Response of Yield and Quality of Soybean (cv. BARI Soybean-6) to Phosphorus and Sulphur Fertilization under Old Brahmaputra Floodplain Soil of Bangladesh

Soybean (Glycine max L.) is one of the leading oil and protein containing crops of the world. It covers 120.30 million ha worldwide with the production of 333.67 million tons (FAO, 2021). Soybean seed contains oil 20-22%, protein 42-45%, carbohydrates 30-35% and total sugar 10-12% and also high amount of the amino acid, thiamin, vitamins, niacin, riboflavin, phosphorus, calcium and iron (Wahhab et al., 2001). It contributes 25% of the global edible oil production (Jaybhay et al., 2021). Soybean seeds are used for preparation of soy tofu, soya milk, soya sprouts, soya nuts, etc. (Sikka et al., 2018). In Bangladesh, soybean occupies 0.051 million ha of land with the production of 0.091 million tones (BBS, 2019). The average yield of soybean in Bangladesh is low (1.2 t ha^-1) compared to world average (BARI, 2007). Bangladesh imports 1.8 million tons of soybean cooking oil each year at the cost of more than 1.5 billion USD and soybean meal with about 25.51 million USD per year (Quaiyum et al., 2015). Among oilseeds in Bangladesh, soybeans are the fourth-ranked crop that covers the 9.82% of total oilseed planted area. The lower production costs, coupled with good market prices, give farmers a premium for soybeans compared to other seasonal crops.
 
Soybean can form a symbiotic association with root rhizobium bacteria and fix atmospheric nitrogen through nodulation. Nodulation in legumes involves a series of biochemical interactions between the bacterium and the plant (Ciafardini and Barbieri, 1997). Phosphorus and sulphur also play a vital role in increasing the nodulation of soybean (Singh and Bansal, 2000). Phosphorus deficiency is now considered one of the major constraints to successful production of legumes in Bangladesh. In soybeans, the demand for P is the greatest during pod and  seed  development  where  more  than  60%  of  P  ends  up  in  the  pods  and  seeds.  It has been proven that P increases weight and number of root nodules and also can enhance the pod yield (Khanam et al., 2016). Phosphorus plays a role in photosynthesis, respiration, energy storage and transfer, cell division and enlargement, it is important for the growth, development and yield of soybean (Kakar et al., 2002). Soybean is a sulphur loving plant and like other oilseed crops, its sulphur requirement is more than that of many other crops for proper growth and yield. The response of soybean to sulphur application has been reported by Akter et al. (2013). However, limited information is available on the role of phosphorus and sulphur on the growth and yield of soybean. Therefore, a field experiment was conducted to find out the optimum doses for maximizing the yield and quality of soybean under the Old Brahmaputra Floodplain soil of Bangladesh.
 

The experiment was conducted at the Agronomy Field Laboratory, Bangladesh Agricultural University, Mymensingh (24^o75 N, 90^o45 E), Bangladesh from January to May 2019. The site belongs to the Sonatola Soils Series of Non-calcareous Dark Grey Floodplain soil under the Old Brahmaputra Floodplain Agro-ecological Zone (AEZ 9) (UNDP and FAO, 1988). The experimental field was a high land having silty-loam soil with pH 6.10. The initial soil (0-15 cm depth) test result showed that the soil contained 0.117% total nitrogen, 0.95 % organic matter, 3.19 ppm available phosphorus, 9.52 ppm available sulphur, 0.092 ppm exchangeable potassium and 233 μS/cm electrical conductivity. The study comprised four levels of phosphorus viz. 0, 20, 40 and 60 kg P ha^-1 and four levels of sulphur viz. 0, 15, 30 and 45 kg S ha^-1. The experiment was laid out in a factorial randomized complete block design with three replications. There were total 48 plots each of 2.5 m × 2.0 m having plot-plot and block-block distance of 0.5 m and 1.0 m, respectively. BARI soybean-6 was used as the test crop.
 
