4.4. Macro- and Micronutrients Enrichment and Their Content
The intensity of the mineralization process of organic compounds is related to the type and amount of plant debris that makes up the soil surface catch [
24]. Plant matter left as mulch contains a lot of organic carbon (C), easily absorbed by soil organisms, which protects minerals from being lost [
21]. The faster the organic matter is broken down, the faster the nutrients are available to the succeeding plants [
24] The effective use of N, absorbed by organic matter, requires the synchronization of N release from harvest residues and N uptake by plants in the next crop [
66].
Sulfur application in the cultivation of garlic increased the dry matter and the content of N, P, K, S, Zn, Mn, and Fe, as well as the total phenolic acids (TPA), antioxidant activity (AA), and glutathione. Simultaneously, it had no significant effect on the level of Ca, Mg, copper (Cu), B, and L-ascorbic acid (LAA) accumulated by plants in bulbs. In the cultivation of garlic with S, the organic mass of CCs quickly decomposed in spring and was mineralized, which favored N uptake by garlic plants. In 2019, in plants cultivated with S, there was a tendency to accumulate N in bulbs with CC plants of clover, radish, buckwheat, and mustard, and in 2020, only with fodder radish. As the mineralization of organically bound N in the soil and its availability for garlic occurred more rapidly with the use of S, in 2019, a similar trend, in the increase in dry matter in garlic bulbs with S and CCs, was expected. This expected tendency was not found because garlic was only cultivated with S and clover with a high and stable dry matter level in bulbs.
Fabaceae plants such as
Trifolium incarnatum are effective CCs, absorb quickly, and release N faster due to their low C:N ratio (range 10–15) as opposed to biomass characterized by a high C:N ratio (
Panacea, mustard > 30), consuming N in the decomposition process [
67].
It is generally believed that applying S to soil may have a synergistic relationship with the bioavailability of N released from organic matter to garlic plants, determining higher yields and a better quality [
48]. The results of many studies indicate the existence of interactions between S and N and their effective positive influence on the yield in
Allium sativum and
Allium cepa [
5,
6,
9,
18]. Literature data show a decrease in the weight of bulbs in
Allium cepa through the use of S [
68], a decrease in the yield of
Allium fistulosum, and an increase in the sharpness of the onion flavor [
7].
In the presented research, the application of S increased the content and uptake of P and K by garlic plants. This factor, which increases garlic’s P levels, signals an additive effect of S. Our findings on the greater availability of P for garlic plants under the influence of S follow other authors’ statements [
10]. Jaggi et al. [
69] found that S reduces the pH of alkaline soil and may improve the P and micronutrient availability for succeeding crops. González-Morales et al. [
70] emphasize that the soil reaction and moisture determine the intensity of uptake of P ions from the soil solution. The effectiveness of P use by plants is influenced by an appropriate S:P:Se balance. The authors mentioned above explained that the absorption of orthophosphoric ions (H
2PO
4− and HPO
42−) by plants largely depends on the S and Se balance in the soil sorption complex. According to Gransee and Führs [
71], the soil’s optimum pH for P uptake is wide (pH ranging from 5 to 7).
This study showed a higher P content in garlic in 2020 because, during intensive plant growth (May–June), more rainfall, and a higher air temperature (June) than in this period in 2019 were recorded. There was a significant interaction effect of S fertilization × type of CC on the P content in garlic bulbs in individual years of research. In 2019, when S was used, the P content in bulbs was only higher when grown with buckwheat. However, in the cultivation with S and clover, the P level in garlic bulbs was lower than in the cultivation without S. Moreover, buckwheat cultivated as a CC combined with S was an efficient source of N (in 2019), P (in 2019 and 2020), and K (in 2020) for garlic plants, as the content of this macronutrient was the highest in bulbs. According to Zarzecka et al. [
72], buckwheat enriches the soil with P and K by releasing organic acids from the roots into the substrate and, consequently, increasing the bioavailability of elements for the succeeding plant. As the soil pH decreases by the release of H
+ ions from the decomposition of organic acids, the absorption of the maximum amount of orthophosphate ions and available forms of K increases. In this study, more P in cloves was determined in cultivation with S (0.42% DM on average) without mineral fertilization than without S (0.38% DM on average). Jiku et al. [
73] determined the P content in garlic cloves at the level of 0.15% DM in cultivation without K fertilization (K0) and 0.35% in cultivation with K 200 kg ha
−1. The above-cited authors also showed that potassium increases the N content in garlic bulbs. With increasing doses of potassium (K0, K200, K175, K250), the content of N (1.5, 2.4, 2.5, 2.1% DM) in garlic bulbs increased.
