Growth, Yield, and Quality of Onion (Allium cepa L.) as Influenced by Intra-Row Spacing and Nitrogen Fertilizer Levels in Central Zone of Tigray, Northern Ethiopia

TABLES OF CONTENT

BIOGRAPHICAL SKETCH

ACKNOWLEDGEMENTS

ABBREVIATIONS AND ACRONYMS

LIST OF TABLES

LIST OF FIGURES

LIST OF TABLES OF APPENDIX

1 . INTRODUCTION

2 . LITERATURE REVIEW
2.1. Description of Onion Crop
2.2. Environmental Requirements of Onion
2.3. Importance and Production Status of Onion in Ethiopia
2.4. Response of Onion to Nitrogen Application
2.4.1. Roles of nitrogen in onion nutrition and growth
2.4.2. Response of onion to nitrogen fertilization
2.5. Effect of Plant Spacing on Onion Yield and Yield Components
2.6. Response of Onion to Interaction Effect of Nitrogen and Plant Spacing

3 . MATERIALS AND METHODS
3.1. Description of the Study Area
3.2. Experimental Materials
3.2.1. Planting material
3.2.2. Fertilizer material
3.3. Treatments and Experimental Design
3.4. Management of Experimental Field
3.5. Soil Sampling and Analysis
3.6. Data Collection
3.6.1. Phenology and growth parameters
3.6.2. Yield and yield components
3.7. Data Analysis

4 . RESULTS AND DISCUSSION
4.1. Physico-Chemical Properties of the Experimental soil
4.2. Phenology and Growth Characters
4.2.1. Plant height
4.2.2. Leaf number per plant
4.2.3. Leaf length
4.2.4. Leaf diameter
4.2.5. Shoot dry matter yield
4.2.6. Dry total biomass yield
4.2.7. Days to maturity
4.2.8. Bolting percentage
4.2.9. Stand count percentage
4.3. Yield and Yield Related Traits
4.3.1. Average bulb weight
4.3.2. Bulb diameter
4.3.3. Bulb neck diameter
4.3.4. Bulb dry matter yield
4.3.5. Total bulb yield
4.3.6. Unmarketable bulb yield
4.3.7. Marketable bulb yield
4.3.8. Under size bulb yield (< 20 g)
4.4. Harvest Index
4.5. Total Soluble Solid
4.6. Correlation Analysis
4.7. Partial Budget Analysis

5 . SUMMARY AND CONCLUSION

6 . REFERENCES

7 . APPENDICES

DEDICATION

I dedicate this thesis manuscript to my mother Guey Weldegerima for her advice and for nursing me with affection and care and for her partnership in the success of my life.

BIOGRAPHICAL SKETCH

The author, Guesh Tekle, was born on 17 June 1984 at Axum, Central Zone of Tigray to his father Tekle Gebregziabher and his mother Guey Weldegerima. He attended Elementary education (grades one to six) at Mytruengi Elementary School from 1993-1996, junior secondary education (grades seven to eight) at Mahbere-Dego School from 1997-1998, and secondary education at Axum Comprehensive Secondary School from 1999-2002. After completing his elementary and secondary education, he joined the College of Agriculture and Veterinary Medicine of Jimma University in 2003, and graduated with the degree of Bachelor of Science (BSc) in Horticulture in July 2006.

Immediately after graduation in 2007, he was employed by South Tigray, Alamata Woreda Office of Agriculture and Rural Development as a vegetable and fruit expert, where he worked up until 10 May 2009. In May 2009, he joined Tigray Agricultural Research Institute (TARI) as a researcher in Horticulture. He continued working at TARI up until he joined the School of Graduate Studies of Haramaya University in 2011 to pursue a study leading to the degree of Master of Science (MSc) in Horticulture.

ACKNOWLEDGEMENTS

Foremost, I would like to express my sincere gratitude to my major adviser Prof. Nigussie Dechassa for the continuous support he provided me throughout the period of my study with great affability, enthusiasm, and immense knowledge. His guidance, comments, suggestions and insightful advice helped me at all stages of my research work and during the writing of my thesis. I would also like to thank my co-adviser Dr. Gebremedhin Woldewahid for his encouragement, insightful comments and advice in preparing the thesis. His suggestions, guidance, insightful ideas, and editorial supports were helpful for me to complete the write up of the thesis.

I would like to take this opportunity to thank the Livestock and Irrigation Value Chain Project (LIVES) for sponsoring my research. I would also like to express my gratitude to Tigray Agricultural Research Institute, which provided with leave of absence to my MSc study and for paying my salaries regularly while I was on the study leave.

My special thanks also go to staff members of Axum Agricultural Research Center specially Nahom Weldu, Haftamu Hailekiros, Atakilti Mekonen, Muruts Belay, Kiros Welday, Tesfuom Fitsum, Fasikaw Belay, Tesfay Araya, Atsede Teklu and others who helped during preparation of the research site, transplanting onion seedlings, data collection, and harvesting. I would also like to express my gratitude to drivers of the Axum Agricultural Research Center particularly for Gebregziabher Atsbeha and Mulualem Tadele for their supporting me with facilitation of transport services starting from transplanting till the final harvesting.

Last, but not least, I would like to thank my family members for their unlimited support and help throughout the period of my study. Above all, I praise and glorify the Almighty and Merciful God for providing me with the patience and stamina to complete my MSc work.

