Response of Nitrogen and Potassium Sulphate Fertilizers on Yield and Yield Component of Onion (Allium cepa var. cepa)

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

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

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

Abstract
A field experiment was conducted at Sirinka Agricultural Research Center, Ethiopia, during the 2009/10 off-season to investigate effect of nitrogen and potassium sulphate on the growth and yield of Onion (Allium cepa 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. Results showed that application of N significantly increased number of leaves, leaf length, plant height, mean bulb weight, bulb diameter, days to maturity and total bulb yield. Application of K significantly increased all parameters that affect N except days to maturity, mean bulb weight, marketable, unmarketable and total bulb yield. Moreover, the interaction effects of N and K2SO4 significantly increased marketable, unmarketable and total bulb yield. The increments of marketable and total bulb yield recorded from the application of 150 kg N ha-1 and 346 K2SO4 ha-1 were about 155 and 124%, respectively, over the control. The result of this study has shown combined application of 150 kg N ha-1 and 346 kg K2SO4 ha-1 is a good compromise for yield of onion bulbs.

Keywords: Bulb, fertilizer, onion and yield.

1. INTRODUCTION
Onion (Allium cepa 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). 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’ (Muhammad, 2004). Onion has great economic importance due to its medical and dietetic values since ancient times. Onion lowers the cholesterol content in blood serum and thus are of value against heart trouble (Griffiths et al., 2002).
Ethiopia has a variety of vegetable 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). At present, the major producers of vegetable crops are small-scale farmers, production being mainly rain fed and few under irrigation (CSA, 2010). Many diverse and complex biotic, a biotic and human factors have contributed to the existing low productivity of onion. Like other crops, low soil fertility is one of the critical factors limiting productivity of onion crop (Lemma and Shimeles, 2003).
Traditionally, farmers maintain or improve farmland soil fertility using different management practices such as fallowing, use of cattle manure, intercropping and crop rotation. The use of some of these cultural practices as a means of maintaining or improving soil fertility is limited to a great extent due to small land holding of farmers (Reijnties et al., 1992). Available statistical data indicate that the average household land holdings in the country in general and in Amahara region in particular are about 1.01 and 0.75 ha, respectively (CSA, 2010). Farmers with small plots of land are unable to maintain the farmland soil fertility through cultural practices, as they are using their land exhaustively (Reijnties et al., 1992). Under such situations, therefore, the use of inorganic fertilizers to optimize productivity becomes indisputable in crop production and hence, onion cannot be an exception.
Nitrogen and potassium are the most important among the elements that are essential to plants. Plants utilize these nutrients in large quantities. The deficiency of these elements is manifested in the detrimental effects on the growth and development of the plants (Tisdale et al., 1995). Furthermore, high mobility of N and high affinity of K for chemical reactions and fixation in the soils put these plant nutrients on the priority list in soil fertility management studies.
Soil fertility studies conducted at different locations in Ethiopia for different crops have shown that on a sandy loam soil in a semi-arid region of Ethiopia, irrigated onion plants benefited from application of 90-120 kg N ha-1 compared to unfertilized crops (Aklilu, 1997). Although the total potassium content of soils is usually many times greater than the amount taken up by crops in a growing season, in most cases only a small fraction is available to plants (Tisdale et al., 1995). In addition, potassium deficiency is more localized than that of the other two primary nutrients (Nitrogen and Phosphorus), so that in some areas there is no response, whereas on other soils large potassium responses are obtained (Anderson, 1973).
A lot is known about soil potassium in different parts of the world. However, little is known about the status of this nutrient in Ethiopian soils (Tekalign Mamo and Haque, 1988). Early indications of favorable potassium supply except in a few acutely deficient soils have led researchers and farmers to ignore needs for potassium in many parts of East Africa (Anderson, 1973). Experiments in the past few years have indicated potassium deficiency to be much more widespread than hitherto known and the need for potassium application increases in proportion to the intensity of cropping even in semi-arid areas where potassium applications traditionally have given least response (Anderson, 1973).
Systematic study on fertilization to improve the yield and yield component of onion is lacking. This is one of the problems of farmers at Sirinka 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 both yield and yield component of the crop. However, available information regarding response of the crop to nutrient application is limited in the study area. 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 nitrogen and potassium fertilizers on the growth and yield of onion. Thus, systematic investigations in to the response of onion to applied N and K2SO4 fertilizers under specific agro-ecologies is very important to come up with relevant recommendations in order to help farmers to increase the productivity and yield component of onion. Objective; To investigate the effects of nitrogen and potassium sulphate on the yield and yield component of onion.

