The Effect of Sulfur Carriers on Nitrogen Use Efficiency in Potatoes—A Case Study

Abstract

The use of sulfur is an important factor in potato production. At the beginning of this study, a hypothesis was put forward according to which sulfur carrier affects yield (TY) and nitrogen efficiency (EN). The three-year study was conducted in a two-factor system: (1) sulfur fertilization, SF (control—without S, elemental sulfur—S⁰, calcium sulfate—CS), and (2) nitrogen fertilization level, NF (0, 30, 60, 90, 120, and 150 kg N·ha⁻¹). In addition to TY, the following EN indicators were analyzed: agronomical efficiency (EA), physiological efficiency (EPh), partial factor productivity (PFP), and recovery (R). For both sources of sulfur, an increase in TY was confirmed. After applying CS, the optimum for the maximum yield was 106 kg N·ha⁻¹, while the application of S⁰ resulted in 134 kg N·ha⁻¹. The impact of SF on the nitrogen economy decreased in the direction of EA = PFP > EF > R and depended on the sulfur carrier. A positive trend was found, associated with the increase in R under the influence of S⁰ and the clearly higher EPh after the application of CS. A particularly strong effect of CS on EA was evident in the range of lower nitrogen doses. The EN values depended on the meteorological conditions during the research years. The strongest variability was subject to EPh, which, as a result of SF, was significantly higher in relation to the control (without S) during the growing season, with an unfavorable distribution of precipitation. The application of CS reduced the unit nitrogen uptake (UU-N). Using path analysis, a direct relationship of Ca accumulation (controlled by N and S) with TY was demonstrated. The conducted research indicates a significant impact of sulfur fertilizers, related to TY and EN, especially visible under conditions of limited nitrogen supply.

1. Introduction

The high yield potential of potatoes and their nutritional value put this plant in the center of interest of agricultural producers around the world [1,2,3]. The production and consumption of potatoes is steadily increasing in developing countries, while in developed countries, despite a downward trend, they are still an important element of the human diet [4,5,6]. In areas where potatoes are consumed in large quantities—for example, in Latin American countries—their contribution to the supply of energy, protein, iron, and zinc is significant [4]. Potato starch ranks third in world production after corn and wheat starch and is used for more than just food purposes [7] In Poland, the use of the yield potential of cultivated potato varieties determined at the level of 48–50 t·ha⁻¹ is about 50% [8]. One of the reasons for this is the belief that potatoes can be grown on sites with low natural soil fertility. Changing this view on a global scale should be the starting point for further research. The average global yield according to the FAO is just over 20 t·ha⁻¹, which is not always explained by soil conditions. Increasingly, views appear according to which the use of yield opportunities of this species is insufficient due to climate change [9,10]. In this context, the forecast reduction in potato production in the coming years comes to just a few percent, and in the next decades over 20% [9]. Climate change should be referred to the increased negative impact of both abiotic factors (related to meteorological conditions) and biotic factors (related to pathogen pressure). Therefore, it is important to prepare the plant to overcome stresses, including nutritional stress related to the control of the nutrient supply to the plant.

Most often, the carriers of sulfur in fertilizers are magnesium sulfates, with varying degrees of hydration. However the use of elemental sulfur can significantly modify the yield efficiency of nitrogen, which leads to a higher yield of potato tubers [11]. A positive reaction of winter wheat and winter oilseed rape to the use of elemental sulfur was demonstrated by Kulczycki [12], while the effects of using the carrier (sulfur) were greater in light soil. The use of elemental sulfur for fertilizing onions can cause an increase in yield at the level of 13–44%, depending on the dose of the nutrient [13]. Calcium sulfate can be another carrier of sulfur. It can be used as a natural gypsum or a product of SO₂ neutralization in CHP (combined heat and power) plants [14]. In agriculture, calcium sulfate (gypsum) is becoming increasingly important and, if properly used, can be an important carrier of sulfur and calcium and provide a strong impulse for more effective improvement of soil fertility, one of the elements of which is nitrogen management [15].

The literature also exposes the role of gypsum formed in the process of flue gas desulfurization to neutralize toxic Al³⁺ in the rhizosphere of plants [16]. This is important due to the fact that many areas in the world have acidic soils. The field tests confirmed the positive effects of calcium sulfate in many crops, such as winter wheat, alfalfa, corn, and on swards—in a mixture of alfalfa and grasses [17,18,19].

The synergism between nitrogen, phosphorus, and potassium in potato production is well recognized not only in systems based on mineral fertilization [20,21], but also under the conditions of combining different sources of ingredients, including manure [22,23]. The problem of using sulfur and calcium to increase the efficiency of nitrogen use by potatoes from various sources, although raised in some works [24,25,26], is still an important research problem. Balanced fertilization of crops with sulfur is not only associated with an increase in nitrogen yield but also increases the contents of defensive compounds against pathogens. This phenomenon is called sulfur-induced resistance [27]. The multidirectional interaction of nitrogen and sulfur has been confirmed in winter oilseed rape, winter wheat, and corn crops [28,29,30]. In the case of corn, the effect of applying sulfur was manifested at higher doses of nitrogen.

The volume of plant production is the result of many production factors determining the implementation of the genetic potential of individual varieties, among which nitrogen management is the key one [31,32,33]. The influence of nitrogen doses on the yield and quality of tubers has been investigated in numerous works in various regions of the world. However, determining the optimal level of fertilization requires the consideration of many environmental conditions [34,35,36]. Varieties currently grown in the world require large doses of nitrogen fertilizer, but on the global scale its effectiveness is low. This means that most of the nitrogen introduced into the soil is not used by plants, which, in turn, creates serious problems for the environment [37]. Sharma and Bali [38] stated that nitrogen dispersion in the environment may be no more than 30%, provided that the available diagnostic tools are used to determine the current nitrogen resources in the plant/soil/fertilizer system, taking into account the dynamics of nitrogen uptake by plants. Hence, the need to determine the required supply of nitrogen and precisely define the conditions affecting its effective use. This applies equally to fertilizers used and soil nitrogen resources, referred to as residuals. Knowledge of the amount of nitrogen to be managed in potato production is the basis for the sustainable management of this important yield-producing nutrient [39]. The next step is to implement measures to convert nitrogen into crop yield. A parallel goal, often considered contradictory, is to minimize the impact of the use of means of production on environmental pollution [40]. Therefore, the concept of yield gap appears, which is defined as the difference between the yield resulting from the influence of the main production factors (i.e., water and nitrogen available to plants during the growing season) and the actual yield [33,41]. Grzebisz et al., in 2018 [40], reported that the yield gap indicates unused nitrogen reserves in crops due to inefficient use of this nutrient, regardless of its pool. For this reason, the proper balance of minerals becomes particularly important [16]. The growth of potato organs during the growing season determines the final yield of tubers. Deficiency of two key growth factors, i.e., water and nitrogen, is the main cause of disturbances in the development of organs conducting photosynthesis (source) for developing tubers (sinc) [11,42]. According to Grzebisz et al. [43], the nitrogen content in the stems is the best predictor of tuber yield. Seasonal variability in the nutritional state of potato plantations, shaping the amount of assimilates produced, affects the structure of their subsequent division among plant tissues [20,44].

