Growth and bulb yield of some onion (Allium cepa L.) varieties as influenced by NPS fertilizer at Dambi Dollo University Research site, Western Ethiopia

Abstract Onions are the main crops in Ethiopian regular nutrition, cultivated year round under rain-fed and irrigated conditions. The low production and productivity of onion crops in Ethiopia as compared to other onion producing countries were due to disease, insect pest, lack of improved varieties, low level use improved varieties of onion and improper agronomic practices used by farmers. A field experiment was conducted at Dambi Dollo University research site for 2 consecutive years (2020 and 2021 cropping season) under rain-fed to evaluate effect of NPS fertilizer rates on performance of onion varieties (Allium cepa L.). The experiment was laid out using Randomized Complete Block Design with three replications in a factorial mixture of four levels of NPS fertilizer (0, 50, 100, and 150 kg ha−1) and three varieties (Adama red, Monarch, and Nafis). Variance Analysis indicated that NPS fertilizer and varieties were expressively affected days to 90% physiological maturity, marketable and unmarketable yield, and total bulb yield of onion crop but not significantly influenced by their interaction. The number of leaves per plant and plant height was significantly influenced by the main effect of varieties, NPS fertilizer rates and their interaction. The highest values for the number of leaves per plant (16.08) recorded at 150 kg ha−1 and monarch variety. Moreover, days to 90% physiological maturity (134.8 days) and total bulb yield (29.35 t ha−1) were noted from the highest NPS application rate of 200 kg ha−1. Similarly, the maximum plant height (16.08 cm) and marketable onion bulb yield (26.41 t ha−1) were obtained from 150 kg ha−1 NPS rate. Hence, based on the highest marketable onion bulb yield gained, Nafis variety of onion crop and 150 kg ha−1 of NPS fertilizer rates should be advised for production in the study area and other areas with similar agro-ecological condition.


PUBLIC INTEREST STATEMENT
Onions, being a main vegetable crop, have a massive potential in developing the Ethiopian economy through improving smallholder farmers, creation of employment opportunities, and source of income. The country has a diverse agro climatic condition, abundant cultivable and irrigable land and favorable policy environment for growing a wide variety of onions. However, due to due to limited knowledge about the NPS on the vegetative growth and yield of onion, many farmers still custom the same amount of NPS fertilizer of all varieties of onion in the study area. Research-based recommendations on the level of NPS fertilizer can increase onion crop production and increase the benefits of local farmers.

Introduction
Onion (Allium cepa L.) belongs to the Allium genus of the family Alliaceae which is believed to be originated in southwestern Asia, which is a breeding ground for diversity, since its first worldwide distribution and cultivated by over 4700 for bulb production purposes (Brewster, 2008). It is a vegetable plant grown because of its bulb (Welbaum, 2015).
Onion is very important in the Ethiopian diet (Rajan & And, 2007). Commonly, all portions of onions can be eaten by people except seeds (H.d & Currah, 2002). Onions play an important role in human nutritional needs and have medicinal properties and are mainly used because of their unique taste or ability to enhance the taste of other foods (Bagali et al., 2012;Randle & Ketter, 1998).
Onions and/or shallot are grown almost in all tropical countries of Africa including Ethiopia Onions are grown under rainy and irrigated conditions.
In 2020/21 during the rainy season, 38,952.58 hectares were planted and approximately 346,048.09 tons of onion bulbs were harvested at an average yield of 8.88 tone ha −1 (CSA, 20/ 21), which is lower than the other countries producing onion. The low productivity of onion crops in Ethiopia as compared to other countries, which are involved in onion producing is due to diseases, pests, lack of improved varieties, low level use improved varieties of onion, and improper agricultural practices by farmers. Nitrogen fertilizer is also responsible for an important part of agriculture-related pollution through nitrate leaching, ammonia volatilization, and nitrous oxide emissions; thus, the efficiency of fertilizer utilization use has to be considered (Lobos Ortega et al., 2016).
Fertilization is one of the influential factors in an effort to increase crop yields. To be able to grow and to produce optimally, shallot plants need sufficient and balanced amount of nitrogen (N), phosphorus (P), potassium (K), as well as other nutrients (Purba, 2014). In general, application of N, P and K fertilizer can increase the growth and bulb yield of onion (Singh et al., 2000). Mineral fertilizers play an important role of onion plant growth and productivity. Many investigators reported that the vegetative growth of onion plants and minerals uptake was increased with increasing the level of NPK fertilizers (Subbo-Rao, 1988). Nutrient requirement of onion depends on the environment of a particular locality or soil type. Split application of 92 kg N ha −1 was found adequate for dry bulb production in the upper awash region. Farmers in the rift valley apply 200 kg ha −1 DAP and 100 kg ha −1 Urea and the Horticultural State Enterprise apply about twice the above concentrations with expectation of high yield. However, the best combination levels for a particular locality or soil type have to be determined for economic production of dry bulb (Dessalegn & Aklilu, 2003).
Proper use of any agronomic practices may enhance the onion cultivation. The use of compound NPS fertilizer varies depending on the location, purpose of the crop, and the variety. However, in the study area, most onion producers use local varieties and in appropriate NPS blended fertilizer such as higher or lower recommended levels contribute for high yield reduction.
In addition, due to limited knowledge about the NPS on the vegetative growth and yield of onion, many farmers still custom the same amount of NPS fertilizer for all varieties of onion in the study area. Research-based recommendations on the level of NPS fertilizer can increase onion crop production and increase the benefits of local farmers. Therefore, it is imperative to evaluate the optimum NPS fertilizer level for release onion varieties in order to increase the onion yield in the study area at different seasons. Therefore, this research was initiated for the following objectives.
• To evaluate effects of different rates of NPS fertilizer on onion varieties.
• To identify the optimum NPS fertilizer and the best performing onion variety for the study area.