In our research, using S had a positive effect on the availability of K for garlic plants. Similarly, Youssif et al. [
10] found a high K accumulation efficiency in garlic leaves when S was used in foliar fertilization. However, in our research, the CC (on average, in 2019 and 2020) did not affect and the availability of this element for garlic plants. The uptake of N, P, and K, depending on S and the type of CC, can be associated with slight differences in the dry matter content of garlic bulbs, as the content of these elements mainly affects the dry weight of plants. It can be assumed that growing conditions, mainly temperature, could have influenced the dry matter content in the tested garlic. The content of dry and organic S compounds in
Allium crops depends on the characteristics of the cultivar, plant organs, length of the growing season, and environmental factors, such as water stress, light quality, duration, and temperature in the growth and root zones [
74,
75]. In our experiment, using S significantly increased the level of K (0.52% DM) in bulbs compared to cultivation without S (0.44% DM). Other authors obtained significantly higher values. According to Jiku et al. [
73], the K content of garlic with the applied K fertilization (K200 kg ha
−1) was 2.88% DM. The average content of this element was also high in garlic leaves and amounted to 17.35–27.99 g kg
−1 DM [
76]. High variability of K in bulbs was found in 14 ecotypes of Greek garlic (ranging from 446 to 675 mg 100 g
−1 FW) [
77]. Driba-Shiferaw [
78] believe that the K content of garlic depends on fertilization and soil properties.
In this study, there was no effect of S on the Ca and Mg uptake by garlic plants and the content of these components in bulbs. There was a tendency to accumulate greater amounts of Ca in the bulbs in an S-free crop with clover. In cultivation with S, a higher Ca level in bulbs was only recorded in 2020 in cultivation with fodder radish and buckwheat. In this situation, Ca’s content and uptake could limit the use of S in cultivation with other CCs. Earlier studies found that sulfate sulfur (S-SO4) can precipitate in the form of Ca, which reduces the availability of Ca to garlic plants [
47,
79,
80]. This statement is also supported by the fact that in cultivation without S, the generally used biomass from CCs increased Ca’s availability in the soil and its absorption by succeeding crops [
81]. In our study, garlic cloves were characterized by a similar Ca content (0.37–0.84% DM) compared to the amount (0.62–0.78% DM) determined by Jiku et al. [
73]. Much smaller amounts of Ca were determined in garlic leaves (7.55–28.96 g kg
−1 DM) [
76]. In the studies by Petropoulos et al. [
77], significant differences in the Ca content (from 163 to 963 mg 100 g
−1 FW) in garlic were associated with varietal differences. In these studies, applying S to the soil had a synergistic effect in cultivation with clover biomass, as the garlic plants accumulated more magnesium (Mg) in the bulbs. Zhao [
49] pointed out that Mg in the soil in a form available to plants only occurs in soil solution. The release of this element from organic matter depends on the precipitation in the season. In our research, a higher Mg content in garlic bulbs was demonstrated in 2020, in which, during the bulbs’ formation period, more favorable humidity conditions for plant growth were noted. The use of S in the cultivation of garlic on bare soil decreased the Mg content in bulbs. Similar results proving that Mg is leached more rapidly from the top layers of bare soil were obtained in earlier studies by Melakeberhan et al. [
82]. In the presented study, garlic bulbs were characterized by low and similar Mg levels in cultivation with S (0.10% DM) and without S (0.11% DM). Jiku et al. [
73] found a higher Mg content in garlic 0.20–0.23% DM in cultivation with mineral fertilization. Piątkowska et al. [
76] showed a higher content of this component in garlic leaves, amounting to 0.85–1.32 g kg
−1 DM. Depending on the garlic ecotype, the Mg content ranged from 23.1 to 63.1 mg 100 g
−1 FW [
77].
Using S had a positive effect on the amount of S in garlic bulbs. Such a phenomenon should be considered favorable due to the improvement in garlic bulbs’ quality [
15,
16]. The basic and primary product in incorporating S into organic compounds in the plant is cysteine, which is a precursor to sulfuric amino acids [
9]. In this study, in both years of research, using S in cultivation with clover and fodder radish, and in 2020, with buckwheat and mustard, increased the S level in bulbs. Our finding that the highest amounts of S in bulbs were found in garlic grown with CCs follows others’ results [
82]. Bloem et al. [
16] believe that the sorption of sulfates by organic matter occurs in humic complexes with aluminum (Al) and iron (Fe). Several factors cause seasonal changes in the availability and bioavailability of S for plants: The rate of mineralization of organic matter, leaching, and sorption processes [
17].
