Sulfur plays a very important role in plant metabolism, being part of amino acids, proteins, chloroplast molecules, coenzymes, sulfolipids, flavonoids, lipids, glucosinolates, polysaccharides, unsaturated compounds, reduced compounds, among other metabolic functions (STIPP and CASARIN, 2010). Performing essential functions for the development and quality of production, it participates in everything from physiological constitution, hormonal control, photosynthesis to plant defense mechanisms, contributing to protection against pests and diseases (CÉSAR, 2012).

            According to Vitti et al., (2015), nitrogen and sulfur participate together in plant metabolism through two main routes: formation of quality proteins; biological fixation of N from the air and incorporation of mineral N into amino acids. The N/S ratio varies from 10-15/1 and is associated with plant growth; in S deficiency, there is formation of low-quality protein (OLIVEIRA et al., 2020). It is found in two forms in the soil, organic and inorganic, with the organic form in greater proportions, associated with microbial life, being converted into products available to plants (MALAVOLTA, 1980).

            S deficiency in agriculture occurs due to low soil fertility (MALAVOLTA, 1989), associated with the small amount of organic matter, increased S export by grains, caused by high productivity of improved varieties, and sulfate leaching (SCHWAAB, 2020). However, with increased productivity, new cultivars and the frequent use of concentrated fertilizers without S in their composition, and the expansion of agriculture to areas with sandy soils and low organic matter content, productivity limitations may occur due to S deficiency (CRUSCIOL et al., 2006).

            Sulfur deficiency significantly compromises plant development (SCHMIDT, 2012), with its requirements varying according to the species and the desired productivity (RHEINHEIMER et al., 2005). It is an element that participates in the composition of ferredoxins, enzymatic complexes involved in photosynthesis and N₂ fixation (in legumes) and in the formation of chlorophyll (CUNHA et al., 2001), and its protein synthesis is interrupted by its deficiency (RAIJ, 1991).

            In Brazil, due to the lack of knowledge and/or financial support, the transition from the conventional tillage system to the no-till system occurred on most properties without meeting some technical requirements, such as performing in-depth correction (0.00–0.20 m) of soil acidity and phosphorus levels. Thus, it is common to detect soils with low acidity on the surface combined with high acidity in subsurface layers. High acidity ends up reducing/limiting the root development of crops and, consequently, the absorption of water and nutrients, negatively impacting grain productivity (PIAS, 2020).

            Although the organic form represents around 90% of total S in most soils (SOLOMON et al., 2005), it is the inorganic form (sulfate anion – SO₄^-2) that is absorbed by plants, which is found in the soil solution or adsorbed on the surface of colloids bound to iron (Fe) or aluminum (Al) (CASAGRANDE et al., 2003). The availability of S forms in the soil is associated with the organic matter content, the depth of the soil profile, the mineralogical composition, the pH, and the drainage conditions (HOROWITZ, 2003).

            An increase in the probability of a positive response of crops to S application has been observed, which may be due to a series of factors, such as the reduction in atmospheric deposition of S, increased use of concentrated fertilizers (NPK) without the presence of S and increased production potential of crops, among others (PIAS, 2020). The sulfur requirements of crops vary greatly according to the species and expected productivity. The group of medium/high requirement crops includes legumes, which, in general, are more demanding than grasses, due to their higher protein content (ALVAREZ et al., 2007; RHEINHEIMER et al., 2005).

            With the use of concentrated fertilizers, sulfur ends up not being applied, gradually reducing its levels in the soil, which can reduce grain productivity (DOMINGUES, et al., 2004). The plant has the ability to absorb sulfur in the form of sulfate (MENDES et al., 2014). If the nutrient is applied in elemental form, it will only be absorbed after being oxidized (HOROWITZ & MEURER, 2006), however, due to its residual effect, in some soils it becomes more efficient than sulfate (NOVAIS et al., 2007).

            One of the sources of S for crops is agricultural gypsum. Nogueira and Melo (2003) found that the levels of available S (SO₄ ^-2) in the 0 to 20 cm layer of the soil increased with the application of gypsum, but there was a displacement of this S in depth in the profile, leaving little residual effect in this layer for the following years. According to the authors, this occurs due to the high solubility of sulfate, and high precipitations can promote the leaching of this nutrient to deeper layers, or even leave the system explored by the roots (BROCH et al., 2011).

