Yield Response of Bambara Groundnut [Vigna subterranea (L.) Verdc.] to Fertilizer Application and Plant Spacing

Bambara groundnut [Vigna subterranea (L.) Verdc.] is an indigenous legume crop known to be originated from North Africa. It is one of the imperative and neglected legume crops that contribute positively to improving global food and nutrient safety (Khan et al., 2021). In Ghana, Bambara is mostly cultivated in the northern and coastal areas, with a yield not exceeding 300 kg/ha (Asante et al., 2021).
 
Because of its ability to survive drought through improved soil water uptake, lower water usage, and cellular hydration maintenance through osmotic adjustment, it is also called a drought-avoiding crop (Blum, 2005). Additionally, the crop has the capacity to preserve leaf turgor pressure by combining osmotic adjustment, decreased leaf area, and efficient stomatal regulation (Berchie et al., 2012).
 
Nucleic acids, proteins, lipids, carbohydrates, and adenylate are a few compounds that include P and are necessary for plant cells to operate (Zhang et al., 2014). Lack of phosphorus (P) in the soil has a negative effect on the growth of legumes because it is necessary for nodule energy conversion and improved N-fixation (Yadav et al., 2017).
 
Plant spacing is an important agronomic practice because it affects light interception, and it is the plant’s green parts’ primary source of energy (Ibeawuchi et al., 2008). Because the crop canopy closes sooner with narrower row spacing than with broader row spacing, it reduces weed development (Knezevic et al., 2003). Shaukat et al., (2012) reported that numbers of pods and seeds per pod increased at a wider row spacing.
 
Dankyi et al., (2005) made reference to poor soil nutrition in crop production as among the causes of declining yields in crops. The quality and productivity of many farmlands in Ghana have been lost through a combination of human-induced natural processes, which affect the capacity of the soil to function optimally. Continuous cultivation of lands has contributed to soil fertility depletion. Soil nutrient deficiency is a major constraint to yield maximization in the tropics, especially small holder farms (Obidiebuba et al., 2012). The aim of the study was, therefore, to evaluate the effects of P based fertilizer and plant spacing on leaf area, leaf area index, seed yield and yield components of Bambara groundnut.

 

The experiment was conducted as an on-farm trial at Nkoranza in the Nkoranza North District in the Bono-East Region and Ejura in the Ejura Sekyedumase Municipality in Ashanti region of Ghana from July to December, 2021. The experimental areas experience bimodal rainfall patterns and have a forest-savanna transition zone and sandy loam soils (Ghana Statistical Service, 2012).
 
The experimental design was a 3x3 factorial with treatments being laid out in a randomized complete block design with three (3) replications. The treatments were made up of three P based complex fertilizer (NPK 11:22:20) application rates, namely 0kg P/ha, 30 kg P/ha and 60 kg P/ha and three spacing; 50 cm x 20 cm, 40 cm x 20 cm and 40 cm x 25 cm (accommodating 10, 12.5 and 10 plants per square metre, respectively). The experimental field was cleared, lined and pegged and demarcated into experimental units or plots. The experimental fields were slashed and tilled manually by hoeing. Hoeing was done twice, first, to earthen-up the soil and secondly, to break the soil clods. Each plot measured 12 m x 12 m with an alley of 1m and 2m between plots and replication, respectively. The plots were labelled prior to planting.
 
Seeds of an early maturing landrace of Bambara groundnut named Black eye obtained from Crop Research Institute-CSIR, Fumesua, were planted in three levels of spacing; 50 cm x 20 cm, 40 cm x 20 cm and 40 cm x 25 cm. Planting was done at two seeds per hill and thinned to one plant per hill on ridges. Phosphorus-based complex fertilizer (NPK 11:22:20) was applied at two weeks after planting at 0 kg/ha, 30 kg/ha and 60 kg/ha. Each plot had equal chance of receiving any of the application rates.  The hand-held hoe was used to control weeds at two and four weeks after planting.
 
Ten soil samples were picked with an auger along the two diagonals of the study field, five from each diagonal and composited. The composite soil sample was characterized using the ordinary procedures described by Motsara and Roy (2008) (Table 1). The climatic data are presented in Table 2. Lengths and breadths of sampled leaves (3^rd and 4^th leaflets on the 5^th compound leaf from the base of the plant) were measured and multiplied to obtain leaf area at 2 weeks interval, from the 2^nd to 8^th weeks after emergence. Means were computed and recorded. After obtaining leaf area, the figure obtained was divided by the ground area covered by the crop  to compute the leaf area index (LAI). This was done manually due to unavailability of LAI tools.




 
The pods in each plot were harvested, dried and weighed. Weights obtained from each treatment/experimental unit were converted to kg/ha to obtain pod weight. One hundred pods were counted from every plot and the weight was taken. The harvested pods were dried and manually threshed. The seeds were separated from the thrash, dried and weighed. Weights obtained from each treatment/experimental unit were converted to kg/ha. Weighing of pods and seeds was done using an electronic balance. Data collected were subjected to ANOVA using the GenStat Statistical Package, current edition (Payne et al., 2009). Means were separated using the LSD at 5% level of significance.

