Quality and Shelf Life of Onion (Allium cepa L.) Bulb as Influenced by Application of Phosphorus and Potassium Fertilizers

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Published on International Journal of Food & Nutrition
Publication Date: June 14, 2019

Muluneh Bekele Etana, Ali Mohammed & Amsalu Nebiyu
Department of Horticulture, College of Agriculture and Veterinary Science, Ambo University
Jimma University College of Agriculture and Veterinary Medicine, Jimma University
Jimma University College of Agriculture and Veterinary Medicine, Jimma University
Ethiopia

Journal Full Text PDF: Quality and Shelf Life of Onion (Allium cepa L.) Bulb as Influenced by Application of Phosphorus and Potassium Fertilizers (Under Jimma Condition, Ethiopia).

Abstract
Onion (Allium cepa L.) is one of the most important vegetable crops produced in Ethiopia. Yield and productivity of the crop has been far below the regional and national standards owing to several factors; absence of location specific fertilizer recommendation being the major among others. In Ethiopia, post harvest loss of vegetables contributed up to 30% yield reduction on vegetables and fruits. Thus, a field experiment was conducted at Jimma University College of Agriculture and Veterinary Medicine Research field in dry season to study the effects of Phosphorus (P) and Potassium (K) fertilizer on quality and storage life of irrigated onion under Jimma condition, South Western Ethiopia. The treatments consisted of factorial combinations of three levels of Phosphorus (0, 46, and 92kg P2O5 ha-1) and four levels of Potassium (0, 40, 80, and 120 kg K2O ha-1) laid out in Randomized complete Block Design with three replications. Data on yield, bulb quality and storage life parameters were recorded and analyzed using GenStat 12.1 version computer soft ware packages. Results of the study revealed that; P and K had shown a highly significant effect on quality parameters like TSS (oBrix), DMC (%) and bulb shape index. Similarly, keeping quality of the Onion bulbs like bulb sprouts (%), weight loss (%), weeks to 50% bulb sprouts and storage rots (%) are highly influenced by application of P and K at different levels. Potassium significantly decreased the bulb rots (%), bulb sprouts (%) and weight loss (%) during the two month storage time at ambient temperature. This can be recommended for the use by potential onion investors or farmers in the study area. Nevertheless, more researches are needed in different locations and on different soils to come up on general recommendation.

Keywords: Jimma, Onion, Phosphorus, Potassium, Bulb Quality, Storage life.

1. Introduction
Onion is an important vegetable crop worldwide, ranking second among all vegetables in economic importance next to Tomato. Onion contributes significant nutritional value to the human diet and has medicinal properties and is primarily consumed for their unique flavor or for their ability to enhance the flavor of other foods (Randle, 1998). The primary center of origin for Onion is Central Asia with secondary center in Near East and the Mediterranean region. From these centers, the Onion has spread widely to other many countries of the world (Astley, 1982). Onion is different from the other edible species of alliums for its single bulb and is usually propagated by true botanical seed. According to FAO among the onion producers, the first is China in terms of area of production. The highest productivity is from Korea Republic (67.25 t/ha) followed by USA (53.91 t/ha), Spain (52.06 t/ha) and Japan (47.55 t/ha). India being the second major Onion producing country in the world has a productivity of 10.16 t/ha only. Onion was introduced to the agricultural community of Ethiopia in the early 1970’s when foreigners brought it in. Though shallots were traditional crop in Ethiopia, Onion is becoming more widely grown in recent years.
Depending on soil P status, cultivar and plant density, application rates of up to 200 kg ha-1 was found to maximize Onion yields (Hegde et al., 1986a; Vachhani & Patel, 1993; McPharlin & Robertson, 1999; El-Rehim, 2000; Singh et al., 2000) and reduce storage loss of bulbs. Increased P levels are also known to improve bulb size and the number of marketable bulbs in shallots (Zahara et al., 1994; Nagaraju et al., 2000). Regardless of the P status of the soil, placement of P-fertilizers in the soil near to the plant would be the most effective method of P supply to Onion plants (Brewster, 1994; Henriksen and Hansen, 2001 and Muluneh et al,. 2019).
In Ethiopia, so far there was a general understanding that Ethiopian soils are rich in K and there was no need for its application based on the research conclusion of some 50 years ago (Murphy, 1968). However, research report indicated that K is removed through deforestation, crop export, leaching of cations and other possible reasons, especially in some highland areas of Southern Ethiopia and possibly in other similar areas of the country (Wassie, 2009). Similarly, a significant higher bulb yield (247.79 q ha-1) and fresh bulbs weight (49.53 g) were registered with application of 150 kg K ha-1 over other levels. Worldwide, post-harvest losses in fruits and vegetables range from 24 to 40% or even greater, reaching up to 50% in developing tropical countries (Raja, 1993). A post-harvest loss in onion has been estimated to reach 30% in Sudan (Hayden, 1989) and 50 to 76% in Nigeria (Denton, 1990). A comprehensive statistics for such losses is not available for Ethiopia.
However, Proper management techniques such as fertilizers, soil moisture and disease control, harvest time and curing enhance Onion produce (Kabir, 2007). Optimization of such practices results in significant decrease in post harvest losses and increase bulb yield in Onion. Decrease in post harvest losses will be instrumental in market stability and exploiting opportunities to export Onion and earn foreign exchange. Best quality Onion can be produced through application of well balanced fertilizers (Murashkina, 2006).
In general, better understanding of the nutrient requirements of onion plant is needed in order to develop management strategies, which optimize fertilizer use of the crop and thereby increase returns with premium bulb qualities to the producers. In the light of the above aspects, the present research was initiated to identify the economical level of potassium and phosphorous fertilization for onion (Allium cepa L.) optimum quality and storage life under Jimma conditions, Southwestern Ethiopia.

