Soybean (Glycine max (L.) Merill) popularly known as “Wonder crop”, “Golden bean” and “Miracle crop” of the 21^st century, is rich in protein (40-42%) and oil content (18-20%) (Patel et al., 2019). In India, soybean has rapidly risen to prominence as the primary oilseed crop from a modest initial cultivation area of 0.03 m ha in 1970, its growth has been swift by 2024, soybean cultivation has expanded to cover 13.25 m ha, yielding a robust production of 13.06 m tonnes with an average yield of 985 kg ha^-1. Zooming on the state of Karnataka, soybean farming extends 0.38 m ha, with a production tally of 0.413 m tonnes. The average yield is slightly better than the national average, standing at 1062 kg ha^-1 (Anonymous, 2024).
       
Among various plant nutrients, Sulphur has emerged as an indispensable secondary macronutrient in plant growth, standing alongside of nitrogen, phosphorus and potassium as a primary nutrient. It is a constituent of proteinaceous amino acids, i.e. cystine, cysteine and methionine. Sulphur is involved in the formation of S-glycosides, coenzyme A, vitamins like biotin and thiamine and iron-sulphur proteins termed ferredoxins. Additionally, sulphur is vital for protection against oxidative damage protein synthesis, chlorophyll formation and facilitating oil synthesis (Chahal et al., 2020).
       
The advantages of sulphur fertilization are most pronounced for oilseeds, followed by pulses and least for cereals (Zenda et al., 2021). However, in India, sulphur deficiencies are notably extensive and dispersed. During the early 1990s, nearly 130 Indian districts reported sulphur deficiencies in soils, but recent data indicates a surge, with around 45% of the country’s districts exhibiting significant sulphur shortages (Meena et al., 2019). Conventionally, sulphur is applied as a form of elemental sulphur, ammonium sulphate, ammonium thiosulphate, ammonium bisulphate, sulphides or calcium sulphate (gypsum). The main drawback of elemental sulphur products is that their oxidation to plant available sulphate is slow often taking more time from the application. Therefore, the application of granular elemental sulphur containing fertilizers quickly disperses into finely divided particles and increases surface area for microbial oxidation process compared to conventional sulphur fertilizers (Lisowska et al., 2022). In these circumstances application of sulphur from suitable sources is one of the key measures to optimize sulphur use for sustainable crop productivity. There are few studies regarding the use of improved sulphur fertilizers for enhancing the productivity and quality of crop species. Some field experiments have been carried out using commercial-grade fertilizers like elemental S, bentonite S, gypsum, phospho-gypsum and pyrites. In this regard present study was aimed to evaluate granular sulphur efficacy in soybean.

This field experiment was conducted for two years 2022 and 2023 during kharif season at Agroforestry unit, Zonal Agricultural Research Station, University of Agricultural Sciences, GKVK Campus, Bengaluru located in the northern part of Bengaluru between Latitude/longitude of 13^o05′21˝N /77^o34′02˝E and at an altitude of 989 m above mean sea level which comes under Eastern Dry Zone (ACZ-V) of Karnataka. During the years in 2022 and 2023, received 864.2 and 941.4 mm rainfall in 75 and 52 rainy days, respectively and 27.3 and 27.5^oC mean maximum temperature, 18.1 and 19.7^oC mean minimum temperature, 86.7 and 81.75 % mean relative humidity and 6 and 7.9 hr day^-1 bright sunshine hours, respectively. The details of meteorological data of an experiment area are presented in Fig 1. The soil of the experimental site was red sandy clay loam having initial slightly acidic pH 6.2, EC 0.19 dS/m, organic carbon 0.49 %, low in available nitrogen, medium in available phosphorus, potassium and sulphur (258.26, 34.58, 230.52 and 20.15 kg/ha, respectively). The eight treatments were laid down in randomized block design with three replications. The treatment details are T₁: RDF + Sulphur 90% Dispersible Granules @ 12.5 kg ha^-1, T₂: RDF + Sulphur 90% Dispersible Granules @ 18.5 kg ha^-1, T₃: RDF + Sulphur 90% Dispersible Granules @ 25 kg ha^-1, T₄: RDF + Sulphur 90% Powder @ 18.75 kg ha^-1, T₅: RDF + Sulphur 90% Granular Bentonite Pastille @ 20 kg ha^-1, T₆: RDF + Sulphur 90% Granular Bentonite Pastille @ 30 kg ha^-1, T₇: University recommendation (RDF+ zinc sulphate @12.5 kg/ha) and T₈: Absolute control. During the experiment farm yard manure (FYM) @ 7.5 t ha^-1 was applied two weeks prior to sowing, recommended dose of fertilizers (25:62.5:25 kg N: P₂O₅: K₂O / ha) and different sulphur products as per treatments were applied at the time of sowing. The soybean crop (Var. JS-335) was sown during the second week of July with spacing of 30 cm × 10 cm. The experimental data on growth attributes (plant height, leaf area, dry matter accumulation and nodules per plant). yield and yield attributing (number of pods per plant, test weight, seed yield per plant, seed and haulm yield) were recorded from five randomly selected and labelled plants at harvest stage of the crop. Protein content in seed was calculated by multiplying nitrogen percentage with factor 6.25 (Simson et al., 1965), Fifteen grams of dried seeds of soybean drawn from the net plot of each treatment were used for estimation of oil content by Nuclear Magnetic Resonance (NMR) method. Sulphur containing amino acids like methionine and cysteine content were estimated by Ultra-High Performance Liquid Chromatography-Mass Spectrometry (UPLC-MS/MS) system. Nutrient use efficiency of nitrogen, phosphorus, potassium and sulphur were quantified using different indices like agronomic efficiency (AE) and partial factor productivity (PFP).
 

