Effect of Nitrogen and Potassium Sulphate Fertilizers on Shelf-life of Onion (Allium cepa var. cepa)

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Published on International Journal of Agriculture & Agribusiness
Publication Date: December, 2019

Solomon Kassaye and Nigussie Dechassa
Sirinka Agricultural Research Centre, Woldia, Ethiopia. P. O. Box 74
Haramaya University, Dire Dawa, Ethiopia. P. O. Box 138

Journal Full Text PDF: Effect of Nitrogen and Potassium Sulphate Fertilizers on Shelf-life of Onion (Allium cepa var. cepa) (Studied at Sirinka, North-Eastern Ethiopia).

Abstract
A field experiment was conducted at the experimental site of Sirinka Agricultural Research Center, Ethiopia, during the 2009/10 off-season. The objectives of the experiment were to investigate effect of nitrogen (N) and potassium sulphate (K2SO4) on shelf life of Onion (Allium cepa L. var.cepa) bulbs. Treatments comprised five by three factorial combinations of N (0, 50, 100, 150 and 200 kg N ha-1) and K (0, 346 and 692 kg K2SO4 ha-1) laid out using a randomized complete block design with three replications. Analysis of the storage data revealed that application of 200 kg N ha-1 significantly increased rotten, sprouted and total loss by 50, 74 and 62% over the control, respectively. On the contrary, application of 692 kg K2SO4 ha-1 significantly decreased rot, sprout and total loss by 25, 15 and 15%, respectively, over the control. However, N and K2SO4 interaction did not influence any of the storage parameters. In addition, increasing the rate of N does not influence storability and shelf life of onion bulbs while K2SO4 decreases storage loss. However, as the study was done using only one cultivar in one location for one season, it would be worthwhile to repeat it in order to arrive at a sound conclusion.

Keywords: Bulb, Shelf life, Storage and Yield.

