Interactive Effect of Phosphorus and Boron on Their Temporal Soil Availability under Black Gram [Vigna mungo (L.) Hepper] Cultivation and Nodulation in Acid Inceptisol

Phosphorus plays an important role in an array of cellular processes, including maintenance of membrane structures, synthesis of biomolecules, formation of high-energy molecules and also helps in cell division, enzyme activation/inactivation and carbohydrate metabolism. In addition, availability of P increases the N-fixing capacity of leguminous plants (Razaq et al., 2017). Boron is also an important micronutrient that plays crucial role in cell division and enlargement, cell wall formation, sugar translocation, metabolism of carbohydrates, metabolism of nitrogen and water relations (Oyinlola, 2007; Marschner, 2012). A key role of B includes the development of floral organs, flower male fertility and pollen tube growth (Gupta and Solanki, 2013). P and B have significant synergetic effect in improving the production, yield and quality of plant species (Kaya et al., 2009; YuFan et al., 2012). The growth and metabolism of plants is more inhibitory than the combined abundance of boron and phosphorus alone, here is synergistic relationship to rise in P uptake by plants under application of B (Huang et al., 2012).
               
Pulses are rich in protein similar to other less expensive vegetables and can be grown as an inter crop and mixed crop. Black gram [Vigna mungo (L.) Hepper] is one of the most important pulse crops grown in India. It contains about 25-26% protein, 60% carbohydrates, 1.3% fat and is the richest in phosphoric acid among the all pulses (Tamang and Sanjay-Swami, 2017). Pulse productivity depends mainly on appropriate nutrient management practices (Kumpawat, 2010). Meghalaya soils are highly acidic and phosphorus and boron are poor in supply. Low availability of phosphorus in these soils is due to fixation of P by Fe and Al oxides. Deficiency of boron in acidic soils of Meghalaya is due to coarse texture and leaching (Takkar, 1996). Keeping in view the above facts, the present study was undertaken to investigate the interactive effect of phosphorus and boron on their temporal availability and nodulation of black gram in acid Inceptisol of Meghalaya.

The field experiment was conducted at research farm of School of Natural Resource Management, College of Post Graduate Studies in Agricultural Sciences, Umiam, Meghalaya in 2019 during the period of June to September. Geographically, the experimental site was located at 91°18' to 92°18' E longitude and 25°40' to 26°20' N latitude with an altitude of 950 m above the mean sea level with agro-climatic zone of mixed subtropical hill and falls in AES-III zone (Choudhury et al., 2012). The experiment comprises of four levels of phosphorus (0, 25, 50, 75 kg ha^-1) and four levels of boron (0, 0.5, 1.0, 1.5 kg ha^-1) with 16 treatment combinations laid in split plot design with three replications. Phosphorus was used as main plot and boron as sub-plot. These are randomly allocated in plots of size 2.1 m × 2 m. Phosphorus and boron were applied through Single Super Phosphate (SSP) and Borax, respectively. The experimental site was located in humid sub tropical climate. The experimental site has received an average temperature of 23.0°C, total rainfall of 1625.5 mm during the cropping period. The physico-chemical properties of the experimental soil exhibited sandy clay loam texture, pH 4.99, EC 0.42, organic carbon 1.32%, available nitrogen 251.35 kg ha^-1, available phosphorus 13.68 kg ha^-1, available potassium 233.24 kg ha^-1 and available boron (0.054 ppm). To assess the temporal availability of phosphorus and boron, representative soil sample from each treatment were collected at 30, 60 days after sowing (DAS) and after harvesting of black gram. The available phosphorus was estimated by using Bray and Kurtz (1945) method whereas available boron was estimated by Berger and Troug (1939) method. Five plants from each treatment were taken at 30, 60 DAS, at maturity and number of nodules were counted and their dry weight and average per plant was worked out. The data obtained was subjected to statistical analysis as described by Gomez and Gomez (1984).

