Foliar Nutrition with Water Soluble Macro and Micro Nutrient Fertilizers for Yield Maximization in Bush Vegetable Cowpea

Cowpea [Vigna unguiculata (L.) Walp] is a leguminous crop of indeterminate growth habit continues to produce flowers throughout its life cycle grown mainly for vegetable, grain and fodder. In India, cowpea was cultivated in an area of 0.5 million ha (Rajpoot and Rana, 2016) and the leading cowpea producing states in India are Rajasthan, Karnataka, Kerala, Tamil Nadu, Maharashtra and Gujarat (Directorate of Pulses development, 2017). Cowpea is the major vegetable crop of Kerala cultivated in an area of 5803.05 ha (FIB, 2021). The recommended dose of fertilizers (RDF) for cowpea cultivation in Kerala was 20 kg N, 30 kg P₂O₅ and 10 kg K₂O ha^-1 (KAU, 2016). Full dose of P and K were applied as basal and remaining half N at 30 DAS which was found inadequate to meet the nutrient requirements of later formed flowers and pods. Hence, under field condition yield potential of the crop was not achieved. In order to realize the genetic yield potential of cowpea, nutrient requirements of later formed flushes also needs to be met. Foliar application is a means of rapid correction of nutrient deficiencies as well as physiological disorders in crop plants. Foliar application was 6, 4 and 20 times more beneficial than soil application in case of N, Zn and B respectively (Dixon, 2003).  Supplementary feeding of nutrients through foliage improves the plant nutrient status and crop yield (Krishnasree et al., 2021).  A significant increase in number of fruits per plant, fruit weight and fruit diameter were observed due to foliar spray of 19:19:19 (2%) twice at vegetative stage followed by 0:52:34 (2%) during flowering stage and 13:0:45 (2%) twice at fruit development stage in comparison to no foliar spray (Narayan et al., 2011). Foliar application of K 2 g L^-1 favourably influenced number of fruits per plant and fruit weight in brinjal (Ramadan and Shalaby, 2016). The highest number of fruits per vine and fruit weight were recorded in foliar application of DAP (1.5%) and boric acid (0.1%) at 35 DAS followed by one more spray of boric acid (0.1%) at 45 DAS in cucumber (Mondal et al., 2017). An enhancement in number of flowers per plant, number of pods per plant, pod length and pod weight were noticed in cowpea due to application of 125 per cent RDF along with 2 per cent DAP spray at flowering and pod formation stages (Kavitha et al., 2019). Foliar nutrition of Zn (100 mg L^-1) and B (100 mg L^-1) recorded significantly higher fruit length and number of fruits per plant in tomato receiving (Saravaiya et al., 2014). Foliar feeding of Zn (0.05%) in combination with B (0.03%) recorded the highest fruit length, fruit diameter as well as fruit weight in tomato (Sultana et al., 2016). With this back ground, the present study was formulated with an objective to assess the effect of foliar nutrition of water-soluble macro and micronutrient fertilizers for yield maximization in bush vegetable cowpea.

Field experiment was conducted during Rabi 2020-21 at Coconut Research Station, Balaramapuram, Thiruvananthapuram, Kerala located at 8°22'52" North latitude and 77°1' 47" East longitude and at an altitude of 9 m above MSL with an objective to assess the effect of water- soluble macro and micronutrient fertilizers on yield maximization in bush vegetable cowpea. The experiment was laid out in randomized block design with 13 treatments. The treatment details are presented in the Table 1. Experimental site was prepared by ploughing the field with a garden tiller twice and was properly levelled and treatment plots having a gross plot area 3.6 m × 3.6 m were laid out. Small bunds were taken all around the treatment plots.  The soil of the experimental field was strongly acidic with a pH of 4.71, normal in electrical conductivity (0.04 dSm^-1), high in organic carbon (1.29%), medium in available N (313.6 kg ha^-1), high in available P (39.23 kg ha^-1) and medium in available K (235.21 kg ha^-1). Bhagyalakshmi, a short duration, multi season, multipurpose, bush vegetable cowpea, highly resistant to anthracnose disease having a yield potential of 6.48 t ha^-1 released from College of Agriculture, Vellanikkara was used as the test crop. Seed rate adopted was 25 kg ha^-1. Before sowing, seeds were primed in 0.05 per cent zinc sulphate for 4 h and dried back to original moisture content. Seeds were dibbled at a spacing of 30 cm × 15 cm. Farm yard manure @ 20 t ha^-1 and lime @ 250 kg ha^-1 was uniformly applied to all plots. Fertilizers were applied as per the treatments. All other agronomic practices were carried out as per POP recommendations of crops for Kerala (KAU, 2016). Observations on growth attributes viz., plant height, number of branches and dry matter production were recorded at 60 DAS and at 80 DAS (final harvest) from the five observation plants which were randomly selected and tagged. Yield attributing characters viz., number of pods per plant, individual pod weight, pod length and girth were also recorded from the observation plants. Tender pods were harvested from the net plot area of each treatment starting from 45 DAS and continued up to 80 DAS. Pod yield per hectare and haulm yield per hectare were calculated from the pod and haulm yield obtained from the net plot area. Economics was worked out based on the prevailing market price of the inputs and price of the tender pods (₹ 20 kg^-1). Analysis of variance technique for RBD (Cochran and Cox, 1965) was used for the statistical analysis of the experimental data and the significance was tested using F test. The treatment means were compared at 5 per cent probability level.
 