At the beginning of this study, it was assumed that both of these nutrients, apart from specific physiological functions, can control nitrogen metabolism at a minimum of three levels: The first concerns the ability of plants to take up nitrogen, which is a function of the availability of the nutrient in soil and the activity of the root system. The efficiency of uptake of ammonium cations and nitrate anions depends on the range of roots penetrating the soil profile, which is associated with calcium activity in meristematic tissues and extended root longevity [45]. This, in turn, causes increased penetration of deeper layers of the soil. The deficiency of calcium cations in the nutrient solution is manifested by a drastic reduction in the length of primary roots and increased proliferation of secondary roots, which requires greater energy expenditure [46]. In addition, Ca²⁺, as an ion accompanying auxins, contributes to increasing nitrogen uptake [47].

The second aspect is the control of the productivity of photosynthetically active organs, i.e., current assimilation measured by the physiological efficiency of nitrogen. The whole cycle of processes aimed at the production of complex nitrogen compounds depends on the supply of secondary nutrients to green organs. Sulfur has an effect on many levels, among which the most important are control of chlorophyll synthesis and photosynthesis efficiency (ferredoxin), nitrogen metabolism (nitrogenase), participation in protein synthesis, and responses to abiotic stress (related to weather conditions) and biotic stress (dependent on the presence of pathogens) conditioned by the presence of glutathione [48,49]. The presence of calcium in the plant in the phase of strong physiological activity is associated with the induction of signals related to abiotic stress, which, in turn, is associated with the activation of appropriate defense reactions of the plant [45,50].

The yield potential of potatoes can be achieved after meeting the third aspect—the remobilization process, which involves the movement of assimilates from the aboveground organs to developing tubers conditioned by a good supply of sulfur [11,49].

The aim of this study was (1) to indicate the role of sulfur and the presence of calcium in sulfur fertilizer, against the background of increasing nitrogen doses, in shaping yield structure and the accumulation of nutrients in yield, and (2) to assess the indicators reflecting nitrogen efficiency.

2. Materials and Methods

2.1. Experiment Description

This research was conducted in 2012–2014 in Rybitwy, Poland (52°31′11.7″ N, 17°20′34.0″ E). The experiment was carried out on soil that originated from sandy loam, classified as Albic Luvisols (Neocambic) [51]. The arable layer (0–30 cm) was characterized by a slightly acidic reaction (

Table 1. Soil characteristics of the experimental plots during the 2012–2014 growing seasons.

Soil Property	2012	2013	2014
pH (1 M KCl)	5.8	5.9	5.8
Nmin (kg∙ha−1) I/	46	51	56
Available P (mg P·kg−1) II/	50 M V/	57 M	53 M
Available K (mg K·kg−1) II/	112 M	116 M	116 M
Available Mg (mg Mg·kg−1) III/	81 VH	80 VH	85 VH
Sulfur S-SO4 (mg S·kg−1) IV/	9.0 L	11.5 L	11.1 L
Organic carbon (g·kg−1)	9.9	10.9	11.3

^I/ Layer 0–90 cm (measured in 0.01 CaCl₂); ^II/ Egner–Riehm method; ^III/ Schachtschabel method; ^IV/ Bradsley–Lancaster method; ^V/ classes of the available nutrient contents: L—low, M—medium, VH—very high.

). The contents of available potassium and phosphorus were at average levels of abundance, while for magnesium a very high abundance was demonstrated. In each year of the study, the amount of sulfate sulfur was classified within the low content range. The mineral nitrogen (N_min) content in individual years of the study in the 0–90 cm layer was in the range of 46–56 kg N·ha⁻¹.

2.2. Weather Conditions

Monthly precipitation totals for the perennial survey area are shown in Figure 1. The growing season of 2012 was characterized by favorable humidity conditions. In June and July, rainfall was twice as high compared to a period of many years. May and August were months where the amount of precipitation was at the level of the long-term average. In the following year (2013), in May, June, and September, precipitation slightly exceeded the values recorded over a period of many years. A significant deficiency of rainfall occurred in July and August. In the 2014 growing season, April, May, and August were characterized by precipitation at a higher level compared to the perennial one. The remaining months (June and July) were clearly distinguished by a lower amount of precipitation in relation to the values obtained over many years. In general, in the period from April to September, the rainfall totals (in mm) in the individual years were as follows: 444 (2012), 370 (2013) and 374 (2014).

In 2012, during the potato growing season, the average temperatures over many years were lower than the monthly averages (Figure 2). The exception was June, where the differences in temperature between the growing season in 2012 and the perennial were small. The average monthly temperatures in 2013 were slightly higher than the temperatures occurring over a period of many years. A deviation from this rule occurred in September, where the temperatures did not differ from the values obtained over many years. In 2014, July recorded a higher temperature than the average temperature obtained over a period of many years. The remaining months, in terms of temperature, did not differ much from the values obtained in previous years.

2.3. Experimental Design

The two-factor experiment was performed in four repetitions. The following experimental factors were investigated:

(A)

Sulfur fertilization (SF):

(1)

Control (without S fertilization);

(2)

Elemental sulfur (S⁰);

(3)

Calcium sulfate (CS).

(B)

Nitrogen fertilization level (NF): 0, 30, 60, 90, 120, and 150 kg N·ha⁻¹.

The S⁰ fertilizer contained 90% S in elemental form, while in the CS fertilizer the sulfur content was 17% S. The second of these fertilizers also contained 21.3% Ca. Both sulfur fertilizers were applied in doses corresponding to 35 kg S·ha⁻¹, about 3 weeks before planting the potatoes. Shortly before planting the potatoes, 125 kg K·ha⁻¹ in the form of potassium salt and 26 kg P·ha⁻¹ in the form of triple superphosphate were introduced into the soil. Nitrogen was applied in doses determined by the experimental scheme in the form of ammonium nitrate before planting. Each time, the forecrop for potatoes was spring barley. In the experiment, potatoes of the starch variety Skawa were grown. The area of one plot was 32 m² (8 m × 4 m). The tests were carried out in four replicates.