In our research, more S was accumulated by plants with S fertilization (0.63% DM) than in cultivation without S (0.41% DM). The obtained results do not differ from those obtained by other authors. In the studies of Bloem et al. [
48], the content of this component ranged from 2.9 to 7.0 mg g
−1 DM, while according to Jiku et al. [
73]), garlic contained 0.43–0.50% DM. The content of this component in garlic leaves was higher, ranging from 2.41 to 6.22 g kg
−1 DM [
76]. Fertilization with S and N (N90S60 kg ha
−1) increased the content of S in garlic bulbs in the range from 3.5 to 5.0 mg 100 g
−1 DM [
19]. Simultaneously, the authors mentioned above suggest that pre-harvest factors, such as cultivar selection, and N and S fertilization determine the size and quality of garlic bulbs yield.
Some have also noted that the yield-generating effect of S in the cultivation of
Allium plants depends on Zn’s availability for plants [
83]. The bioavailability of Zn for garlic plants is largely determined by the soil’s texture and the content of organic carbon in the soil [
10].
In this work, using S increased the bioavailability of Zn to garlic plants. In 2019, the CCs fodder radish, buckwheat, and mustard generated abundant organic matter, with a high organic carbon content, which, it should be assumed, affected the subsequent use of Zn by garlic plants. In 2020, such a beneficial effect of S combined with CCs on Zn’s bioavailability for garlic was not recorded. As shown by the research conducted by White and Broadley [
84], the content of Zn in plants is strongly correlated with the content of this element in the soil and the soil pH. The content of Zn in plants decreases with an increasing soil pH. Zn uptake by plants may reduce S and Fe oxides in the soil [
85]. In the presented research, when S was used, plants accumulated on average more Zn (22.26 mg kg
−1 DM in bulbs and 20.5 mg kg
−1 DM on average) in cultivation without S. The results obtained in this study confirm the regularities observed by other authors who report a high level of Zn in
Allium plants, in leaves of
A. sativum 9.32–13.78 mg kg
−1 DM [
76], in bulbs (0.55–1.18 mg 100 g
−1 FW) [
77], in pseudo-steams
A. porrum (11.96–23.97 g kg
−1 DM) [
86].
In our research, no subsequent influence of S and CCs on the Cu concentration in bulbs was found. In the first year, the factor influencing the solubility, migration, and bioavailability of Cu for garlic plants was S with CC of oilseed radish. It is generally believed that Cu is a slow-moving element, strongly absorbed by plant roots, both with deficiency and excess of the element [
83]. It is believed that low-weight organic compounds released during the decomposition of organic matter increase Cu’s mobility, and thus its uptake from the soil solution [
87]. The above-cited authors have shown that Cu uptake from soil by plants only decreases under conditions of strong absorption by Al, Fe oxides, and Mn oxides, and by the organic matter under alkaline conditions. Since S lowers the soil pH, the absorption of Cu indirectly increases because of the dissolution of compounds that chelate it strongly [
88]. In our research, the slight differences in the interaction of S fertilization × type of CC do not indicate such an action of the organic substance limiting the absorption of Cu by garlic plants because this micronutrient content in bulbs was even recorded in 2019 and 2020. Using S increased the Cu content (2.50 mg kg
−1 DM on average) in garlic compared to cultivation without S (2.31 mg kg
−1 DM on average). However, higher values were given by Golubkina et al. [
86] in pseudo-steams of
A. porrum 3.46–7.18 g kg
−1 DM.
In bulbous vegetables, Mn positively affects the yield and increases the N use efficiency by plants [
89]. In this study, the consequence of S used in the first year was a low Mn level in bulbs cultivated with radish and in the second year, in those cultivated with clover. Moreover, using S in the cultivation of garlic without CCs lowered the Mn level in bulbs. Although, in the second year of research, using S with CCs of buckwheat and mustard increased Mn’s concentration in bulbs, the movement of this element and its uptake could be limited by S. The antagonism of S towards Mn limits
Allium cepa plants’ availability in a controlled greenhouse experiment [
5]. In soil, using S may only indirectly affect Mn’s availability for plants, as it may lower the soil pH and, thus, conducive the bioavailability of this element. The availability of Mn-available forms for plants also depends on the soil moisture status because strong soil moisture promotes accessibility to plants, and drought caused Mn’s transition from the Fe form to the form of dioxide unavailable to plants [
90]. Under the conditions of our experiment, the soil moisture level could have affected a slightly higher level of Mn in garlic bulbs in 2020 because of greater rainfall during the period of intense weight gain of the bulbs (May–June).
