Effect of Plant Spacing and Np Fertilizer Levels on Growth, Seed Yield and Quality of Onion (Allium cepa L.) at Shewa Robit, Northern Ethiopia
Getachew Amare1, *, Wassu Mohammed2, Tamado Tana2
Identifiers and Pagination:Year: 2020
First Page: 12
Last Page: 22
Publisher Id: TOBIOTJ-14-12
Article History:Received Date: 06/09/2019
Revision Received Date: 27/11/2019
Acceptance Date: 01/01/2020
Electronic publication date: 13/02/2020
Collection year: 2020
open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
The shortage of quality seed is the major limiting factor of onion production in the study area. Inappropriate use of NP fertilizers and plant spacing is the major source of the problem.
The experiment was conducted to determine optimum plant spacing and NP fertilizer for high yield and quality seed production of onion.
The treatments were composed of plant spacing 10×20×40 cm, 20×30×50 cm, 10×40 cm, 10×50 cm, 10x30 cm and NP fertilizer levels 86.25 P2O5 & 85.5 N kg ha-1, 115 P2O5 & 114 N kg ha-1, 143.6 P2O5 & 142.5 N kg ha-1, 69 P2O5 & 142 N kg ha-1 and control in a 5 x 5 factorial arrangement of RCBD with three replications.
The highest seed yield per hectare (879.4 kg) and per plot (663.6 g) was obtained from 115 P2O5 & 114 N kg ha-1 fertilizers and plant spacing 10×30cm gives the highest seed yield per plot (561.7g) and per hectare (748.9 kg). The highest germination percentage was obtained by the interaction effect of 10×30 cm and 143.6 P2O5 & 142.5 N kg ha-1. Whereas, highest seed vigor index I and II were obtained from the interaction of 115 P2O5 &114 N kg ha-1 and 20×30×50 cm spacing.
Plant spacing of 10×30 cm followed by 20×30×50 cm and 115 P2O5 &114 N kg ha-1 fertilizers could be recommended for high yield and quality onion seed production in the study area.
Onion (Allium cepa L.) belongs to the family Amaryllidaceae. It is believed to have originated in Afghanistan, the area of Tajikistan and Uzbekistan, western Tien Shan and India . The most recent estimations have reported that there are about 750 species in the genus Allium, among which onion, Japanese bunching onion, leeks and garlicare the most important edible Allium crops .
Onions are highly beneficial in nutritional terms, providing a rich source of Vitamins B1, C and E and certain trace elements. The carbohydrate content ranges from 5% to about 11%; 100 g of edible portion provides about 36 kcal energy values . Onions are also advantageous in a way that, the bulbs can be harvested and sold either ‘green’ to be used in salads , while the mature bulbs are cooked or eaten raw as a vegetable .
The world onion production in 2017 was about 97,862,928 tons of dry bulbs from 5,201,591 hectares of land with an average yield of 18.8 t ha-1. China is the biggest onion producer followed by other major onion producing countries India, the USA, Iran, Egypt, Russia and Turkey .
The estimated total area under onion in Ethiopia was about 31,673.21 hectares, from which 293,887.5 tones were produced in 2018 with an average yield of about 9.27 t ha-1 . This indicates the productivity of onion in Ethiopia (9.27 t ha-1) is far below the world average (18.8 t ha-1). The factors affecting onion production could be inappropriate spacing, poor fertilizer application and unavailability of quality seeds together with other agronomic practices [5, 6].
The seed yield and quality of onion are influenced by many factors, but cultivar, soil and climate, seedling age, bulb weight, spacing, fertilization, date of planting and seed quality are very important. Mirshekari and Mobasher  reported that plant spacing significantly affected onion seed production. According to Verma et al.  interaction between bulb sizes and spacing (heaviest bulb and narrow spacing) produces higher total seed yield per plant. Asaduzzaman et al.  also reported plant spacing showed a significant effect on the number of flower stalks and its length, number of umbels per plant, number of seeds per umbel and seed weight per umbel. In Ethiopia, reports show higher onion seed yield is obtained through bulb planting at 30 cm space between rows on the ridge, 20 cm between plants and 50 cm between furrows on double row spacing .
Fertilizer application affects the yield and quality of onion seed production . Nitrogen has been found to increase the number of umbels per plant, number florets per umbel, umbel size and seed yield . However, the reports regarding the role of phosphorous on seed yield are contradictory. Chakrabarti et al.  reported that phosphorous application alone or in combination with nitrogen had no effect on seed yield but Ahmed and Abdella  reported phosphorous alone had no effect on seed yield, however, if it is applied in combination with nitrogen, it increases the number and size of umbels and seed yield significantly.
In Ethiopia, 46 P2O5 kg ha-1 and 87 N kg ha-1 are used for bulb production. Whereas, the recommendation for seed production of onion is the application of 100-150 kg ha-1 Urea and 200-250 kg ha-1 of DAP which is equivalent to 114 N kg ha-1 and 115 P 2O5 kg ha-1 [10, 15].
Bombay Red is one of the most popular onion varieties in Ethiopia. Its area coverage and production across the country are increasing from time to time and it accounts for the largest area coverage among onion varieties .