The land was prepared thoroughly and the field was laid out consequently. The individual plots were prepared and the seeds were sown (40 kg ha^-1) continuously with 30 cm line spacing on 19 January, 2019. Urea- MoP-boron were applied @ 60-100-10 kg ha^-1, respectively. Phosphorus and sulphur fertilizers were also applied as per treatment specification. The full dose of urea, MoP and boron were applied during final land preparation. Intercultural operations were done to ensure the normal growth of the crop. Before harvesting, ten plants were randomly selected from each plot for collecting data on plant characters and yield components. The crop was harvested on 2 May, 2019 when the color of the leaf turned yellow and dropped off. The harvested plants were dried, threshed, cleaned and seeds were dried in the sun for 2-3 days to 9% moisture basis. The grain and straw were weighed and subsequently converted to t ha^-1. The harvest index (%) was calculated by the empirical formula given below:



Total nitrogen in the seeds was determined by the Micro Kjeldhal method (AOAC, 1984) and the protein percentage was calculated by multiplying the factor 6.25. The oil content was estimated by Soxhlet apparatus method following the  procedure of Singh et al. (1960). The collected data were compiled and analyzed statistically using the analysis of variance (ANOVA) technique with the help of the computer package program MSTAT-C and the mean differences were adjudged by “Duncan’s multiple range test” (DMRT) (Gomez and Gomez, 1984).

Crop characters, yield components and yield of soybean
 
Crop characters, yield components and yield of soybean were significantly influenced by P fertilization except for the number of branches plant^-1 and seeds pod^-1 (Table 1). The tallest plant (51.58 cm) was recorded in 40 kg P ha^-1 which was at par with 20 kg P ha^-1 and 60 kg P ha^-1 while the lowest one (49.25 cm) was recorded in control. The progressive enhancement of plant height due to phosphorus application was reported by Afzal et al. (2010) who also found similar results for plant height. However, the number of branches plant^-1and seeds pod^-1 did not show significant variation where numerically the highest number of branches plant^-1 (8.50) and seeds pod^-1 (2.66) were found when the crop was fertilized with @ 60 kg P ha^-1 showing the lowest in control. The highest number of pods plant^-1 (48.33), the heaviest seeds (9.11 g), the highest seed yield (1.71 t ha^-1), stover yield (1.87 t ha^-1) and harvest index (47.90 %) were found when the crop was fertilized with 60 kg P ha^-1while, 40 kg P ha^-1 gave the statistically similar heaviest seeds (9.05 g) and stover yield (1.87 t ha^-1). The control treatment gave the corresponding lowest values for yield and yield contributing parameters. Application of sufficient phosphorus enhanced the performance of the yield attributes of soybean as reported by Singh and Menon (2021) and who also found a significant increase of yield upto 60 kg P ha^-1. Similar observation was reported elsewhere (Singh et al., 2001:  Kumari et al., 2018; Suman et al., 2018).


 
Sulphur fertilization significantly impacted all the vegetative and yield characters of soybean except the number of seedspod^-1 (Table 1). Application of 15, 30 and 45 kg S ha^-1 statistically produced the tallest plants and the highest number of branches plant^-1. The shortest plant (48.50 cm) and lowest number of branches plant^-1 (7.83) were found in control. These findings are similar to those of Ghosh et al. (1997) who reported that sulphur application enhanced the plant height and branches plant^-1. Though the heaviest seeds (9.95 g) was found in 30 kg Sha^-1, the highest number of pods plant^-1 (48.33), the highest seed (1.51 t ha^-1) and stover yield (1.84 t ha^-1) were found in 45 kg S ha^-1 whereas sulphur had no significant influence on the number of seeds pod^-1. The highest seed yield might be due to the cumulative effect of the yield contributing characters because of sulphur fertilization. Farhad et al. (2010) reported that the number of pods of soybean increased significantly due to increased levels of sulphur. Shubhangi et al. (2014) found that sulphur application had increased 100-seed weight of soybean. Babhulkar et al. (2000) obtained higher seed yield with 45 kg S ha^-1 and Kumari et al. (2018) observed increased seed yield with successive S application up to 45 Sha^-1. Tomar et al. (1997) observed that increasing the doses of sulphur increased the stover yields of soybean. The highest and lowest harvest index were observed in 15 kg S ha^-1 (45.91%) and control (43.44 %) treatments, respectively.
 
