *	0.20 n.s.	5.15 *	6.8 *

Different letters in the same column represent significant differences at p ≤ 0.05; ***, **, and * indicate significance at p < 0.001, <0.01, and <0.05, respectively; n.s.—not significant.

). The phosphorus content in these stages increased by 4.38% on average after the sulphur application, and the phosphorus uptake increased by 9.22% compared to the variant without sulphur. In the BBCH 30–31 stage, nitrogen fertilisation (NF) at a dose of 120 kg ha⁻¹ significantly reduced the content of this element by 3.0% on average compared to lower doses of nitrogen and the control fertilisation. At this stage, phosphorus uptake by spring wheat dry matter was the lowest in the control sample, and nitrogen fertilisation (NF), regardless of the dose, significantly increased the uptake of this element. At the BBCH 55–59 stage (inflorescence emergence), the content of phosphorus in the whole plant yield did not increase significantly after nitrogen application; however, up to a dose of 80 kg N ha⁻¹, phosphorus uptake increased significantly in spring wheat plants. After each dose of nitrogen fertilisation (NF), the content of P in straw was progressively and significantly reduced, decreasing by 17.43% on average compared to the control variant. Phosphorus uptake by straw was significantly higher after the application of 80 and 120 kg N ha⁻¹ compared to 40 kg N ha⁻¹ and the control variant. After applying 80 and 120 kg N ha⁻¹, nitrogen fertilisation (NF) caused a decrease in phosphorus content in the grain; however, the same doses increased phosphorus uptake by grain. Nitrogen fertilisation (NF), after the application of 80 and 120 kg N ha⁻¹, resulted in a significant increase in phosphorus uptake in the total yield (straw + grain) compared to the 40 kg N ha⁻¹ dose and the control variant. The total phosphorus uptake by straw and grain was, on average, 28.42 kg ha⁻¹ (Table 5).

The interaction between SFxNF did not significantly differentiate the content of phosphorus in the BBCH 30–31 stage and in grain of the spring wheat. Further, the interaction SFxNF did not significantly differentiate the uptake of this element in any of the phases examined. A significant interaction between S and N fertilisation was also observed for the phosphorus content in whole-plant biomass (BBCH 55–59) and in straw (BBCH 89–92). In BBCH 55–59 in plots without sulphur, increasing the nitrogen fertilisation to 80 kg ha⁻¹ resulted in a decrease in the content of phosphorus. However, in plots fertilised with S, nitrogen fertilisation at doses of 80 and 120 kg ha⁻¹ resulted in a significant increase in phosphorus content. In the variant without sulphur, as well as with sulphur fertilisation, each increase in the N fertilisation dose significantly reduced the element in straw.

The most favourable season for phosphorus accumulation and uptake during the BBCH 30–31 stage occurred in 2011 (second season of May), which was defined as dry and warm (k = 0.7). In BBCH 30–31 in other seasons, defined as wet (k = 2.6 and 2.0, respectively), phosphorus content and uptake were significantly lower. The most favourable season for phosphorus accumulation at the BBCH 55–59 stage was 2010, which was defined as relatively dry (k = 1.1 and 1.0, respectively). The most favourable season for phosphorus content in straw at the BBCH 89–92 stage occurred in 2011, for which the entire growing season was defined as optimal bordering on relatively humid (k = 1.6). Phosphorus uptake by straw was favourable in the 2010 and 2011 seasons, for which the Sielianinow hydrothermal coefficient was the same (k = 1.6). The most favourable seasons for phosphorus content in grain at the BBCH 89–92 stage occurred in 2010 and 2011, for which the Sielianinow hydrothermal coefficient was the same (k = 1.6),and the entire growing season was defined as optimal bordering on relatively humid (k = 1.6) (Table 5).

3.3. Silicon

The silicon content in biomass and its uptake by dry plant mass at each phase examined in the BBCH stage were significantly higher after sulphur fertilisation (SF) (

Table 6. The influence of nitrogen and sulphur application on the silicon content (mg kg⁻¹ or g kg⁻¹) and uptake (mg kg⁻¹ or kg ha⁻¹) by spring wheat (2009–2011).