Despite an increase in the area of coverage, the productivity of Bombay Red onion variety in Ethiopia is much lower than the expected production level. The bottleneck problem for high production and productivity is limited availability of quality seed and lack of other associated technologies. The major problem affecting quality seed production throughout the country is that the farmers have little knowledge of the optimum amount of N and P fertilization as well proper spacing for the bulb to seed onion seed production method . Farmers in North Shewa, particularly in Shewa Robit Woreda also face the problem similar to other farmers in the country. Due to this, farmers do not apply fertilizers that are recommended by FAO  for seed production or blanket recommendation for bulb production by the Ethiopian Institute of Agricultural research , instead, they apply fertilizers as they think good for high seed production. In addition, they are using high plant spacing per unit area assuming that they can produce higher onion seed. Therefore, there was a need to undertake research to identify the appropriate plant spacing and NP fertilizer combination for maximum onion seed production. Thus, this study was conducted with the following objective.
To determine the effect of plant spacing and NP fertilizer levels on growth, seed yield and quality of onion.
2. MATERIALS AND METHODS
The experiment was conducted at Shewa Robit Research Station of Debre Berhan Agricultural Research Center, which is located at about 225 km Northeast of Addis Ababa. It is located at 110 55’ N latitude and 370 20’ E longitudes at an altitude of 1380 meters above sea level (m.a.s.l.). The area has an average annual rainfall of 1007 mm, and annual mean minimum and maximum temperatures of 16.5 and 31°C, respectively . Bombay Red onion variety was used as experimental material. The planting material was medium-sized bulbs of uniform diameter and they were sorted, which were free from insect, disease and mechanical injuries (4-5 cm). Urea fertilizer as nitrogen fertilizer and TSP (triple superphosphate) as phosphorous fertilizer were applied to the experiment.
The experiment was laid out in a Randomized Complete Block Design (RCBD) in a factorial arrangement with three replications. Five levels of nitrogen and phosphorous fertilizers combinations were 86.25 P2O5×85.5 N kg ha-1; 143.6 P2O5 ×142.5 N kg ha-1 and 115 P2O5× 114 N kg ha-1 which were 25% below, above and FAO  recommendation for Central Rift Valley, respectively, 69 P2O5× 142 N kg ha-1 (farmers practice) and with zero fertilizer application as a control were used as treatments. The five spacing treatments were set as 10 × 20×40 cm double row spacing recommended for bulb production, 20 × 30 × 50 cm (spacing recommended by FAO  for Central Rift Valley) and single row spacing of 10 × 40 cm (spacing commonly used by farmers); 10 × 50 cm (one level above the farmers) and 10 × 30 cm (one level below farmers) respectively. The five fertilizer combinations and spacing were combined in the factorial arrangement and the experiment was laid out as a randomized complete block design with three replications. Each replication was arranged as 25 plots corresponding to the 25 treatment combinations. The plot sizes were 2.5 × 3 m = 7.5 m2 and adjacent plots and blocks were 1 m and 1.5 m apart, respectively. Data were collected from the middle rows of each plot left one plant at both ends of rows.
The data collected were Days to 50% flowering, Days to maturity, Plant height (cm), Flower stalk diameter (cm), Number of umbels per plot, Umbel diameter (cm), Number of seeds per umbel, Seed weight per umbel (g), Seed yield per plot (kg), Seed yield per hectare (kg/ha), 1000 seeds weight (g), Germination percentage , Seed vigour index I (Germination (%) x Seedling length (cm) (Seedling Root length + Shoot length) and seed vigour index II (Germination (%) x Seedling dry weight (g)) and Speed of germination .
The collected data were subjected to analysis of variance using SAS statistical computer software version 9.1.3 program for factorial arrangement in RCBD. The analysis of variances, were performed following Duncan’s Multiple Range Test .
The partial budget analysis was conducted for economic analysis of fertilizer application and it was carried out for combined seed yield data. The potential response of crop towards the added fertilizer and price of fertilizers during planting ultimately determined the economic feasibility of fertilizer application. The economic analysis was computed using the procedure described by CIMMYT . The data were Gross average seed yield (kg ha-1) (AvY), Marginal Rate of Return (MRR %), Gross Field Benefit (GFB), Net Benefit (NB) and Adjusted Yield (AjY).
2.1. Soil Analysis
Surface soil (0-30 cm depth) samples were collected by using auger from 10 spots of the experimental field before planting. These samples were composites to yield one representative sample. The samples were subjected to air drying and ground that allowed them to pass through 2.0 mm sieve before laboratory analysis. The soil samples were analyzed for texture, pH, CEC, organic matter content, total N and available P.
