Foliar Nutrition in Spring Blackgram [Vigna mungo (L.) Hepper] in Lower Gangetic Plains of West Bengal

One of the popular food legumes of tropical India is blackgram. Consumer response to this widely grown food legume reflects its contribution in food and nutritional security. Blackgram and its resilience in difficult weather condition makes it a good choice in spring or summer rice fallows (Kumar et al., 2015). Therefore, the crop also fits well in rice-pulse cropping system under receding soil moisture. The main productivity constraint emerges from the fact that it predominantly suffers growing in marginal lands under rainfed condition. Farmers have an inclination to spend less on legumes and end up with poor management practices. Moreover, indeterminate growth habit subjects the crop to constant competition between vegetative stage and sink partitioning for photo assimilates in the respective period (Marimuthu and Surendran, 2015) and this has been an identified major impediment to productivity. Owing to all these reasons for a short duration indeterminate crop, scheduling soil nutrients do not always have pronounced response. At the onset of reproductive growth marked by declined nodulation, there is need of proper fertilization. Unavailability of sufficient nutrients at the critical phenophase of flowering and pod development observed nutrient stress (Thanunathan et al., 2018) together with flower and fruit drop (Ganapathy et al., 2008). Consequently, though blackgram produces large number of flowers, a smaller number of flowers convert into pods. Excessive flower abscission followed by poor pod setting occurs due to less efficient partitioning of assimilates and aggravated nutrient demand at the pod stage which may be attended by replenishing macro and micronutrients.
       
Soil application of fertilizers entails heavy cost in production, transport and mobilization which the farmer has to bear. Credit worthiness of small holding farmer is coming down and with emerging policies, enterprising agriculture and corporate farming leaves the farmer less room for grain production. Rationalization of costs in plant nutrition led to studies in foliar application of major nutrients. 
       
Foliar nutrition has better efficacy in faster uptake and significant less amount of losses (Karuku et al., 2017). Time lag observed between application and absorption of nutrients by plants is very less. Foliar nutrition is less contributing to environmental concerns. While soil applications have reported 30-60% efficiency the same for foliar application can reach values upto 85% subject to nutrient formulations. Foliar application has an edge in economics, quality and yield. Sink partitions are more favoured by foliar nutrition (Maheswari and Karthik, 2017). Potentially, foliar nutrition provides a simple solution for nutrient administration in pulse crops, more so, in the later stage of plant growth when soil application is either ineffective or expensive (Kuepper, 2003).
       
Identifying stages of crops responding more to foliar nutrition holds the key for better and efficient utilization by the crop (Krishnaveni et al., 2004). So, foliar application of nutrients not only alleviates the nutrient deficiency but it improves the reproductive efficiency (Shankarappa et al., 2020). With this background, the present experiment aimed to investigate the efficacy of foliar nutrition towards growth and yield of spring blackgram in the lower Gangetic plains of West Bengal, India.

The two year trial was reported from a study undertaken in the spring seasons of 2018 and 2019 respectively, in “AB” block farm, Bidhan Chandra Krishi Viswavidyalaya, Kalyani, Nadia, West Bengal, India enjoying bearings of 22.5°N latitude and 89.0°E longitude at an elevation of 9.72 m. The soil of the experimental field was analyzed for physico-chemical properties. The available nitrogen of the experimental field was (301.50 and 321. 61 kg ha^-1), phosphorus (32.60 and 35.75 (kg ha^-1) and potassium (100.42 and 106.45 kg ha^-1).
       
The experiment had eight foliar treatments (T₁: Absolute control (No spray), T₂: Control (water), T₃: CaCl₂ @ 0.1%, T₄: KCl @0.2%, T₅: KNO₃ @ 2%, T₆: Salicylic acid @ 100 ppm, T₇: Complex NPK (19:19:19) 2% and T₈: 2% DAP) fitted in randomized block design (RBD), replicated three times. All the treatments were given as foliar spray at flowering and pod initiation stages.
       