Description of the study area
The study of field experiment was conducted at the Research site of Dambi Dollo University for two consecutive years (2020 and 2021) under rain-fed conditions. The research site is located in Kellem Wollega Zone in the western part of Oromia, Ethiopia, which is 652 km away from Addis Ababa to West direction at an altitude of 1500-1740 meter above sea level. The area gets rainfall of 850-1200 mm yearly. The minimum and maximum temperatures of the area are 15 and 28 °C, respectively. The study are soil was marked sandy loam (Degefa et al., 2021).

Description of treatments used and design of experiment
In this study, the experiment contained 15 treatments as a grouping of five combined NPS fertilizer (0, 50, 100, 150, and 200 kg −1 ha) as the first factor which were 50% below, above, and (FAO (Food and Agriculture Organization), 2017) recommendation for Central Rift Valley, respectively, and three different varieties of onions as second factors (Adama red, Monarch, and Nafis) arranged as a Completely Randomized Block Design (RCBD), which is replicated three times. The total plot size of 2 m × 1.1 m with spacing of 20 cm distance between a row and a total of 10 rows were used and contains 12 plants per row. The total experimental area was 25 m × 12 m (300 m 2 ). The net plot size from which data were taken is 1.6 m × 1.1 m (1.76 m 2 ) leaving one outer row on each side of the plot and row length of 1.1 m at both ends as a border. The distance between the plots and the blocks were 0.5 m and 1.5 m, respectively.

Experimental procedures and management
The experimental field was plowed three times for good growth. The field repeatedly cultivated as a recommendation for the crop to control weed control. Seed beds were well prepared prior to sowing of seeds in the nursery. After 55 days of sowing, transplant of seedlings to the main field were done when the seedlings produce three to four true green leaves.
The recommended, inter spacing of 20 cm on the ridge (Lemma & Shimelis, 2003) was maintained for all plots. The seedling was transplanted by hand at a recommended spacing (20 cm ×10 cm) by placing two seedlings per hill, with a depth of 7-10 cm depend moisture status of the soil during transplanting and later thinned to one plant after survival of the plant. Only strong and healthy seedlings were transplanted.
The recommended Urea fertilizer is applied in the formats specified in three divisions (1/4 th when replanting, ½ in the active growth stage, and 1/4 th at the beginning of building) but different NPS fertilizer applied per side dressing before transplanting in 10 cm depth below ground to place the seedling roots (EARO, 2004).At physiological maturity when 70 % tops fell or leaves senesced, plants were harvested and used for determining bulb yield and yield-related traits (EIAR, 2004).
Agronomic practices such as cultivation, watering, pests and disease control are carried out according to regional recommendations. When the plants reach physiological maturity and 70% of the leaves are cut off, the plants are collected to determine yield components. Crops in the outer border rows and at both ends of each harvested row have been left to avoid edge side effects.

Properties of the experimental soil
The physico-chemical properties of the experimental soil is explained in the following Table 1.