Fertilisation		Silicon
		BBCH
S	N	30–31		55–59		89–92
		Stems + Leaves		Whole Plants		Straw		Grain		Straw + Grain
		Content	Uptake	Content	Uptake	Content	Uptake	Content	Uptake	Uptake
		g kg−1	kg ha−1	g kg−1	kg ha−1	g kg−1	kg ha−1	mg kg−1	g ha−1	kg ha−1
0S (N × S)	0	20.03 a *	6.35 a	15.50 a	8.88 a	11.80 e	53.45 a	11.97 a	42.68 a	53.49 a
	40	20.43 a	7.44 a	16.90 a	11.16 a	12.90 d	62.88 a	14.83 a	54.62 a	62.94 a
	80	22.90 a	8.38 a	18.10 a	14.31 a	13.40 d	86.01 a	15.63 a	74.15 a	86.09 a
	120	25.97 a	9.56 a	19.28 a	15.74 a	14.23 cd	99.85 a	15.77 a	76.29 a	99.92 a
50S (N × S)	0	21.70 a	7.42 a	17.380	10.78 a	13.63 d	66.99 a	14.30 a	52.09 a	67.04 a
	40	23.13 a	8.97 a	19.45 a	13.68 a	15.03 c	78.60 a	15.70 a	60.72 a	78.66 a
	80	25.70 a	10.11 a	22.01 a	19.46 a	17.03 b	118.9 a	16.67 a	80.10 a	119.0 a
	120	26.43 a	10.22 a	23.10 a	20.54 a	18.40 a	132.4 a	18.72 a	94.58 a	132.5 a
F-value, p		1.8 n.s.	2.0 n.s.	3.2 n.s.	3.6 n.s.	5.3*	3.8 n.s.	1.1 n.s.	2.4 n.s.	3.8 n.s.
Mean (S)	0S	22.33 b	7.93 b	17.45 b	12.52 b	13.08 b	75.55 b	14.55 b	61.94 b	75.61 b
	50S	24.24 a	9.18 a	20.48 a	16.12 a	16.03 a	99.22 a	16.35 a	71.87 a	99.29 a
F-value, p		22.6 **	55.5 ***	119.2 ***	70.0 ***	143.6 ***	78.3 ***	13.4 *	28.2 **	78.4 ***
Mean (N)	0N	20.87 c	6.88 d	16.44 d	9.83 c	12.72 d	60.22 d	13.13 c	47.39 d	60.27 d
	40	21.78 c	8.21 c	18.17 c	12.42 b	13.97 c	70.74 c	15.27 b	57.67 c	70.80 c
	80N	24.30 b	9.25 b	20.05 b	16.88 a	15.22 b	102.5 b	16.15 ab	77.12 b	102.5 b
	120N	26.20 a	9.89 a	21.19 a	18.14 a	16.32 a	116.1 a	17.24 a	85.44 a	116.2 a
F-value, p		36.4 **	61.5 ***	56.5 ***	81.2 ***	40.2 ***	96.3 ***	12.6 *	87.0 ***	96.5 ***
Mean (Y)	2009	22.65 b	8.00 c	16.71 c	12.35 b	12.66 c	74.08 b	15.96 a	68.25 a	74.15 b
	2010	24.23 a	8.59 b	18.01 b	13.45 b	13.94 b	81.97 b	15.13 a	63.35 a	82.00 b
	2011	22.99 b	9.08 a	22.18 a	17.17 a	17.06 a	106.1 a	15.25 a	69.11 a	106.21 a
F-value, p		5.7 *	13.9 **	140.7 ***	46.1 ***	113.4 ***	52.0 ***	1.1 n.s.	3.7 n.s.	52.0 ***

Different letters in the same column represent significant differences at p ≤ 0.05; ***, **, and * indicate significance at p < 0.001, <0.01, and <0.05, respectively; n.s.—not significant.

). It should be emphasised that the silicon content and uptake by grain were almost a thousand times lower than in the spring wheat phytomass in the previous stages, i.e., BBCH 30–31 and BBCH-55–59, and in straw at BBCH 89–92. The silicon content increased by an average of 13.02% after sulphur application, and its uptake increased by 14.41% compared to the variant without sulphur. As the nitrogen dose (NF) increased, the silicon content and its uptake at each phase by the spring wheat plants increased. The SFxSN interaction significantly differentiated the silicon content in whole plants (BBCH 55–59) and in straw (BBCH 89–92). In the variant without sulphur, as well as with the addition of sulphur, each increase in the nitrogen dose caused an increase in the silicon content in whole plants (BBCH 55–59) and in straw. The SFxSN interaction in BBCH 30–31 and in grain did not significantly differentiate the content of silicon and did not significantly differentiate the uptake of silicon in any phase.

The highest silicon content by the leaves and stems (BBCH 30–31) was recorded in the 2010 season, and its highest uptake in the 2011 season. The highest silicon content and uptake by whole plants (BBCH 55–59) and straw (BBCH 89–92) were recorded in the 2011 season. At that time, during the BBCH 55–59 stage (from the end of the third week of May to the end of the first week of June), it was relatively dry (k = 1.0), which translated into better silicon accumulation in the spring wheat phytomass. For straw, the season was defined as optimal bordering on relatively humid (k = 1.6), which had a beneficial effect on the silicon concentration in the spring wheat straw. The growing seasons did not differentiate the silicon content in the grain; the uptake of silicon by the grain was the highest in 2011 (Table 6).