ABBREVIATIONS AND ACRONYMS

Abbildung in dieser Leseprobe nicht enthalten

LIST OF TABLES

1. Treatment combinations, number of plants m-2, number of plants per plot and plantpopulation per hectare

2.Chemical characteristics of the experimental soil

3. Main effect of intra-row spacing and nitrogen fertilizer levels on plant height of onion 29

4 Interaction effect of intra-row spacing and nitrogen fertilizer levels on leaf number perplant, leaf length, leaf diameter and shoot dry weight yield of onion

5. Interaction effect of intra-row spacing and nitrogen fertilizer levels on dry totalbiomass, days to maturity and bolting percentage of onion

6. Main effect of intra-row spacing and nitrogen fertilizer levels on stand countpercentage of onion

7. Interaction effect of intra-row spacing and nitrogen fertilizer levels on average bulbweight, bulb diameter, neck diameter and bulb dry weight yield per plant of onion

8. Interaction effect of intra-row spacing and nitrogen fertilizer levels on marketable,unmarketable and total bulb yield of onion

9. Interaction effect of intra-row spacing and nitrogen fertilizer levels on marketablebulb size distribution of onion

10. Interaction effect of intra-row spacing and nitrogen fertilizer levels on under sizedbulb yield, harvest index and total soluble solid yield of onion

11. Partial budget and MRR analysis for fertilizer rate and intra row spacing trial onmarketable yield of onion

LIST OF FIGURES

1. Map of the Study Area

LIST OF TABLES OF APPENDIX

1. Mean squares of analysis of variance for leaf length (LL), leaf diameter (LD), plantheight (PH), leaf number per plant (LN), bolting percentage (BP) and stand countpercentage (SCP)

2. Mean squares of analysis of variance for yield and yield related traits of onion

3. Mean squares of analysis of variance for marketable and unmarketable size distributionof onion

4. Mean squares of analysis of variance for shoot dry weight per plant (SDWPP), harvestindex (HI), Dry total biomass (DTB), Total soluble solids (TSS), Days to maturity(DTM) and Bulb dry weight per plant (BDWPP)

5. Simple correlation between yield, yield components and growth characters

GROWTH, YIELD, AND QUALITY OF ONION (ALLIUM CEPA L.) AS INFLUENCED BY INTRA-ROW SPACING AND NITROGEN FERTILIZER LEVELS IN CENTRAL ZONE OF TIGRAY, NORTHERN ETHIOPIA

ABSTRACT

Haphazard and inappropriate plant spacing and poor soil fertility management practices are among the major factors constraining onion production in the Central Zone of Tigray. Therefore, a field experiment was conducted in Axum district from October to March 2014 to assess the influence of intra-row spacing (2.5, 5, 7.5, 10 and 12.5 cm) and nitrogen rate (0, 41, 82 and 123 kg N ha - 1 ) on growth, bulb yield, and quality of onion. The experiment was laid out in a randomized complete block design (RCBD) of factorial arrangement with three replications. The main effects of nitrogen rate and intra-row spacing influenced only the plant height and stand count significantly (P < 0.01). The tallest plants (46.70 cm) were obtained from plants treated with 82 kg N ha - 1 as well as those spaced at 7.5 cm intra-row spacing (43.78 cm). The highest stand count (90.33%) at harvest was recorded from plots that received 82 kg N ha - 1 and spaced at 12.5 cm (98.04%). Nitrogen rate and intra-row spacing interacted to significantly (P < 0.01) to influence all parameters. Thus, increasing the rate of nitrogen across the increasing intra-row spacing significantly prolonged days to maturity, enhanced average bulb weight, bulb diameter, bulb neck diameter, leaf number per plant, leaf diameter, shoot dry matter, and dry total biomass yield. In general, the highest values of these parameters (126.67, 123.85 g, 6.05 cm, 1.35 cm, 12.57, 1.38 cm, 3.22 g and 13.08 g, respectively) were attained in response to the application of 123 kg N ha - 1 and 12.5 cm intra-row spacing. However, their least values (100, 23.99 g, 2.33 cm, 0.68 g, 6.60, 0.47 cm, 0.73 g, and 2.72 g, respectively) were obtained at 0 kg N ha - 1 and 2.5 cm intra-row spacing. The highest value of bolting (31.95%) was observed from the application of 0 kg N ha - 1 and plant spacing of 2.5 cm. Increasing the N rate across the increasing intra-row spacing increased the yields of over-sized bulbs whereas decreasing the yields of small-sized, under-sized bulbs and unmarketable bulb yield. The highest yield of over-sized bulbs (4.03 t ha - 1 ) was recorded at 123 kg N ha - 1 and the intra-row spacing of 12.5 cm. Whereas, the highest yield of medium sized (28.27 t ha - 1 ) and large sized bulb yield (8.03 t ha - 1 ) was obtained both at 82 kg N ha - 1 and plant spacing of 5.0 cm and 7.5 cm, respectively. The total and marketable bulb yields increased markedly across the increasing rate of nitrogen and intra-row spacing only up to 82 kg N ha - 1 and 5.0 cm intra-row spacing which attained maximum values of (39.51 t ha - 1 and 39.69 t ha - 1 ) , respectively and beyond which their yields decreased significantly. However, total bulb yield decreased as nil nitrogen level interacted with across the increasing of intra-row spacing. Thus, the lowest total (18.89 t ha - 1 ) and marketable bulb yields (17.93 t ha - 1 ) were obtained from plants that received no nitrogen at the intra-row spacing of 12.5 cm and 2.5 cm, respectively. The highest value of harvest index (79.98%) was produced at 82 kg N ha - 1 at the plant spacing of 5.0 cm and 7.5 cm. However, the highest total soluble solid (13.57 0 Brix) was obtained at 123 kg N ha - 1 and intra-row spacing of 2.5 cm. In conclusion, as the partial budget analysis revealed that the highest net benefit with low cost of production was obtained in response to the application of 82 kg N ha - 1 and the intra-row spacing of 10 cm and was optimum for producing the crop in the study area.

Key words: Onion, intra-row spacing, nitrogen, marketable yield, bulb size distributions

1 . INTRODUCTION

Onion (Allium cepa L.) is one of the most important vegetable crops commercially grown in the world. It probably originated from Central Asia between Turkmenistan and Afghanistan where some of its relatives still grow in the wild. Onion from Central Asia, the supposed onion ancestor had probably migrated to the Near East (Grubben and Denton, 2004; Bagali et al., 2012).

