Growth and Yield of Blackgram [Vigna mungo (L.) Hepper] Crop as Influenced by Humic Acid Application

Pulse crops play an important role in food security of country like India where malnutrition is common and ever increasing and they are the quickest means to augment protein production in developing countries. To increase the public awareness about the nutritional benefit of pulse crop and as a part of sustainable food production aimed towards food as well as nutritional security the year 2016 was assigned as “international year of pulse”.
       
Blackgram [Vigna mungo (L.) Hepper] ranked third among all pulses in India and contributes to 10% of the national pulse production from an area of 13% of total pulse crop area. Blackgram is rich in nutrients such as protein (25-28%), oil (1-1.5%), carbohydrate (60-65%), essential mineral (K, Ca, Zn), various vitamins (Vit C, B), fibre (3.5-4.5%) and antioxidant compounds that can help to boost the human health (Lavanya et al., 2022 and Patial et al., 2022). It is also rich in amino acids like methionine, cysteine and lysine. Both production and area under this crop lay decreased during last decade. The targeted production and productivity are possible by way of harnessing the yield gap by growing pulses in new niches i.e., precision farming, quality inputs, soil test based integrated nutrient management and mechanized method of pulse cultivation complimented with generous governmental policies and appropriate funding support to implementing stake holders. Maintenance of soil organic material at a satisfactory level is needed for sustainable and high productivity of crops over long periods (Govindasamy, 2002). Humic acid (HA) is the major component of most of the organic fertilizers and the most active components of soil and organic matter might help to overcome these  production constraints in the pulse crops. Humic substances arise from chemical and biological degradation of plant and animal residues and from synthetic activities of micro-organisms (Schnitzer, 1968). It enriches the soil and facilitates fertilizer nutrients to reach their maximum potential in improving plant growth (Nithila et al., 2013).
               
Humic acid helps to increase oxygen uptake resulting in better plant growth. Root growth is improved and consequently uptake of nutrient and water is more efficient. Foliar application of humic acid may reduce the nitrogen application to the soil (Sani, 2014). Humic substances also increase microbial population in soil and help in reducing metal poisoning. MacCarthy et al., (1990) and Singaravel et al., (1993) stated that the auxin like growth promoting activity of humic acid is the reason of accumulation of more dry matter in the plant. The HA molecule also raised the effect of fertilization based on N, P, K on plants (Pollhamer, 1993). Keeping all these in view, the present experiment was undertaken to study the influence of humic acid application on growth and yield of blackgram under new alluvial zone of West Bengal.

The field experiment was conducted at experimental farm, Bidhan Chandra Krishi Viswavidyalaya situated in the new alluvial zone of West Bengal, India, during autumn (August-November) season of 2018 and 2019. The experimental farm was situated at 22.93° N latitude, 88.53° E longitude and an altitude of 9.75 m above mean sea level, in the sub-tropical sub-humid climatic zone of eastern India. The soil properties of the experimental site were neutral in reaction (pH 6.72), medium in organic carbon content (0.48%), low in available nitrogen (198.26 kg/ha), high in available phosphorus (31.15 kg/ha) and medium in available potassium (172.52 kg/ha). The experiment was laid out in randomized complete block design (RCBD) with twelve treatments and each treatment was replicated thrice with the variety Pant U-31. The treatments were comprised of; T₁ = Control (sowing of dry seed); T₂ = Seed priming (soaking) in water; T₃ = Seed priming in 0.5% humic acid solution; T₄ =  Seed priming in 1% humic acid solution; T₅ =  Seed priming in 2% humic acid solution; T₆ =  Soil application of humic acid @1 kg/ha (basal); T₇ = Soil application of humic acid @ 2 kg/ha (basal); T₈ =  Soil application of humic acid @1 kg/ha + 10 t compost/ha; T₉ =  Soil application of humic acid @ 2 kg/ha + 10 t compost/ha; T₁₀ = Foliar application of  0.01 % humic acid solution (at 25 DAS); T₁₁ = Foliar application of 0.05 % humic acid solution (at 25 DAS); T₁₂ = Foliar application of  0.1 % humic acid solution (at 25 DAS). Seed priming was done for 4 hours, thereafter the seeds were dried under shade to remove the surface water. Foliar application of humic acid was done at 25 days after sowing. Different agronomic operation such as weeding, thinning, plant protection measures were same for all the plots.
       
The crop was raised with only one pre-sowing irrigation for uniform crop stand. Seeds were manually sown (hand dibbled) @ 30 kg ha^-1 in 2-3 cm depth. Row to row distance was 30 cm within a plot size of 4 m x 3 m. A normal dose of fertilizer was applied @ 10 kg N, 60 kg P₂O₅ ha^-1 and 40 kg K₂O ha^-1 by using fertilizers like urea, single super phosphate and muriate of potash, respectively. Thinning and gap filling was done at seedlings 20 days after sowing (DAS) so as to maintain optimum and uniform plant population in all the plots.
       