The soil analysis result showed that the experimental site had a soil pH of 7.9 with 0.145% total nitrogen, 1.55% and 2.68% organic carbon and organic matter content, respectively. It has 2.32 mg P per kg of soil, 48.6 cmol (+)/kg and 10.68% available phosphorus, CEC and carbon to nitrogen ratio, respectively. The soil was 8, 50 and 42% sand, silt and clay, respectively, which gave silt clay soil texture. The total nitrogen was low as it was below 0.15% as per the description given by Bruce and Rayment , organic matter was moderate and the level of organic carbon was high as per the description of Charman and Roper . The available phosphorus was very low as it was < 5 mg P/ kg soil . The CEC was very high as per Metson , who indicated that CEC > 40 cmol (+) /kg is high.
3. RESULTS AND DISCUSSION
3.1. Phenology and Growth of Bombay Red Onion Variety
3.1.1. Days to 50% Flowering
Days to 50% flowering was significantly (P<0.01) affected by the main effect of plant spacing. The earlier days to 50% of flowering (71.53 days) was attained in plants grown at a single row spacing of 10 × 50 cm followed by 10 × 30 cm closer spacing, with no significant difference between them. While the delayed flowering was at about 74 days in plants grown at a spacing of 10 x 40 cm (Table 1).
The reason for early flowering on wider plant spacing might be due to less competition for light, moisture and nutrients, which consequently resulted in vigorous growth and early flowering than those which compete for essential growth factors on closer spacing. The result was in agreement with Mengistu and Yamoah  as they reported, when plant spacing increased from 133333 to 400000 ha-1the days to flowering also increased from 158 to 160 days. On the other hand, it is in contrast with Gray et al. , who reported that flowering was a few days later as plant spacing increased from 10 to 80 plants/m2. Menberu et al.  also reported that varieties planted at 25 cm intra-row spacing attained 50% flowering within a short period of time (42 days) than 40 cm spacing, indicating that densely populated plants attained 50% flowering earlier than sparsely populated plants. El-Naim  also confirmed that closer spacing could reduce vegetative growth and enhance flower formation.
3.1.2. Days to Maturity
Days to maturity was significantly (P<0.01) affected by the main effect plant spacing. The days to maturity was early (133.5 days) when grown at 10 × 30 cm followed by 10 × 50 cm (133.9 days) plant spacing with the non-significant difference between the two plant spacing. The maturity was delayed when grown at 10 x 40 cm single row spacing which was followed by 10 x 20 x 40 cm and 20 x 30 x 50 cm double row spacing (Table 1).
Rumpel and Kaniszewski  and Brewster  found that delayed days to maturity was observed from sparsely populated plants, while plants grown in closer spacing or in high plant population matured early. This is because the low plant spacing per unit area had adequate space and they were not severely competing for resources for their growth and development. Accordingly, it resulted in delaying of seeds maturity. In accord with the above findings, the present study result for days to maturity showed that closer spacing 10 × 30 cm was earlier to mature but with no significant difference with wider spacing 10 × 50 cm and 20 x 30 x 50 cm double row spacing.
3.1.3. Plant Height
Significant (P<0.05) differences were observed in plant height due to the main effect of plant spacing and NP fertilizers.
The tallest plants (87.3 cm) were found in double row spacing of 10 x 20 x 40 cm and single row spacing of 10 x 50 cm with no significant difference in between. On the other hand, the shortest plants (77.18 cm) were recorded from 20 x 30 x 50 cm double row spacing and narrower spacing of 10 × 40 cm (Table 1). This might be due to the preference of the onion crop for wider double row and single row spacing which give a small number of plants per unit area. The taller plant height observed in plants grown under single row spacing of 10 x 50 cm might be due to less competition to light, moisture and nutrients as they grow far apart.
The current result was in accord with the reports of Law-Ogbomo  and Jilani et al.  who reported that the onion bulbs planted on wider spacing of 30 × 20 cm produced the tallest plant, which was much higher than the shortest plants produced from the closest spacing of 25 × 15 cm. The authors described the closer spacing resulted in competition among plants for nutrient and light thus resulting in short plants, while, the plants grown in wider spacing produced more green leaf and extra food which promoted the plant height.
|Spacing (cm)||Days to 50% Flowering||Days to Maturity||Plant Height (cm)||Flower Stalk Diameter (cm)||Number of Umbel Per Plot|
|10 x 20 x 40||72.07bc||134.9ab||87.13a||1.449bc||1562a|
|20 x 30 x 50||72.53b||134.5abc||77.18c||1.549a||505c|
|10 x 40||73.53a||135.2a||79.31bc||1.421c||1213b|
|10 x 50||71.53c||133.9bc||86.31ab||1.497ab||1050b|
|10 x 30||72.33bc||133.5c||85.49ab||1.308d||1472a|
|Fertilizer rate (P2O5 and N kg ha-1)|
|0 x 0||73.00a||135.07a||76.01b||1.334c||1118a|
|86.25 x 85.5||72.33a||134.53a||82.59ab||1.435b||1189a|
|115 x 114||71.87a||133.80a||87.65a||1.559a||1218a|
|143.6 x 142.5||72.07a||133.93a||84.63a||1.439b||1153a|
|69 x 142||72.73a||134.67a||84.53a||1.457b||1122a|
The NP fertilizer rate of 115 P2O5 and 114 N kg ha-1 gave the highest plant height of 87.65 cm followed by 143.6 P2O5 and 142.5 N kg ha-1. Increasing fertilizer from zero to 115 P2O5 and 114 N kg ha-1 resulted in the increased height of the plant. Whereas, further increase in fertilizers above those described levels decreases the height of the plants (Table 1). Gupta and Sharma  and Ali et al.  reported that an increase in nitrogen fertilization from the control increases the height of the plant up to the certain stage at which the growth ceases or become decreased due to the toxicity of the fertilizer which is similar to results of the present study. The increase in growth with a rise in nitrogen up to a certain optimum level might be due to the availability of more nutrients, which help the maximum vegetative growth of onion plant and the effect of N contributing to the higher rates of vegetative growth and stem elongation.