Temporal soil available phosphorus
 
The effect of phosphorus and boron on temporal soil available phosphorus (kg ha^-1) is presented in (Table 1). The maximum soil available phosphorus (18.89, 16.17, 14.02 kg ha^-1) was obtained at phosphorus level 75 kg ha^-1 at 30, 60 DAS and after harvesting, whereas the lowest available phosphorus was recorded at 30, 60 DAS and after harvesting at control as (14.15, 16.17 kg ha^-1); (10.36, 13.06 kg ha^-1); (9.29, 10.77 kg ha^-1) at P and B, respectively. Further, the scrutiny of data depicted that available phosphorus showed decreasing trend after 30 days of black gram sowing. The interaction effect of phosphorus and boron on soil available phosphorus at 30, 60 DAS and after harvest was found to be significant. The lowest soil available phosphorus at 30 DAS was found at control (P₀B₀) as 13.58 kg ha^-1 and significantly highest value was found at P₇₅B_1.5 as 19.54 kg ha^-1 in phosphorus within boron and boron within phosphorus as 19.10 kg ha^-1, at P₇₅B₁, whereas lowest soil available phosphorus was found in control i.e. P₀B₀ as 9.85 kg ha^-1 and significantly highest soil available phosphorus was found at P₇₅B_1.5 as 16.67 kg ha^-1 in phosphorus within boron and in boron within phosphorus as 16.10 kg ha^-1 at P₇₅B₀.5 at 60 DAS and the lowest soil available phosphorus after harves was found at control i.e. P₀B₀ as 8.83 kg ha^-1 and significantly highest soil available phosphorus was found at P₇₅B_1.5 as 14.52 kg  ha^-1 in phosphorus within boron and 12.53 kg ha^-1 at P50B1 in boron within phosphorus. The per cent increase from control to significantly highest soil available phosphorus at 30 DAS was 43.92, 40.70. There was an increase in 69.24, 63.42 per cent from control to significantly higher soil available phosphorus at 60 DAS and per cent increase from control to significantly highest value after harvest was 64.37, 41.81. The results obtained are in agreement with those reported by Kamboj et al., (2018) after green gram cultivation and Phogat et al., (2021) after black gram cultivation. The results are also in close conformity with the results obtained by Tamang and Sanjay-Swami (2021) who assessed the temporal availability of phosphorus at 20, 40 DAS and after harvesting of black gram in acid Inceptisol of Meghalaya and reported that available phosphorus in soil increased with each successive increasing level of phosphorus application, however it showed decreasing trend with time intervals of 40 DAS and after harvesting of black gram at each successive dose of 40, 60 and 80 mg P kg^-1 soil over 20 mg P kg^-1 soil in the presence of sulphur fertilization.
 

 
Temporal soil available boron
       
Maximum soil available boron of 0.104, 0.097, 0.077 ppm was obtained with boron application @ 1.5 kg ha^-1 at 30, 60 DAS and after harvesting (Table 2) whereas, the lowest boron was found in control as 0.072, 0.062, 0.051 ppm at phosphorus levels and at boron as 0.049, 0.042, 0.035 ppm, respectively. Further, the scrutiny of data revealed that available boron showed decreasing trend after 30 days of black gram sowing. The interaction effect of phosphorus and boron on soil available boron at 30, 60 DAS and after harvest was also observed to be significant. The lowest soil available boron at 30 DAS was found in control (P₀B₀) as 0.045 ppm and significantly highest value was found at P₂₅B_1.5  as 0.104 ppm in phosphorus within boron and boron within phosphorus as 0.112 ppm at P₇₅B_1.5, whereas the lowest soil available boron at 60 DAS was found in control  i.e. P₀B₀ as 0.039 ppm and significantly highest soil available phosphorus was found at P₂₅B_1.5 as 0.097 ppm in phosphorus within boron and in boron within phosphorus as 0.103 ppm at P₇₅B_1.5 at 60 DAS and the lowest soil available phosphorus after harvest was found at control i.e. P₀B₀ as 0.028 ppm and significantly highest soil available phosphorus was found at P₇₅B_1.5 in phosphorus within boron and boron within phosphorus as 0.084 ppm. The per cent increase from control to significantly highest soil available boron at 30 DAS was 131.85, 148.14 with an increase in 150, 167.24 per cent from control to significantly higher soil available boron  at 60 DAS and per cent increase from control to significantly highest value after harvest was 198.81 per cent. Similar results were obtained by Dhakal et al., (2009) after harvest of cauliflower (Brassica oleracea var. Botrytis) with the application of boron and phosphorus in the soils of Rupandehi district of Nepal. The results of the present investigation also confirms the findings of Kamboj et al., (2018) who determined the soil availability of phosphorus and boron after harvest of green gram grown with four level of boron (0, 0.25, 0.5 and 1.0 mg kg^-1) along with five level of phosphorus (0, 25, 50, 75 and 100 mg P₂O₅ kg^-1 soil) in pots and reported that with each graded level of boron application, soil boron and phosphorus concentration increased. Similarly, application of phosphorus increased the availability of soil boron and phosphorus concentration.
 