1. INTRODUCTION
Onion (Allium cepa L.var.cepa) is a member of the Amaryllidaceae family and it is one of the most important vegetables in the world, whose utility is ranked second to tomatoes (Mogren et al., 2007). Recent estimate indicate that there are about 750 species in the genus Allium, among which Onion, Japanese bunching onion, leek and garlic are the most important edible Allium crops (Rabinowitch and Currah, 2002). Onion is believed to have originated in Afghanistan, the area of Tajikistan and Uzbekistan,Western Tien Shan and India while Western Asia and the areas around the Mediterranean Sea are considered secondary centres of development (Malik, 2000).
Onions are popular vegetables among most of the world’s population. They are valued for their distinct pungent or mild flavours and form essential ingredients of the cuisine of many regions. Since onions are used by rich and poor alike, and are often called the ‘poor man’s food’. Onion has its own distinctive flavour and is widely used as a condiment in preparation of soups, meat dishes, salads, sandwiches and is cooked alone as vegetable. It is also processed as pickle, chutney, sauces and consumed in dehydrated form (Muhammad, 2004).
Ethiopia has a variety of vegetables and root and tuber crops adaptable to specific locations and altitudes due to great variety of climate and soil types. Land acreage under onion production is estimated to be about 17588 hectares and the national average yield which is 9.63 t ha-1 (CSA, 2010) as compared to world average of 17.30 t ha-1 (FA0, 2010).
Post-harvest losses in onion are classified as quantitative that is reduction in weight through loss of moisture, loss in dry matter by respiration and qualitative i.e., quality is deteriorated by changes in nutritional values and taste. Post-harvest losses can by reduced by improving production technique such as fertilizer management, harvesting practices, and curing just after harvesting (Kabir, 2007). Optimizations of such practices results in significant decrease in post-harvest losses 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 onions can be produced through application of well-balanced fertilizers (Murashkina et al., 2006).
Nitrogen application can affect onion quality and influence storage loss. Late-season and high dose application of nitrogen, even during bulbing, increases growth of leaf blades (Brewster, 1994); delaying ripening of bulbs (Schwartz and Bartolo, 1995) and adversely affecting storability (Coolong et al., 2008). Potassium plays a pivotal role in plant growth and development. Like other vegetable crops, onion is very responsive to potassium fertilization. It has a crucial role in the energy status of the plant, translocation and storage of assimilates and maintenance of tissue water relation. In addition, K plays a key role of crop quality. It improves size of fruit and stimulates root growth. It is necessary for the translocation of sugars and formation of carbohydrates.
Research done in other countries showed that different onion cultivars at the rate of 0, 40, 120 and 180 kg N ha-1 reported storage loss over 144 days with a storage loss of 32.13-50.07% (Shukla et al., 1989). Moreover, Singh et al. (1994) conducted trials on onion cv ‘Pusa Red’. N was applied at the rate of 0, 40, 80,120, 160 and 200 kg N ha-1. They concluded that plant mortality increased and storage quality of onion bulbs at room temperature decreased with increasing rates of nitrogen. Moreover, Lordachcu et al. (1984) grew onion cv. ‘Gigant der stullgar’ in 3 years trials at different nitrogen and potassium levels and stored the bulbs after harvest at 1°C for up to 240 days. They reported that the least storage losses were found in bulbs grown at 80:40 kg N: K2O ha-1. Ghulam et al. (2010) studied the influence of different K doses i.e. 0, 25, 50, 75 and 100 kg K2O ha-1 on post-harvest quality of onion using cultivar ‘Swat-1’. The results indicated that significant minimum disease incidents, bulb sprouting and weight loss at room temperature were at 100 K2O ha-1. The mean loss of weight in bulbs stored at room temperature was 47.70%. Similarly, sprouting in bulbs stored at room temperature was 18.70%. It was noted that with increase in potassium dozes, decrease in weight loss and sprouting was observed.
Considering this systematic study on fertilization to improve quality of onion is lacking. This is one of the problems of farmers at Sirinka (the study site) as well as many parts of Ethiopia. Hence, considering that Ethiopian soils are deficient in N and K and realizing the importance of fertilizers in onion production, the use of inorganic fertilizers is important for enhancing quality of the crop. In addition, fertilizer practices in the study area have been mainly based on blanket recommendations. Moreover, very little information is available in the country with regard to the influence of potassium fertilizers on the growth, yield and quality of onion. Thus, systematic investigations in to the response of onion to applied N and K fertilizers under specific agro-ecologies is very important to come up with relevant recommendations in order to help farmers to increase the productivity and quality of onion. With this background, the proposed study was conducted with the following Objectives. Objectives; To determine the level of nitrogen and potassium sulphate that may lead to prolonged shelf life of onion bulbs.

2. MATERIALS AND METHODS
2.1 Field experiment
The field study was conducted during the off-season from September 2009 to May 2010 for fieldwork and storage study from May 11 to August 10, 2010 at Sirinka Agricultural Research centre located at 11021′ N latitude and 39038′ E longitude and at an altitude of 1680 masl The mean annual rainfall was 1204.60 mm and average annual minimum and maximum temperatures were 11.200C and 25.600C, respectively (SARC, 2010). Onion cultivar ’Adama Red’ was used in the experiment. Urea and potassium sulphate were used as a sources of nitrogen and potassium, respectively. The treatments comprised a factorial combination of five levels of nitrogen (0, 50, 100, 150 and 200 kg N ha-1) and three levels of potassium sulphate (0, 346 and 692 kg ha-1). The experiment was laid out as a randomized complete block design with three replications.
Seeds were sown in the nursery on 2 November 2009 and seedlings were transplanted from the nursery to the field on 4 January 2010 at the spacing of 0.20 m between rows and 0.10 m between plants. In each plot, there were ten rows and the total number of plants in each row was 30. Nitrogen was applied in three splits i.e., at transplanting, 15 and 30 days after transplanting. All plots received basal dressing of phosphorus at the rate of 92 kg P which is recommended for onion (Lemma and Shimeles, 2003). DAP was used as a source of phosphorous. All the required potassium was applied at the time of transplanting. Weeding, cultivation and other recommended agronomic and plant protection practices were done at the appropriate time following the practice of the research centre. Harvesting was done during first week of May 2010, when the bulbs were fully matured and about 70% tops of the bulbs were dried. Bulbs were pulled out by hand and weight was recorded separately in each plot.