The crop onion is a popular vegetable and its bulb is used raw, sliced for seasoning salads, and cooked with other vegetables and meat. Onion bulbs are essential ingredients in many African sauces and relishes. The leaves, whole immature plants called ‘salad onion’ or leafy sprouts from germinating bulbs are used in the same way. In some parts of West Africa, leaves still green at bulb harvest are propounded, and then used to make sun-dried and fermented balls, which are used later for seasoning dishes. Sliced raw onions have antibiotic properties, which can reduce contamination by bacteria, protozoa or helminths in salads (Grubben and Denton, 2004).

Onions are day length sensitive, several onion types exist depending upon the latitude at which they grow. It is estimated that around the World, over 3,642,000 ha of onions are grown annually. On a worldwide scale, around 80 million metric tons of onions are produced per year. China is by far the top onion producing country in the world, accounting for approximately 28% of the world’s onion production, followed by India, USA, Iran, Egypt, Turkey, Russia, Pakistan, Netherlands and Brazil. The worldwide onion exports are estimated at around 7 million Metric tons. The Netherlands is the world’s largest onion exporter with a total of around 220,000 Metric tons followed at a distance by India (FAO, 2013).

Onion has economically important role in Ethiopia. The country has enormous potential to produce the crop throughout the year both for domestic use and export market. Ever since the crop is distributed to different parts of the country, it is widely cultivated as a source of income by many farmers in many parts of the country as a whole. Onion production also contributes to commercialization of the rural economy and creates many off-farm jobs (Lemma and Shimeles, 2003; Nikus and Mulugeta, 2010). Onion production in the country is increasing from time to time. During the 2013/2014 cropping season, the total area under onion production was estimated to be 24, 375.7 ha with an average yield of about 9.02 tons per hectare and estimated a total production of greater than 2, 19, 735.27 tons (CSA, 2014).

Nutrients play a significant role in improving productivity and quality of vegetable crops. Onions are the most susceptible crop plants in extracting nutrients, especially the immobile types, because of their shallow and unbranched root system; hence they require and often respond well to addition of fertilizers (Brewster, 1994; Rizk et al., 2012). Therefore, optimum fertilizer application and cultivation of suitable varieties with appropriate agronomic practices in specific environment are necessary for obtaining good yield of onion.

Nitrogen (N) and phosphorus (P) are often referred to as the primary macronutrients because of the probability of plants being deficient in these nutrients and the large quantities taken up from the soil relative to other essential nutrients (Marschner, 1995). Nitrogen plays an important role for optimum yield of onion and is found to be essential to increase the bulb size and yield. Increasing nitrogen application rates significantly enhances plant height, number of green leaves per plant and weight of bulb, marketable yield and also total soluble solids (Nasreen et al., 2007; Al-Fraihat, 2009).

In addition to nitrogen, plant spacing is an important factor determining onion yield and quality. An essential aspect of any crop production system is the development of a crop canopy that optimizes the interception of light, photosynthesis, and the allocation of dry matter to harvestable parts. A crop canopy is commonly managed by manipulating row spacing and plant population; as plant density increases, yield per unit area increases and will approach an upper limit, the plateau. Then, the yield per unit area declines since yield per plant tends to decrease with further increase in the plant density because of competition for growth factors between adjacent plants (Silvertooth, 2001). Thus, spacing is an important factor for the production of onion since it affects both bulb yield and quality. Plating density greatly influences quality, texture, taste and yield of onion even within a particular variety (Saud et al., 2013). Yield responses to plant population need to be known for practical purposes, as planting density is a major management variable used in matching crop requirements to the resources by the environment (Smith and Hamel, 1999). Coleo et al. (1996) reported that the highest commercial bulb yield was recorded at a higher planting density, but the highest proportion of large bulbs and average bulb weight at lower planting density.

The enhancement of onion production and productivity can be related to different growth factors. Onion dry bulb production depends on nutrient requirements, location of production, variety, soil type, agronomic practices etc. Thus, research should be undertaken to determine specific application rates for individual fields since it is important to avoid over fertilization with nitrogen or phosphorus, as this will contribute to increased pest attacks and stimulation of succulent growth that may predispose the plant to damage by field or storage pathogens Ware and McCollum (1980). On the other hand, under fertilization should also be avoided lest low yield and quality of the crop are obtained.

The use of appropriate agronomic management has an undoubted contribution to increased crop yields. One of the major problems to onion production is improper agronomic practice used by farmers. The optimum level of any agronomic practice such as plant population, planting date, harvesting date, and fertilizer of the crop varies with environment, purpose of the crop and cultivar. Optimum plant spacing and nitrogen recommendations have been formulated for onion particularly in the Rift Valley region of Ethiopia, which is double row spacing of 10 cm between plants and 20 cm between rows and application of 46 kg N ha-1 and 92 kg P2O5 ha-1 (Lemma and Shimeles, 2003; Nikus and Mulugeta, 2010). However, these recommendations cannot be directly adopted for the soil and growing conditions of the Central Zone of Tigray, which are different from the conditions in the Rift Valley region. This means that, it is very difficult to give general recommendations that can be applicable to the different agro-ecological zone (Upper Awash Agro-Industry Enterprise, 2001). Therefore, to optimize onion productivity in the study area, a specific package of recommendation of nitrogen fertilizer and plant spacing is required (Gupta et al., 1994; Lemma and Shimeles, 2003).