2.4. Sampling of Plant Material and Chemical Analysis

The harvest was brought in from an area of 10.8 m² from each plot. In order to determine the yield structure, tubers were divided into the following fractions (FY, mm): 0–30, 36–50, 51–60, and more than 60. The nutrient contents were calculated on a dry matter basis.

The collected plant material was dried at 60 °C and then ground and burned at 550–600 °C. The obtained ash was then dissolved in 33% HNO₃. The calcium concentration was measured by atomic absorption spectrometry—flame type. The content of sulfur in the plant was determined by the nephelometric method (Specord 40, Jena, Germany). Nitrogen content was determined by the Kjeldahl method (Kjeltec 8200, FOSS, Hillerod, Denmark). The results were expressed on a dry matter basis.

2.5. Calculated Indicators

The following parameters of fertilizer nitrogen efficiency were calculated:

Agronomical efficiency: EA = (TY_N − TYc)·D⁻¹, kg·kg⁻¹

(1)

Physiological efficiency: EPh = (TY_N − TYc)·(TU-N_N − TU-Nc)⁻¹, kg·kg⁻¹

(2)

Partial factor productivity: PFP = TY_N∙D⁻¹, kg·kg⁻¹

(3)

Recovery: R = (TU-N_N − TU-Nc)·D⁻¹·100, %

(4)

where:

• D—Nitrogen dose (kg N·ha⁻¹);
• TY_N—Yield of fertilized plants (kg·ha⁻¹);
• TYc—Yield of plants from the control (not fertilized) (kg·ha⁻¹);
• TU-N_N—Total nitrogen uptake by fertilized plants (kg N·ha⁻¹);
• TU-Nc—Total nitrogen uptake by plants from the control (not fertilized) (kg N·ha⁻¹).

This study also analyzed the unit intakes of nitrogen (UU-N), sulfur (UU-S), and calcium (UU-Ca), which were calculated as follows:

UU-N = TU-N·TY⁻¹, kg·t⁻¹

(5)

UU-S = TU-S·TY⁻¹, kg·t⁻¹

(6)

UU-Ca = TU-Ca·TY⁻¹, kg·t⁻¹

(7)

where:

• TY—Tuber yield (t·ha⁻¹);
• TU-N—Total nitrogen uptake (kg·ha⁻¹);
• TU-S—Total sulfur uptake (kg·ha⁻¹);
• TU-Ca—Total calcium uptake (kg·ha⁻¹).

2.6. Path Analysis

The method of path analysis was used in this study. This method entails determining the impact of cause and effect variables, on the basis of which it is established how the cause determines the effect. Analysis based on path factors is used in systems of high complexity, in terms of both the number of variables and their interrelationships. Each elementary path that connects two variables in a cause-and-effect system can have a number assigned, called the path coefficient. This is a bare quantity and can take any real quantities [52]. The adopted procedure allows one to trace the interaction between independent variables in shaping the dependent variable. This paper presents diagrams showing the relationship between yield (TY, dependent variable) and the uptake of total nitrogen (TU-N), calcium (TU-Ca), and sulfur (TU-S), which are independent variables.

2.7. Statistical Calculations

Analysis of variance was used to assess the impact of experimental factors (i.e., year, sulfur fertilization, and nitrogen fertilization level) and their mutual interactions on the tuber yield and nitrogen productivity parameters. Means were separated by honest significant difference (HSD), and Tukey’s method was applied when the F-test showed significant factor effects at p < 0.05. Pearson’s correlation and linear regression were used in order to analyze the relationships between the examined characteristics. For all statistical analyses, STATISTICA 12 software was used (StatSoft Inc., Tulsa, OK, USA, 2013).

3. Results

3.1. Yield and Yield Structure

The average yield of potatoes during the experimental period ranged from 41.65 (2014) to 51.61 (2012) t∙ha⁻¹ (

Table 2. Yield and yield structure of potatoes.

Factor	Level of Factor	Total Yield TY (t·ha−1)	Yield in Fraction FY (mm)
			0–35	36–50	51–60	60<
			t·ha−1
Year (Y)	2012	51.61 a	3.12 b	10.41 b	21.78 a	16.30 b
	2013	49.94 b	1.97 c	9.80 b	11.27 c	26.90 a
	2014	41.65 c	4.06 a	12.40 a	13.17 b	12.02 c
p		***	***	***	***	***
S fertilization (SF) kg S·ha−1	Control	45.68 b	3.14	10.34	14.36 a	17.84
	S0	48.63 a	2.88	11.13	15.56 ab	19.06
	CS	48.89 a	3.12	11.15	16.45 b	18.17
p		***	n.s.	n.s.	**	n.s.
N fertilization (NF) kg N·ha−1	0	36.31 d	2.46	10.33	13.49 b	10.03 d
	30	45.83 c	3.04	11.10	15.77 ab	15.92 c
	60	49.45 b	3.64	11.85	15.84 ab	18.12 bc
	90	50.52 bc	2.91	10.69	15.75 ab	21.17 ab
	120	52.20 a	3.26	10.27	16.16 a	22.50 a
	150	52.10 a	2.98	11.00	15.53 ab	22.59 a
p		***	n.s.	n.s.	*	***
Source of variation for interactions
Y × SF		n.s.	n.s.	n.s.	n.s.	n.s.
Y × NF		***	*	*	n.s.	***
SF × NF		**	n.s.	n.s.	n.s.	n.s.
Y × SF × NF		*	n.s.	n.s.	n.s.	n.s.

Similar letters means a lack of significant differences using Tukey’s test; ***, **, and * indicate significant differences at p < 0.001, p < 0.01, and p < 0.05, respectively; n.s.—non-significant. Legend: Control—without S, S⁰—elemental sulfur, CS—calcium sulfate.