            However, the elementary S (S⁰) has been gaining visibility in Brazil. This is the immediate product of the oxidation of hydrogen sulfide (H₂S), being a stable form (LUCHETA, 2010). Horowitz (2003) reports that the oxidation of S⁰ It is mainly carried out by soil microorganisms. These are affected by management and soil characteristics such as temperature, texture, aeration, pH and fertility (organic matter and nutrients). Another factor that affects the oxidation of the elemental S source added to the soil is its specific surface area, where the greater the specific surface area, associated with the presence of oxidizing microorganisms, the greater the oxidation rate, transforming elemental S into S-sulfate (GERMIDA & JANZEN, 1993). Elemental sulfur stands out from sulfate salts because it has low solubility and contains a high concentration of S (> 85%), compared to 12% of simple superphosphate and 24% of ammonium sulfate (STIPP & CASARIN, 2010). In addition to its high concentration, the attraction of elemental S is its relatively low cost, allowing formulations with high levels of N, P or K (CANTARELLA et al., 2007).

            Several groups of microorganisms can oxidize elemental S in the soil, and they are divided into: a) chemoautotrophs, such as bacteria of the genus Thiobacillus; b) photoautotrophs and c) heterotrophs (bacteria and fungi). In most aerobic (well-oxygenated) soils, chemoautotrophic and heterotrophic organisms are the most important (GERMIDA and JANZEN, 1993).

            In a simplified way, the sulfur cycle (Figure 1) occurs as follows: since sulfur in its elemental form cannot be used by higher organisms, for its assimilation to become possible, microorganisms must oxidize elemental sulfa to sulfates. Photopigmented bacteria of the genera can participate in this process. Chlorobium and Pelodityon, however the most active in this process are the non-photopigmented ones, especially those of the genus Thiobacillus, which can generate sulfuric acid during the process. The sulfate generated can be assimilated directly by plants, algae and various heterotrophic organisms, being incorporated into sulfur amino acids. The same sulfate can also be dissimilated, forming H₂S.

Figure 1: Sulfur cycle

Source: The Chemistry Studies Portal (PEQ)

            The main contribution of S to the soil occurs indirectly, mainly through nitrogen, phosphoric and potassium fertilizers. The most common forms of sulfate fertilizers are: ammonium sulfate (24% of S), simple superphosphate (12% of S), agricultural gypsum (14-18% of S), potassium sulfate (18% of S) and potassium and magnesium sulfate (22% of S). The most concentrated form of application of S as fertilizer is directly in the form of S.⁰ (100% S), as S⁰ bound to bentonite (90% of S) or S⁰ suspended in clay (40 – 60% of S). Some fertilizers can also be coated with S⁰, as is the case with S-coated urea⁰ (10-20% of S) (LUCHETA, 2010).

ILSA SOLUTIONS FOR SUPPLYING SULFUR IN SOIL

            As discussed in the text, sulfur is a secondary macronutrient required in large quantities in several crops, such as legumes. This element plays several roles in plant metabolism, including hormonal control for cell growth and differentiation, helping plants defend themselves against pests and diseases, being an important component in the formation of amino acids and proteins, and improving the nutritional quality of cereals.

            However, in some cases, the application of agricultural gypsum solely for the purpose of supplying sulfur is not economically viable, since the sulfur in agricultural gypsum is in the form of sulfate and this compound is highly soluble in water, which causes the element to move deeper into the soil profile and be lost through the leaching process, reducing the availability of the nutrient for absorption by plants. In this case, the application of sulfur in its elemental source increases the residual power of the nutrient, which enhances its use.

            S-TIME (https://ilsabrasil.com.br/produtos/s-time) from ILSA Brasil is a high-efficiency fertilizer that contains organic nitrogen based on AZOGEL and sulfur in its elemental form. The exclusive S-TIME technology allows AZOGEL to act as a fertilizer conditioner and a potentiator of the oxidation of elemental sulfur. AZOGEL has in its composition a high concentration of organic carbon which will increase the activity of soil microorganisms responsible for the oxidation of sulfur (bacteria of the genus Thiobacillus), around the fertilizer pellet, and consequently increase the use of the nutrient by the plants, which will be made available gradually and according to the plants' needs throughout the production cycle.

            In the experiment carried out in 2021 in partnership with the Federal University of Lavras (UFLA), under the coordination of Ph.D professor Luiz Roberto Guimarães Guilherme and team, we sought to evaluate the oxidation kinetics of elemental S to S-sulfate, after the application of fertilizers containing S in a Red Latosol. In addition, the supply of S to the Triticum aestivum L. (wheat) was also evaluated.

            Nine treatments and three replicates were used to conduct the experiments. The treatments consisted of eight fertilizers containing sulfur (S) and one control (without fertilizer application) (Table 1). Products 3, 4 and 5 refer to the S-TIME fertilizer from ILSA Brasil and differ from each other mainly in the granulometry of the S source that constitutes them (325, 100 and 20 mesh), also presenting a small variation in the nitrogen (N) and S contents. All fertilizers were macerated in a mortar and agate pestle and passed through a sieve with a 1 mm mesh diameter in order to standardize the granulometry, prior to addition to the soil.