3.4. Ratio of Si:P

Sulphur fertilisation (SF) and nitrogen fertilisation (NF) significantly increased the Si:P ratio in each phenological phase (BBCH 30–31, BBCH 55–59, and BBCH 89–92—straw) (

Table 7. The influence of nitrogen and sulphur application on Si:P or P:S ratio in biomass of spring wheat (t ha⁻¹) (2009–2011).

Fertilisation		Ratio
		Si:P			P:Si	Si:P
		BBCH
S	N	30–31	55–59	89–92
		Stems + Leaves	Whole Plants	Straw	Grain	Straw + Grain
0S (N × S)	0	5.001 a *	2.618 a	2.769 d	358.1 a	2.194 e
	40	5.239 a	2.975 a	3.095 cd	282.4 a	2.601 d
	80	6.145 a	3.321 a	3.264 cd	262.6 a	2.816 cd
	120	7.356 a	3.713 a	3.526 c	256.9 a	3.215 c
50S (N × S)	0	5.604 a	3.054 a	3.090 d	309.7 a	2.485 d
	40	6.071 a	3.306 a	3.503 c	274.3 a	3.008 c
	80	6.864 a	3.665 a	4.007 b	256.9 a	3.676 b
	120	7.490 a	3.820 a	4.495 a	219.7 a	4.040 a
F-value, p		0.7 n.s.	1.8 n.s.	7.9 *	0.90 n.s.	4.9 *
Mean (S)	0S	5.935 b	3.157 b	3.163 b	290.0 a	2.707 b
	50S	6.507 a	3.461 a	3.774 a	265.1 a	3.302 a
F-value, p		9.1 *	34.6 **	130.3 ***	4.96 n.s.	83.6 ***
Mean (N)	0N	5.302 c	2.836 d	2.930 d	333.9 a	2.339 d
	40	5.655 c	3.140 c	3.299 c	278.3 b	2.804 c
	80N	6.505 b	3.493 b	3.635 b	259.7 bc	3.246 b
	120N	7.432 a	3.766 a	4.010 a	238.3 c	3.628 a
F-value, p		25.0 **	61.6 ***	74.7 ***	13.43 *	73.0 ***
Mean (Y)	2009	6.117 b	3.308 b	3.009 c	269.2 a	2.626 c
	2010	7.042 a	2.806 c	3.313 b	284.8 a	2.866 b
	2011	5.505 c	3.813 a	4.084 a	278.7 a	3.521 a
F-value, p		22.3 **	125.9 ***	143.1 ***	0.66 n.s.	67.3 ***

Different letters in the same column represent significant differences at p ≤ 0.05; ***, **, and * indicate significance at p < 0.001, <0.01, and <0.05, respectively; n.s.—not significant.

). This means that at these stages, the spring wheat plants accumulated more silicon than phosphorus as a result of sulphur (SF) and nitrogen (NF) fertilisation. Although no significant interaction between sulphur fertilisation and nitrogen fertilisation was observed in BBCH 30–31 or BBCH 55–59, in the treatments without and with sulphur, each higher dose of nitrogen favoured an increase in the Si:P ratio. The SFxNF interaction significantly increased the Si:P ratio in straw, meaning that straw accumulated more silicon than phosphorus as a result of the interaction between sulphur and nitrogen.

The highest Si:P ratio was observed in biomass at BBCH 30–31 (the second decade of May) in the 2010 season, which was classified as wet (k = 2.0). The highest Si:P ratio was observed in biomass at BBCH 55–59 (from the end of the third week of May to the end of the first week of June) in the 2011 season, which was defined as relatively dry (k = 1.0). The highest Si:P ratio was recorded in straw in the 2011 season (k = 1.6).

Because the silicon content and uptake by grain in spring wheat phytomass was almost a thousand times lower than in the previous stages, it is described separately. The silicon content and uptake by grain were significantly higher after sulphur fertilisation (SF) and nitrogen fertilisation (NF). However, no significant interaction between the tested factors was found. The growing season did not differentiate the silicon content or its uptake by grain. Sulphur fertilisation (SF) and nitrogen fertilisation (NF) significantly narrowed the P:Si ratio. The interaction between sulphur fertilisation and nitrogen fertilisation also showed a tendency to narrow the P:Si ratio as the N dose increased in the experimental plots to which sulphur was applied. The growing season did not change the P:Si ratio (Table 7).

3.5. Correlations Between Studied Traits

Numerous significant correlations were found between the studied traits. The yield of biomass in the BBCH 30–31 stage had a highly significant positive correlation with phosphorus uptake (r = 0.7443), silicon content and uptake (r = 0.4639 and r = 0.8028, respectively). The yield of biomass in BBCH 55–59 stage had a highly significant positive correlation with phosphorus uptake (r = 0.8427), silicon content and uptake (r = 0.7080 and r = 0.8992, respectively) and Si:P ratio (r = 0.6669). The yields of straw and grain in BBCH 89–92 stage had highly significant negative correlations with the phosphorus content (r = −0.7527 and r = −0.5459, respectively), and highly significant positive correlations with all other traits (

Table 8. Selected correlation coefficients between biomass yield and phosphorus and silicon content, phosphorus and silicon accumulation, and Si:P ratio (2009–2011).