3.1.4. Flower Stalk Diameter
Flower stalk diameter was significantly (P<0.01) influenced by the main effect of plant spacing and NP fertilizers (Table 1). The highest flower stalk diameter was recorded by 20 x 30 x 50 cm double row spacing followed by plants grown at 10 x 50 cm, which were statistically apart. Plants grown at plant spacing of 10 x 30 cm had small (1.30 cm) flower stalk diameter. The highest flower stalk diameter (1.56 cm) was recorded from NP fertilizers 115 P2O5 and 114 N kg ha-1 and the lowest (1.334 cm) was from control (Table 1). This result was in agreement with the report by Gusmao et al.  and Mozumder et al.  as they reported that wider spacing showed higher flower stalk diameter than closer spacing. This might be due to the fact that plants grown in wider spacing or low plant populations had adequate space and they might not severely compete for resources that were important for their growth and development and consequently resulted in high flower stalk diameter.
3.2. Seed Yield and Yield Components
3.2.1. Number of Umbles Per Plot
The number of umbels per plot was significantly (P<0.01) affected by the main effect of plant spacing. Plant spacing of 10 x 20 x 40 cm double row and 10 × 30 cm had 1562 and 1472 umbels per plot, respectively, which were the highest in the experiment. On the other hand, the smallest number of umbels per plot (505) was recorded for the plants grown at a spacing of 20 × 30 × 50 cm double row spacing (Table 1).
The result agreed with the works of Asaduzzaman et al. , Rumpel and Kaniszewski  and Abedin et al.  who reported that as the plant spacing decreased the number of umbels per plot also increased dramatically.
3.2.2. Umbel Diameter
Umbel diameter was significantly (P<0.01) influenced by the main effect of plant spacing and significantly (P<0.05) affected by NP fertilizers (Table 2).
The highest umbel diameter (6.366 cm) was obtained when plants were grown at 20 × 30 × 50 cm double row spacing which was followed by 10 x 20 x 40 cm double row spacing. Accordingly, the larger umbel diameter (6.267 cm) was obtained with the NP application rates of 115 P2O5 and 114 N kg ha-1, which were followed by 143.6 P2O5 and 142.5 N kg ha-1. The diameter of the umbel was small when plants were grown at 10 × 30 cm spacing and zero fertilizer application (Table 2).
These results were in accord with the work done by Firoz et al.  and Khan et al.  who found significantly large umbels from sparsely populated plants than densely populated. This might be due to a higher supply of food materials to the umbel by the plants grown at wider spacing with decreased competition for moisture, light and nutrients. According to Ali et al. , phosphorous and nitrogen fertilization showed a significant effect on umbel diameter. This was in agreement with the result of this study where NP fertilizers showed significant effect on umbel diameter. This might be due to the fact that flower setting and seed formation are highly controlled by phosphorous and the application of nitrogen increased the vegetative growth, produced good quality foliage and promotes carbohydrate synthesis thereby produces larger umbel diameters .
3.2.3. Number of Seeds Per Umbel
The number of seeds per umbel was significantly (P<0.01) affected by the main effect of plant spacing and NP fertilizers (Table 2). The highest number of seeds per umbel (910.2) was recorded for plants that were grown at 20 × 30 × 50 cm double row spacing followed by 10 × 50 cm spacing. Whereas, the small number of seeds per umbel was recorded from plants grown in plots with 10 x 20 x 40 cm double row spacing. The highest number of seeds per umbel (914.6) was recorded from plants that received 115 P2O5 and 114 N kg ha-1 followed by 143.6 P2O5 and 142.5 N kg ha-1, which were statistically apart with the former. While the lowest number of seeds per umbel was from control with no significant difference from plants that received 86.25 P2O5 and 85.5 N kg ha-1 (Table 2).
This result was in line with Ali et al. , who found that the highest number of seeds per umbel (95.28) was obtained from 150 N kg ha-1 and the lowest from the control. Abu-Sarra  also reported that the highest number of seeds per umbel was recorded from optimum fertilization and the lowest from lower nitrogen levels. This might be due to the role of nitrogen in the reduction of flower abortion and the effect of phosphorus on flower and seed production. Asaduzzaman et al.  also reported that as plant spacing increased the number of seeds per umbel also increased significantly which was in agreement with the current study result. This might be due to the role of wider spacing for a better supply of nutrients, light and moisture which might give excellent growth and development of a large number of seeds per umbel.