2. Materials and Methods
2.1 Description of the Experimental Site
The experiment was conducted under field condition at Jimma University College of Agriculture and Veterinary Medicine research field under irrigation condition. Jimma University College of Agriculture and Veterinary Medicine is geographically located 346 km southwest of Addis Ababa at about 70, 33’N latitude and 360, 57’ E longitude and an altitude of 1710 meter above sea level. The analysis of soil samples from the top 30 cm depth of the experimental site before the experiment revealed that the soil contains 1.46% organic carbon; 1.42% total nitrogen, 2.80ppm available phosphorus, 53.1μS/cm electrical conductivity and a pH value of 5.94.The mean maximum and minimum temperatures are 26.80C and 11.40C, respectively and the mean maximum and minimum relative humidities are 91.4% and 39.92%, respectively. The mean annual rainfall of the area is 1500mm (BPEDORS, 2000).

2.2 Experimental design and layout
Onion seedlings were raised in the nursery on a well prepared seedbed whose dimension was 5m × 1m. The seeds were sown in rows marked 15cm interval across the length of the seed bed and the beds were covered with dry grass mulch until emergence. Complete germination of the seeds took place within 7 to 10 days of sowing and seedlings were thinned out after three weeks in order to maintain optimum plant population and to keep them vigorous. Watering of the seed bed was done always in the morning and afternoon using watering can. The seed beds were watered before uprooting the seedlings in order to minimize the damage of the roots. Healthy, uniform and 51 days old seedlings were transplanted to the prepared field at spacing according to the EARO, 2004 recommendation. All the twelve treatment combinations were randomly assigned and there were 10 plants in each row and 60 plants per plot with three replications. During the course of the study Mancozeb was applied to prevent the damage of disease at rate of 4.0 kg ha-1 mixed in 600 liter of clean water. All other agronomic management practices were provided as per the recommendation equally for all the treatments (Getachew, 2009).
Finally, bulbs from the central four rows were harvested after 60% neck-break and used for analysis. Curing of bulbs was done for ten days under partial shade and ten sample bulbs were used for storage. Naturally ventilated house was constructed from wire mesh wall and corrugated iron sheet roofing then kept in boxes made of wire mesh to record data on storage life of onion bulb. Daily storage room temperature and relative humidity was recorded using digital sling Psychrometer (AZ8706 model, China). The storage time was from the month of May to July for three months under the average monthly temperatures and relative humidity of 17.23oc and 16.72oc, 75.32% and 77.65%, respectively.