Variables (n = 24)	P Content	P Uptake	Si Content	Si Uptake	Si:P Ratio
Yield of biomass in BBCH 30–31 (air D.M.)	0.2502	0.7443	0.4639	0.8028	0.1492
Yield of biomass in BBCH 55–59 (air D.M.)	0.0050	0.8427	0.7080	0.8992	0.6669
Yield of straw in BBCH 89–92 (air D.M.)	−0.7527	0.8612	0.6374	0.8807	0.6878
Yield of grain in BBCH 89–92 (air D.M.)	−0.5459	0.9758	0.6650	0.9108	0.6989

Explanatory notes: significant for α = 0.05: R = 0.4060, significant for α = 0.01: R = 0.5168.

). The phosphorus content in the total phytomass correlated negatively with phosphorus uptake (r = −0.2187) and silicon uptake (r = −0.7331). However, a significantly positive correlation was observed between the phosphorus content and silicon content (r = 0.2888) in spring wheat plants regardless of the developmental stage analysed. Phosphorus uptake had a highly significant negative correlation with the silicon content (r = −0.5571) and positively with the Si:P ratio (r = 0.8069), and the silicon content had highly significant negative correlations with the silicon uptake (r = −0.2537) and the Si:P ratio (r = −0.3770) in the spring wheat phytomass. The silicon uptake had a significant negative correlation with the Si:P ratio (r = −0.3804) (

Table 9. Selected correlation coefficients between phosphorus and silicon content and phosphorus and silicon accumulation and Si:P ratio (2009–2011).

Variables (n = 96)	P Uptake	Si Content	Si Uptake	Si:P Ratio
P content	−0.2187	0.2888	−0.7331	0.1028
P uptake	-	−0.5571	0.1090	0.8069
Si content	-	-	−0.2537	−0.3770
Si uptake	-	-	-	−0.3804

Explanatory notes: significant for α = 0.05: R = 0.1946, significant for α = 0.01: R = 0.2673.

).

4. Discussion

This article presents the results of a three-year field experiment investigating the effect of progressively increasing doses of nitrogen (0, 40, 80, 120 kg N ha⁻¹) and sulphur (0, 50 kg S ha⁻¹) on biomass yield and phosphorus and silicon content and uptake in spring wheat. The studies were conducted in three phenological stages: (1) stem elongation, BBCH 30–31—leaves; (2) inflorescence emergence, BCH 55–59—whole plants, and (3) full ripeness, BBCH 89–92—straw and grain.

4.1. Biomass

The biomass yield of spring wheat in each growth stage depended significantly on nitrogen and sulphur fertilisation. The weather conditions in the growing seasons studied also had a significant impact. After sulphur application, the magnitude of the tested traits increased with each increase in N dose level. In particular, the use of 80 and 120 kg N ha⁻¹ in soil enriched with 50 kg ha⁻¹ of S had a very favourable effect This effect of a yield-changing factor, in this case, a fertiliser, indicates the additive effect of sulphur. It manifests under conditions of a moderately weak deficiency factor, in accordance with the rules defined by the law of diminishing returns, known as Mitscherlich’s law [40]. In general, the additive interaction of components manifests when there is a constant index increase in mass (yield) as a result of the application of the second factor.

Sulphur among the macronutrients essential for plant growth; plants need about equal quantities of sulphur and phosphorus for optimal growth [5]. Sulphur performs a variety of vital biochemical functions within the plant, which are closely related to those of nitrogen. In its reduced form, sulphur plays an important role in protein structure and the regulation of protein synthesis; in the function of cofactors like coenzyme A, biotine, and thiamine; in secondary compounds like alliin and glucosinolates; and in the to stress conditions [7]. In plant proteins, the atom ratio of N:S is 36:1, which is equivalent to about 15:1 on a weight basis. The N:S ratio is a significant indicator of the quality of wheat grain, mainly in the case of baking varieties. Correct proportions of nitrogen and sulphur range from 12:1 to 15:1, depending on the rate of fertilisation with these elements [5,28]. An insufficient S supply can decrease utilisation of nitrogen from mineral and organic fertilisers and thus increase nitrogen leaching. The varying demand for S in different areas depends, among other factors, on the crop species, soil type, soil hydrology, soil texture, climatic conditions, irrigation, and the application of organic manures [5].