More Mn was accumulated by plants in cultivation without S (6.33 mg kg
−1 DM) than with S (6.14 mg kg
−1 DM). Less Mn was determined in pseudo-steams in another species of
A. porrum in the range from 6.93 to 23.15 mg kg
−1 DM. It is presumed that, besides the differences in species characteristics, the differences in the content of this component were also influenced by the cultivation conditions, mainly the soil. In shaping the Fe content in plants, S’s role is emphasized, which reduces the amount of this element in conditions of S deficiency or increases the level of Fe in the plant with a good S supply [
70]. In the cultivation of onions, S antagonizes Fe and Zn [
5]. The solubility and availability of Fe for plants depend on the soil pH, and as the soil pH increases, the amount of Fe available to plants decreases. The availability of Fe in the soil is also strongly influenced by oxidation and reduction reactions, the course of which depends on soil moisture [
91]. In our research, CCs combined with S had a different effect on the Fe content in garlic bulbs. In 2019, using S and the CCs fodder radish, buckwheat, and mustard increased the bioavailability of Fe in the cultivation of garlic, as the level of this element in the bulbs was higher than in those cultivated without S. In 2019, the application of S and clover reduced the Fe in bulbs, and in 2020, increased the content of this element. The positive effect of S application on the Fe level in bulbs was also visible in the second year with garlic cultivation without using plant biomass. Based on these results, it is difficult to generalize unequivocally because deficiencies of this component are usually observed in alkaline soils due to the appearance of sparingly soluble iron hydroxides. In these studies, the soil’s pH (pH
H2O 6.1–6.3) before the experiment was set up was slightly acidic, so using S could have contributed to lowering the soil pH and increased the concentration of this element in garlic cloves. In the cultivation with S, cloves of garlic contained more Fe (28.26 mg kg
−1 DM) than in the cultivation without S (23.81 mg kg
−1 DM). Much lower values were obtained by Petropoulos et al. [
77] in fresh plant material (2.89–5.45 mg per 100 g) and also Piątkowska et al. [
76] in the dry matter of garlic leaves (34.83–85.71 g kg
−1).
In these studies, in 2019, the consequence of S was a low level of B in garlic bulbs, especially in plants cultivated with clover and fodder radish, and in those cultivated without CCs. In 2020, there was no effect of S on the B content in onions. Since, in June 2019, there was a shortage of rainfall, it can be assumed that using S in such weather conditions contributed to a reduction in the B content in garlic bulbs. Coolong et al. [
5] showed that the uptake of B by
Allium cepa plants may be difficult during drought and S fertilization. Similarly, Ozturk et al. [
92] showed that the availability of B to plants is limited by drought and the alkaline pH of the soil. The uptake of B by plants may also be influenced by the content of organic matter in the soil, as a greater amount usually increases the overall abundance of this element [
93].
Under conditions of the experiment, in cultivation without S, B was determined in bulbs much more (7.40 mg kg
−1 DM) than with S (6.47 mg kg
−1 DM). A lower B content characterized garlic from the Kushtia province in Bangladesh; the content of this component in bulbs was 21–23 µg g
−1, in leaves 26–28 µg g
−1 [
73]. According to Hatwal [
94], the use of S in the form of ZnSO
4 (0.4%), or elemental (25 kg ha
−1), and vermicompost (15 t ha
−1) increases the garlic yield. It has a positive effect on the content of garlic bioactive substances. In this study, an increase in secondary metabolites’ content in garlic bulbs was expected due to S. However, it can be seen that this was not found for the LAA content (for two-year mean values). In previous studies, Sałata et al. [
25], whilst investigating the cultivation of garlic for bunch harvesting, found an average reduction in LAA levels of 10% in bulbs and leaves cultivated with serradella, buckwheat, and millet CCs compared to those cultivated in bare soil. In shaping the level of secondary metabolites, the role of environmental factors is emphasized. According to Maggio et al. [
95], the levels of antioxidants such as LAA increase or decrease, responding to environmental stress. In our research, the produced biomass reduced soil heating in various ways, thus reducing evaporation and promoting water absorption and infiltration. On this basis, it can be assumed that the LAA level was more influenced by the weather factor and the type of mulch than by S. A higher LAA content was determined in garlic bulbs in 2019; in this year, during the spring-summer period, there were more favorable thermal conditions than in the same period in 2020. In 2019, the application of S with fodder radish and buckwheat and without CCs lowered the LAA levels in bulbs. In 2020, the combined use of S and mustard biomass resulted in a higher LAA level in garlic bulbs.