). The variability of yields in the research years resulted from meteorological conditions in the individual growing seasons (Figure 1 and Figure 2). Among the examined fertilization factors, the effects of both sulfur fertilization (SF) and nitrogen dose (NF) were confirmed. The positive effect of sulfur in shaping the yield of potatoes occurred regardless of the carrier of this element in the fertilizer. The average yield increase under the influence of sulfur fertilizer was 7%. Table 2 shows that the average yield increased to 120 kg N∙ha⁻¹. The SF × NF interaction allows the optimal nitrogen dose to be determined for the experimental conditions. The yield reaction to the level of nitrogen fertilization depended on the type of sulfur fertilizer (Figure 3). When calcium sulfate (CS) was used, the maximum yield of potatoes (TY) over the three-year study period was harvested after the application of 106 kg N∙ha⁻¹, while after the application of elemental sulfur (S⁰) the maximum occurred at the fertilization level of 134 kg N∙ha⁻¹. In turn, for the treatment without sulfur (control), the maximum value was 120 kg N∙ha⁻¹, according to the following equations:

TY–control = −0.0012 NF² + 0.2903 NF + 33.606; for n = 6, R² = 0.972, and p ≤ 0.01

TY–S⁰ = −0.0008 NF² + 0.2154 NF + 39.162; for n = 6, R² = 0.963, and p ≤ 0.01

TY–CS = −0.0013 NF² + 0.2762 NF + 39.215; for n = 6, for R² = 0.952, and p ≤ 0.05

where TY is the total yield (t·ha⁻¹) and NF is the nitrogen dose (kg N·ha⁻¹).

A question arises about the relationship between NF and SF in the individual years of the study. In 2012, the highest yields were recorded, with a small reaction to nitrogen fertilization, but conditioned by the use of sulfur fertilizers and dependence, especially in the range of low doses of nitrogen. The regression correlation between NF and TY was confirmed only when no sulfur was applied (Figure 4). This growing season featured a creeping reaction of plants to the use of S⁰ and a significant increase in TY only after the application of the highest dose of nitrogen (150 kg N·ha⁻¹). For CS, the maximum TY occurred at a dose of 60 kg N∙ha⁻¹.

In 2013, although a reduced yield level in comparison to 2012 was statistically proven, the average harvest reduction was only 1.7 t·ha⁻¹. Nevertheless, the strongest reaction of plants to the dose of nitrogen was recorded. The determined optima for nitrogen doses, depending on sulfur fertilization, were 116 kg N·ha⁻¹ (without S) and 112 kg N·ha⁻¹ (for S⁰ and CS). In the last year of the study (2014), the lowest levels of TY were found, while limiting the impact of NF. This was evidenced by the flattening of the course of the TY curves, regardless of the type of sulfur carrier. Under these conditions, nitrogen doses of 126, 110, and 135 kg N·ha⁻¹ should be recommended for the control (without S), S⁰, and CS, respectively.

For all tuber fractions, variability in years was recorded (Table 2). Interestingly, for the tuber fraction with the largest diameter (over 60 mm), the highest yield was obtained in 2013, and it was 65% higher than in 2012 and more than twice as high as in 2014. In the growing season, when the highest yields were recorded (2012), the majority of the dry matter of the tubers was contained in the 51–60 mm fraction. Only for this fraction (F 51–60) was the effect of sulfur fertilization proven, but compared to control plants the effect of CS turned out to be stronger. The opposite effect occurred for the largest tubers, with the positive effect of S⁰ in this case being considered only in the category of a trend. For nitrogen doses, the greatest effect was confirmed for tuber fractions with a diameter greater than 60 mm (for p < 0.001). The effect of NF on yield for FY 51–60 was significant, but in this case p < 0.05. In each year of the study, TY was associated with the dry matter of the largest tubers (

Table A1. Correlation matrix of total yield and yield structurel n = 18.

Year		FY 0–35	FY 36–50	FY 51–60	FY 60<
2012	TY	0.19	−0.09	0.57 **	0.66 **
	FY 0–35	1.00	0.01	−0.16	0.11
	FY 36–50		1.00	−0.06	−0.63 **
	FY 51–60			1.00	0.01
2013	TY	−0.03	0.06	0.36	0.94 ***
	FY 0–35	1.00	−0.03	0.41	−0.24
	FY 36–50		1.00	0.38	−0.20
	FY 51–60			1.00	0.04
2014	TY	0.47 *	0.76 ***	0.76 ***	0.73 ***
	FY 0–35	1.00	0.62 **	0.09	0.11
	FY 36–50		1.00	0.51 *	0.22
	FY 51–60			1.00	0.39

Note: ***, **, and * indicate significant differences at p < 0.001, p < 0.01, and p < 0.05, respectively. Legend: TY—total yield, FY—fraction of tubers (0–30, 36–50, 51–60, and 60< mm).

). In addition, in 2012, TY was positively correlated with FY 51–60, but the relationship between this fraction and F 36–50 was inversely proportional. In the last year, a positive dependence of yield on all tested tuber fractions was confirmed.

3.2. Nitrogen Efficiency Indicators

All analyzed nitrogen efficiency indicators showed variability between years (

Table 3. Nitrogen efficiency indicators.

Factor	Level of Factor	EA (kg·kg−1)	EPh (kg·kg−1)	PFP (kg·kg−1)	R (%)
Year (Y)	2012	172 c	224 b	790 a	83 b
	2013	342 a	284 a	780 a	123 a
	2014	213 b	282 a	646 b	80 b
p		***	***	***	***
S fertilization (SF) kg S·ha−1	Control	210 b	239 b	706 b	92
	S0	252 a	260 ab	748 a	105
	CS	266 a	292 a	762 a	91
p		***	**	***	n.s.
N fertilization (NF) kg N·ha−1	30	440 a	329 a	1527 a	154 a
	60	281 b	272 a	824 b	112 b
	90	199 c	284 ab	561 c	72 c
	120	163 d	220 bc	435 d	76 c
	150	130 e	213 c	347 e	64 c
p		***	***	***	***
Source of variation for interactions
Y × SF		n.s.	**	n.s.	*
Y × NF		***	n.s.	***	n.s.
SF × NF		***	*	***	n.s.
Y × SF × NF		n.s.	n.s.	n.s.	n.s.

Similar letters means a lack of significant differences using Tukey’s test; ***, **, and * indicate significant differences at p < 0.001, p < 0.01, and p < 0.05, respectively; n.s.—non-significant. Legend: Control—without S, S⁰—elemental sulfur, CS—calcium sulfate, EA—agronomical efficiency, EPh—physiological efficiency, PFP—partial factor productivity, R—recovery.

).