2.2 Storage experiment
Cured onion bulbs from the field were collected. Each treatment of five kg marketable onion bulbs was replicated three times and completely randomized by putting marketable bulbs on wooden shelves 0.50 m above the floor of the store. The bulbs were thus subjected to natural ventilation. These were periodically re-randomized to avoid spatial variation within the store. The store with 5 m length, 5 m width and 8 m height with thatched roofing and the shelves were made from wooden materials. The bulbs stored on shelves in a storehouse at ambient atmospheric condition from May 11 to August 10, 2010 for about three months. The average daily maximum and minimum temperatures during the three-month storage periods were 29.69 0C and 15.80 0C, respectively, and the average daily relative humidity was 38-52% (Figure 1 and 2).

Figure 1. Relative humidity data of the storage house for the months May 11 to August 10, 2010.

Figure 2. Temperature data of the storage house for the months May 11 to August 10, 2010

2.3 Data Collection and Measurements
In the storage study, the following observations were recorded in 15 days interval for the three months storage period.

Percentage of weight loss of bulbs: percentage of weight loss of bulbs was determined using the methods described by Waskar et al. (1999). The weight loss data was calculated from 5 kilogram of onion bulbs, which was randomly taken per treatment.

Percentage of weight loss (WL) was calculated using the formula:
WL (%) = (Wi-Wf)*100
Wi
Where: Wi = initial weight
Wf = final weight

Percentages of bulbs rotten: percentages of bulbs rotten were taken as cumulative data based on the number of bulbs rotten in the biweekly assessment. The rotten bulbs were discarded after each count to avoid double counting.

Percentages of bulbs sprouted: percentages of bulbs sprouted were taken as cumulative data based on the number of bulbs sprouted in the biweekly assessment. Bulbs that sprouted and were rotten at the same time were classified as sprouted. The sprouted bulbs were discarded after each count to avoid double counting.

2.4 Statistical Analysis
The data were subjected to Analysis of Variance (ANOVA) and correlation coefficients were calculated for selected parameters using SAS (Statistical Analysis Software) software version9.0 (SAS Institute, 2002). LSD (Least Significant Difference) test is used to separate means whose treatment effect is significant.