3.2.4. Seed Weight Per Umbel
Seed weight per umbel was significantly (P<0.05) affected by the main effect of NP fertilizer application and plant spacing. The highest seed weight per umbel was recorded from plants grown at spacing of 20 x 30 x 50 cm double row spacing, 143.6 P2O5 and 142.5 N kg ha-1 and 115 P2O5 and 114 N kg ha-1 fertilizers application with the non-significant difference among them. The lowest seed weight per umbel was recorded from the plants that were grown under zero NP fertilizer application (Table 2).
High seed weight per umbel under high NP fertilizers might be due to the role of nitrogen in the buildup of carbohydrate and different metabolites and the role of phosphorous in seed formation and development . Small seed weight per umbel under the highest plant spacing could be due to the effect of competition for moisture, light and nutrients.
The result was in agreement with the study by Ali et al.  who reported that seed weight per umbel was significantly different due to the effect of nitrogen and phosphorus dose. In accord with this study, Begum et al.  and Tesfu and Yamoah  also reported that the highest seed weight per umbel was recorded from wider spacing than narrower spacing.
3.2.5. Seed Yield Per Plot and Per Hectare
Highly significant (P<0.01) differences were exhibited for seed yield per plot and per hectare due to the main effects of NP fertilizers and plant spacing. The highest seed yield per plot and per hectare was obtained from the plants that were grown at 20 x 30 x 50 cm double row spacing and 10× 30 cm single row spacing with the non-significant difference between the two plant spacings. The lowest seed yields per plot and per hectare were recorded from plant spacing of 10 x 20 x 40 cm double row spacing. On the other hand, the highest seed yields per plot and per hectare were obtained from the rate of fertilizer 115 P2O5 and 114 N kg ha-1 followed by 143.6 P2O5 & 142.5 N kg ha-1. The lowest seed yield per plot and per hectare was recorded from control or plants which did not receive fertilizer (Table 2).
This indicated that the plots with very high and low plant spacing were efficient in giving higher seed yield per plot and hectare. This might be due to less competition for growth essentialities under low plant spacing; therefore, it can favor vigorous growth and highest yield. Whereas, when the plant spacing is closer, the total seed yield per plot and per hectare will be higher; since low seed yield per plant is compensated by higher plant spacing per unit area. The current study result was in accord with the studies by Jilani et al.  and Ali et al. , who reported significantly higher seed yield per plot from plants grown under closer spacing, increased fertilizer applications and from plants grown under wider spacing with optimum fertilizer application.
3.3. Seed Quality Parameters
3.3.1. Thousand Seeds Weight
The main effect of NP fertilizer levels significantly (P<0.05) affected 1000 seeds weight. The highest 1000 seeds weight was obtained from plants that received NP fertilizers of 143.6 P2O5 and 142.5 N kg ha-1 followed by 115 P2O5 and 114 N kg ha-1. Plants that did not receive fertilizer (control) produced the lowest 1000 seeds weight (Table 2). In this experiment, only the control treatments produced seeds slightly lower than the world average for 1000 seed weight (3.6 g) , while all other treatments produced more than the average seed weight for the onion. Generally, as both N and P2O5 rates increased the seed weight was also increased. This might be due to the fact that nitrogen and phosphorous are basic components of starch and carbohydrate of the seeds of plants. These elements are actively involved as a building block of the seed materials thereby increased the weight of the seed of the onion .
In agreement with the present study, Ozer  and Ozden  reported that higher rates of fertilizer application produced heavyweight seeds. On the other hand, the current study results were in contrast to Ali et al.  and Ali et al.  who reported that nitrogen and phosphorous fertilizers did not have a significant effect on 1000 seeds weight.
|Spacing (cm)||Umbel Diameter(cm)||Number of Seeds Per Umbel||Seed Weight Per Umbel(g)||Seed Yield Per Plot(g)||Seed Yield Per Hectare (kg)||1000 Seed Weight (gm)|
|10 x 20 x 40||6.144ab||753.7b||2.811c||383.8b||511.7c||3.654a|
|20 x 30 x 50||6.366a||910.2a||3.516a||545.3a||727.1ab||3.694a|
|10 x 40||5.656cd||824.7ab||3.243abc||505.9ab||674.1ab||3.609a|
|10 x 50||5.975bc||904.1a||3.333ab||463.2ab||612.3bc||3.689a|
|10 x 30||5.533d||795.3b||2.954bc||561.7a||748.9a||3.685a|
|Fertilizer rate (P2O5 and N kg ha-1)|
|0 x 0||5.734b||820.1bc||2.899b||281.3d||375.0c||3.529b|
|86.25 x 85.5||5.809b||756.1c||2.984b||374.8cd||499.8c||3.697ab|
|115 x 114||6.267a||914.6a||3.293ab||663.6a||879.4a||3.634ab|
|143.6 x 142.5||6.045ab||895.0ab||3.636a||640.7ab||853.8a||3.873a|
|69 x 142||5.818b||802.2bc||3.046b||499.5bc||666.1b||3.598ab|
3.3.2. Germination Percentage
Germination percentage was significantly (P<0.01) affected by the interaction effect of NP fertilizers and plant spacing. Plant spacing of 10 × 30 cm and 143.6 P2O5 and 142.5 N kg ha-1 fertilizers gave the highest germination percentage of seeds (97%) followed by the same spacing with 142 N and 69 P2O5kg ha-1 (96.67%) with no significant difference between the two means. The lowest germination percentage (82.0) was obtained when 20 × 30 × 50 cm double row spacing was interacted with the control fertilizer application (Table 3).