 
The data were subjected to Fisher’s method of “Analysis of Variance” (ANOVA) as outlined by Panse and Sukhatme (1967).

Growth attributes of soybean
 
Application of granular sulphur sources significantly maximize the growth attribute (plant height, leaf area, dry matter accumulation and nodules per plant) in soybean compared to control (Table 1). Significantly highest plant height of 25.68, 63.95 and 67.98 cm was recorded at 30, 60 DAS and at harvest, respectively with treatment receiving RDF + Sulphur 90% dispersible granules at 25 kg ha^-1 (T₃). However, it was found at par with treatments T₂ and T₆. There was increase in plant height of 26.3, 18.38 and 20.26 per cent over control at 30, 60 DAS and at harvest, respectively. The increase in plant height was attributed to the higher availability of sulphur in the soil that led to improved metabolic activity and biosynthesis of proteins which appears to have encouraged meristematic activity (Das et al., 2022). Similarly, significantly higher leaf area of 456.60 and 1793.86 cm² plant^-1 was recorded in T­₃ over control, which was at par with T₂ and T₆. The significant increase in leaf area was attributed to a higher number of trifoliate leaves per plant. Improved nutrient availability and sulphur absorption enhanced chlorophyll synthesis, as sulphur is a component of the succinyl Co-A enzyme involved in this process. This activation at the cellular level promoted greater photosynthate production and meristematic activity. These findings are supported by Mir et al. (2013) and Bangar et al., (2014). Dry matter accumulation was significantly influenced by application of granular sulphur at all stages in soybean over control. Combination of RDF along with sulphur dispersible granules at 25 kg/ha recorded 4.49, 27.57 and 48.05 g/plant, respectively at 30, 60 DAS and at harvest. Application of RDF along with sulphur might have played a crucial role in chlorophyll synthesis and photosynthetic efficiency, thereby increasing the plant’s ability to produce carbohydrates and accumulate dry matter. It also activates several enzymes involved in metabolic and growth processes, particularly in meristematic tissues were active cell division and expansion occurs. This observation aligns with findings of Devi et al., 2012. Similar trend was observed in case of number of nodules per plant. At 30 and 60 DAS, T₃ has recorded higher number of root nodules plant^-1 of 14.61 and 27.73, respectively followed by T₂ over control. 23.9 and 29 per cent increase in root nodules were observed over control. Sulphur has several oxidising functions in plant nutrition and a constituent of Fe-S protein called ferredoxin, which plays a pivotal role in increasing root nodulation and atmospheric nitrogen fixation. The observed increases could be due to enhanced root growth facilitated by greater sulphur availability, which is consistent with the findings of Yadav et al., (2013).


 
Yield attributes and yield of soybean
 
Application of sulphur 90% at 25 kg ha^-1 through dispersible granules along with RDF in soybean crop recorded a significantly higher number of pods plant^-1 (91.67), test weight (14.92 g), seed yield/plant (27.06 g plant^-1), seed yield (3273 kg ha^-1) and haulm yield (3448 kg ha^-1) which was found at par with T₂ and T₆ (Fig 2 and Table 2) compared to control (74.80, 12.35 g and 19.20 g plant^-1, 2219 kg ha^-1 and 2633 kg ha^-1, respectively). Sulphur played a crucial role in upsurge of growth characteristics, which in turn boosted chlorophyll content and the rate of photosynthesis. Consequently, the number of pods plant^-1 and test weight increased, contributing to a higher seed yield plant^-1 (Verma et al., 2013 and Yadav et al., 2013). Sulphur application increased the seed yield of soybean by 47.46 per cent over absolute control showing the indispensability of sulphur nutrition in oilseed crops. Sulphur fertilization played a pivotal role in enhancing yield by promoting vegetative growth for better photosynthesis, fortifying reproductive structures and producing assimilates for valuable sinks leading to a boost in seed yield. The results are in close conformity with the findings of Dheri et al., (2021) and Movalia and Savalia (2021). Better growth in terms of plant height, leaf area, dry matter accumulation, higher nutrient availability and uptake by soybean crop resulted in significantly higher haulm yield. These results were accordance with the findings of Patel et al. (2022) and Manoj et al., (2022).