This indicates a strong relationship between meteorological conditions and the efficiency of nitrogen fertilizers. In general, the highest values of all indicators were recorded in the 2013 growing season. However, for PFP, the average value for 2013 did not differ significantly from that obtained in 2012, just like EF in 2013 and 2014. The positive effect of sulfur fertilizers on EA and PFP was unequivocal in relation to the control (without S), with the type of fertilizer having no impact on these indicators. In the case of EF, a stronger effect of CS was revealed, while the introduction of sulfur into the fertilization system did not alter the nitrogen recovery (R). After applying S⁰, the average R value exceeded 100%, which should be referred to the priming effect. As shown in Table 3, all indicators used in the studies decreased with the dose of nitrogen, and the greatest variation in values was recorded for PFP.

In further considerations, attention should be paid to the interactions between experimental factors. The influence of sulfur fertilizers on nitrogen efficiency, in connection with the meteorological conditions in different growing seasons, was revealed especially in relation to EPh—a parameter describing the metabolic efficiency of nitrogen intake in shaping the useful yield (Figure 5). The strongest effect of sulfur fertilization on EPh occurred in the last year of the study (2014). Compared to the control, EPh increased under the influence of S⁰ and CS by 51 and 42%, respectively.

Linking the dose of nitrogen fertilizer with the application of sulfur is very important for economic and environmental reasons, both related to the dispersion of nitrogen in the environment. The obtained test results clearly confirm the strong interaction of both fertilizing agents. EA was particularly strongly differentiated in the range of lower nitrogen doses, which should be directly related to the high yield-generating effects resulting from the use of SF (Figure 6). In this way, the importance of the fertilizers used in conditions of limited nitrogen supply was confirmed. Due to the prices of nitrogen fertilizers, the obtained research results should be considered crucial in the effective fertilizer management of the farm. For EA, the relationship between SF and NF changes exponentially. However, it is worth noting that for doses of 120 and 150 kg N·ha⁻¹, the curves determined for variants of sulfur fertilization coincide. A similar pattern of relationships was noted for PFP, with the differentiation between sulfur fertilizers (S⁰ and CS) proven only for the smallest nitrogen dose of 30 kg N·ha⁻¹ (Figure 7). When considering the relationship between EF and nitrogen dose against SF, a difference between the sulfur fertilizers is visible. Compared to CS, in variants with elemental sulfur (S⁰), the system shows some stability, related to the lower variability of values in the dose range of 30–150 kg N·ha⁻¹, as described by function 2°. After applying CS, the EPh values decreased in a linear fashion (Figure 8).

3.3. Accumulation of Nitrogen, Calcium, and Sulfur

The assessment of potato plantations during the harvest period was based on the accumulation of three minerals contained in the fertilizers used, namely, nitrogen, calcium, and sulfur (

Table 4. Total uptake (TU) of nutrients.

Factor	Level of Factor	TU-N (kg N·ha−1)	TU-Ca (kg Ca·ha−1)	TU-S (kg S·ha−1)
Year (Y)	2012	177.8 a	4.19 a	21.64 a
	2013	145.3 b	3.41 b	17.46 b
	2014	138.4 c	2.85 c	12.91 c
p		***	***	***
S fertilization (SF) kg S·ha−1	Control	148.9 b	3.30 b	15.20 b
	S0	160.6 a	3.58 a	18.53 a
	CS	152.1 b	3.58 a	18.28 a
p		**	***	***
N fertilization (NF) kg N·ha−1	0	104.0 d	2.53 c	12.57 c
	30	137.0 c	3.31 b	17.11 b
	60	157.8 b	3.63 a	17.05 b
	90	155.3 b	3.75 a	18.51 ab
	120	182.4 a	3.87 a	18.89 ab
	150	186.8 a	3.82 a	19.89 a
p		***	***	***
Source of variation for interactions
Y × SF		**	*	n.s.
Y × NF		**	***	*
SF × NF		n.s.	n.s.	n.s.
Y × SF × NF		n.s.	n.s.	n.s.

Similar letters means a lack of significant differences using Tukey’s test; ***, **, and * indicate significant differences at p < 0.001, p < 0.01, and p < 0.05, respectively; n.s.—non-significant. Legend: Control—without S, S⁰—elemental sulfur, CS—calcium sulfate, TU-N—total nitrogen uptake, TU-Ca—total calcium uptake, TU-S—total sulfur uptake.

).

The influence of experimental factors on total uptake (TU) was proven for all analyzed nutrients. The obtained TU levels should be related to the yield and closely related to the meteorological conditions (highest values in 2012). Under the influence of nitrogen fertilizer, the accumulation of this element increased significantly to a dose of 120 kg N·ha⁻¹; in the case of sulfur, the maximum occurred for the highest dose of nitrogen. The least effect of nitrogen fertilization occurred with respect to calcium accumulation (TU-Ca), as a significant increase in TU-Ca was noted up to a dose of 60 kg N·ha⁻¹. The applied sulfur fertilizers increased the accumulation of calcium and sulfur. Interestingly, this increase did not depend on the chemical formulation of the fertilizer. For nitrogen accumulation, however, only the addition of elemental sulfur was proven.

Unit sampling is defined as the quantity of an ingredient needed to produce a certain weight of the crop.

Table 5. Unit uptake (UU) of nutrients.

Factor	Level of Factor	UU-N (kg N·t−1)	UU-Ca (kg Ca·t−1)	UU-S (kg S·t−1)
Year (Y)	2012	3.43 a	0.0810 a	0.35 b
	2013	2.87 b	0.0680 b	0.42 a
	2014	3.31 a	0.0682 b	0.31 c
p		***	***	***
S fertilization (SF) kg S·ha−1	Control	3.24 a	0.0712	0.33 b
	S0	3.28 a	0.0731	0.38 a
	CS	3.10 b	0.0728	0.37 a
p		**	n.s.	***
N fertilization (NF) kg N·ha−1	0	2.83 c	0.0684 b	0.34
	30	3.00 bc	0.0716 ab	0.37
	60	3.22 b	0.0730 a	0.37
	90	3.10 b	0.0741 a	0.37
	120	3.50 a	0.0741 a	0.36
	150	3.60 a	0.0733 a	0.38
p		***	**	n.s.
Source of variation for interactions
Y × SF		***	***	n.s.
Y × NF		**	n.s.	**
SF × NF		***	n.s.	n.s.
Y × SF × NF		***	n.s.	n.s.

Similar letters means a lack of significant differences using Tukey’s test; *** and ** indicate significant differences at p < 0.001 and p < 0.01, respectively; n.s.—non-significant. Legend: Control—without S, S⁰—elemental sulfur, CS—calcium sulfate, TU-N—total nitrogen uptake, TU-Ca—total calcium uptake, TU-S—total sulfur uptake.

shows the variation in unit uptake (UU) by year for all components. Unlike the case of TU, sulfur fertilization had no effect on UU-Ca, and the UU-S values were at the same level regardless of the nitrogen dose. One of the important elements of the assessment of plantation nitrogen management is the interpretation of UU-N related to the system of fertilization with secondary nutrients.