 
 
Number of nodules and their dry weight
 
Increased number of nodules and dry weight was observed with the advancement of crop development stages from 30 DAS to 60 DAS, whereas from 60 DAS to maturity, the number of nodules and dry weight decreased (Fig 1 and 2). It is also observed that increasing phosphorus and boron doses increased the number of nodules and dry weight in black gram. Under different phosphorus levels, significant number and dry weight of nodules were observed at 50 kg P₂O₅ ha^-1 with 17.61 nodules and their dry weight as 26.14 mg plant^-1. Similarly, under different boron doses, the significant higher number of nodules (18.23) and their dry weight (26.46 mg plant^-1) was observed at 1.5 kg B ha^-1. Similar pattern was observed at 60 DAS and at maturity. At 60 DAS under different phosphorus levels, significant number and dry weight of nodules were observed at 50 kg P₂O₅ ha^-1 with 19.41 nodules and their dry weight as 27.88 mg plant^-1. Similar to this under different boron doses, the highest number of nodules (20.03) and their dry weight (27.92 mg plant^-1) was observed at 1.5 kg B ha^-1. At maturity, the significant number and dry weight of nodules of black gram were 17.78 and 25.70 mg plan^t-1 at 50 kg P₂O₅ ha^-1, whereas  with different boron doses, the highest number of nodules (17.99) and their dry weight (25.73 mg plant^-1) was observed at 1.5 kg B ha^-1. The interaction effect of phosphorus and boron on number of nodules and their dry weight was significant at all crop development stages. Within the same level of boron, increasing phosphorus doses increased the number of nodules and their dry weight, but significantly highest number of nodules at 30 DAS and at maturity was found at P50B1 with 18.22, 18.38 whereas at 60 DAS, it was found at P₅₀B_0.5 with 19.54. Similarly, with same level of phosphorus, increasing boron doses increased the number of nodules but the significant highest number of nodules at 30, 60 DAS and at maturity was observed at P₇₅B₁ with 19.71, 21.97, 19.66. The lowest number of nodules was observed at control i.e. P₀B₀ with 11.25, 13.14, 9.48 at 30, 60 DAS and at maturity. The percent increase in number of nodules in P₅₀B₁ over control at 30 DAS and at maturity was 62.03 and 93.99 per cent, whereas at 60 DAS, the per cent increase in number of nodules in P50B0.5 over control was 48.74 per cent. The per cent increase in P₇₅B₁ over control at 30 DAS, 60 DAS and harvest was 75.25, 67.24, 107.49 per cent. The results obtained are in conformity with Hellsten and Huss-Danell (2000), Laltlanmawia et al., (2005). The dry weight of nodules at different crop development stages in control (P₀B₀) was observed to be 18.07, 19.26 and 17.06 mg plant^-1, respectively. Further, within the same level of boron, increasing phosphorus doses increased the dry weight of nodules, but significantly highest dry weight of nodules was found at P₅₀B_0.5 at 30 and 60 DAS as 25.92 and 27.88 mg plant^-1 whereas at maturity at P₅₀B₁ as 26.71 mg plant^-1. Similarly, with same level of phosphorus, increasing boron doses increased the dry weight of nodules but the significant highest number of nodules was observed at P₇₅B_0.5 as 26.35, 28.60 mg plant^-1 at 30 and 60 DAS whereas at maturity at P₇₅B₁ as 27.89 mg plant^-1. The per cent increase in P₅₀B_0.5 at 30 and 60 DAS over control was 43.43 and 44.77 per cent whereas at maturity, the increase in the dry weight of nodules in P₅₀B₁ was 56.58 per cent. The dry weight of nodules increased in P₇₅B_0.5 at 30 and 60 DAS over control was 45.78, 48.48 per cent, respectively whereas at maturity, it was 63.51 per cent in P₇₅B₁. The increase in number of nodules per plant in black gram (Vigna mungo L.) was also reported by Parashar et al., (2020) which is attributed to better root biomass with increasing applications of the essential nutrients, especially phosphorus, being the constituent of nucleic acid and different forms of proteins, which might have stimulated cell division resulting in increased growth of plants and thereby nodulations.
 

 

The soil available phosphorus showed significant increment with succeeding application over control, despite this, it showed declining trend with advancement of crop development/time. The addition of boron had significant impact on available phosphorus content in soil. Similarly, the soil available boron showed significant increment with consecutive addition of B besides control and it showed declining trend with time. The addition of P has significant impact on available boron content in soil. The number of nodules and their dry weight increased significantly with increase in phosphorus and boron doses and optimum values for number of nodules were recorded with combined application of 50 kg P₂O₅ ha^-1 and 1.0 kg B ha^-1, whereas for dry weight of nodules with 50 kg P₂O₅ ha^-1 and 0.5 kg B ha^-1.

Authors are thankful to the School of Natural Resource Management, College of Post Graduate Studies in Agricultural Sciences, Central Agricultural University, Umiam (Barapani), Meghalaya for providing necessary facilities for conducting the experiment and laboratory analysis.

None.