Effect on growth attributes
 
Foliar nutrition of macro and micro nutrients had significant effect on growth attributes viz., number of branches per plant and DMP per plant. Compared to POP, foliar nutrition treatments registered higher values for number of branches per plant and DMP per plant. The treatment T₁₂ recorded the highest number of branches per plant both at 60 DAS and at final harvest. At 60 DAS higher dry matter production was recorded in treatment T₂ which was on par with T₅ and T₈. At final harvest T₅ registered higher DMP and was statistically at par with T₈, T₁₃ and T₁₂. Better expression of growth attributes in these treatments might be due to the quick access of plants to nutrients due to foliar application (Dey et al., 2017). Increased availability and uptake of nutrients might have resulted in enhanced photosynthesis and translocation of photosynthates from source to the growing plant parts leading to better expression of growth attributes in these treatments. The results obtained are in consonance with the observations of Haleema et al., (2018) who observed that foliar application of Zn and B resulted in enhanced vegetative growth due to increase in photosynthetic and metabolic activity, cell division and cell elongation. Elhindi et al., (2016) reported that foliar application of KNO₃ improved the plant growth by enhancing the physiological process viz., osmoregulation, stomatal conductance, photosynthesis, protein synthesis, solute transport and nitrate metabolism. Significant increase in dry matter production was observed in chilli owing to foliar application of water-soluble fertilizers (Muthumanickam and Anburani, 2017). Banasode and Math (2018) opined that foliar application of water-soluble fertilizers significantly influenced the uptake of nutrients. Better uptake of nutrients increased the physiological process like photosynthesis, chlorophyll formation, protein synthesis, nitrate metabolism and carbohydrate synthesis which ultimately results in increased DMP. Foliar application of solubor also contributed to higher DMP in T₅. Boron plays an important role in increasing the permeability of cell membrane which led to better transportation of sugars and carbohydrates ultimately leading to higher DMP (Thakur et al., 2019).
 
Effect on yield attributes
 
Compared to POP recommendation of fertilizers (T₁), foliar application of macro and micro nutrients had significant effect on pods per plant and individual pod weight. Fageria (1992) opined that number of pods per plant is the most important yield determining character in pulses. Rapid availability of nutrients under foliar application leads to accelerated rate of physiological processes which ultimately improved the plant growth and number of pods per plant (Khanda et al., 1999).  Krishna and Kaleeswari (2018) opined that foliar application of macro and micro nutrients increased the number of flower buds and their retention by maintaining optimum bio-physiological conditions compared to control (without foliar application). Treatment T₅ recorded the highest number of pods per plant and it was statistically on par with T₂, T₆ and T₁₁. Premsekhar and Rajashree (2009) reported that foliar application of 19:19:19 significantly enhanced the number of fruits per plant in tomato. Higher number of pods per plant observed in T₅ and T₁₁ was also due to the favorable influence of B in enhancing the fruit setting percentage by enhancing the pollen viability (Praveena et al., 2018), pollen germination and elongation of pollen tube (Narayanamma et al., 2009).  Better expression of growth attributes (Table 2) and higher availability (Table 4) and uptake of nutrients also contributed to higher number of pods per plants in treatments T₅, T₂, T₆ and T₁₁. Meena et al., (2017) opined that foliar application of nutrients improved the source-sink relation leading to less intra plant competition for nutrients.
 