Nitrogen is considered one of the most important yield-forming elements and has been widely described in the literature [4,10,13,14,16]. The beneficial effect of sulphur on spring wheat yield has been confirmed by numerous studies [17,18,19,20,21,22,23,24,67]. Grzebisz [40] believes that sulphur deficiency leads to stunted plant growth but, like with nitrogen, this growth limitation primarily affects above-ground organs. The negative effects of sulphur deficiency first negatively affect photosynthetic processes, carbohydrate synthesis, and protein synthesis, ultimately reducing the growth of the entire plant (primarily the leaves). A reduction in leaf chlorophyll content, combined with reduced protein synthesis, manifests in the form of net chlorosis. In this study, the addition of sulphur improved the intensity of the green colour of the plants. The beneficial effect of sulphur on the yield of spring wheat has been confirmed by numerous other studies [18,19,67]. According to Klikocka et al. [68], in wheat cultivation, sulphur deficiency inhibited plant growth and development and led to changes in the chemical composition of the vegetative organs and grain. The beneficial effect of sulphur on wheat yield was also confirmed by Brodowska et al. [69].

In this study, a positive effect of nitrogen fertilisation on straw yield was found for all nitrogen doses. Sulphur supplementation also had a beneficial effect on straw yield. These results are confirmed in the literature [70]. Studies by other authors show that sulphur fertilisation resulted in a significant increase in cereal straw yield [22,71,72,73]. Other studies have found that the dry matter yield of barley increased with each successive stage of development [74]. This phenomenon was confirmed in the present study, as the lowest dry matter yield of spring wheat leaves and stems was obtained in the stage from the beginning of stem elongation to the first joint stage (BBCH 30–31), and this amount was more than twice as high in the stage between mid- and full-heading (BBCH 55–59). Therefore, it can be concluded that the dry matter yield increases with the vegetation period of the plant. According to other researchers, an increase in nitrogen fertilisation dose increases the dry matter yield of plants during their vegetation period [75,76].

This original research shows that pre-sowing supplementation with sulphur at a dose of 40 kg S ha⁻¹ resulted in a significant increase in the dry matter yield of leaves and stems of spring wheat in the tillering stage to the first stage (BBCH 30–31) and in the stage between mid- and full-heading (BBCH 55–59) compared to the control. Based on the data obtained, it can be concluded that sulphur fertilisation leads to an increase in the dry matter yield of the plant during the growing season. Similar results were obtained in an experiment with spring rye [77].

The weather conditions during the 2011 growing season had the most favourable effect on biomass yield. In the 2011 season, Sielianinow’s hydrothermal coefficient was k = 1.6, and the conditions were defined as optimal bordering relatively humid (k = 1.6). The total amount and distribution of precipitation play an important role in grain yield. Dmowski et al. [78] demonstrated the dependence of yield on the number of days with precipitation between March and July. Grain yields achieved with 80 days of precipitation were approximately 0.7 t ha⁻¹ higher than with 58 days of precipitation. Research by Radzka et al. [79] shows that April droughts are favourable for spring wheat yields, while those in May and June reduce yields. Dmowski and Dzieżyc [80] also found that spring wheat has the highest rainfall requirements in June. The optimum during this period is approximately 100 mm, and under good wheat complex conditions, this factor increased the yield by 0.98 t ha⁻¹, while under rye complex conditions, it increased the yield significantly by 0.41 t ha⁻¹. According to Grzebisz [81], the critical stage of water demand for cereals falls between the stem elongation stage (BBCH 31) and the grain filling stage (BBCH 71). Water deficiency during this period reduces the number of grains per unit area, which usually means a reduction in grain yield. In contrast, the weight of 1000 grains is determined after the grains have set and depends mainly on the availability of water (apart from nitrogen and potassium) and temperature (and sulphur) [40]. According to Grzebisz [40], excessively high temperatures cause intensive hydrolysis of leaf proteins, thus degrading chlorophyll, which contains nitrogen. As a result of this phenomenon, the leaves bind less carbon dioxide and the grain-filling period is shortened, resulting in 1000 times lower grain weight. This period is also critical for sulphur. The significant influence of weather conditions was observed in relation to biomass yield and analysed in three developmental stages of the vegetation period of spring wheat. Woźniak and Staniszewski [82] also point out, following other authors, that (1) grain yield and quality are particularly negatively affected by rainfall deficiencies during the heading, flowering, and grain-filling stages; (2) warm and moderately humid summer months increase the protein content of the grain; (3) cool and humid weather during this period increases alpha-amylase activity, which leads to grain sprouting; and (4) a moderately dry and warm growing season promotes protein accumulation in winter wheat grain, resulting in a high gluten content and high Zeleny sedimentation index in particular.