Table 1. Description of treatments and nitrogen (N), phosphorus (P) and sulfur (S) levels contained in the fertilizers used in the experiment.

            The amount of each fertilizer applied to the soils, focusing on the macronutrients nitrogen (N), phosphorus (P), potassium (K) and S, was based on the fertilization recommendations for pots proposed by Malavolta (1980), with some modifications to meet the objectives of the study. Thus, amounts of fertilizers were applied to the soil aiming to meet the requirements of 400 mg dm-3 of N, 330 mg dm-3 of P, 200 mg dm-3 of K and 150 mg dm-3 of S.

 RESULTS:

            As observed in figure 2 in general, considering all the periods evaluated, products (2 and 6) together with Product 7 were those that presented the lowest S-SO levels.₄^2-. On the other hand, the highest levels of S-SO were observed for Product 1.₄^2-, with the maximum value (200 mg dm-3) recorded at 119 days. During this period (119 days) high levels of S-SO were also observed.₄^2-for Products 3, 4 and 5, with an average of 148, 159 and 148 mg dm-3, respectively.

            Most treatments presented the highest S-SO levels.₄^2-in 119 days, following the decreasing order (Figure 2) in mg dm-3: Product 1 (200), Product 4 (159), Product 5 (148), Product 3 (148), Product 8 (128), Product 7 (119), Product 6 (105), Product 2 (82) and control – without application (25).

 Figure 2. S-sulfate (S-SO) levels₄^2-) available in typical dystrophic red latosol at 0, 7, 14, 21, 35, 49, 63, 91, 119 and 147 days after the application of different fertilizers with sulfur.

            At the end of the experiments, the S-SO levels₄^2-of soil samples from the 0-20 and 20-40 cm layers were evaluated and these showed significant differences between treatments (Figure 3). For 0-20 cm, the highest S-SO content₄^2- was found for Product 1, followed by Product 5, then Products 3, 4, 6, 7 and 8 in which there was no significant difference and Product 2 and the control (without application) (Figure 3a). Regarding the 20-40 cm layer, the control treatment did not differ from the treatments Product 2, 7 and 8 (Figure 3b).

Figure 3. S-SO contents₄^2- after wheat cultivation for 30 days in a typical dystrophic red latosol with the application of different sulfur fertilizers. Results of ANOVA and Tukey tests (P < 0.05). Different letters correspond to significant differences between treatments.

CONCLUSIONS:

            And the conclusions of the experiment suggest that after the application of fertilizers with S, the highest oxidation rate of elemental S to S-sulfate in the typical dystrophic Red Latosol occurred at 119 days.

            Products 1, 3, 4 and 5 presented the highest levels of S-sulfate (S-SO₄^2-) at 119 days and, when the wheat was planted (147 days), the highest levels were for Products 1, 4 and 5. However, Products 4 and 5 presented higher levels in relation to S in the dry mass of the aerial part of the wheat.

            Product 5 stood out in the present study, as it also presented lower S-SO leaching.₄^2- between the 0-20 and 20-40 cm layers at the end of the two experiments.

            This study showed that fertilizers containing N and S using the AZOGEL® matrix as raw material (S-Time N 10 | S 20) are as efficient as similar traditional products currently available in the Brazilian fertilizer market. Furthermore, no comparative advantage was observed in the use of elemental S with smaller particle sizes as raw material for the manufacture of the aforementioned fertilizers under the conditions of the present study (i.e., homogeneous mixing of the products in the surface layer of the soil and initial reaction pH around 5.0 to 5.5).

            In view of this, S-Time proves to be an ally, contributing to the nutritional balance of the plant when we talk about applying S and thus increasing the final yield of the crop.

Bibliographic references

ALVAREZ, VH et al. Sulfur. In: NOVAIS, RF et al. (ed.). Soil fertility. Viçosa, MG: Brazilian Society of Soil Science; chap. 10, p. 596–635. 2007.

BROCH, Dirceu Luiz et al. Soybean grain yield in cerrado region influenced by sulphur sources. Agronomic Science Journal, v. 42, n. 3, p. 791, 2011.

CANTARELLA, Heitor; TRIVELIN, Paulo Cesar Ocheuze; VITTI, AC Nitrogen and sulfur in sugarcane cultivation. Nitrogen and sulfur in Brazilian agriculture, 2007.

CASAGRANDE, JC et al. Phosphate and sulfate adsorption in soils with variable electrical charges. Brazilian Journal of Soil Science, v.27, p.51-59, 2003.