The test variety of the crop ‘Pant Urd-31’ was sown in the 3^rd week of February, 2018 and 2019. A spacing of 25 cm × 10 cm was maintained. A blanket basal dose of urea, single super phosphate and muriate of potash supplied major nutrients @ 20:40:20 Kg ha^-1 (N: P: K). The crop was harvested during 2^nd week of May, 2018 and 2019. Observations were collected through random sampling of 10 plants from each treatment at 30 DAS, 45 DAS and 60 DAS to record height, dry matter, nodule number and LAI. At the time of harvest, number of pods plant^-1, seeds pod^-1, test weight (g) and finally, the grain yield and stover yield were recorded.
 
Chlorophyll extraction was done in 80% (v/v) acetone followed by observations at wavelengths 645 nm and 663 nm in UV VIS spectrophotometer. Chlorophyll contents were analyzed from the leaf samples collected at 30 DAS and 60 DAS following Arnon (1949). Protein content of seed was also measured from the seed samples after harvesting.
       
The cost of production (Rs. ha^-1) and the gross monetary return (Rs. ha^-1), net revenue (Rs. ha^-1) and benefit: cost ratio (B:C) from the produce of various treatments were computed from prevailing market prices of inputs in West Bengal (India) and implied labour cost for incidental and application.
 
The data were analyzed comparing the means of various treatments on different parameters through the analysis of variance (ANOVA) method to test the significance (p≤0.05) (Gomez and Gomez, 1984). The data were analyzed by analysis of variance (ANOVA) and significant differences were judged through F test. The F value was compared for the determination of least significant differences (LSD) at p≤0.05.

Growth parameters
 
T₆ produced tallest plants (38.97 cm), followed by T₇ (Table 1). Significantly highest values of dry matter (196.82 g m^-2 and 318.66 g m-2 respectively) at 45DAS and 60DAS were recorded with T₇. Crop growth rate (CGR) at 30-45 DAS, varied between 10.33 g^-2day^-1 with the application of T₇ to 7.45 g m^-2day^-1 in T₁. Many researchers reported that foliar nutrition results in higher photosynthesis due to greater absorption, assimilation and translocation of nutrients (Dalei et al., 2014). The increments in the total dry matter production (TDM) per unit area which is otherwise known as CGR is the pre-requisite for higher yields as well as a crucial indicator that reflects the crop growth and metabolic activity.
 

 
At 45DAS and 60 DAS, maximum number of nodules plant^-1 (33.70 and 22.65) was recorded in T₇ (Table 1). The nitrogen and phosphorus present in complex 2% NPK (19:19:19) is likely to improve root development and increase activity of Rhizobium spp. in the rhizosphere region leading to a greater number of nodules (Geetha and Velayutham, 2009). At 45 DAS, the highest LAI (3.79) was recorded with T₇. At 60DAS, the highest LAI (4.33) was observed in T₇ followed by T₈ (Table 1).
. 
Yield attributes and yield
 
The maximum pods plant^-1 (29) was observed in T₇ (Table 2). The maximum seeds pod^-1 (7) was recorded with T₇ and T₆. The highest test weight (38.71) was recorded in T₇. Enhancement of translocation of assimilates towards the desirable sink i.e., pods with the effective foliar treatment might be another possible reason of more pod development. Increased nutrient absorptions through leaves might improve functional activity of root nodules, leaf area, and biomass production eventually improving seed formation. Shedding of flowers was drastically reduced in foliar treatments which had resulted in greater pods and yield (Maheswari and Karthik, 2017).
       
Grain yield of blackgram was influenced by different foliar nutrition. The highest grain yield (1200 kg ha^-1) was recorded with T₇ followed by T₆ (Fig 1). The stover yield was found to be highest (2234 kg ha^-1) in T₂. The maximum harvest index was observed in T₇. Foliar nutrition of NPK might have delayed leaf senescence with high photosynthetic rate during pod development, thereby led to higher yield. The increased HI under foliar nutrition might be due to enhanced assimilate translocation to sink (Thakur et al., 2017).
 