3. RESULTS AND DISCUSSION
3.1 Effect of nitrogen and potassium sulphate on storage rot loss
Significant (p<0.01) effect of nitrogen application was observed on percentage of bulb rotting at different days after storage. At the end of three months storage, increasing the level of nitrogen from 0 to 50, 100, 150 and 200 kg N ha-1, increased loss of bulbs due to rotting by about 27, 35, 39 and 50%, respectively. The highest percentage of bulbs rotting (25%) were observed in plots fertilized with the highest rate of nitrogen, while the minimum rotting (16%) was recorded in the control treatment (Table 1). The results are in agreement with those of Mozumder et al. (2007) who indicated that rapid deterioration of bulbs were observed in response to increasing N fertilization up to 175 kg N ha-1, while being lower in the control treatment. The results are also in conformity with that of Madan and Sandhu (1985) and Singh and Kumar (1994) who concluded that the plant mortality increased and storage quality of bulbs decreased with increasing N from 0 up to 200kg N ha-1. Similar results were also reported by Dhankar and Singh (1991) and Sebsebe (2006) who found that increasing the rate of applied N from 0 up to 150 kg N ha-1 led to significant increases in storage loss of onion. Generally, application of high doses of N resulted in higher storage losses. This might be due to the phenomenon that higher rates of N encourage plants to produce large bulbs with soft succulent tissues that make them susceptible to attack by disease-causing microorganisms and produce bulbs with thick necks that are difficult to cure. This, in turn, would reduce storage life (Jone and Mann, 1963; El-Tantawy and El-Beik, 2009). Table 1. Effect of N and K2SO4 on rotten loss of marketable bulbs of onion in ambient storage at Sirinka. Treatments Rotten loss (%) 15 DAS 30DAS 45DAS 60DAS 75DAS 90DAS N (kg ha-1) 0 1.72d 2.86d 4.04d 8.60d 14.57c 15.74c 50 2.56c 3.60d 5.71c 10.66c 16.43c 19.95b 100 4.51b 5.02c 6.60c 14.04b 16.84bc 21.25b 150 5.10b 6.39b 8.09b 15.80ab 19.64ab 21.88ab 200 5.44a 7.92a 9.32a 16.17a 20.90a 23.60a LSD (5%) 0.82 1.01 1.15 2.06 2.98 2.35 K2SO4 (kg ha-1) 0 5.22a 7.11a 9.61a 14.85a 18.22 21.24 346 3.58b 4.62b 5.84b 13.12b 17.54 21.03 692 2.80c 3.75c 4.80c 11.20c 17.27 19.19 LSD (5%) 0.63 0.78 0.89 1.59 NS NS CV(%) 21.83 20.17 17.70 16.31 17.44 11.86 Means within a column for a factor sharing common letter(s) are not significantly different at 5%; NS=non- significant; *, **=significant at 5% and 1%, respectively; DAS=days after storage. The analysis of variance showed that rotten loss percentage was significantly (P<0.01) decreased with application of potassium sulphate compared to control treatment. The three months storage assessment result indicated that application of 346 and 692 kg K, depressed loss of bulbs due to rotting by about 12 and 25%, respectively. The highest percentage of bulb rotting (15%) was observed in the control treatment whereas the lowest (11%) was recorded in plots fertilized with the highest rate of potassium sulphate (Table 1). The results obtained in this study in accord with those recorded of Mozumder et al. (2007)and Ghulam et al. (2010) who concluded that the maximum rotten loss of bulbs was observed in the control treatment (0 K2SO4 ha-1), while the minimum was recorded for the rate of 208 kg K2SO4 ha-1. Agrees with that of Nosov (2007) and Mandal et al. (2008) who reported that application of potassium sulphate fertilizer reduced percent disease index by more than 10% compared with nil potassium sulphate treatment. This might be the presence of sulfur in potassium sulfate, which may have enhanced the disease tolerance in the plants. Sulfur was highly toxic to many fungal pathogens (Cooper and Williams, 2004). Generally application of potassium sulphate significantly decreased loss of bulbs due to rotting. This might be due to the role of potassium sulphate in decrease of storage losses, enhancement of shipping quality and extending of shelf life of onion (Singh and Verma, 2001).