4.2. Phosphorus

The phosphorus content of spring wheat biomass was ambiguously affected by nitrogen and sulphur fertilisation and their interaction. In the BBCH 30–31 stage, the phosphorus content in leaves and stems was reduced after the application of the highest nitrogen dose (120 kg ha⁻¹). The opposite trend was observed in the case of phosphorus uptake, which increased after the application of the lowest nitrogen dose—40 kg ha⁻¹—and was positively correlated with phytomass yield (r = 0.7443). In the next developmental stage studied, BBCH 55–59, nitrogen fertilisation did not differentiate the phosphorus content. However, phosphorus uptake increased significantly as the nitrogen dose increased up to 80 kg N ha⁻¹. In contrast, increased fertilisation caused a decrease in phosphorus content in straw and spring wheat grain. The opposite trend was observed in the case of phosphorus uptake, where a significant increase in uptake occurred up to a fertilization dose of 80 kg ha⁻¹. Plants in the BBCH 55–59 stage and grain contained the most phosphorus. The lowest phosphorus content was recorded in straw in the BBCH 89–92 stage. Grzebisz reports that young plants contain the most phosphorus [40]. With age, the content of this element in vegetative organs decreases. A significant part of the phosphorus absorbed by the plant remains in the vacuole as a reserve. In the leaves of young plants, up to 80% of the total phosphorus content may be in an inorganic form. The inorganic phosphorus content in the plant increases in proportion to the concentration of this element in the environment [40]. Spring plants have a high demand for phosphorus during two specific periods of vegetation: (1) initial growth and, (2) seed setting and growth. Therefore, phosphorus accumulates progressively in plants throughout the entire vegetation period [40].

The literature devotes considerable attention to the issue of nitrogen fertilisation of high-quality wheat and its impact on nutrient content [25]. As described above, our own research has shown that the use of nitrogen in fertilisation causes an increase in phosphorus in spring wheat biomass. Kozera et al. [20] found a similar relationship in grain. However, Brzozowska [83] did not observe a significant effect of nitrogen fertilisation on the phosphorus content in winter wheat grain. According to Nogalska et al. [84], the main factors influencing the content and uptake of macroelements are the wheat cultivar and grain yield.

In this study, the addition of sulphur during fertilisation did not differentiate the phosphorus content or uptake by leaves in the BBCH 30–31 stage. However, in the subsequent BBCH 55–59 and BBCH 89–92 stages, the whole plants, straw and grain contained and absorbed significantly more phosphorus after the addition of sulphur than in plants the control plots.

As reported by Barczak and Nowak [21], use of sulphur to fertilise barley caused a slight decrease in P in the oat grain compared to variants without sulphur. Skwierawska et al. [22] also found that mineral fertilisation with added sulphur does not differentiate the P content in spring barley grain. Many authors have also reported the beneficial effect of sulphur fertilisation on the content of available phosphorus in the soil [85] and on its uptake and utilisation by plants [38,86,87]. Maize fertilised with additional sulphur absorbed on average of 9 kg more phosphorus per hectare from the soil than maize fertilised with NPK alone [86]. The addition of sulphur in NPK fertilisation in spring triticale cultivation also resulted in increases in the average phosphorus uptake by the grain of 1.6 and 3.2 kg ha⁻¹, respectively, for traditional and simplified soil cultivation [85]. The beneficial effect of sulphur on phosphorus utilisation by maize and winter triticale is also attributed to the use of single superphosphate compared to triple superphosphate [88].

The phosphorus content and uptake by spring wheat biomass depended on weather conditions in the analysed growing seasons.

It is assumed that precipitation and the associated soil moisture conditions play a major role in the release of phosphorus through the mineralisation of soil organic matter [85,89,90]. Khan et al. [91] also found that an adequate water supply supports phosphorus uptake. In situations of insufficient rainfall, plants may not be able to absorb enough water for growth. This stress directly affects root growth and function, as well as the availability and transport of phosphorus and other nutrients in the soil, resulting in reduced uptake and utilisation efficiency. Temperature is one of the most important physical factors in the phosphorus uptake process. Grzebisz and Potarzycki [34] and Dibb et al. [91] argue that low soil and air temperatures reduce the availability of phosphorus for plants, which is one of the reasons for disturbances in phosphorus uptake by plants in early spring, even in sites rich in this nutrient. According to these authors, this is due to (1) a slower rate of organic phosphorus mineralisation, (2) lower activity of the microorganisms involved in making phosphorus available to plants, (3) slower plant metabolism, and 4) lower solubility of mineral phosphorus compounds. Other authors have also described the influence of temperature on phosphorus uptake [92,93].