The morphological characters viz., plant height, leaf area index (LAI) and total dry aerial biomass production were recorded. Total leaf area was calculated by area-weight relationship. For dry aerial biomass the above ground plant samples were dried in a hot air oven at a temperature of 65-70°C for 2-3 days till constant mass was obtained. The total chlorophyll content was also estimated (Lichtenthaler et al., 1983). Data on pods m^-2, seed pod^-1, seed index, grain yield (kg ha^-1) and harvest index (%) were recorded at the time of harvest. Pods were dried, threshed and weighed with the help of an electronic balance. Grain yield were recorded from the net plot area and converted to kg/ha. Quality parameters like seed protein content obtained by multiplying the nitrogen content with the conversion factor of 6.25 (equivalent to 0.16 g nitrogen per gram of protein) used for converting nitrogen content into protein content.
               
The experimental data recorded for various parameters under study were subjected to statistically analysed ANOVA given by Gomez and Gomez (1984) to draw a valid conclusion. The variation in the treatments mean was tested by using critical difference (CD) values at 5% level of significance.

Plant height
 
Plant height is an important index of plant growth at a given time during the growth period. From Table 1 it is revealed that the plant height has positive respond to different humic acid application doses and methods. Soil application of humic acid @ 2 kg ha^-1 along with farm compost @ 10 t ha^-1 (T₉) recorded higher plant height (i.e., 45.62 cm) in 2018 and also in 2019 (51.07 cm). However, the minimum plant height was observed in T₁ in 2019 but in 2018, T₂ (receiving N-P-K fertilizers plus seed priming was done with plain water) shows minimum plant height. The plant height was significantly influenced by soil application of humic acid along with compost during both the years.
 

       
Soil application and foliar spray of humic acid (HA) significantly increased plant height over other seed treatment and control where no humic acid applied and there is no significant difference between seed priming with control in both the year of experiment. Plant height was recorded more at higher dose of HA at soil with compost. However, seed priming with higher doses of HA (2%) in some cases shows better result than the seed priming with lower doses of HA. The positive effect of humic acid in improving plant height was probably due to synergistic and beneficial effect due to the adequate nutrient availability thus resulted in greater elongation (Singeravel et al., 1993). The increased plant height can also justify by Prasad and Prasad (1994) that auxin derivatives are involved in cell division and differentiation ads also expansion through increasing the plasticity thereby enhancing growth and development.
 
Leaf area index (LAI)
 
Leaf area Index (LAI), an important growth factor of blackgram crop and data regarding LAI are shown in Table 1. Humic acid treatment adapted in blackgram crop has significant effect on its LAI in both the year of experiments. It is clear that (Table 1) in comparison to other treatments the highest LAI (3.38 and 3.52 respectively in 2018 and 2019) was obtained from T₉ treatment. However, both the control (T₁ and T₂) treatment gives the lower value of LAI in both the year of experiments. The combine effect of HA and compost improved the LAI significantly. Most of the growth stages seed priming was at par with the control and the two control treatments (T₁ and T₂) were at par in both the years.
       
Higher LAI values might be due to adequate supply of nitrogen that had produced larger leaves and reflected in more photosynthetic area coupled with the increased growth rate, respiration rate and metabolic activity due to humic substances (Saravanan et al., 1989).
 
Dry aerial biomass
 
Dry aerial biomass is one of the most important parameters and has a marked influenced on final yield realization of a crop. The mean data on dry aerial biomass of blackgram as influenced by different treatments of humic acid is tabulated in Table 1. Soil application of HA with or without compost had a profound effect in increasing the total aerial dry matter production over control and treatments receiving N-P-K and seed priming (T₂). T₉ treatment recorded the highest value of dry aerial biomass in both the years of experiments where HA at 2 kg ha^-1 was applied in soil along with farm compost at 10t ha^-1; the second highest value of dry aerial biomass was recorded in treatment T₈ where HA at 1 kg ha^-1 was applied in soil along with farm compost at 10 t ha-1 and the lowest value was recorded in control treatment (T₁).
       
Data revealed that foliar application of HA significantly increased the dry aerial biomass over in both 2018 and 2019 and on an average, there was an increase of 37.86 and 37.18% over control in 2018 and 2019, respectively. The results of this experiment are similar with the finding of Khazaie et al., (2011). HA was capable of increasing the total amount of dry weight in soybean, peanut and clover plants (Tan et al., 1983).
 
Yield attributes
 
Yield is complex character determined by several traits internal plant processes and environmental factors. The number of pods per unit area is the most important yield component of any pulse crop. Data pertaining to pod number as influenced by different treatments was tabulated in Table 2. Sole application of humic acid along with compost had significance influence on the number of pods m^-2 of blackgram crop over control. The highest number of pods m^-2 was recorded in T₉ (597.34 and 604.23 in 2018 and 2019, respectively). There was no significant difference between T₉ with T₈ (humic acid @ 1 kg ha^-1 + 10 t compost ha^-1).
 