Smaller UU-N combined with efficiency indicators means that fertilization costs can be reduced. Taking into account the sulfur fertilization system, it should be stated that under the influence of calcium sulfate, UU-N was significantly smaller compared to the controls and S⁰. In this context, the interaction between SF and NF was analyzed (Figure 9).

The effect of including CS in the fertilization strategy turned out to be a reduction in the nitrogen requirements of the plantation, visible in the dose range of 30–90 kg N·ha⁻¹. This explains the increase in EPh under the influence of calcium sulfate, which compared to the variant with elemental sulfur was 19% (doses of 30 and 60 kg N·ha⁻¹) and 36% (90 kg N·ha⁻¹).

3.4. Linking Yield to Nutrient Accumulation

We looked for the relationship between tuber yield and the accumulation of nutrients during the harvest period. The correlation analysis did not allow us to determine the nutrient whose accumulation was most strongly associated with yield. Therefore, the analysis of path coefficients was used, not only taking into account the direct influence of independent variables (x) on the dependent variable (Y), but also describing indirect influences, i.e., interactions between variables x (Figure 10). The component most strongly associated with yield turned out to be calcium, which was particularly visible in 2012 and 2013. In addition, in both growing seasons, an indirect effect was revealed, i.e., the impact of TU-N and TU-S accumulation on TU-Ca. This interaction was the weakest in the year with a rainfall deficit, in which the yield level obtained was the lowest.

4. Discussion

4.1. Influence of Experimental Factors on Yield Effects

Full use of the yield potential of each crop is the result of many agrotechnical factors. One of the most important is the management of secondary nutrients [16,53]. The research undertaken here assumed a different yield reaction of potatoes to nitrogen fertilization depending on the level of sulfur supply to the plant.

An important result of this research turned out to be variability between years, resulting from meteorological conditions. If we were to refer this to the distribution of rainfall, it should be stated that in the first year of the study there were splendid conditions for potato vegetation, especially in June and July (Figure 1). This is a critical period for potato growth, when tubers are set and grow intensively, with the maximum growth rate in calendar terms at the turn of July and August. The seasonal development of potato tubers is a function of weather conditions during the growing season and nutrient supply [21,54]. According to Begum et al. [55], potatoes should be provided with adequate humidity during the period of stolon formation and tuber development. Water scarcity during this period—called tuber formation—leads to disruptions in assimilate production [56]. As a consequence, the plant suffers from “hunger for sugars” in the stolons, which, in turn, leads not only to a reduction in the number of tubers, but also to the death of those already set. This means that water deficiency in the period immediately preceding flowering (June, BBCH 40-44) is a strong stress factor for the plant, the effects of which are irreversible in the later stages of development [57]. In the critical months of 2014 (June and July), there was a mere 89 mm of rain in total, with the average air temperature for July more than 3 °C higher compared to previous years. As a result, weather conditions in 2014 caused a decrease in yield compared to the average of 2012–2013, by 18%, which is in line with predictions [9].

During the research period, the smallest share of large tubers (with a diameter greater than 60 mm) and the largest share of tubers with a diameter of less than 35 mm occurred in 2014. The period of interruption of tuber growth due to drought resulted in the production of not only undersized but also distorted tubers [55]. The results obtained in our research are confirmed by the literature, because according to Klikocka and Sachajko [24], for the research area, the amount of precipitation in May and June determines the number of small tubers, while the July and August rainfall affects the final weight of the large tubers.

Humidity conditions shaped a different reaction to the most important yield factor, which is nitrogen (Figure 4). A nutrient that combines plants’ resistance to stress resulting from rainfall deficiency and the importance of nitrogen in the production and transport of assimilates to developing tubers is potassium [21]. A good supply of K during potato bulking ensures the tuber sink capacity. In each year, our research was carried out on soil with an average potassium content and a fertilizer dose of 150 kg K₂O·ha⁻¹, which was sufficient for adequate nitrogen efficiency. Therefore, the depression of the yield in 2014 cannot be explained by nutritional stress.

Selected yield characteristics are presented in

Table A2. Yield characteristics in the years (2012, 2013, and 2014) of the research.

Year	Indicator	Factor			Mean for Year
		Control	S0	CS
2012	x (t∙ha−1)	49.7	51.9	53.2	51.6
	SD (t∙ha−1)	5.30	2.22	2.47	3.71
	V (%)	10.7	4.3	4.6	7.2
	Min. (t∙ha−1)	40.6	50.0	50.0	40.6
	Max. (t∙ha−1)	55.7	56.3	57.0	57.0
2013	x (t∙ha−1)	48.0	50.8	51.0	49.9
	SD (t∙ha−1)	10.28	11.31	10.60	10.18
	V (%)	21.4	22.3	20.8	20.4
	Min. (t∙ha−1)	28.7	30.2	30.8	28.7
	Max. (t∙ha−1)	56.2	59.6	57.8	59.6
2014	x (t∙ha−1)	39.4	43.2	42.4	41.7
	SD (t∙ha−1)	6.13	4.82	4.42	5.15
	V (%)	15.6	11.2	10.4	12.4
	Min. (t∙ha−1)	28.5	34.3	33.7	28.5
	Max. (t∙ha−1)	46.2	47.7	45.3	47.7

Legend: Control—without S, S⁰—elemental sulfur, CS—calcium sulfate, x—mean, SD—standard deviation, V—coefficient of variation (V = SD·x⁻¹·100).

. The above-described relationships were confirmed, among others, by the values of the standard deviations (SD) and coefficients of variation (V), which reached their highest average values in 2013. In the first year of the study, the variability of yield in the control (without S) was clearly higher, which results from the positive effect of calcium sulphate and elemental sulfur on the yield, on the control not fertilized with nitrogen (Figure 4). This may be a prerequisite for the thesis that both sulfur fertilizers have a particularly strong impact on yield in the event of nutritional stress resulting from lack of nitrogen, but only in conditions with optimal distribution of precipitation during the growing season.