 

       
Foliar application of macro and micro nutrients had significant effect on individual pod weight. Treatment T₂ recorded significantly higher pod weight and it was on par with T₅, T₆, T₈, T₉, T₁₀, T₁₁ and T₁₃.  Higher pod weight recorded in these treatments might be due to better partitioning of assimilate from source to sink. This was clearly evident from the data on harvest index (Table 3). Guievence and Badem (2000) reported that foliar application of nutrients promotes better uptake of nutrients and water leading to improvement in photosynthesis and better partitioning of assimilates to the edible parts. Better sugar translocation triggered by the foliar application of B (Meena, 2010) might have contributed to higher pod weight in T₅, T₆ and T₁₁. The increase in pod weight observed in T₅, T₆ and T₁₁ are in consonance with the results of Pandav et al., (2016) in brinjal and Harris et al., (2018) in chilli. Haque et al., (2011) opined that higher levels of B resulted in better uptake of water, synthesis of sugars and translocation of metabolites to sink tissues. Favourable effect of Zn in sugar translocation (Singh and Tiwari, 2013), IAA synthesis (Shnain et al., 2014) and reduction in flower drop (Ali et al., 2008) also contributed to higher individual pod weight in T₆, T₁₀ and T₁₃.
 

       
Foliar application of macro and micro nutrients did not contribute significantly to pod length and pod girth. This might be due to the fact that these characters are highly associated with the genetic makeup of the variety.
 
Effect on yield and harvest index
 
Foliar application of macro and micro nutrients significantly enhanced the pod yield compared to POP recommendation (T₁). Among the treatments, the highest pod yield was recorded in the treatment T₅ which was statistically on par with T₂, T₆ and T₁₁. Higher pod yield registered in these treatments was due to the production of higher number of pods per plant and pods with higher weight (Table 3). Foliar nutrition of 19:19:19 resulted in increased availability and assimilation of major nutrients (Singhal et al., 2015) and the favourable influence of B in various physiological processes viz., calcium metabolism, auxin synthesis, sugar metabolism, translocation of solutes and protein synthesis (Thakur et al., 2019) led to the formation of higher number of pods per plant which finally contributed to higher pod yield in T₅. Tariq and Mott (2007) revealed that B deficiency during flowering prevents the growth of the pollen tube leading to pollen sterility, flower drop and poor seed setting. Foliar nutrition of P enhances cell division (Reddy et al., 2017) leading to production of higher number of branches (Table 2) and pods per plant which finally contributed to higher yield in T₂. Yield enhancement in T₁₁ might be due to the favourable influence of K and B. Potassium promotes the water use efficiency of plants and assimilate translocation towards sink (Zhao et al., 2001). Vekaria et al., (2013) revealed that foliar nutrition of KNO₃ 0.4 per cent significantly increased the grain yield of green gram by 18.4 per cent compared to control. Higher yield registered in T₆ was due to the better expression of growth and yield attributes (Table 2 and 3). Similar observations were also made by Wasaya et al., (2017) in maize and Ali et al., (2015) in tomato.
       
Foliar application of macro and micro nutrients also had significant effect on haulm yield and the highest haulm yield was registered by T₁₃, whereas the lowest haulm yield was recorded in T₁. Compared to T₁, all the foliar nutrition treatments registered higher haulm yield. This might be due to the fact that foliar application of primary nutrients along with Zn and B enhanced the crop growth (Krishna and Kaleeswari, 2018). Adequate availability of N favours vegetative growth and formation of dark green leaves which would actively participate in photosynthesis (Thakur et al., 1991). Favourable influence of boron on meristematic activity and cell division also contributed to higher dry matter production. In addition, B increases the permeability of cells thereby permitting increased translocation of sugars and carbohydrates (Thakur et al., 2019). Zinc had a major role in the synthesis of auxin, a plant growth promoter crucial for better growth (Singh and Tiwari, 1989). Reduced Zn availability leads to shortened internodes and chlorosis of leaves which reduces assimilatory surface area available for photosynthesis (Yadav et al., 2014).
       