4.3. Silicon

The silicon content and uptake by spring wheat plants in all analysed growth stages was significantly higher after the application of each dose of nitrogen fertilisation and sulphur supplement. However, no interaction between nitrogen and sulphur on the content and uptake of this element was observed. The growing seasons also did not differentiate the silicon content and uptake by dry grain weight. It should be emphasised that the silicon content and uptake by grain were almost a thousand times lower than in the previous studied stages of spring wheat phytomass, i.e., BBCH 30–31 and BBCH-55–59, and in straw in BBCH 89–92. This phenomenon is confirmed by studies by other authors, who found a high silicon content in leaves and stems and a very low content in grain [94]. Other studies also report that most of the silicon is deposited in the cell walls of roots, leaves, stems and husks, where it can form a thin layer of silica gel (SiO⋅nH₂O) layer [94,95]. Ma et al.’s [96] study on the grains of 401 barley cultivars and show that the silicon content in the grain is genetically determined. The cited studies showed that more than 80% of the total silicon was found in the husk, and its amount ranged from 15.343 to 27.089 mg kg⁻¹ in the tested cultivars. Similar results were obtained in the current study, where the average silicon content in the grain was 15.45 mg kg⁻¹. Plants show significant variation in silicon content in above-ground organs, ranging from 0.1% to 15%, depending on the species, age, and growing conditions. The lower limit of the content of this element corresponds to the values recorded for macronutrients such as P, S, and Mg. The upper range of the content exceeds the level recorded for N and K [40]. The results of our experiment were similar. In the BBCH 30–31 and BBCH 55–59 stages, the phytomass of spring wheat contained an average of 21.18 g kg⁻¹ of silicon. In contrast, in previously published studies, the same spring wheat contained an average of 28.34 g kg⁻¹ of total nitrogen [42].

Nitrogen and phosphorus fertilisation reduces the concentration of silicon in plants. This is explained by the indirect effect of nitrogen and phosphorus: when plants have a good supply of these elements, the transpiration rate is significantly reduced, and the amount of transpiration affects the uptake and transport of silicon. Since nitrogen or phosphorus fertilisation reduced the silicon content only at doses that caused significant yield increases, in this case, its impact can also be explained by the “dilution effect” [97]. However, some studies show that there is a decrease in silicon content in barley and beans under the influence of nitrogen fertilisation despite a slight increase in yield, which may indicate that silicon participates in ionic balance in the plant [97].

Studies conducted by Batyqi et al. [98] on the fertilisation of spring oats with liquid sulphur fertiliser (5 L ha⁻¹, in the form of lignosulfonate formulation) proved that the use of S resulted in an increase in the silicon content in the grain. The silicon content also depended on the oat cultivar used. Weather conditions in the analysed growing seasons did not affect the silicon content in the grain, which is also consistent with our study on spring wheat.

Some studies show that silicon uptake by plants depends on the soil water content. Plants absorb larger amounts of silicon from moister soil [99,100,101]. The silicon content in grain showed a significant negative correlation with the Pálfai Drought Index (PaDI), indicating increased silicon accumulation under more severe drought conditions, while its correlations with other mineral nutrients were generally weak or specific to a given element [102].

4.4. Si:P Ratio

Since the silicon content was higher than the phosphorus content in phytomass in stages BBCH 30–31, BBCH 55–59 and BBCH 89–92 in straw, the Si:P ratio was calculated. In the case of grain, the opposite pattern was observed; the grain contained a thousand times less silicon than phosphorus, so the P:Si ratio was calculated for a better illustration.

Sulphur fertilisation (SF) and nitrogen fertilisation (NF) significantly increased the Si:P ratio in biomass in the BBCH 30–31 and BBCH 55–59 stages. This means that spring wheat plants in these stages accumulated more silicon than phosphorus as a result of sulphur (SF) and nitrogen (NF) fertilisation. Although no significant interaction between sulphur and nitrogen fertilisation was observed, at sites without sulphur and at those fertilised with sulphur, each higher dose of nitrogen favoured an increase in the Si:P ratio. The highest Si:P ratio was observed in biomass in BBCH 30–31 stage (second week of May) in the 2010 season, which was classified as wet (k = 2.0). In contrast, in the BBCH 55–59 stage, the largest Si:P ratio was found for 2011. This stage, i.e., from the end of the third week of May to the first week of June, was defined as relatively dry (k = 1.0). Sulphur fertilisation (SF), nitrogen fertilisation (NF), and the interaction between these factors significantly increased the Si:P ratio in straw (BBCH 89–92). This means that straw accumulated more silicon than phosphorus as a result of sulphur fertilisation (SF) and nitrogen fertilisation (NF) and their interaction. The highest Si:P ratio was recorded in straw in the 2011 season (k = 1.6). With regard to spring wheat grain, it was found that sulphur fertilisation (SF) and nitrogen fertilisation (NF) significantly narrowed the P:Si ratio. The interaction of sulphur fertilisation and nitrogen fertilisation also showed a tendency to narrow the P:Si ratio as the N dose increased when sulphur had been applied. The growing seasons did not change the P:Si ratio.