Onion is one of the most important vegetable crops cultivated mostly under irrigated conditions in Axum District in Central Zone of Tigray Region. There are many production constraints responsible for low yield per unit area in the Districts. According to Problem Appraisal of Axum Agricultural Research Center (Unpublished, 2012), different agronomic practices are undertaken to produce the crop in the District. For example, growers use different levels of inorganic fertilizers for production of different vegetables under irrigation particularly for onion production. The farmers often apply between 100-200 kg ha-1 of DAP and 50-150 kg ha-1 Urea. However, a few of the farmers use higher doses of these fertilizers, and a significant number of farmers use small doses of N fertilizer in the form of Urea. This shows no specific nitrogen levels are applied by smallholder farmers in the District. However, the blanket recommended rates of fertilizers are 200 kg ha-1 of DAP and 100 kg ha-1 Urea (Nikus and Mulugeta, 2010). Farmers in the district also grow onion using double row planting method at the spacing of 40 cm between furrows and 20 cm between rows on the ridge. However, the different growers use different spacing for improving the yield of onion.

Onion is traditionally grown at the recommended spacing of 40 cm x 20 cm x 10 cm in Ethiopia (Lemma and Shimeles, 2003). However, farmers in Axum District use narrower intra-row spacing. Farmers’ reasons for using narrow plant spacing are to minimize the number of oversized bulbs produced in wider plant spacing and for producing bulbs with higher yields and optimum sizes per unit area and for effective use of the limited irrigable land. Moreover, oversized bulbs have little market demand in the local markets of Axum and its environs. The reasons for less market demand of large onion bulbs have not been documented, but may be attributed to the poor shelf life and other quality attributes of large bulbs. At present, onion growers in the Central Zone of Tigray produce onions with the application of blanket recommendation of nitrogen fertilizer rates and intra-row spacing or using N rates and spacing which they feel as best for obtaining higher yields. Therefore, the present study was initiated with the following objectives:

- To assess the effect of intra-row spacing and nitrogen fertilizer level on growth, yield and quality of onion; and
- To identify the appropriate intra-row spacing and nitrogen fertilizer rate that improves yield and quality of onion.

2. LITERATURE REVIEW

2.1. Description of Onion Crop

Onion (Allium cepa L.) belongs to the family Alliaceae or Amaryllidaceae which is one of the most important monocotyledonous crops. It belongs to the genus Allium and recent estimations accept about 750 species in the genus Allium, among which onion, Japanese bunching onion, leeks, and garlic are the most important edible Allium crops. And about 60 taxonomic groups at sub-generic, sectional and sub-sectional rank (Baloch, 1994; Rabinowitch and Currah, 2002). Onion from central Asia, the supposed onion ancestor had probably migrated to the Near East. Then it was introduced to India and South-East Asia; and into the Mediterranean area and from there to all the Roman Empire (Grubben and Denton, 2004).

Onion is a cross-pollinated cool season vegetable crop. It is the oldest known vegetable. Onion is an indispensable and important vegetable item which is used in every kitchen therefore its constant demand always remains throughout the year. Besides its high food value, it is also a good source of income for vegetable growers. It can be eaten as green leaves, bulbs that are mature and immature which can be eaten as fresh and also can be used in preparation of different dishes. The pungency of the onion bulbs is due to the presence of a volatile oil that is allylpropyl disulfide (Baloch, 1994). The onion has its own distinctive flavor and used in soups, dishes, salad and sandwiches and is cooked alone as a vegetable. It is consumed at its young green stage or after its full development and maturity when it is harvested in the form of a dry bulb. The mature bulbs contain some starch, appreciable quantities of sugar, some protein, and vitamins A, B and C (Jilani et al., 2010).

Onion is a shallow rooted, biennial crop which is grown as annual. The leaves are long, hollow with widening, overlapping bases. The tubular leaf blades are flattened on the upper surface, and the stem of the plant also is flattened. Roots arise from the bottom of the growing bulb. Leaf initiation stops when the plant begins to bulb. The base of each leaf becomes one of the “scales” of the onion bulb, so the final bulb size depends in part on the number of leaves present at bulb initiation. The leaf base begins to function as a storage organ at bulb initiation, so the size of the leafy part of the plant also influences bulb size. Thus, the more leaves present and the larger the size of the plant at the onset of bulb initiation, the larger will be the bulbs and the greater will be the crop yield (Hamasaki et al., 1999).

The onion develops distinct bulbs depending on the varieties. These bulbs are varying in size (small, medium and large). Bulb weight may be one kg in some Southern European cultivars, and the shape covers a wide range from globose to bottle like and to flattened disk-form. The color of the membranous skins may be white, silvery, buff, yellowish, bronze, rose red, purple or violet. The color of the fleshy scales can vary from white to bluish-red. There is also much variation in flavor and keeping or storage ability of the bulbs ((Baloch, 1994; Rabinowitch and Currah, 2002).

2.2. Environmental Requirements of Onion

Onion can be grown in a wide range of climatic environments, but it thrives best at mild climate without excessive rainfall or extremes of heat and cold. Onion is a cool season crop that has some frost tolerance but is best adapted to a temperature range between 13 and 24 0C. Optimum temperatures for early seedling growth are between 23 and 27 0C; growth is slowed at temperatures above 30 0C. Acclimatized plants are able to tolerate some freezing temperature. Best production is obtained when cool temperature prevails over an extended period of time, permitting considerable foliage and root development before bulb formation starts. After bulb formation begins, high temperature and low relative humidity extending into the harvest and curing period are desirable (Purseglove, 1985; Rubatzky and Yamaguchi, 1997; Jilani et al., 2010).

Onions can be grown on a wide range of soils, varying in texture from coarse-grained sands to clays. Lighter soils are easy to manage. Soils should be 45-60 cm deep and well drained. Soils with high water holding capacity are better able to provide moisture to the shallow rooting system but must also drain well to be suitable. Growth is retarded when available soil moisture is low, but onions are also sensitive to a high water table or water logging. Uniform moisture availability about 400-800mm per crop is conducive to large bulb size and high yields. Favorable soil pH is about 6.5–8.0 in mineral soils (Rubatzky and Yamaguchi, 1997; Savva and Frenken, 2002).