 
Chlorophyll content
 
The highest value (2.91) of chlorophyll content was observed in T₇ (Table 2). Chlorophyll synthesis depends on mineral nutrition (Li et al., 2018). Foliar application of nitrogen is reported to improve leaf chlorophyll content since it is a structural component of chlorophyll (Tucker, 2004). Phosphorus also has influence in the formation as well as stability of the pigment in plants (Bojovic et al., 2005). Many researchers found out that potassium has a vital role in increasing chlorophyll content in the crops (Gairola et al., 2009). Potassium helps in the uptake of iron and magnesium; those are associated with chlorophyll synthesis. In this way, foliar nutrition with 2% N: P: K-19:19:19 resulted in increased chlorophyll content.
 

 
Correlation analysis
 
An attempt has also been made to recognize the parameters that are most significantly associated with grain yield and/or harvest index of blackgram under the influence of foliar nutrition (Table 3). The results revealed that final grain yield ha^-1 was highly correlated (at p≤0.01) with several growth or physiological parameters like dry matter accumulation (DMA) both at 45 and 60 DAS, CGR both at 30-45 DAS and 45-60 DAS, nodule number plant^-1 at 60 DAS. Besides, high positive correlations of harvest index with DMA both at 45 and 60 DAS, CGR at 30-45 DAS, and nodule numbers plant^-1 both at 45 DAS and 60 DAS were found.
 

       
High positive correlation of grain yield or harvest index with dry matter production both at 45 and 60 DAS might indicate that high dry matters were produced during this period due to foliar nutrition and at latter stage significant amount of stored photosynthates partitioned towards grain. Further, the higher positive correlation between CGR and grain yield might be due to increased dry matter production emerging from improved physiological processes like regulation of stomatal opening, chlorophyll synthesis, enzymatic activity and biochemical processes in response to foliar nutrition (Marschner, 2012).
       
 
Further, most interestingly high positive correlations of both grain yield and harvest index with nodules plant^-1 particularly at 60 DAS (0.939 and 0.934, respectively) might indicate the necessity of balanced nutrition at latter growth stages of this crop. Foliar nutrition at proper crop growth stages might also result in spreading of root system and as such it helps in forming more effective nodules by providing more site for rhizobia infection and increasing their proliferation in rhizosphere (Raj et al., 2018). Positive correlation between grain yield and nodule number plant^-1 has been reported by earlier researchers (Goni, 2000).
 
Protein content
 
The maximum value (24.85%) of protein content was observed with T₇ (Fig 2). Greater protein content is generally associated with increasing N content as it is the key constituent of amino acids i.e. building blocks of proteins. Hence, improved N uptake and assimilation of foliar applied N is therefore likely to improve the seed protein content (Fageria et al., 2009).
 

 
Economics
 
The data (Table 4) revealed that the highest cost of cultivation was required for T₃. But the maximum economic benefits were obtained with T₇ followed by T₆. The highest B:C ratio was observed in T₇.
 

In conclusion, foliar nutrition is effective in improving crop growth, grain yield, and quality of blackgram. The improvement of dry matter production and yield of blackgram would be attributed to enhanced nodulation as well as better mobilization of nutrients or partitioning of photosynthates due to foliar application of combined nutrients. It is evident that spraying of complex NPK 19:19:19 @ 2% at flowering and commencement of pod development can improve the overall performance of spring blackgram and therefore promote its adoption in the rice-based cropping systems by fetching satisfactory yield and economic benefits.

The support and facilities provided by the AICRP-MULLaRP, Bidhan Chandra Krishi Viswavidyalaya, Kalyani, West Bengal, India for conducting this study is duly acknowledged.

None.