According to the reports by Asaduzzaman et al. , inter and intra row spacing had a tremendous effect on seed germination percentage in which the highest germination was mostly recorded from wider spacing than closer spacing. Ali et al.  and Ahmed  reported that phosphorus and nitrogen fertilization had a significant effect on seed germination percentage in which mostly higher germination percentage was recorded from high fertilizer application.
Unlike the reports of those researchers and similar to the report by Gray et al. , the result of this study showed that closer spacing with high phosphorus and nitrogen application gave higher seed germination percentage than wider spacing and high NP fertilizer applications. The reason behind this could be the interaction of closer plant spacing and higher fertilization increased the proportion of seed from primary umbels, thus improving seed quality. This is because the embryos of seeds from primary umbels are larger than seeds derived from subsequent umbels and due to the active involvement of the minerals in the development of the embryo and in buildup and activation of seed germination hormones.
Seed germination percentage of 90-94% is considered as the highest and 70% as a minimum germination requirement for most onion cultivars under the Ethiopian condition . In this study, 48% treatments (12 out of 25) produced seeds with >90% germination which could be categorized as highest germination percentage and none of the treatments produced seeds with <80% germination indicating all the treatment combinations produced seeds either with highest or above the minimum germination requirement of onion seeds in the country.
3.3.3. Speed of Germination
The main effect of NP fertilizers and its interaction with plant spacing had a highly significant (P<0.01) effect on the speed of germination (Table 4).
The fastest rate of germination (30.20 seed germination per day) was recorded by the interaction of 142 N and 69 P2O5 kg ha-1 fertilizers combination with 10 × 40 cm plant spacing followed by plants in plots without fertilizer application and 20 x 30 x 50 cm double row spacing (Table 4). Those fast rates of germination might be due to the small-sized seeds produced from those treatments and the ability of those small seeds to germinate early. This result was in agreement with Ndor et al.  and Cuocolo and Barbieri  as they explained that the earlier germination of the small seeds could be that, the seeds required less moisture and nutrients to germinate and small seeds also have small surface area and can imbibe enough moisture at a short time and commenced the process of germination.
3.3.4. Seed Vigour Index
The interaction effect of plant spacing and NP fertilizers showed a highly significant (P<0.01) effect on seed vigour index I and II (Table 5).
18.104.22.168. Seed Vigour Index I
The highest seedling vigour index I (947.2) was obtained from the interaction of 20 x 30 x 50 cm double row plant spacing with 115 P2O5 and 114 N kg ha-1 followed by 10 × 50 cm and 143 P2O5 and 142 N kg ha-1. On the other hand, the lowest (102.3) seedling vigour index I was recorded for seeds that did not receive fertilizer and grown at 20 x 30 x 50 cm double row spacing (Table 5).
According to Singh and Singh  and Ashenafi et al. , the highest seed vigour index I was recorded from plants that were grown in wider spacing and optimum fertilization. This could be due to the fact that in large seed size there is more food reserve in the cotyledon of the seed to sustain the seedling growth and make seedlings to be vigorous than the smaller seed sizes whose food reserve could be exhausted very soon. The result of this study agreed with the above reports as the highest seedling vigour index I was observed from the interaction of wider spacing of 20 x 30 x 50 cm double row spacing and 10 × 50 cm in combination with the optimum and highest rate of fertilizers combinations.
22.214.171.124. Vigour Index II
The highest vigour index II (46.73) was recorded from the interaction of 20 x 30 x 50 cm double row spacing and 115 P2O5 and 114 N kg ha-1 fertilizers. Whereas, the lowest vigour index II (9.2) was obtained from the interaction of 10 x 20 x 40 cm double row spacing and without fertilizer application (control) (Table 6). This report was in agreement with Ashenafi et al.  and Singh and Sachan , who reported that wider spacing with optimum fertilizer application produced large-sized seeds and gave high seed vigor index II. The result of this study agreed with the above reports as the highest seedling vigour index II was observed from the interaction of wider spacing 20 x 30 x 50 cm double row spacing and 115 P2O5 and 114 N kg ha-1 NP fertilizers. Whereas, the lowest was observed from the interaction of control and closer 10 x 20 x 40 cm double row spacing and control.
3.4. Partial Budget Analysis
The partial budget analysis was performed using CIMMYT  description and guidelines for economic analysis of fertilizer application. As the rate of NP fertilizer application increased, each additional kilogram of the fertilizer had an effect on seed yield. To estimate the total costs, the mean current prices of Urea and TSP were collected at the time of planting and the market price of onion seed was taken at harvest.