Light and temperature influence the process of bulbing. Both factors must be at optimum for the initiation of the bulbs. Cool conditions with long days are normally important for production, although there are cultivars that tolerate warm conditions and short day-lengths. Cool conditions are usually required during the first part of the season, when the plants start to form bulbs. Warm and dry weather is needed for harvesting and curing. Each cultivar differs in its sensitivity to day-length (Savva and Frenken, 2002).

The onions are grouped into short-days and long-days depending on the day length requirements. The bulbs that acquire day length of 11.5 hours are categorized into short-day group and those take 14 hours or more for bulb formation fell into long-day group. Onion also requires varying day length and temperature for the purpose they produced. A relatively high temperature and long photoperiod are required for bulb formation, and for seed production, temperature is of immense importance than day length. Onion bulbs have specific temperature requirement for seed and bulb production (Baloch, 1994). Light intensity, light quality, and other factors interact with temperature and day length to influence the bulbing response of onion cultivars. With warm weather and bright days, onions bulb at shorter day lengths than when the days are cool and over cast (Hamasaki e t al., 1999).

Onion dry bulb are established either by direct sowing to the field, by transplanting seedling or from dry sets depending on the growing conditions of the specific regions. Sowing seeds directly into the soil where the crop is to be grown is potentially the most economical method of raising an onion crop, particularly where the availability of labor for transplanting is limited and its cost is high or where the availability of facilities for raising transplants is limited (Brewster, 1994). Sets and transplants are used in areas where the season is not long enough for proper bulb development. Transplants have the advantage on economic use of seed, selecting superior (healthy and vigorous) seedlings. It saves weeding and watering effort during the early weeks of onion growth it enables the farmers attend to the seedlings in a compact area (Lemma and Shimeles, 2003).

2.3. Importance and Production Status of Onion in Ethiopia

The production of vegetables is becoming important with the expanding irrigated agriculture and with the growing awareness on the importance of the sector as source of income, improved food security, sources of raw materials for industries, employment opportunity because it demands large labor force. The expansion of water harvest schemes in small farmers sector and irrigated agricultural development projects have made significant contribution to the development of the sector. The success of production depends on the adoption of improved technologies such as cultivars that have acceptable standard and high value in the local use and export markets (Lemma et al., 2006).

Ethiopia has a great potential to produce onion throughout the year both for local consumption and for export. It grows best at an altitude of between 700-2200 meters above sea level. Onion is a rapidly becoming popular among producers and consumers. Its popularity among producers is because of the advantage of high yield potential, availability of desirable cultivars for various uses, ease of propagation by seed, high domestic (bulb and seed) and markets in fresh and processed forms (Lemma and Shimeles, 2003). Onion contributes substantially to the national economy, apart from overcoming local demands. With the growing irrigate agriculture in the country, there is a great potential for extensive onion seed and dry bulbs production in the different production belts of the country.

Specifically to onion production and improvement, the Ethiopian Agricultural Research Institute has made efforts to generate different improved varieties. As a result of this effort the varieties Adama Red, Bombay Red, Red Creole, Melkam, Mermiru Brown, Nasik Red and Nafis are made available to farmers (Lemma and Shimelis, 2003; MoARD, 2010). It is widely produced by small farmers and commercial growers throughout the year for local use and export market. Onion is important in the daily Ethiopian diet and all the plant parts are edible, although the bulbs are widely used as a seasoning or a vegetable in various dishes. Onion is valued for its distinct pungency and form essential ingredients for flavoring varieties of dishes, sauces, soup, sandwiches, snacks as onion rings etc. It is popular over the local shallot because of its high yield potential per unit area, availability of desirable cultivars for various uses and ease of propagation by seed (Lemma, 2004).

Onion is considered as one of the most important vegetable crops produced on large scale in Ethiopia. It also occupies an economically important place among vegetables in the country. The area under onion is increasing from time to time mainly due to its high profitability per unit area and ease of production, and the increases in small scale irrigation areas. The crop is produced both under rain-fed in the “Meher” season and under irrigation in the off season. In many areas of the country, the off season crop (under irrigation) constitutes much of the area under onion production. Despite areas increase, the productivity of onion is much lower than other African countries. The low productivity could be attributed to the limited availability of quality seeds and associated production technologies used, among the others (Nikus and Mulugeta, 2010).

2.4. Response of Onion to Nitrogen Application

2.4.1. Roles of nitrogen in onion nutrition and growth

Onion being among the high nitrogen demanding vegetables, its productivity depends on use of optimum fertilizer rates and if not adequately fertilized, considerable yield losses are apparent. Among all nutrients, nitrogen is the most important and also the most limiting to crop production. Efficient N use is important for the economic sustainability of cropping systems (Brewster, 1994; Fageria and Baligar, 2005). Excessive use of N fertilizers is a concern, since large amounts of N can remain in the soil after crop harvesting (Neeteson et al., 1999). In a temperate climate, usually ≤ 50% of N applied is effectively used by plants, while a considerable part is lost by leaching and contaminates ground and surface waters (Fageria and Baligar, 2005).

Mineral fertilizers are one of the principal factors that materially set up onion growth and production. Onion plants take up large amounts of the three primary nutrients, i.e. nitrogen, phosphorus and potassium (Kandil et al., 2013). Marschner (1995) also stated that nitrogen and phosphorus are often referred to as the primary macronutrients because of the large quantities taken up from the soil relative to other essential nutrients.

2.4.2. Response of onion to nitrogen fertilization

Onion, compared with most crops, is usually the weakest crop plant in terms of extracting nutrients, especially the immobile types, because of their shallow and unbranched root system (Brewster, 1994). Thus, the crop is a heavy feeder, requiring ample supplies of N; hence it requires and often responds well to addition of fertilizers. However, excess application of nitrogen causes excessive vegetative growth, delayed maturity, increase susceptibility to diseases, reduces dry matter contents and storability and ultimately reduces yield and quality of bulbs (Brewster, 1994; Sørensen and Grevsen, 2001).