3.2 Effect of nitrogen and potassium sulphate on sprout loss Nitrogen fertilization had significant (P< 0.01) main effect on bulb sprouting at all assessment periods. At the end of three months storage, increasing the level of nitrogen from 0 to 50, 100,150 and 200 kg N ha-1, increased loss of bulbs due to sprouting by 27, 43, 61 and 74%, respectively. The highest percentage of loss (20.81%) due to bulb sprouting was observed in the plots fertilized with the highest rate of nitrogen, while the minimum (11.98%) was recorded for the plots fertilized with no nitrogen (Table 2). The results of this study are consistent with those obtained by Bhalekar et al. (1987) and Sebsebe (2006) who reported that the highest incidence of sprouting loss was recorded for bulbs harvested from the treatment of 150 kg N ha-1, while the least was observed for bulbs harvested from plots not fertilized with nitrogen. Similar results were also reported by Bottcher and Kolbe (1975) and Batal et al. (1994) who showed that nitrogen application increased sprouting of onion bulbs under normal storage conditions. This result is also supported by that of Celestino (1961) who reported that, under common storage conditions, bulbs obtained from plots fertilized with 60 to 120 kg N ha-1 above 100 kg N ha-1 sprouted twice as much as those obtained from plots not fertilized with nitrogen. Generally higher rates of nitrogen increased sprouting of bulbs. This might be due to higher concentration of growth promoters than inhibitors in the bulbs of N fertilized plants that keep it growing and high dose of N produced thick-necked bulbs that had higher sprouting in storage due to greater access to oxygen and moisture to the central growing point (Dankhar and Singh, 1991). Table 2. Effect of N and K2SO4 on sprout loss of marketable bulbs of onion in ambient storage at Sirinka. Treatments Sprout loss (%) 15 DAS 30DAS 45DAS 60DAS 75DAS 90DAS N (kg ha-1) 0 1.66e 3.20e 6.98d 9.26d 9.92d 11.98d 50 2.96d 5.69d 7.89d 11.24c 12.21c 15.17c 100 5.27c 7.38c 9.34c 13.05b 15.39b 17.19b 150 6.43b 9.54b 12.16b 16.64a 18.01a 19.31a 200 7.39a 11.63a 14.43a 17.58a 18.85a 20.81a LSD (5%) 0.68 0.95 1.06 1.24 1.82 1.55 K2SO4 (kg ha-1) 0 5.67a 8.75a 11.38a 14.95a 16.62a 18.23a 346 4.56b 7.56b 10.00b 13.41b 15.00b 17.00b 692 4.00c 6.16c 9.10c 12.30c 13.00c 15.45c LSD (5%) 0.53 0.74 0.82 0.96 1.41 1.20 CV (%) 14.88 13.12 10.80 9.50 12.70 9.50 Means within a column for a factor sharing common letter(s) are not significantly different at 5%; NS=non- significant; *, **=significant at 5% and 1%, respectively; DAS=days after storage. Potassium sulphate fertilization had significant (P<0.01) main effect on bulb sprouting at all storage assessment periods. At the end of three months’ storage period, application of 346 and 692 kg K2SO4 ha-1, decreased loss of bulbs due to sprouting by 7 and 15%, respectively. The highest percentage of bulbs sprouting (18.23%) was observed in the control treatment, while the minimum (15.45%) was recorded in plots fertilized with the highest rate of potassium sulphate (Table 2). These results are corroborated by those of Ghulam et al. (2010) who reported that the maximum sprouting of bulbs was recorded for bulbs harvested from plots that received no potassium sulphate whereas the minimum was recorded for bulbs that received potassium sulphate at the rate of 208 kg K2SO4 ha-1. These results confirm also those of Masalkar et al.(2005) who reported that sprouting of bulbs in storage declined with successive increase in potassium sulphate supply. High percentage of sprout loss was observed for bulbs produced without potassium sulphate application. This might be attributed to the phenomenon that lower uptake of potassium sulphate by the plants during growth may lead to early physiochemical changes in bulbs like sugar formation and may cause early sprouting (Celestino,1961). Minimum percentage sprout loss was obtained from bulbs produced under potassium sulphate application most probably due to increased concentration of inhibitors apparently by facilitating the release of potassium ions from the guard cells (Sundarasan, 1979).