Regardless of weather conditions, the obtained yield effects, on the one hand, testify to the high productivity of the site; on the other hand, they lead to the question of a diverse and relatively small response to the increasing level of nitrogen fertilization. In experiments performed in Polish soils, the optimal dose of nitrogen was assumed to be in the range of 100–120 kg N∙ha⁻¹ [58,59]. For Cohan et al. [60], the nitrogen dose ensuring 95% of the maximum yield of potato tubers was set at 125 kg N∙ha⁻¹, while for the control (without nitrogen) the authors recorded a yield of 50 t∙ha⁻¹, which represented a 25% increase in yield under the influence of the optimal nitrogen dose. In our own research, the control yield level ranged from 32.6 to 38.2 t∙ha⁻¹, and such increases amounted to 50, 34, and 30% for the control (without S), calcium sulfate (CS), and elemental sulfur (S⁰), respectively. The relationship system is a second-degree function, which is consistent with the research of Fontes et al. [34], who showed that the response of several varieties to the increasing level of nitrogen fertilization was best described by the quadratic function. In our own research, these maxima were conditioned by the application of sulfur fertilizers (Figure 3 and Figure 4). The introduction of CS fertilization to the system turned out to be an important factor, allowing us to limit the doses of nitrogen fertilizers, which resulted from both the physiological functions of sulfur [48,49] and the calcium content in the plant [16,45,50,61]. High concentrations of calcium in potato vines between the 89th and 110th days of vegetation can be treated as a condition for high yield of potatoes [26]. It is also possible that this fertilizer affects the processes controlling nitrogen metabolism in the soil. This statement indirectly resonates with the results of research by Skwierawska et al. [62], who observed an increase in the content of nitrate nitrogen in the surface layer of soil fertilized with sulfate sulfur at a dose of 40 kg S∙ha⁻¹. However, such an effect was not obtained when elemental sulfur was introduced into the soil. In this context, the question of the maximum value for variants fertilized with elemental sulfur, set at 134 kg N∙ha⁻¹, becomes important. Cultivated plants absorb sulfur in the form of sulfate anions, which means that when elemental sulfur is used, S⁰ must be oxidized to SO₄²⁻. This process is controlled by sulfur bacteria in the soil. In cultivated soils, oxidation is carried out mainly by bacteria of the genus Thiobacillus and heterotrophic bacteria [63]. It cannot be ruled out that these bacteria stimulated by elemental sulfur, completing their metabolism, competed for nitrogen with plants, which, in turn, translated into greater fertilizer needs. In addition, some chemoautotrophs (e.g., Thiobacillus denitrificants) have the ability to denitrify, obtaining energy from the oxidation of sulfur compounds and causing depletion of nitrates in the environment, i.e., fast-acting ions that are easily absorbed by plants [64]. This statement is confirmed in Figure 4, because in the three-year study period the highest value of maximum yield on plots fertilized with elemental sulfur was recorded in 2012 (150 kg N∙ha⁻¹). This was the year with the most rainfall and, therefore, conditions promoting denitrification.

In this study, the positive yield-forming effect of sulfur at a dose of 35 kg S∙ha⁻¹ was confirmed. While in the literature the issue of fertilizing potatoes with sulfur has been addressed in many studies [25,65,66,67,68,69], the problem of the recommended dose of this nutrient in potato cultivation is constantly discussed. According to the research of Singh et al. [56], the maximum increase in potato yield under the influence of sulfur fertilization was 17%. This work shows that the use of sulfur doses up to 45 kg S∙ha⁻¹ resulted in a strong increase in yield, but after the introduction of 60 kg S∙ha⁻¹ there was a downward tendency. Exactly the same average yield increase was obtained in our own research on a plot that was not fertilized with nitrogen; however, the yield-generating effect of sulfur decreased with the increase in the level of nitrogen fertilization. In an experiment performed by Sharma et al. [25], the yield and quality of tubers increased up to a dose of 45 kg S∙ha⁻¹. Sanli et al. [69] recorded the largest increases in potato yield in the dose range 40–60 kg S·ha⁻¹. An older study by Aulakh [65] indicated that the optimal dose of CS for potatoes was 25 kg S∙ha⁻¹. This means that the dose adopted in the studies—at the level of 35 kg S∙ha⁻¹—can be considered to be close to optimal, although the adopted fertilization strategy must always take into account the sulfate sulfur content of the soil. This content was low in each year of our study. It should be emphasized that the significant effects of both tested fertilizers (CS and S⁰) on the fraction of tubers with a diameter of 51–60 mm may lead to the conclusion that the positive effect of sulfur fertilization is associated with controlling the size of large tubers, which is consistent with the results of Klikocka and Sachajko [24] and Sanli et al. [69].

The positive effect of sulfur fertilizers, similarly to nitrogen fertilizers, was determined by the distribution of precipitation. Calcium sulfate was particularly effective in the wet year (2012). This observation can be explained by conditions promoting CS solubility in the initial period of potato growth [70]. Elemental sulfur showed a strong effect in extreme years in terms of precipitation (2012 and 2014), while in both growing seasons the maximum yield occurred after applying the highest dose of fertilizer.

4.2. Nitrogen Management

The studied indicators of nitrogen efficiency showed variability over the years. The maximum values of EA, EPh, and R occurred in 2013. The PFP values obtained in 2012 and 2013 were at a similar level. These results indicate that in a year with highly humid conditions (2012), both agronomic (EA) and physiological (EF) efficiencies were lower compared to the other years of research. Grzebisz et al. [33], in conducting a complex assessment of efficient nitrogen management, paid attention to water use efficiency (WUE) as the main factor responsible for the yield. It is clear that WUE is closely related to the rate of ion transfer towards the roots [71]. On the one hand, in a year with an even distribution of precipitation, ensuring adequate soil moisture throughout the growing season, the solubility of all fertilizers used (not only nitrogen) and the ion mobility were higher. On the other hand, in these conditions, soil resources could be a significant source of nitrogen for plants, since the rate of nitrogen mineralization during the growing season is a function of temperature and humidity [72]. The effective use of residual nitrogen was the result of better availability of phosphorus and calcium—nutrients responsible for the development of the root system [50,73]. In this way, the plants increased the range of the impact zone related to the inter-rows and deeper layers of the soil profile. As a result, the synergistic interaction of many environmental factors resulted in lower EA and EPh of fertilizer nitrogen in the first year, compared to 2013 and 2014. The observed decrease in the values of all indicators under the influence of increasing nitrogen doses is a normal phenomenon that has been well documented in the literature. Fontes et al. [34] reported that the agronomic efficiency of nitrogen in the dose range of 50–200 kg N∙ha⁻¹ decreased by 1.9–3.7 times, depending on the tested variety. According to the research of Cohan et al. [60], nitrogen efficiency indicators for a potato can be differentiated by variety. This is an argument for the precise determination of plant fertilization needs using various diagnostic tools [38].