A significant interaction effect of phosphorus and sulphur on all the plant characters and seed yield but  number of seeds pod^-1 were observed (Table 2). Almost all the treatment combinations gave the statistically tallest plants while control produced the shortest one. The highest number of branch plant^-1 (9.33), pods plant^-1 (51.00) and the heaviest 100-seed (11.00 g) were found in 60 kg P×30 kg S ha^-1 and the lowest values were found in control (0 kg P×0 kg S ha^-1). The highest seed yield (1.78 t ha^-1) was recorded in  60 kg P×30 kg S ha^-1which was as good as 60 kg P×45 kg S ha^-1. The highest stover yield (2.28 t ha^-1) was recorded in control while 0 kg P×15 kg S ha^-1 produced the lowest stover yield (1.43 t ha^-1). The result exhibits that 60 kg P along with 30 kg Sha^-1gave the highest the harvest index (49.45 %) which was statistically identical with 20 kg P×30 kg Sha^-1. The results are in agreement with the findings of Shubhangi et al. (2014) who reported that phosphorus and sulphur interaction had significant effect on harvest index. The lowest seed yield (1.18 t ha^-1) and harvest index (43.14 %) were found in control. Better performance of different yield attributes of soybean resulted in the highest seed yield. Soybean yield rise owing to different levels of S and P fertilization was observed by some other researchers (Kumar et al. 2017, Kumari et al. 2018 and Suman et al. 2018).


 
Quality parameters of soybean
 
Phosphorus and Sulphur had a significant effects on the protein and oil content of soybean individually and in interaction (Table 1 and 2). The highest protein (42.00%) and oil content (20.60%) were found in 40 kg P ha^-1 which was statistically identical with 60 kg Pha^-1 (Table 1). The findings are similar to those of Deliboran et al. (2011) and Dubey and Khan (1991), who reported that application P increased total protein and oil content substantially in soybean. The control treatment gave the corresponding lowest values. Treatment of 15 kg S ha^-1 was enough to produce the highest protein content (42.65%) while the highest oil content (21.02%) was found in 30 kg S ha^-1 which was at par with 60 kg S ha-1. In both cases, the lowest protein (39.93%) and oil content (19.15%) were found in the control. The interaction of 60 kg P×30 kg S ha-1 produced the highest protein (44.50%) and oil content (21.50%) while the lowest protein (35.80%) and oil content (18.20%) were found in control (Table 2). Sulphur had a remarkable influence on the protein and oil synthesis of soybean. A similar observation was reported elsewhere (Tomar et al. 1997; Babhulkar et al. 2000; Haq et al. 2005).
 
 

The yield components, seed yield, protein and oil content of soybean were significantly influenced by phosphorus and sulphur application. The highest seed yield, protein and oil content in seeds of BARI Soybean-6 were recorded when the crop was fertilized with 60 kg P ha^-1 along with 30 kg S ha^-1. Therefore, application of P @ 60 kg ha^-1 with S @ 30 kg ha^-1 might be suggested for higher yield and seed quality of BARI Soybean-6 under Old Brahmaputra Floodplain Soil of Bangladesh.
 

The authors are thankful to the Ministry of Science and Technology, Govt. of the People’s Republic of Bangladesh for providing the grant (39.00.0000.09.02.90.18-19/09/BS-437) to carry out the research work.
 
Conflict of interest: None.