Soleymani and Shahrajabian (2012) showed that the highest and the lowest marketable yield were obtained in to the application of 300 kg N ha-1 and 0 kg N ha-1, respectively. Negash et al. (2009) also reported that increasing the rate of N fertilization from 0 to 138 kg ha-1 increased total bulb yield from 19.26 t ha-1 to 32.24 t ha-1. Similarly, increasing the rate of nitrogen application from 0 to 138 kg ha-1 significantly increased marketable bulb yield from 18.82 t ha-1 to 31.90 t ha-1 which was 69.5% higher than the control. Jilani et al. (2004) reported that with increase in dose of nitrogen up to120 kg N ha–1 the marketable and total bulb yield was increased, but below this level the total yield t ha–1 began to decrease. A significant increase in total bulb yield in response to nitrogen fertilizer levels was also observed by (Balemi et al. 2007).

Bolting is triggered in response to exposure of the onion plant to conditions like low temperature or limited N supply which induces flowers to emerge before bulb are adequately grown to suppress flower initiation (Yamasaki and Tanaka, 2005). Al-Fraihat (2009) also, stated that highest percentage of bolting was obtained from plants fertilized with the lowest level of nitrogen (100 kg N ha-1). Abdissa et al. (2011) also showed that nitrogen fertilization significantly reduced bolting in onion. The authors reported that ratio of bolting percentage per plot decreased by about 11 and 22% in response to the fertilization of 69 and 92 kg N ha-1, respectively as compared to the control.

Yield is composed of marketable and unmarketable dry bulbs. The marketable product typically depends on onion cultivar (Lemma and Shimeles, 2003). According to the authors, marketable bulb weight can be grouped into different size of bulb categories: Oversized (above160 g), large (100-160 g), medium (50-100 g), small (20-50 g) and under-sized below (20 g). These sizes can be preferred by consumers or users according to their purpose for planting material, food, as well as for processing. EARO (2004) stated that the different bulb size category was indicated for different varieties under the Ethiopian conditions: Bombay Red (85-100 g), Adama Red (60-80 g), Red Creol (80-100 g), Melkam (70-90 g), Mermiru Brown (70-90 g) and Dereselign (85-100 g). This showed that bulb size category is also highly dependable on variety in addition to intra-row spacing.

With regard to unmarketable bulb yield of onion, it is related to the under sized bulb which is below 20 gram, diseased, decayed, physiological disorder such as thick necked, splits and bolters. Disorders are influenced by location, season, cultivar, and management practice. On the other hand, thick necked also occurs mainly when some of the proportion of bulbs fail to complete bulbing in which the leaves continue growing. Under this condition, the neck does not get soften and the bulb does not become dormant. Heavy and continuous watering and late application of nitrogen contribute to this disorder (Lemma and Shimeles, 2003). The onion bulb size increased significantly with the application of different doses of nitrogen. Application of higher nitrogen of 120 kg ha-1 recorded the maximum bulb size while the minimum bulb size was recorded in control (Jilani et al., 2009).

Nitrogen significantly affected yields of various onion bulb size categories. Onion fertilized with different N levels decreased the yield of small sized bulbs, but increased the yield of large sized bulbs. Small sized bulbs decreased by 61.8% when N application was increased from 0 to 138 kg ha-1. On the hand, when N fertilization increased from 0 to 138 kg ha-1 the increased large size bulbs increased from 12.58 t ha-1to 25.67 t ha-1, respectively, resulting in 104% increment (Negash et al., 2009).

Nitrogen fertilization increased the bulb yield of onion and yield components. Increasing nitrogen levels from 0 to 120 kg ha-1 resulted in progressive increase in bulb yield. Application of 120 kg N ha-1 increased the number of leaves per plant and plant height over the control as well as lower levels of nitrogen. There was an increase in diameter and weight of bulbs due to application of nitrogen up to 120 kg N ha-1 and thereafter decreased (Nasreen et al., 2007). A report also described by Morsy et al. (2012) indicated that 120 kg N ha-1 appeared higher values of plant height, number of leaves per plant, bulb diameter and days to maturity as compared to adding of 90 kg ha-1.

Al-Fraihat (2009) stated that with increasing application of nitrogen fertilizer from 100 kg N ha-1 to 200 kg N ha-1 in the first and second growing seasons, the TSS value increased from 13.75% to 14.70% and 13.90% to 15.07% during the first and second growing seasons, respectively. Morsy et al. (2012) also showed application of 120 kg N ha-1 led to the highest values of TSS whereas, application of 90 kg N ha-1 resulted in the lowest values of TSS in both seasons. Moursy et al. (2007) also indicated that increasing the level of N fertilizer to 80 kg N ha-1 resulted in about 8.5% increase in the TSS as compared to the level of 40 kg N ha-1.

The different N levels affected the leaf diameter and length of onion. The application of 150 kg N ha-1 gave the highest value with regard to leaf diameter. Generally, with increasing nitrogen level from 0 kg ha-1 to 150 kg ha-1 the leaf diameter of onion increased from 0.81 cm to 1.00 cm (Kokobe et al., 2013). Al-Fraihat (2009) reported the highest level of nitrogen significantly increased plant height and number of green leaves per plant as compared to the control treatment.

Nitrogen fertilization significantly extended the number of days required for onion crop to attain its physiological maturity. Regardless of the rate, N fertilization extended physiological maturity by about 6 days over the unfertilized treatment (Abdissa et al., 2011). A report by Meena et al. (2007) also described the delay in maturity of onion bulb due to application of enhanced level of nitrogen.

Generally, considering the status of the soil, additional nitrogen fertilizer levels application may be necessary in order to meet the crop N requirements. The amount of N needed is usually based on soil organic matter content, crop uptake and yield levels. Nitrogen uptake levels by onion crops may vary from less than 50 kg to more than 300 kg ha-1, depending on cultivar, climate, plant density, fertilization and yield levels (Soujala et al., 1998).