CÉSAR, F. R. C. F. Effect of elemental sulfur on the efficiency of natural phosphates. 2012. 90f. Dissertation (Master's)–Luiz de Queiroz School of Agriculture, Piracicaba, SP. 2012.

CUNHA, MK; SIEWERDT, L.; JÚNIOR, PS; SIEWERDT, F.; Nitrogen and Sulfur Doses in the Production and Quality of Forage from Natural Grassland of Planossolo in Rio Grande do Sul. Rev. bras. zootec., v.30, n.3, pp 651-658, 2001.

CRUSCIOL, CAC; SORATTO, RP; SILVA, LM da.; LEMOS, LB Application of sulfur as a top dressing on common bean in a no-tillage system. Bragantia, v.65, n.3, p.459-465, 2006.

DOMINGUES, MR; BUZETTI, S.; ALVES, MC; SASSAKI, N. Sulfur and zinc doses in corn crops in two cultivation systems in the recovery of a degraded pasture. Científica, Jaboticabal, v. 32, p. 147-151, 2004.

GERMIDA, JJ; JANZEN, HH Factors affecting the oxidation of elemental sulfur in soils. Fertilizer Research, Wageningen, Netherlands, v.35, p.101- 114, 1993.

HOROWITZ, N. Oxidation and agronomic efficiency of elemental sulfur in Brazilian soils. 2003. Thesis (Doctorate) – Postgraduate Program in Soil Science, Faculty of Agronomy, Federal University of Rio Grande do Sul, Porto Alegre, 2003.

HOROWITZ, N.; MEURER, EJ Oxidation of elemental sulfur in tropical soils. Ciência Rural, Santa Maria, v. 36, p. 822-828, 2006.

LUCHETA, Adriano Reis. Microbiological oxidation of elemental sulfur in soil. Doctoral Thesis. University of São Paulo (USP). 2010.

MALAVOLTA, E. Elements of Mineral Nutrition of Plants. São Paulo: CERES, 1980 p. 251.

MALAVOLTA, E. ABC of Fertilization. São Paulo: Publisher: Ceres, p. 292. 1989.

MENDES, MARCELO CRUZ et al. Nitrogen dose associated with elemental sulfur in topdressing in corn crop under no-tillage. Brazilian Journal of Corn and Sorghum, v. 13, n. 1, p. 96-106, 2014.

NOGUEIRA, MA; MELO, WJ Soybean-available sulfur and arylsulfatase activity in soil treated with agricultural gypsum. Brazilian Journal of Soil Science, v. 27, n. 04, p. 655-663, 2003.

NOVAIS, RF de; SMYTH, TJ; NUNES, FN Phosphorus. Soil fertility. Viçosa, MG: Brazilian Society of Soil Science, p. 471-550, 2007.

OLIVEIRA, Rafael José et al. Oxidation of elemental sulfur in different sources and doses of fertilizers. Brazilian Journal of Development, v. 6, no. 5, p. 27735-27745, 2020.

PIAS, Osmar Henrique de Castro. Soil acidity management and sulfate fertilization in the no-till system. (Doctoral Thesis) Federal University of Rio Grande do Sul, 2020.

RAIJ, BV Soil fertility and fertilization. Piracicaba-SP. CERES, POTAFOS. 343p. 1991.

RHEINHEIMER, DS; ALVAREZ, JWR; OSORIO FILHO, BD; SILVA, LS;BORTOLUZZI, EC Crop response to sulfur application and sulfate levels in a sandy soil under no-tillage. Ciência Rural, Santa Maria, v. 35, p. 562- 569, 2005.

SCHMIDT, F. Morphological and metabolic modifications in tropical forage grasses and legumes related to sulfur supply. Thesis (Doctorate) Piracicaba, 2012. 162p.

SCHWAAB, Jarriel et al. Response of soybean crops to doses of sulfur. 2020.

STIPP, Silvia Regina; CASARIN, Valter. The importance of sulfur in Brazilian agriculture. Agronomic information, v. 129, n. 1, p. 14-20, 2010.

SOLOMON, D. et al. Sulfur speciation and biogeochemical cycling in long-term arable cropping of subtropical soils: evidence from wet-chemical reduction and S K-edge XANES spectroscopy. European Journal of Soil Science, vol. 56, p.621-634, 2005.

VITTI, Godofredo Cesar; OTTO, Rafael; SAVIETO, Julia. Sulfur management in agriculture. Agronomic information, n. 152, p. 02-12, 2015.

Authors

• Agr Eng. Dr. Angélica Schmitz Heinzen
• Agricultural Eng. Msc. Carolina Custodio Pinto
• Agricultural Eng. Msc. Thiago Stella de Freitas