The result of the economic analysis showed that the highest net benefit 358, 120.8 birr ha-1, highest rate of marginal returns 3587% and highest benefit-cost ratio 5.34 were obtained from the plant spacing 10 × 30 cm and 115 P2O5 & 114 N kg ha-1 NP fertilizers. It was followed by 20 × 30 × 50 cm double row spacing and 115 P2O5 & 114 N kg ha-1 NP fertilizers which had 329,615.20 birr hs-1 net benefit, 3463.9% rate of marginal return and 4.885 benefit-cost ratio. The result revealed that the use of those plant spacing and fertilizer applications was cost-effective as compared to the others (Table 7). The cost of onion seed was exceptionally high 400 birr per kg from normal market which was 260 birr per kg when the produce was collected and this was one reason for high net benefit recorded in this study.
|Spacing (cm)||Fertilizer Rates (P2O5 and N kg ha-1)|
|0 & 0||86.25 & 85.5||115 & 114||143.6 & 142.5||69 & 142|
|10 x 20 x 40||85.00jk||84.00kl||95.00abc||91.00fg||80.00m|
|20 x 30 x 50||82.00l||88.00hi||91.00fg||93.00cdef||86.00 ijk|
|10 x 40||82.33l||88.00hi||94.33bcd||92.33def||95.33ab|
|10 x 50||85.33jk||85.00jk||95.00abc||89.00gh||94.00 bcde|
|10 x 30||92.00ef||82.67l||86.67ij||97.00a||96.67a|
|LSD (5%) =||2.13|
|CV (%) =||1.5|
|Spacing (cm)||Fertilizer Rates (P2O5 and N kg ha-1)|
|0 x 0||86.25 x 85.5||115 x 114||143.6 x 142.5||69 x 142|
|10 x 20 x 40||28.65 cdef||26.75 ghijk||29.67abc||26.25efghij||24.57m|
|20 x 30 x 50||30.03 ab||27.56 efgh||25.07lm||28.68 bcdef||27.00 ghij|
|10 x 40||28.57cdef||27.35fghi||28.00 defg||26.03 ijkl||30.20a|
|10 x 50||27.71 efg||26.31hijkl||29.06 abcd||25.86 jklm||29.17 abcd|
|10 x 30||29.31abcd||25.49 klm||25.72 jklm||28.55 cdef||28.83 bcde|
|LSD (5%) = 0.6033|
|CV (%) = 3.0|
|Spacing (cm)||Fertilizer Rate (P2O5 and N kg ha-1)|
|0 & 0||86.25 & 85.5||115 &114||143.6 & 142.5||69 & 142|
|10 x 20 x 40||726.4m||869.0f||817.0h||813.9h||780.1j|
|20 x 30 x 50||102.3r||572.0q||947.2a||637.0n||624.4o|
|10 x 40||744.0l||817.4h||883.3e||597.7p||110.1r|
|10 x 50||826.0g||833.0g||880.0e||935.0b||920.0c|
|10 x 30||794.7i||791.0i||895.0d||570.0q||770.7k|
|LSD (5%) =||3.622|
|CV (%) =||0.7|
|Spacing(cm)||Fertilizer Rates (P2O5 and N kg ha-1)|
|0 & 0||86.25 & 85.5||115 & 114||143.6 & 142.5||69 & 142|
|10 x 20 x 40||9.2t||18.75 n||22.49 efg||20.76 kl||22.28 fgh|
|20 x 30 x 50||22.04 hi||22.58 ef||46.73 a||16.02 q||18.37º|
|10 x 40||16.03q||20.49l||31.15 b||17.07p||20.83 k|
|10 x 50||19.26m||21.84i||24.11d||21.25j||22.17 ghi|
|10 x 30||13.2r||16.97p||22.66e||26.67c||10. 20s|
|LSD (5%)= 1.636|
|CV (%)= 1.0|
|N and P2O5 Rate (kg ha-1)||Spacing (cm)||Average Seed Yield (kg ha-1)||Adjusted Seed Yield (kg ha-1)||Gross Field Benefit (ETB ha-1)||Total Cost (ETB ha-1)||Net Benefit (ETB ha-1)||Marginal Return||Marginal Rate of Return (%)||Benefit Cost Ratio|
|0 × 0||10 ×20×40||267||240.3||96120||60000||36120||0.602||60.2||0.602|
|85.5 × 86.25||10 ×20×40||459||413.1||165240||65578||99662||11.39||1139||1.52|
|114 × 115||10 ×20×40||662.||595.8||238320||67464.8||170855.2||18.04||1804||2.53|
|142.5 × 143.75||10 ×20×40||560.||504||201600||69330.2||132269.8||10.30||1030||1.907|
|142 × 69||10 ×20×40||611.||549.9||219960||67039.2||152920.8||16.59||1659||2.28|
|0 × 0||20×30×50||364.||327.6||131040||60000||71040||1.184||118.4||1.184|
|85.5 × 86.25||20×30×50||518.||466.2||186480||65578||120902||8.939||893.9||2.015|
|114 × 115||20×30×50||1103.||992.7||397080||67464.8||329615.2||34.639||3463.9||4.885|
|142.5 × 143.75||20×30×50||804.||723.6||289400||69330.2||220069.8||15.972||159.72||3.174|
|142 × 69||20×30×50||847.||762.3||304920||67039.2||237880.8||23.701||2370.1||3.548|
|0 × 0||10×40||435.||391.5||156600||60000||96600||1.61||161||1.61|
|85.5 × 86.25||10×40||479.||431.1||172440||65578||106862||1.839||183.9||1.629|
|114 × 115||10×40||837.||753.3||301320||67464.8||233855.2||18.386||1838.6||3.466|
|142.5 × 143.75||10×40||713||641.7||256680||69330.2||187349.8||9.726||972.6||2.70|
|142 × 69||10×40||906.||815.4||326160||67039.2||259120.8||23.08||2308||3.865|