The highest HI was recorded in treatment T₇. This was due to lower haulm yield registered in the treatment compared to other foliar treatments (Table 3). Enzyme activation and biosynthesis of enzymes, growth hormones mediated by Zn also helped in better vegetative growth of plants (Rout and Das, 2003) which in turn led to the synthesis of greater proportion of carbohydrates through photosynthesis. Arora et al., (1990) has described the physiological role of Zn in effecting translocation of assimilates towards sink tissues. Deficiency of Zn causes accumulation of carbohydrates in plant leaves (Marshner and Cakmak, 1989). Boron plays a key role in translocation of sugars from leaves to pods (Thakur et al., 2019). Better partitioning of assimilates due to B application might have led to higher HI. Adequate supply of N, P and K through 19:19:19 and Zn and B led to better utilization of photosynthates for reproductive growth instead of excessive vegetative growth (Muthumanickam and Anburani, 2017).
 
Effect on post-harvest nutrient status
 
Foliar application of macro and micro nutrients significantly influenced the available nutrients in the post-harvest soil. All the foliar nutrition treatments registered higher availability of N compared to T1 might be due to enhanced biological N fixation as a result of better vegetative growth. The treatments T₁, T₃ and T₉ registered higher P availability. The reason for the increased availability of P observed in T₃ and T₉ might be due to the better root growth which would enable the crop to tap nutrients from the deeper soil layers. Addition of organic matter through FYM and decomposition of fallen leaves from the cowpea plant also paved for better availability of nutrients. Similar observations were also made by Raj (2019) in cowpea. Lower P uptake by the crop resulted in higher post-harvest P status in T₁. The treatment T₆ recorded the lowest available K might be due to higher K uptake by the crop. Similar results of reduced availability of K due to increase in uptake was reported by Kuwar et al., (2014) in water melon.
       
Foliar application of macro and micro nutrients significantly enhanced the availability of Zn and B in soil. The highest available Zn was reported in T₁₁ and was statistically on par with T₈, T₉ and T₁₀. The highest available B was registered by the treatment T₂ and was statistically comparable with T₅, T₆, T₇, T₁₀, T₁₁ and T₁₂. Similar results of increased availability of Zn and B in the soil was reported by Mini and Mathew (2019) following the foliar application of micro nutrient mixture in okra. Application of 0.25 per cent borax at flowering and fruit initiation stages of tomato recorded the highest soil available boron (Sathya et al., 2013).
 
Effect on economics
 
Foliar nutrition of water-soluble macro and micronutrient fertilizers brought significant increase in net return and benefit cost (B:C) ratio. Foliar nutrition of water-soluble macro and micronutrient fertilizers along with RDF resulted in an increase in net return of Rs 13,852 to 49,712 ha^-1. Among the treatments, treatment T₅ registered the highest net return (82,651 Rs ha^-1) which was statistically on par with T₂, T₆ and T₁₁. Benefit cost ratio was found to be the highest in T₅ and it was statistically on par with T₂, T₆ and T₁₁.   Higher net return and B:C recorded in these treatments was owing to higher green pod yield registered in the treatment (Table 5). Singhal et al., (2015) reported that foliar feeding of water-soluble nutrients significantly improved the okra yield and recorded higher net income. Saravaiya et al., (2014) observed an increase in net income and B:C ratio due to foliar application of B 100 mg L^-1 in tomato. The control treatment (T₁) registered the lowest net return and B:C ratio due to lesser pod yield recorded in the treatment.
 

It could be concluded from the results that foliar application of water-soluble macro and micronutrient fertilizers along with RDF (N:P:K @ 20:30:10 kg ha^-1) significantly improved the growth and yield attributes of bush vegetable cowpea. Among the treatments, RDF + foliar application of 19:19:19 (0.5%) and solubor (0.025%) at 45 DAS, RDF + foliar application of 19:19:19 (0.5%) alone at 45 DAS, RDF + foliar application of 19:19:19 (0.5%),  solubor (0.025%)  and  zinc sulphate 0.05 per cent at 45 DAS and RDF + foliar application of potassium nitrate (0.5%) and solubor (0.025%) at 45 DAS recorded higher dry matter production, number of pods per plant, green pod yield, harvest index, net return and B: C ratio. However, considering the highest green pod yield, net return and B:C ratio, the treatment T₅ (RDF + foliar application of 19:19:19 (0.5%) and solubor (0.025%) can be recommended for yield maximization in bush vegetable cowpea.