2.5. Effect of Plant Spacing on Onion Yield and Yield Components

Plant population refers to number of plants per square meter (plants m-2) or hectare (plants ha- 1) and is important in onion production since it has an influence on growth, yield and quality of onion bulbs (Brewster, 1994). Plant and row spacing are considered important to the optimum plant population which may be reflected in higher yield and quality. Onion bulb size can be controlled to a certain extent by plant population. In order to produce large bulbs (> 70 mm in diameter) a plant population of between 25 and 50 plants m-2 is required, for medium bulbs (25-50 mm) between 50 and 100 plants m-2 and for small bulbs (< 50 mm) more than 100 plants m-2 are required (Brewster, 1994).

According to Dorcas et al. (2012) reported that with increasing plant density of onion from lower 100,000 plants ha-1 to higher plant density of 500,000 plants ha-1 then average bulb weight and bulb diameter decreases from 58.22 g to 40.04 g and 4.56 cm to 2.83 cm respectively. The authors also reported that highest and lowest yield was obtained in the higher plant density of 500,000 plants ha-1 and lower plant density of 100,000 plants ha-1. Yemane et al. (2013) indicated that with increasing intra-row spacing from 5 to 10 cm, statistically bulb diameter and bulb neck diameter of onion increased from 4.66 to 5.63 cm and 1.48 1.74 cm respectively. Dawar et al., (2005) indicated that as plant population increased from 40 to 80 plants m-2 onion neck diameter declined significantly. Jilani et al. (2009) indicate that bulbs of thick neck of onion were found in plots of lowest plant density (20 plants m-2). Bulb neck diameter decreased as population density increased. Mean bulb weight and plant height decreased as population density increased (Kantona et al., 2003).

Khan et al. (2002) reported that various plant spacing leads to the increase in plant height, onion bulb size, and weight of the bulbs, bulbs ha-1 and yield of the bulbs. Khan et al. (2003) reported that wider spacing (20 x 10 cm) produced higher size of plant height, leaf length and number of leaves, bulb length, diameter and weight of onion. On the contrary, highest yield was observed at the closest spacing and the lowest yield at widest spacing. Yamane et al. (2013) also indicated that as intra-row spacing increased from 5 to 10 cm, marketable bulb yield in t ha-1 decreased from 34.49 to 28.10. Seck and Baldeh (2009) reported that plant density has an impact on marketable bulb size and the higher the plant density the smaller the marketable size. Kantona et al. (2003) also reported that as plant density increased number of marketable bulbs increased.

Sikder et al. (2010) evaluated three intra-row spacing (20x20, 20x15 and 20 cmx10 cm) of onion. Based on this, the maximum yield were recorded from 20 cm x 10 cm spacing and the narrow plant spacing produced comparatively lower values on fresh weight of leaves per plant, plant height, leaves number per plant, bulb diameter and fresh weight of bulb. Stoffela (1996) also found that as number of rows per bed increased, marketable onion yield linearly increased and mean bulb size decreased. Latif et al. (2010) showed that yield of onion bulbs produced at the spacing of 20 cm x 10 cm was recorded as the highest compared to 20 cm x 20 cm spacing. Mahadeen, (2008) also reported that narrow intra-row spacing produced higher yield.

According to Balraj et al. (1998) with increase in plant spacing, the bulb weight and size increased, but the yield ha-1 decreased. Kumar et al. (1998) indicated that the spacing has a direct effect on the quality and production of onion. Lower planting density was the best with regard to leaf length. Latif et al. (2010) indicated that the numbers of leaves per plant, bulb weight, foliage dry weight, plant height was highest when the plants were grown at wider spacing of 20 x 20 cm. However, yield per unit area was higher in the narrow spacing. Nasir et al. (2007) also stated that the highest leaf number per plant was recorded at lower planting density. Planting of onion at 20 and 25 cm spacing produced larger bulbs compared with planting at 10 and 15 cm spacing (Mahadeen, 2008). Jilani (2004) reported that onion plants from the lowest plant population (20 plants m-2) recorded the highest number of leaves and leaf length.

According to Jan et al. (2003), the highest yield (40.44 t ha-1) was found at spacing of 17 x 4.5 cm, and the lowest yield (19.95 t ha-1) at 27 x 14.5 cm spacing. Yemane et al. (2013) also indicated that the highest total bulb yields were achieved at 5 and 7.5 cm intra-row spacing, respectively as compared to the 10 cm intra-row spacing. Dereje et al. (2012) also indicated that total yield per hectare increased as plant density increased although yield of the individual plants and their components were significantly reduced suggesting a compensation of higher plant densities on yield in shallot.

Kantona et al. (2003) observed that onion yield increased from 17.4 to 39.5 t ha-1 as plant population per square meter increased from 50 to 150. Yemane et al. (2013) mentioned that the highest unmarketable bulb yield of onion was produced by the narrow intra-row spacing. Dereje et al. (2012) also reported that high unmarketable yield of shallot was recorded in closely spaced plants. Seck and Baldeh (2009) also concluded that plant density has an impact on marketable bulb size. The smaller the marketable size is an issue for high plant densities and needs to be improved.

According to Nasir et al. (2007) maximum weight of small and medium sized of onion was obtained at higher population density, However, the highest weights of large bulbs were found at the lowest planting density. Dawar et al. (2007) also reported that maximum weight of medium and small sized bulb was achieved at higher planting density of 80 plants 4m-2. However, maximum weight of large bulbs was found at the lowest planting density of 40 plants 4m-2. Rumpel et al. (2000) showed that yield of medium bulbs increased with density but, the yield of large bulbs decreased as plant density increased. Stoffella (1996) also mentioned that percentage of small and medium sized bulbs increased and percentage of large bulbs decreased as intra-row spacing decreased. Yemane et al. (2013) stated that the highest percentage of small and medium size bulbs yield was scored at narrow intra-row spacing of 5 cm as compared to 7.5 cm and 10 cm. However, as the intra-row spacing increased from 5 to 10 cm, the percentage of large size bulbs increased from 9.3 to 20.3%.