 
Profitability of soybean
 
A perusal of data presented in Table 2 showed that RDF with sulphur 90% granular bentonite pastille at 30 kg ha^-1 application recorded a higher cost of cultivation (₹ 42709 ha^-1) due to higher cost of bentonite pastille. Lower cost of cultivation was observed in absolute control (₹ 39709 ha^-1) due to absence of cost on fertilizers. Gross returns, net returns and benefit-cost ratio were observed highest with the application of RDF + Sulphur 90% dispersible granules at 25 kg ha^-1 application (₹127803 ha^-1, ₹ 85969 ha^-1 and 3.05, respectively) followed by T₂. The higher returns were recorded due to higher returns obtained with higher seed yield. These results are in conformity with Prusty et al., (2020) in soybean and higher B:C ratio due to higher profit obtained per rupee invested (Agarwal and Singh, 2014) and Patel et al., (2022).
 
Quality parameters of soybean
 
A reference to data presented in Table 3, there was no significant difference in protein and oil content with application of granular sulphur. However, numerically higher protein (37.91%) and oil (21.95%) content was recorded in T₃ treatment, significantly higher methionine (13.02 mg per g seed) and cysteine content (0.13 mg per g seed) content was obtained under same treatment which was at par with T₂ and T₆ treatment. The higher protein content was due to sulphur application which plays a role in enhancing nitrogen assimilation where, nitrogen is a primary component of amino acid, protein and enzyme constituents which leads to higher protein content in seed. Similar results were reported by Nevase et al., (2016) and Mamatha et al., (2018). Significantly higher oil content might be due to direct involvement of sulphur in sulfhydryl (-SH) linkages in the lipid biosynthesis and these linkages are constituents of essential amino acids such as methionine and cysteine (Kumar et al., 2017 and Manoj et al., 2023).


 
Agronomic efficiency and partial factor productivity
 
Indices of nutrient use efficiency in terms of agronomic efficiency and partial factor productivity are presented in Table 4. Significantly higher AEN (42.14 kg kg^-1), AEP (16.86 kg kg^-1) and AEK (42.14 kg kg^-1) was recorded with application of dispersible granules at 25 kg ha^-1 along with RDF, followed by T₂ (39.56, 15.82 and 39.56 kg kg^-1, respectively) and T₆ (30.42, 12.17 and 30.42, respectively). Significantly higher AES (58.68 kg kg^-1) was observed in T₂. Partial factor productivity of nitrogen (130.9 kg kg^-1), phosphorus (64.5 kg kg^-1) and potassium (130.9 kg kg^-1) was significantly maximum with T₃ treatment followed by T₂ (128.3, 64.5, 128.3 kg kg^-1, respectively) and T₆ (119.2, 59.5 and 119.2 kg kg^-1, respectively). T₁ treatment recorded significantly higher partial factor productivity of sulphur (244.88 kg kg^-1) followed by T₆ treatment (225.61 kg kg^-1). Significantly higher efficiency might be due to ability of plants to produce higher yield per unit nutrient applied. These results were in conformity with the findings of Perveen et al., (2021) and Islam et al., (2012).


 
Simple linear regression
 
Correlation reveals the relationship between different variables, however with diverse variables it does not show how strong the relationship exists. Henceforth, for quantifying the degree of influence of diverse variables over dependent factor, such as seed yield, linear regression among an illustrative variable and a described variable is used (Sanam et al., 2021). Between the total NPK and S uptake and yield, linear regression is revealed in Fig 3. It unveils the greatest impact of nutrient uptake and seed yield of soybean under different sulphur sources. The uptake of total nitrogen documented 97% of the seed yield. Likewise, total phosphorus, potassium and sulphur of 81%, 95% and 85%, respectively.

The effect of granular sulphur sources along with RDF has proven to be a promising approach for enhancing soybean growth, yield, quality and economic viability. Application of granular elemental sulphur particles viz., dispersible granules which contains higher nutrient content and disperse quickly into finely divided particles which results in higher sulphur content in soil and improves the overall performance of soybean crop. Further, it can be concluded that the application of sulphur 90% dispersible granules at 25 kg ha^-1 along with RDF could be recommended for better yield and net returns to the farming community which recorded the highest benefit-cost ratio.

Authors are grateful to University of Agricultural Sciences, Bangalore (Karnataka) and AICRP for Dryland Agriculture, UAS, GKVK, Bengaluru for providing facilities in smooth conduct of the experiment.
 
Disclaimers
 
The authors have made every effort to ensure the accuracy and integrity of the data and information presented in this research paper. However, they do not accept any liability for any direct, indirect, or consequential loss or damage arising from the use of the information contained herein.

The authors state that there is no conflict of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish or preparation of the manuscript.