Phenological and growth variables
Days to physiological maturity: It was recorded as the actual number of days from the date of sowing until, 90% of the plant leaves senesced and reach maturity in each net plot (EARO, 2004. Plant height: It was measured in centimetre from 10 randomly selected plant samples per net plots as the height from the base of the plant to tip the plant using meter tape. The average of 10 sampled plants was considered as plant height.
Leaves number per plant: It was recorded 80 days after transplanting and counted 10 randomly selected plants.
Unmarketable bulb yields (t ha −1 ): were determined by classified as: under sized (<20 g and >160 g), contaminated, rotten, and disordered physiologically (thick-necked and divided bulbs). These bulbs weighed and expressed as unmarketable bulbs from net plot area and later extrapolated to per hectare basis.
Total bulb yield (t ha −1 ): The sum of marketable and unmarketable bulb yield was computed in tons per hectare.

Statistical analysis of data
The data collected were subjected to analysis of variance (ANOVA) using general linear model of Genstat 16 th edition statistical package. Treatment means that exhibited significant difference was separated using Duncan Multiple range test (DMRT) at the 5 % level of significance.

Cost benefit analysis
To consolidate the statistical analysis of the agronomic data, economic analysis was done for each treatment. For economic evaluation, cost and return were calculated. To estimate economic parameters, onion was valued at an average open market price per kg and cost of chemical fertilizer inputs.

Days to 90% physiological maturity
The result from the variance analysis showed days to physiological maturity of onion varieties was very significant (P < 0.001) affected by the impact of varieties and NPS fertilizer level in both testing years. Conversely, these two factors did not significantly affect days to physiological maturity (Table 4).
The late days to maturity (132.9 days) were observed from variety Monarch, and the early days to maturity (103.5 days) were observed in the variety Nafis in the 2021 growing season. This means that the varieties of onion Nafis took 103.5 days to reach maturity, which was 29 days and 24 days before the Monarch and Adama red, respectively ( Table 4). The variation in maturity may be due to differences in the variety of onion. The variability in maturity between onion varieties may be due to their genetic diversity and reaction of the varieties in the environment. This finding is concomitant with the results of Ketema et al. (2013); Kahsay et al. (2014) and Tesfalgn and Wassu (2015) who reported that Bombay Red matured significantly earlier than Adama Red.
The late days to maturity (135 days) of onion crops were noted in the high application of NPS fertilizer at 200 kg ha −1 , while the early days to 90% physiological maturity (104.2 days) were obtained due to non-use of NPS fertilizers in the 2021 growing season (Table 4).
This means that as NPS fertilizer levels increase at 0-200 kg ha −1 , the number of days to 90% physiological maturity has increased. This may be due to an increase in the levels of NPS fertilizer, which contributes to increasing the amount of nitrogen fertilizer in NPS which has encouraged the growth of onion plants. This result was consistent with the finding of Gupta and Sharma (2000) who reported that nitrogen promoted plant growth and greenness thus slowing the ripening of Onion plants.
Moreover, this result is concomitant with the finding of Yamasaki and Tanaka (2005) and Tadesse (2012) who reported that a high nitrogen application level are significantly delayed maturity. Early maturation at the control level may be due to insufficient nutrient intake that slows plant growth; thus, it entered the fertile phase and matured earlier.

Plant height
Plant height was very important (P < 0.01) affected by the significant effect of NPS fertilizer levels, varieties and significantly (P ≤ 0.05) its interaction ( Table 2).
The highest plant height (70.25 cm) was found in monarch varieties with a maximum of NPS of 150 kg ha −1 , which was statistically equivalent to Adama red variety at 200 kg ha −1 and a monarchy variety at 200 kg ha −1 NPS rates. Whereas the lowest plant height (43.21 cm) was recorded from Adama red variety and Nafis variety at 0 kg ha −1 NPS application rates at both growing season ( Table 2).
The difference in onion crop yields due to increased NPS fertilization may be due to increased nitrogen fertilizer content in NPS fertilizers, which contributes to overgrown crop growth and crop height. And the height of the plants depends on the genetic makeup of the onion plants.
Possible cause of crop failure may be due to increased nitrogen utilization, which contributes significantly to amino acids, cell proliferation, cell proliferation, chlorophyll synthesis, and protein synthesis that promotes onion growth. This result agrees with Nasreen et al. (2007); Gustafson (2010) and Birhanu et al. (2014), who reported that the use of nitrogen fertilizer increased vegetative growth of onions due to its ability to increase the rate of photosynthesis.