Minimum planting density attained the highest number of leaves which decreased with increasing planting density. Minimum plant population (20 plants m-2) had larger bulb diameter against smaller bulb diameter of higher plants density (40 plants m-2) (Jilani et al., 2009). A report by Hyder et al. (2007) who indicated that plant height, bulb length, bulb diameter and days to harvest were the most important yield contributing factors. There is indirect effect on bulb yield of each trait. Plant height revealed a positive indirect effect on yield and was favorable through bulb length, bulb neck thickness, TSS in Brix and dry matter content. Akoun (2005) reported that bulb diameter was greatest (8.18cm) at the lowest population density. Seid et al. (2014) indicated that lowest leaf width (0.73 cm) of garlic was recorded in higher plant density.

Bosch and Olivé (1999) in Spain conducted two experiments, one under natural light condition and another one under black neutral shade, with the aim of investigating an influence of plant population (20, 40, 80 and 160 plants m-2) on bolting percentage using a long day cultivar. Based on this, under natural light condition, as plant population increased from 20 to 160 plants m-2, number of bolters significantly increased from 8 to 75.

Onions have a high harvest index with 70 to 80% of the shoot dry weight found in the bulb at maturity. As compared to other crops, onions are poor at intercepting radiation, average at converting radiation to dry matter but good at partitioning the dry matter to harvestable material (Brewster, 1990). Dereje et al. (2012) reported that lower harvest index of shallot in wider intra-row spacing. Kabir and Sarkar (2008) also reported highest value of harvest index of mungbean recorded from closer spacing probably due to the reduced vegetative biomass.

The ideal spacing and plant population are those that maximize yield, vegetable quality and profits to farmers without excessively increasing costs. An essential aspect of any crop production system is the development of a crop canopy that optimizes the interception of light, photosynthesis, and the allocation of dry matter to harvestable parts. A crop canopy is commonly managed by manipulating row spacing and plant population; as plant density increases, yield per unit area will approach an upper limit, plateau, and then decline while yield per plant tend to decrease with increasing plant density because of competition for growth factors between adjacent plants (Silvertooth, 2001).

Generally, yield of onion increases with an increase in plant population because plant densities allowed the canopy to close quickly reducing the ability of weeds to compete, but only up to an optimal limit and yield will decrease beyond this optimum. Appropriate spacing enables the farmers to keep appropriate plant population in their field. Hence, a farmer can avoid over and less population in a given plot of land, which has negative effect on yield. Therefore, to avoid nutrient competition due to inappropriate use of plant spacing and N fertilizer, sufficient spacing between plants and rows and optimum amount N fertilizer application is vital to get highest yield in a given plot of land (AVRDC, 2004).

2.6. Response of Onion to Interaction Effect of Nitrogen and Plant Spacing

Islam et al. (1999) reported that interaction effect of spacing and N levels on bulb yield of onion and most of the characters. The highest spacing in association with high nitrogen level up to 180 kg ha-1 increased number of leaves per plant and splitted bulbs. The highest bulb yield (31.6 t ha-1) was obtained from the lowest spacing (20x10 cm) along with nitrogen level of 120 kg ha-1 but the large sized bulbs were obtained from the combination of higher spacing (20 x 20 cm) and at 120 kg N ha-1.

According to Naik and Hosamani (2003) the narrow spacing of 15 x 10 cm gave the maximum bulb yield of onion and decreased the bulb yield with widening spacing. The highest bulb yield was recorded with treatment interaction of closer spacing (15 x 10 cm) and higher levels of nitrogen (150 kg ha-1). The bulb diameter was highest in the wider spaced crop (15 x 20 cm) followed by 15 x 15 cm than narrow spacing. Similarly, this parameter was also increased with increase in nitrogen levels and the bigger sized bulbs were found in the plots applied with 150 kg N ha-1. Average bulb weight was increased with increase in nitrogen levels. The highest bulb weight was found in the plots applied with 150 kg N ha-1 (Naik and Hosamani, 2003). The TSS was increased with increase in nitrogen levels and the maximum (10.15%) was recorded in the bulbs applied with 150 kg N ha-1 followed by 100 kg N ha-1 (9.15%). Shojaei et al. (2011) also reported that the highest mean bulb weight was produced by the plants treated with higher nitrogen and lower population density. The increase in N fertilization level and plant population also resulted in the increase in yield from 3 to 10 t ha-1.

Maximum number of leaves per plant was produced by the treatment interaction of higher nitrogen (150 kg N ha-1) with wider (15 cm spacing). Mean values of root diameter in response to different nitrogen levels showed superiority in 200 kg N ha-1 over 100 kg N ha-1 and 0 kg N ha-1. It would be observed from the means of interactions that 200 kg ha-1 with 10 cm spacing produced maximum root yield per hectare. Nitrogen dose of 200 kg ha-1 when interacted with 15 cm spacing produced maximum total biomass per plant followed by 200 kg N ha-1 with 10 cm spacing (Pervez et al. 2004).

Interaction effect of different intra-row spacing (10, 15, 20 and 25 cm) and levels of nitrogen fertilizer (0, 50, 100 and 150 kg N ha-1) showed that an increase in nitrogen dose up to 100 kg ha-1 resulted in the increase of yield of onion bulbs 40.83 t ha-1 by interacting with 15 cm intra- row spacing. But, further increase in N level up to 150 kg ha-1 did not significantly increase in bulb yield. The lowest bulb yield was recorded from the control plots when interacted with wider intra-row spacing of 25 cm (Aliyu et al., 2008). The authors reported that treatment combinations of 0 kg N ha-1 and 10 cm intra-row spacing gave lower values of average bulb weight, bulb diameter and leaves number per plant.

[...]