2.3 Statistical analysis
The data were analyzed using GenStat versions 12.1 (2009) with the REML variance component analysis. Mean differences were tested following least significant difference (LSD) at (P<0.05). 3. Results and Discussions 3.1 Quality Parameters of Onion Bulb 3.1.1 Total soluble sugars content (TSS) Regarding the total soluble sugars content (TSS), the main applications of P and K had shown a highly significant (Table 1) difference. The highest TSS value (10.39 oBrix) and (10.22 oBrix) were recorded in the main application of P and K at the rate of 92 and 80kg ha-1 respectively; while the minimum TSS value (9.11 oBrix) and (9.03 oBrix) were recorded in both P and K control levels. Table 1: Main effects of Phosphorous and potassium fertilization on TSS of onion bulb 3.1.2 Bulb shape index Considerable variation was observed in the result of bulb shape index. The shape of onion bulb can vary from flat to globe to torpedo which is in different markets having different requirements. The onion bulb shape was assessed by the bulb shape index; this was determined by the ratio of bulb length to diameter. The result of this study revealed that application of P and K at different levels had shown a highly significant (P<0.001) difference on the bulb shape index, while their interaction did not (Table 2). This result also showed that the null and lower application of P and K fertilizers increased the percentage of shape rejects as compared to the plot received higher levels of P and K (92 kg P ha-1 and 120 kg K ha-1), respectively. Similarly, Geremew, (2009) reported as bulb shape of onion is affected by mineral nutrients. Table 2: Bulb shape index as influenced by main effects of P and K fertilization Regardless of levels, higher application of P at (92 kg ha-1) and K at (120 kg ha-1) increased the bulb shape index by about 3% and 2.5% over control, respectively (Table 2). The reason why P and K fertilization increased the bulb shape index of onion may be because of their vital role in plant growth and development. Kimani et al. (1993) reported as bulb shape difference is among onion cultivars and affected by growing environment and also further explained that globe shaped (shape index=1) are preferred by the consumers. 3.1.3 Dry matter contents Regarding the dry matter contents, main application of P and K had shown a highly significant effect, while their interaction did not (Table 3). The increasing levels of P and K encouraged bulbs with a significantly higher dry matter contents as compared to the unfertilized plot. The maximum dry matter content of onion bulb (10.30%) and (10.42%) recorded with higher application of P and K at rate of 92 and 120 kg ha-1, respectively. The minimum dry matter contents (9.54%) and (9.20%) detected in control respectively (Table 3). This finding is in consistent with the result of Mojsevich (2008) who reported that with the increase of doses of the main fertilizer N, P and K 70, 45, 70 kg ha-1 to N, P and K 110, 75, 110 kg ha-1 caused the increase of dry matter content in bulbs from 14.6% to 15,5%. 3.2 Shelf Life Parameters of Onion Bulb 3.2.1 Bulb storage rots percentage P and K application had a highly significant (p<0.05) effect on the bulb rotting percentage during the storage time; while P and their interaction did not (Table 4). Application K had shown a highly significant (p<0.001) difference on the bulb storage rot percentage of onion plants (Table 4). The findings indicated that the maximum application of K at 120 kg ha-1 showed a significantly decreased in the bulb rot percentage which is about 53.5% when compared with the unfertilized plots. The results of these finding are supported by the reports of Singh and Dhankar (1989) recorded that rotting percentage was reduced considerably during storage in the bulbs produced by the application of 100 kg K2O ha-1. Similarly, Nandi et al. (2002) also recorded that the lowest rotting (7.60 %) with application of K at 180 kg ha-1 as compared to control. Table 4: Storage rots of bulb (%) as influenced by applications of P and K fertilizers. 3.2.3 Bulb Sprouts percentage Sprouting is physiological change that occurs on bulbs of onion in storage. P and K application had shown a significant (P<0.001) difference on percentage bulb sprouts. K application significantly decreased the sprouts percentage (Table 5). The maximum sprout of bulb recorded with control treatment, while the minimum sprouts occurred in the treatment of K at 40 kg ha-1 and further application had no significant effect on the bulb sprouts percentage. Similarly, Nandi et al. (2002) also recorded that the lowest sprouting (20.00%) with application of K at 180 kg ha-1 compared to control. Masalkar et al., (2005 a) also reported that sprouting of bulbs in storage had declined with successive increase of K.

4. Summary and Conclusions
The quality and storage life of onion plants were affected with application of P and K at different levels. Higher dry matter content (10.30% and 10.42%) at higher levels of P (92kg/ha) and K (120kg/ha) application, respectively; higher storage rot percentage; lower storage rot percentage (2.05%) at higher levels of K (120kg/ha); higher bulb weight loss; lower bulb weight loss (34.99%) at higher levels of K (120kg/ha); lower bulb sprouts percentage (7.59%) at 80kg/ha levels of K. In general, from marketable yield, post-harvest quality and storability point of view, P and K fertilization was very sound; especially for our country farmers where their production is once in a year. If these methods are integrated and well applied, year round production of this crop may not be required. In addition, problem of market glut could be stabilized with balanced costs from stored bulbs dispatch. Therefore, the result of this study has shown that P and K fertilization have a sound and promising impact for post-harvest quality that could be applied for onion production. However, this study was done using one cultivar under one location for one season alone, so it’s difficult to give general recommendation.

5. Acknowledgment
We would heartedly like to thank and praise the Lord Almighty God in giving us strength and wellbeing to successfully complete the study. We sincerely thank Jimma University for all necessary support. We also want to thanks all the data collectors who help us in data collection and for all their concern and moral support. Finally, all the reference materials used in this article are dully acknowledged