Number of leaves per plant
Number of leaves per plant were significantly affected (P < 0.01) by the main effect of NPS fertilizer levels and varieties and their interaction showed significant differences (p < 0.01) in the number of leaves per plant (Table 3).
The large number of leaves per plant (17.30) was recorded in a monarchy variety with a fertilizer rate of 150 kg ha −1 in 2021 growing season, and the minimum number of leaves (6.37) was recorded at 0 NPS kg ha −1 applied at a Nafis variety in the 2020 growing season. The use of increased NPS fertilizer increases the number of onion leaves. The reduced leaf yield at a low levels of fertilizer may be due to a lack of sufficient nutrients and to a combination of growth. Increased in leaf number per plant with high NPS fertilizer may be due to the availability of macro and micronutrients that allow the leaves to grow vigorously. Nitrogen and sulfur fertilization had the potential to increase nutrient uptake and thus improve plant nutrient uptake.
The result of the present work is consistent with the finding of Nasreen et al. (2007) who reported that the consumption of 120 kg N ha −1 significantly increased the number of leaves in the onion plant and a further increase of nitrogen supply to 160 kg ha −1 tends to reduce it. Yohannes and Pant (2011) report that nitrogen fertilization had a significant impact on the

Marketable yield
The result from the variance analysis showed that the marketable yield of the onion variety was signed (P < 0.01) affected by NPS fertilizer levels and significantly the varieties. On the other hand, these two factors did not significantly affect marketable bulb ( Table 4).
The highest marketable yields (25.07 t ha −1 ) were obtained from the Nafis variance, while the lowest commercial yields (17.25 t ha −1 ) were obtained from monarch varieties in the 2021 growing season (Table 4). This difference in yield between onions may be due to differences in genetic diversity. Consistent with this result, Kasech and Rahel (2018) reported that variety Nafis provided the highest marketable bulb yield (36.24 t ha −1 ).
Also, the highest commercial yield (27.52 ha −1 ) of onions was harvested at NPS application rates of 150 kg ha −1 and 200 kg ha −1 , and the lowest commercial yield (9.20 t ha −1 ) obtained for non-use of NPS fertilizer in the 2021 growing season (Table 4). Increasing the NPS fertilizer level from 0 to 150 kg ha −1 increases the marketable yield of onion crop to a higher level but otherwise reduces the yield.
Significant decrease in the maximum marketable yield due to an increment of NPS level of more than 150 kg ha −1 may be due to overcrowding or total nutrient uptake, which may lead to overgrowth of vegetation components in terms of crop growth and development of bulbs that contribute for yield reduction. In line with this result, Muluneh et al. (2018) found that high yield bulbs were recorded using complete NPS fertilizer estimates.

Non-marketable bulb crop
The result from the variance of analysis showed that unmarketable yields of onion variety was very important (P < 0.01) affected by the significant impact of NPS fertilizer and significantly by variety. Conversely, these two factors did not significantly affect unmarketable bulb yield (Table 4).
The least unmarketable bulb yield of onions (1.32 t ha −1 ) was obtained from the red Adam variety while the lowest unmarketable bulb yield (0.80 t ha −1 ) was obtained from the Nafis variance in the 2021 growing season (Table 4). Similar to this effect, Kibebew et al. (2021) reported that a high yield of unsold bulb was achieved in the Adam variety.
The highest unmarketable yield (1.94 ha −1 ) of onion was harvested at zero NPS application rates in the 2021 growing season, and the lowest unmarketable yield (0.53 t ha −1 ) was obtained at the 200 kg ha −1 fertilizer application rates in the 2021 growing season (Table 4). As the combined NPS fertilizer levels increase from 0 kg ha −1 to 200 kg ha −1 , the non-marketable yield of onion variation has been reduced. This may be due to an increasing in NPS fertilizer levels and an increase in the amount of nitrogen fertilizer, which reduces the yield of onion tuber that can be sold for the progressive role, and it plays in increasing the new bulb weight ratio. This finding was similar to the results of Kibebew et al. (2021) who reported that the maximum amount of unsold onion bulb yields was obtained at zero kg per Hectare of NPS fertilizer.