Numerous reports on the relationship between phosphorus and silicon can be found in the literature [33,50,97]. The mechanism of the interaction between phosphorus and silicon uptake by plants is described in detail by Kulus and Ciereszko based on current publications [33]. Generally, according to these authors, silicon may modify phosphorus mobility in soils, promoting more efficient uptake and utilisation in plant tissues [33]. In aquatic cultures, the addition of silicon increased the phosphorus content in rice plants. Under soil conditions, greater utilisation of soil phosphorus by plants has often been observed under the influence of soluble silicates [98]. Initially, it was thought that silicon could partially replace phosphorus in plants, but it was later found that it increases phosphorus uptake. It has been shown that this is not a direct effect on phosphorus uptake by plants but an indirect effect through silicon’s impact on soil phosphorus availability [98]. The current study showed a significant positive correlation between phosphorus and silicon contents in spring wheat grain (r = 0.2888). A high negative correlation was also demonstrated between the silicon content in grain and its phosphorus uptake (r = −0.5571). According to some literature, it is not silicate ions but silicic acid that reduces the activity of aluminium ions in solution, thus preventing the precipitation of phosphates [99]. Silicon can also increase the utilisation of phosphorus from fertilisers in which phosphate less readily available. In acidic habitats with an excess of toxic aluminium, manganese and iron, silicon improves growing conditions. In zinc-deficient sites, it improves the balance of this element in relation to phosphorus. The clear response of plants to silicon under alkaline conditions can be explained by the mechanism of silicon displacing adsorbed phosphorus that was previously unavailable to plants [50].

Under drought conditions, silicon reduces water stress through improved root growth, reduced evaporation and increased photosynthetic efficiency. In agricultural practice in tropical and subtropical regions, and now also in Europe, many plants, apart from rice and sugar cane, are fertilised with silicon. Under certain conditions (in acidic organic soils), yield increases can reach up to 50% [40,45]. The reason for these good results is the very low content of soluble silicon in the soil, which is quickly washed out of these soils (a process called desilication) [40].

Schaller et al. used soil amorphous silica (ASi) to fertilise Tybalt spring wheat and demonstrated its usefulness in alleviating drought stress in wheat cultivation [102]. They found that fertilising soil with ASi can be a means of improving crop yields during drought. Amorphous silica increases soil moisture and thus improves the availability of soil water to plants thanks to a faster stomatal response. Another aspect of using ASi during drought is the increased concentration of phosphorus in plants. Silicon is able to mobilise phosphorus from resources in the soil that are unavailable to plants, thereby improving the availability of phosphorus to plants. According to the authors cited, managing silicon resources in the soil is extremely important for maintaining crop production under drought conditions [102].

In summary, silicon enhances phosphorus uptake primarily by increasing soil P availability [33,103]. This effect is mediated through a range of biochemical mechanisms and has been reported in several plant species, including rice [104], maize [46], and wheat [105]. Silicon stimulates the release of root organic acids, such as malate and citrate, which compete with phosphate for adsorption sites on soil minerals, thereby reducing P fixation and increasing the pool of bioavailable Pi [106]. Moreover, Si can increase P availability by forming complexes with Al³⁺ and Fe³⁺ ions, modifying soil pH, and influencing microbial community dynamics [102].

The experiment presented in this study has been partially described in earlier works [18,68]. However, the research results presented here are original and unpublished. The results presented and described in this paper on the effects of nitrogen and sulphur fertilisation on the content and uptake of phosphorus and sulphur by spring wheat fill a significant gap in this topic. However, it does not answer all questions, and primarily concerns the interactions of the studied elements. The obtained research results confirm the findings of other authors regarding the yield-enhancing importance of nitrogen and sulphur and their influence on phosphorus and silicon content in wheat biomass. However, it was noted that the literature on the subject lacks studies on the effects of nitrogen and sulphur on the interrelationship between phosphorus and silicon. This work aims to fill this gap. This work aims to fill this gap, but further research on this subject is needed.

5. Conclusions

Studies on the fertilisation of Tybalt spring wheat grown in Cambisols soil with nitrogen and sulphur showed their significant impact on the content and uptake of phosphorus and silicon and the Si:P ratio. Effects of nitrogen and sulphur fertilisation and weather conditions eon the growth of spring wheat green weight, straw, and grain were observed. A significant positive correlation between phosphorus and silicon content was proven, showing that the two elements do not act antagonistically towards each other. However, silicon is taken up more strongly by plants under conditions of nitrogen and sulphur fertilisation (negative correlation), as evidenced by the increase in the Si:P ratio and the fact that plants accumulated, on average, 3.5 times more silicon than phosphorus. The highest silicon content was found in the green parts of plants in the BBCH 30–31 and BBCH 55–59 stages, with nearly half as much measured in straw in BBCH 89–92 and a thousand times less found in grain (also in BBCH 89–92).

Based on this research, use of nitrogen fertilisation up to the most favourable dose of 80 kg ha⁻¹ and sulphur supplementation at a dose of 50 kg ha⁻¹ Is recommended Under the soil and climatic conditions studied in this experiment, this system guarantees favourable spring wheat yields and an optimal environment for the accumulation of phosphorus and silicon by plants.