3.3 Effect of nitrogen and potassium on total weight loss The total weight loss of onion bulbs at different periods of storage was significantly (P < 0.01) influenced by N fertilization. The three months storage result showed that increasing level of nitrogen from 0 to 50, 100, 150 and 200 kg N ha-1 increased total weight loss by 28,40, 50 and 62%, respectively. The highest percentages of total weight loss of marketable bulbs were obtained from bulbs produced fertilized with 200 kg N ha-1 whereas the lowest was recorded from 0 level of nitrogen application (Table 3). This finding also supports the earlier findings of Bhalker et al. (1987) and Dhankar and Singh (1991) who reported that increasing N from 0 up to 150 kg N ha-1 increased weight losses of stored bulbs. These results are also in accord with that of Mozumder et al. (2007) who reported that rapid deterioration was observed in bulbs produced with the N fertilizer rate of 175 kg N ha-1. The increased bulb weight loss due to increased level of N supply may be attributed to the higher moisture content in the bulbs which may lead to decrease shelf-life due to rapid metabolic activity and moisture loss and shrinkage in storage (El-Tantawy and El-Beik, 2009). Table 3. Effect of N and K2SO4 on total weight loss of marketable bulbs of onion in ambient storage at Sirinka. Treatments Total weight loss (%) 15 DAS 30DAS 45DAS 60DAS 75DAS 90DAS N (kg ha-1) 0 3.55e 6.12e 11.06e 18.11d 25.30d 26.86d 50 5.69d 9.35d 13.63d 22.16c 29.44c 34.27c 100 9.94c 12.46c 15.97c 27.35b 33.03b 37.60b 150 11.69b 15.99b 20.29b 32.69a 38.46b 40.34ab 200 13.00a 19.60a 23.78a 34.00a 40.55a 43.56a LSD (5%) 0.99 1.37 1.51 2.34 3.50 3.23 K2SO4 (kg ha-1) 0 10.88a 15.86a 21.00a 29.80a 34.84 39.48a 346 8.14b 12.18b 15.84b 26.52b 33.42 36.47b 692 7.30c 10.06c 14.00c 24.27c 31.80 33.63c LSD (5%) 0.76 1.06 1.17 1.82 NS 2.50 CV (%) 11.63 11.19 9.21 9.04 10.86 9.15 Means within a column for a factor sharing common letter(s) are not significantly different at 5%; NS=non- significant; *, **=significant at 5% and 1%, respectively; DAS=days after storage. Weight loss percentage was significantly (P<0.01) decreased with application of potassium sulphate compared to control treatment. At the end of the assessment period, the decrease of weight loss was 8 and 15% more in case using 346 and 692 kg K2SO4 ha-1 application rate, respectively. The highest percentages of bulb weight loss was observed in the control treatment whereas the minimum was recorded in plots fertilized with the highest rate of potassium sulphate (Table 4). These findings are supported by those of Dhanker and Singh (1991) and Ghulam et al. (2010) who reported that the minimum weight loss of bulb was obtained at 208 kg K while the maximum was noted in control treatment. The findings of the present study support those of lordachcu et al. (1984) and Madan and Sandhu (1985) who reported that application of 125 kg K2SO4 ha-1 improved storage quality of onion. The low weight loss in potassium fertilized plots could be attributed to the role of potassium in enhancing translocation of photosynthates to storage organs, which are bulbs, in this case (Brewster and Butler, 1989).

4. SUMMARY AND CONCLUSION
Crop growth, development and their subsequent yield are governed by the availability of optimum levels of water and nutrients and favorable environmental conditions. The maximum yield achievement by crop relies on the application of the correct level of fertilizers. In addition to increment of yield, improvement of the shelf life has to be achieved during the production stage, coupled with appropriate post-harvest handling practices. Therefore, the present study was conducted to determine the level of nitrogen and potassium sulphate that may lead to prolonged shelf-life of onion (Allium cepa var.cepa) bulbs using five levels of N (0, 50, 100, 150 and 200 kg N ha-1) and three levels of K2SO4 (0, 346 and 692 kg K2SO4 ha-1) using a randomized complete block design with three replications. The study was conducted during the off-season, from September 2009 to May 2010 at Sirinka Agricultural Research Center, Wello North-East Ethiopia on a sandy clay loam soil. After harvest, five kg of marketable onion bulbs were stored from each treatment for three months at SARC in ambient storage atmosphere of 29.69oC maximum and 15.370C minimum average temperatures and 38-52% relative humidity during the storage period. Analysis of storage experiment revealed that fertilization of N significantly increased percentage of rotten, sprouted and total weight loss during all part of the assessment periods with the highest loss recorded at 200 kg N ha-1, while the minimum at control. At the end of three months storage, application of 200 kg N ha-1 result in 50, 74 and 62% higher rotting, sprouting and total weight loss over the control, respectively. On the contrary, fertilization of K2SO4 significantly decreased percentage of rotten, sprouted and total weight loss almost during all part of the assessment periods with the highest loss recorded at control and the minimum at 692 kg K2SO4 ha-1. At the end of three months storage, application of 692 kg K2SO4 ha-1 significantly decreased rotten, sprout and total weight loss by 25, 15 and 15%, respectively, over the control. On the other hand, N and K2SO4 interaction did not influence rotten, sprouted and total weight loss during all part of the assessment periods. In conclusion, the result of this study has showed that increasing the rate of N does not influence storability and shelf life of onion bulbs while K2SO4 ha-1, decreases storage loss. However, as the study was done using only one location for one season, it would be worthwhile to repeat it in order to arrive at a sound conclusion.

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