The total yield of the bulb
The result from the analysis of variance showed that the total yield of bulb of onion variety was very significant (P = 0.001) affected by the main effect of NPS fertilizer rates and significantly by variety. On the contrary, these two factors did not significantly affect the total bulb yield of onions (Table 4). The variety of onions Nafis produced the highest bulb yield (26.28 t ha −1 ), and the lowest bulb yield (20.21 t ha −1 ) was recorded in an Adama red variety in the 2021 growing season. This yield difference between onions may be due to differences in the genetic makeup of onion plants. This result has been in agreement with Kasech and Rahel (2018), who reported that variety Nafis provide the highest total bulb yield (36.28 t ha −1 ). The present result agrees with findings of Tibebu et al. (2014) who obtained the highest total bulb yield in different Nafis than other species.
The maximum total bulb yield (30.35 ha −1 ) of onions was harvested at the input rates of NPS 200 kg ha −1 , and the minimum total bulb yield (11.42 t ha −1 ) obtained from 0 kg ha −1 NPS fertilizer application rates in the 2021 growing season ( Table 4). As the combined NPS fertilizer levels increased from 0 kg ha −1 to 200 kg ha −1 , the total bulb yield of onion variety was increased. This may be due to an increase in bulb size and bulb weight due to the use of NPS fertilizers, which may increase photosynthesis, and subsequently, enhanced growth and expansion of vegetative growth as a whole, and ultimately significantly higher carbohydrate to the bulbs at maturity. This result is inconsistent with the findings of Tibebu et al. (2014), who reported that high onion bulb yields in response to nitrogen use. Woldeyohannes et al. (2007) also reported a steady increase in the yield of onion bulbs due to an increase in nitrogen levels in the NPS from 0 to 100 kg N ha −1 .

Economic analysis
From the final experimental data, the gross yield for all treatments was obtained. Then, the recommended level of 10% was adjusted to obtain net yield. Net yield was multiplied by the market price to obtain gross field benefit. Costs and benefits were calculated for each treatment. All variable costs were calculated based on the current market price especially for fertilizers. Purchasing costs for Urea and NPS were taken as Birr 1,500/Qt and 1,600/Qt, respectively.
The selling price of onion at the local market at the harvest time was taken as Birr 20.00/kg. Variable costs were summed up and subtracted from gross benefits. This was taken as the net benefit.
The total cost applied for control treatment was assumed to be 50 ETB for one plot and for one hectare total 138,888.89 ETB were obtained. The plot receives fertilizer was calculated as the expense of urea is about 28.8 ETB, and the variable cost of NPS is 14.4 ETB. The total cost of this treatment 93.2 ETB. Thus, the total cost of one hectare of onion field receives was NPS and urea was estimated to be 258,888.89 ETB (3.6 m 2 = 93.2 ETB). The gross margin of this onion field is estimated to be 300,000.00 ETB ((3.6 m 2 = 108 ETB). Therefore, the net benefit of this experiment was assumed to be total sell minus total cost, which is 300,000 minus 258,888.89 equals to 41.111.11 ETB. Based on this cost benefit analysis use of NPS, urea fertilizer for onion in the study area is recommended.

Conclusion
In conclusion, the major impact of the variety and fertilizer level of the NPS greatly affects all phenological, growth variables and yield contributing traits of onion crops at both growing season. Analysis of variance showed that day to 90% physiological maturity of the onion crop, marketable and unmarketable bulb yield, and over-all bulb yield of onion crop were significantly affected by main effect of varieties and NPS fertilizer rates but not affected significantly by their interaction. Both the height of the plant and the number of leaves per plant were greatly influenced by the interaction of the variety and fertilizer level of the NPS. Of the three tested onion varieties tested, the highest marketable onion bulb yield is harvested in the variety of Nafis. Similarly, an increase in the NPS fertilizer rate of 0 kg ha −1 to 200 kg ha −1 was increases the number of leaves per plant, plant height, days to 90% physiological maturity and total bulb yield but reduces yield of unmarketable bulb of onion plant. Therefore, based on the highest marketable bulb yield obtained, Nafis varieties of onion plants and 150 kg ha −1 of NPS fertilizer level should be recommended for production in the study area. Moreover, in the absence of Nafis variation, Adama red variety with an NPS level of 150 kg ha −1 can be considered as alternative variants to be recommended for the farmers in the study area based on the research finding conducted in 2020 and 2021 cropping season.

Funding
The authors received no direct funding for this research.