       
The number of pods m^-2 recorded in other treatments of humic acid either through seed priming or foliar spray had some positive effect and the number of pods produced was more than the control treatment T₁. In 2018, T₁ recorded minimum number of pods m^-2 (491.04) and in 2019 the minimum number of pods m^-2 (501.91) were recorded at T₂. Hu-shui and Wang (2001) recorded significantly a greater number of pods plant^-1 in soybean when the crop received foliar spray of HA and komix. Pre-care chickpea seeds with use of HA led to the increased number of pod plant^-1 (16.7) than non-pre cared seeds (10.2) (Ulukan et al., 2012). Increase in pod yield may be attributed to mineralization of nutrients, which leads to improve growth and better partitioning of assimilates to various metabolic sinks (Talavia et al., 2007).
       
Application of humic acid through different methods had no significance differences on the number of seeds pod^-1 (Table 1) among the treatments in autumn season of both the year of experiments. The highest number of seeds pod^-1 was recorded in T₉ (5.79) where humic acid was applied along with farm compost in 2018 and the corresponding figure in 2019 was 5.95. The treatment T₉ was closely followed by T₆ in 2018 and in 2019, T₉ and T₈ were statistically at par. The minimum number of seeds pod^-1 was recorded (5.79) in the treatment T₁ in 2018 and in 2019, T₁₂ recorded the minimum number (5.48) of seeds pod^-1. However, in most occasions there was no significant difference and the treatments were statistically at par.
       
Yield attributes like seed index (100 grain weight) represents the development and plumpness of grains and is an important index of grain yield. From the Table 2 it is clear that the effect of various humic acid treatments had no significant influence on the seed index of blackgram in both the year of experiments. During 2018 the seed index varied from 3.79 g at T₅ to 3.90 g at T₉. However increased dose of humic acid had no significant effect on the seed index. The seed index varied from 3.59 g at T₆ to 3.84 g at T₉ in 2019. Gaikwad et al., (2012) observed that foliar sprays of humic acid @ 400 ppm followed by 350 ppm increased the 100-grain weight (g) in maize.
       
Grain yield, an end product of interaction between yield components, differed significantly among treatments tested in the investigation. The effect of various treatments had significant influence on the grain yield (Table 3) of blackgram during both the years. The seed yield was significantly superior in all humic acid treated plots over T₁ in 2018. Increased dose of humic acid had a little effect on the seed yield over the lower dose in corresponding method of application. The maximum seed yield recorded was 1295.53 kg ha_^-1 and 1298.47 kg ha_^-1 in 2018 and 2019, respectively in T₉ which was at par with T₈ in both the year of experiment. The minimum seed yield was recorded in T₁ in 2018 (956.17 kg ha_^-1) and in T₂ in 2019 (1019.40 kg ha_^-1) where no HA was applied.
 

       
A critical examination of the data revealed that seed priming with normal water (T₂) brought no significant increase in seed yield of blackgram. Data recorded further revealed that soil and foliar application of humic acid had better influence on seed yield of blackgram than seed priming with humic acid solution. Increase in seed yield of wheat due to spraying humic acid at the stage of development of wheat branch was also reported by (Xudan, 1986) and Ayosou et al., (1996) reported humic acid caused remarkable increase of seed yield in barley.
       
Harvest index (HI) is an indicator of efficiency of crop plants to translocate manufactured food material at source level to the sink or grains. The data pertaining to harvest index are showed (Table 3) that harvest index was significantly affected by different HA treatments in both the years of experimentation. During 2018 the harvest index varied from 0.345 at T₇ to 0.390 at T₁ (control). While in 2019 the harvest index varied from 0.362 at T₁₁ to 0.412 at T₁.
       
Application of humic acid (HA) in blackgram (Table 3) has considerable effect on protein content of seed in both the years. Minimum values of protein content of seed were recorded in T₁ in the year 2019 but in the year 2018 lowest value was obtained in T₂. Similarly seed priming treatments with HA solution (treatment T₃ to T₅) were statistically at par with T₁ and T₂.
       
The increase in protein content of seed was noticed due to the foliar application of HA over control; the treatments (T₁₀ and T₁₁) were at par with T₁ and T₂. Moreover, foliar application of HA with different doses could not bring any significant variation in protein content of seed among themselves but foliar application of HA at higher dose (0.1% solution) showed significant variation in seed protein content during both the years of experimentation.
               
The protein content increased with the application of HA was reported in maize, sugarcane (Santhi et al., 2003), Soybean (Ashraf et al., 2005) and Kaya et al., (2005) reported that humic acid spraying solution (foliar) has increased protein content in green bean plant.

HA is one of the active major present in organic fertilizer as well as soil and organic matter. This study established that application of humic acids increased overall crop performance and productivity of blackgram. Among the different methods, soil application of humic acid was found to be the best, particularly when applied with compost. Therefore, soil application of humic acid @ 2 kg ha^-1 + 10 t compost ha^-1 (T₉) might be the best option for increasing productivity and quality of blackgram.

The authors declares no conflict of interest.