Sulfur fertilization significantly shaped the four analyzed indicators of nitrogen efficiency (Table 3). It is necessary to emphasize the positive tendency associated with the increase in R under the influence of S⁰ and the clearly higher EF after the application of CS. The first observation can be explained by the increase in microbiological activity in the soil after the introduction of elemental sulfur [63]. The higher efficiency of conversion of nitrogen into TY expressed by EPh may be the result of the interaction of sulfate sulfur with nitrogen, but also with calcium [16,61,74].

The effect of using elemental sulfur was an increase in the use of nitrogen from nitrogen fertilizers (Table 3). The average use of nitrogen by plants was above 100% (108), which clearly indicates the occurrence of the priming effect discovered by Löhnis in 1926 [75]. This phenomenon is the increased release of mineral nitrogen from organic compounds under the influence of mineral forms of nitrogen carried into the soil [76]. The obtained results for R (Table 3) can also be explained by the ability of some bacteria of the genus Thiobacillus to fix atmospheric nitrogen [77]. In the longer term, after the depletion of elemental sulfur resources and the partial death of the bacteria (as a consequence of their mineralization), an additional nitrogen pool for potatoes could be released. However, the greater use of nitrogen from mineral fertilizers did not translate into an increase in yield (compared to SC), which indirectly confirms the thesis about the availability of soil nitrogen (i.e., not derived from fertilizers); however, this had already occurred in the final stage of potato growth and had no impact on the yield.

Compared to the treatments fertilized with S⁰, the range of variation in EPh under the influence of the nitrogen dose was clearly greater when CS was used and described by the linear function (Figure 8). Importantly, for a dose of 90 kg N∙ha⁻¹ after the application of CS, the average EPh value was 36% higher than after the introduction of S⁰ into the soil. This explains the yield-generating effects recorded over the three-year study period (Table 2; Figure 3 and Figure 4).

4.3. Relationship between Nitrogen Accumulation and the Accumulation of Sulfur and Calcium

The assessment in terms of total uptake (TU) and unit uptake (UU) combines nutrient content and biological yield. In general, sulfur fertilization increased TU-N, TU-S, and TU-Ca, with the exception of the CS treatment, in which nitrogen accumulation did not differ from the control (without S). According to Barczak and Nowak [78], the application of sulfur, regardless of the chemical form of the fertilizer and dose, increased the nitrogen and sulfur content in potato tubers compared to the control, while reducing the amount of calcium. Our research showed that the use of sulfur had a positive effect on calcium management, because TU-Ca did not depend on whether S⁰ or CS was used, and at the same time it was significantly greater than the control (i.e., not fertilized with sulfur) (Table 4). This is a very important effect of this research, because calcium compounds present in the covering tissues cause greater stability of neighboring cells, creating a physical barrier for the pathogen [71,79], and they also trigger metabolic pathways leading to increased synthesis of proteolytic enzymes that act destructively on fungal hyphae [80,81]. The positive effect of calcium was also manifested in the context of unit uptake of nitrogen (UU-N) in the range up to 90 kg N∙ha⁻¹ (Table 5). In crop production, the effect of reducing UU-N is a lower demand for nitrogen to produce a unit of yield, and this positively affects both the economic and ecological effects associated with nitrogen dispersion.

Similarly to calcium, sulfur should also be included among the nutrients responsible for the health of tubers. Sulfur fertilization increases the potato’s resistance to infestation with the Rhizoctonia solani fungus, as well as Streptomyces scabies infection [82]. This nutrient, along with copper and zinc, can reduce susceptibility to late blight of potatoes, caused by Phytophthora infestans. The greater accumulation of sulfur in tubers under the influence of fertilization gives grounds to indicate the health-promoting effects of both tested fertilizers. However, unlike the case of UU-N, the use of sulfur fertilizers caused an increase in UU-S. It should be noted that the use of sulfur fertilizers—regardless of the chemical form—significantly limited UU-N in the year with a deficit and unfavorable distribution of precipitation (2014), in the dose range up to 60 kg N∙ha⁻¹. However, in 2012 and 2013, elemental sulfur fertilization resulted in an increase in UU-N not only compared to the CS treatment, but also in relation to the controls (without S) (Figure A1). Therefore, the impact of S⁰ on the nitrogen distribution should be considered in relation to the precipitation regime during the growing season.

Under conditions of increased calcium supply, antagonism with nitrogen may occur [16]. In comparison with the data of Grzebisz [83,84], the obtained UU-Ca values (Table 5) were very low, which should be associated with the generally low calcium contents in Polish soils, which were formed as a result of glacier activity [85]. When compared to aerial parts, the calcium content in tubers is much lower [21]. Low mobility of Ca²⁺ cations in the xylem from roots to leaves, bypassing tubers [74], becomes a physiological factor limiting the accumulation of this element in the useful parts of the plant.

During the harvest period, a direct relationship between calcium accumulation and yield was shown (Figure 10). Importantly, the indirect effects of nitrogen and sulfur on calcium accumulation were revealed. This means that, under the experimental conditions, nitrogen and sulfur acted synergistically in relation to calcium as a minimum nutritional factor.

5. Conclusions

The effect of the level of nitrogen fertilization (NF) on the tuber yield was determined by sulfur carriers. After the application of calcium sulfate (CS), the nitrogen dose determining the maximum yield was more than 20% lower than for the treatment fertilized with elemental sulfur (S⁰). The values of the nitrogen efficiency indicators depended on the meteorological conditions in the research years. EPh was subject to the strongest volatility (under the influence of sulfur carriers), especially in the growing season with unfavorable rainfall distribution. This means a particularly strong interaction of nitrogen and sulfur under conditions of abiotic stress resulting from water deficit in the key stages of plant development.

A particularly significant effect of CS on EA was evident in the range of lower nitrogen doses. Calcium—more precisely, TU-Ca—turned out to be the factor most strongly associated with yield. The effect of using CS in the fertilization strategy was lower nitrogen demand of plantations (UU-N), visible in the dose range of 30–90 kg N·ha⁻¹. This is of great economic and ecological importance, associated with less nitrogen dispersion in the environment. Due to the prices of nitrogen fertilizers, the obtained effects are crucial for the effective fertilizer management of the farm. The effect of introducing S⁰ fertilization into the system turned out to be an increase in the intensity of processes aimed at releasing residual nitrogen from the soil. However, in case of this fertilizer, greater expenditure on fertilizer nitrogen is necessary.