|0 × 0||10×50||350||315||126000||60000||66000||1.1||110||1.1|
|85.5 × 86.25||10×50||504.||453.6||181400||65578||115822||8.931||893.1||1.766|
|114 × 115||10×50||614||552.6||221040||67464.8||153575.2||11.73||1173||2.276|
|142.5 × 143.75||10×50||627.||564.3||225720||69330.2||156389.8||9.68||968||2.255|
|142 × 69||10×50||967.||870.3||348120||67039.2||281080.8||30.55||3055||4.19|
|0 × 0||10×30||460.||414||165600||60000||105600||1.16||116||1.16|
|85.5 × 86.25||10×30||539.||485.1||194040||65578||128462||4.098||409.8||1.958|
|114 × 115||10×30||1181.||1062.9||425160||67039.2||358120.8||35.87||3587||5.34|
|142.5 × 143.75||10×30||627.||564.3||225600||69330.2||156269.8||5.43||543||2.253|
|142 × 69||10×30||938.||844.2||337680||67464.8||270215.2||22.0||2200||4.005|
The results of this research indicated that the application of NP fertilizers significantly (P<0.01) affected flower stalk diameter, seed yield per plot, seed yield per hectare and the number of seeds per umbel. It significantly (P<0.05) affected plant height, umbel diameter, seed weight per umbel and 1000 seeds weight. The highest seed yield per hectare (879.4 kg) and per plot (663.6 g) was obtained from 114 N and 115 P2O5 kg ha-1 fertilizer levels. On the other hand, the highest 1000 seeds weight (3.87 g) was also recorded from 143.6 P2O5 and 142.5 N kg ha-1. The plant spacing significantly (P<0.01) affected days to 50% flowering, flower stalk diameter, umbel diameter, number of umbels per plot, number of seed per umbel, seed yield per plot, seed yield per hectare and significantly (P<0.05) affected plant height, number of umbels per plot and seed weight per umbel. The highest seed yield per plot (561.7 g) and per hectare (748.9 kg) was recorded from plant spacing of 10 × 30 cm. Whereas, the highest number of seeds per umbel (910.2) was obtained from 20 × 30 × 50 cm double row spacing and 114 N and 115 P2O5 kg ha-1 fertilizer levels (914.6). On the other hand, the highest seed weight per umbel (3.516 g) was obtained from 20 × 30 × 50 cm double row spacing and 143.6 P2O5 and 142.5 N kg ha-1 (3.636 g). The interaction effect of NP fertilizers and plant spacing only affected germination percentage, speed of germination, seed vigor index I and II in which highest germination percentage was obtained from the interaction of 10 × 30 cm and 143.6 P2O5 and 142.5 N kg ha-1 whereas the highest seed vigor indexes I and II were obtained from 115 P2O5 and 114 N kg ha-1 and 20 × 30 at 50 cm double row spacing.
On the basis of one growing season research results, it is possible to conclude that the better crop phenology and growth parameters were obtained from 10 × 40 cm and 115 P2O5 and 114 kg N ha-1 fertilizers. The highest seed yield and yield components and seed quality attributes were obtained from plant spacing 20 x 30 x 50 cm double row spacing and 115 P2O5 and 114 N kg ha-1combination followed by plant spacing 10 x 50 cm and 143.6 P2O5 and 142.5 N kg ha-1.
According to the partial budget analysis of the marginal rate of return, net benefit and benefit-cost ratio, plant spacing 10 × 30 cm with 115 P2O5 and 114 N kg ha-1fertilization was the most profitable which was followed by 20 × 30 × 50 cm double row spacing and 115 P2O5 and 114 N kg ha-1 fertilization. Thus, based on partial budget analysis and response of Bombay Red onion variety to plant spacing and NP fertilizers combination for the studied characters, it is suggested that 10 × 30 cm with 115 P2O5 and 114 N kg ha-1 followed by 20 x 30 x 50 cm double row spacing and 115 P2O5 and 114 N kg ha-1 fertilizer application were better than all the other treatments and can be recommended for Bombay Red onion variety for high yield and quality seed production in the study area.
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The authors declare no conflict of interest, financial or otherwise.
Ethiopian Ministry of Education, Debre Berhan University, Madawalabu University and Debre Berhan Agricultural Research Center are acknowledged for their supports.