Vegetative growth
 
Leaf area was significantly (P<0.05) affected by treatments with plants under wider row spacing of 50 cm x 20 cm treated with 60 kgP/ha recording the largest leaf area. Closer row spacing of 40 cm x 20 cm with 12.5 plants per unit area and treated with no fertilizer P (0 kgP/ha) recorded the least leaf area value (Table 3 and Table 5). The 60 kgP/ha with 40 cm x 20 cm treatment combination differed significantly (P<0.05) from the other treatment combinations, except 60 kgP/ha under 40 cm x 25 cm and 0 kgP/ha with 50 cm x 20 cm treatment combinations. The 0 kgP/ha x 40 cm x 20 cm and 30 kgP/ha x 40 cm x 20 cm treatment combinations had the lowest leaf area index of 0.01. All other treatment combinations were similar in leaf area index.






 
The increase in plant growth in the form of leaf area and leaf area index following application of 60 kgP/ha could have smothered the growth of weeds in the field. This could have conserved soil moisture, reduced pest problems and nutrient mining through erosion and leaching, thus, making nutrients available to Bambara plants for proper growth and development.
 
Yield and yield components
 
Weight of hundred pods was significantly (P<0.05) affected by plant spacing and fertilizer application (Tables 4 and 6). Wider row spacing with 60 kgP/ha gave the highest 100-pod weight (125.67 g) and differed significantly from the other treatments, except the 60kgP/ha under 40 cm x 20 cm, which was similar to 60 kgP/ha with 40 cm x 25 cm, 30 kgP/ha x 20 cm x 40 cm and 0 kgP/ha x 20 cm x 50 cm treatment combinations. The least weight of hundred pods (104.17 g) was noticed in 0 kg/ha x 40 cm x 20 cm.


 
Pods per plant varied (22.7 to 35.3) due to plant spacing in Bambara groundnut. Wider row spacing of 50 cm x 20 cm produced more pods than 40 x 20 cm row spacing. Increasing the plant density per unit area from 10 numbers (50 x 20 cm or 40 x 25 cm) to 12.5 (40 x 20 cm) decreased the number of pods per plant. Though 50 x 20 cm and 40 x 25 cm spacing accommodated equal number of plants (viz., 10) per square metre, they differed in number of pods per plant. Row spacing of 50cm x 20cm seems better (35.7 pods/plant) under 50 x 20 cm spacing than increasing the inter-plant spacing from 20 to 25 cm, but decreasing the row spacing from 50 to 40 to accommodate the same level of plants (10 plants per square metre) produced only 31.7 pods/plant (Table 4 and Table 6).
 
Application of fertilizers had limited impact on the Bambara groundnut during the season tested, but the impact was well pronounced only under closer spacing, that is under 40x20 cm (12.5 plants/m^-2). The pods per plant had increments from no fertilizer to 60 kg P based complex fertilizer per hectare. Wider spacing of 50 cm x 40 cm without any fertilizer application produced pods equal to 60kg P applied to other two spacings viz., 40 x 20 and 40 x 25, which needs further investigation to draw any valid conclusion.
 
The dry seed yield of Bambara groundnut for different plant spacings and rate of fertilizer application was very similar to that of number of pods observed. The Bambara groundnut seed yield was found significantly affected by wider row spacing of 50 cm x 20 cm more than 40 cm x 20 cm and 40 cm x 25 cm. It was also observed that merely altering the inter-plant spacing from 20 to 25 cm under 40 cm row spacing, which resulted in reduction of number of plants from 12.5 to 10 per unit area, was seen with significant seed yield improvement by more than 50 per cent without any other manipulation like fertilizer application compared to 40 x 20 cm without any fertilizer application (Table 4 and Table 6).
 
Increasing the plant density from 10 to 12.5 per unit area decreased the Bambara seed yield from 3566kg/ha under 50 cm x 20 cm to 1884 kg/ha for 40 cm x 20 cm; the decrease measured as 47 per cent. The reduced seed yield under 40 x 20 cm spacing was compensated by fertilizer application. To compensate the seed yield reduction under 40 x 20 cm spacing compared to 50 x 20 cm spacing under no fertilizer regime, a fertilizer level of 60 kg P based fertilizer per hectare was needed. In other words, there is no advantage for fertilizer application when the plant spacing was wider (10 plants/unit area). Seed yield reduction under wider spacing when 30 kg P based fertilizer was applied could not be justified, which needs further investigation to draw any valid conclusion.

Results of the study revealed that leaf area, leaf area index, yield components and seed yield of Bambara groundnut were significantly affected by plant spacing and P based fertilizer. Wider spacing of 50 cm x 20 cm row was better than 40 cm x 20 cm and 40 x 25 cm. Fertilizer application had mixed responses, with no significant impact when the plant density was 10 m^-2, whereas under a plant density of 12.5 m^-2, there was a response to 60 kg P based fertilizer application, which needs further study. 
 

All authors declare that they have no conflict of interest.