2. MATERIALS AND METHODS
The field study was conducted during the off-season from September 2009 to May 2010 for field study from May 11 to August 10, 2010 at Sirinka, which is located at 437 km North East of Addis Ababa. Sirinka is 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 11.20 0C and 25.60 0C, respectively (SARC, 2010) (Appendix Table 2). Onion cultivar ‘Adama Red’ was used as a test crop. Urea (46% N)] and potassium sulphate (52% K2O and 18% S)] 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. A 3 m x 2 m plot was used for each experimental unit. The blocks were separated by 2 m space whereas the space between each plot within a block was 1 m. 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. Data like plant height, number of leaves per plant, leaf length, days to maturity, bulb diameter, mean marketable bulb weight, marketable bulb yield, unmarketable bulb yield and total bulb yield were recorded from ten randomly sampled plants for each plot from the middle eight rows.

2.1 Soil Sampling and Analysis
Soil sampling was done before planting and at harvesting. Soil samples were taken randomly in a zigzag pattern from the experimental plots at the depth of 0-30 cm. Sixteen soil cores were taken by an auger from the whole experimental field and combined to a composited sample in a bucket. The soil was broken in to small crumbs and thoroughly mixed. From this mixture, a sample weighing one kilogramme was filled in to a plastic bag for analysis. After harvest, soil samples were collected from each experimental unit in a similar way (Warren, 2004). The soil samples were air dried and sieved through a 2 mm sieve. Then, soil pH was determined by diluting the soil in a 0.01 M CaCl2 solution in the ratio of 1 soil volume to 2.5 volume of the CaCl2 solution. Thus, twenty-five ml of the 0.01 M CaCl2 solution was added into soil sub-samples each weighing 10 g. After equilibrating for 2-3 hours, the suspensions was filtered and the pH measured by a glass electrode. Texture of the soil was determined by sedimentation method. Total nitrogen of the soil was determined by the micro-Kjeldhal procedure (Dewis and Freitas, 1970). Organic carbon was determined by the method of Nelsen and Sommers (1982). Available phosphorus content of the soil was determined by extraction with 0.5 M NaHCO3 (Olsen et al., 1954). Phosphorus in the extracts was determined with atomic absorption spectrophotometer calorimetrically according to the molybdenum blue color method of Murphy and Riley (1962). Exchangeable potassium was determined with a flame photometer after extracting K from the soil with 0.5 N ammonium acetate (Pratt, 1965).

2.2 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 version 9.0 (SAS Institute, 2002). LSD (Least Significant Difference) test is used to separate means whose treatment effect is significant.
3. RESULTS AND DISCUSSION
3.1 Selected Soil Chemical Properties of the Experimental Field
Results of the soil analysis of the study area showed that the soil to be sandy clay loam in texture with neutral (pH 6.8 and 6.9) before planting and after harvest, respectively. According to the limit suggested by Walkley and Black (1934), the OC (1.43%) or (2.46%) OM and OC (1.19%) or (2.07%) OM content of the soil is rated as very low, before planting and after harvest, respectively. According to the rating suggested by Landon (1992) the N content (0.15 and 0.09%), before planting and after harvest, respectively, was as low. According to the rating suggested by Olsen et al. (1954), the P content (15.50 and 18.50 ppm) before planting and after harvest, respectively, was rated as medium. According to the rating suggested by Landon (1991), the CEC (39.13 and 32.97 cmol (+) kg-1) and K (1.01 and 1.21cmol (+) kg-1) before planting and after harvest, respectively rated as high (Table 1).

Table 1. Soil physical and chemical properties of the experimental site before planting and after harvest.

3.2 Effect of Nitrogen and Potassium Sulphate on Phenological and Growth Parameters
Days to maturity
Nitrogen significantly (P<0.01) prolonged days to maturity. Increasing N level from 0 to 50 kg N ha-1 did not prolong days to maturity. Moreover, plants supplied with the highest three levels of N were in statistical parity in terms of days to maturity and prolonged days to maturity by 3, 5 and 6%, respectively, as compared with the unfertilized control. However, application of potassium sulphate did not influence days to maturity (Table 2).
Similar results were reported by Khan et al. (2002) who illustrated that high N levels above 100 kg N ha-1 delayed bulb maturity. In agreement with the result of this study, Kumar et al. (1998) also observed that N at the rate of 150 kg ha-1 recorded the highest number of days with regard to the time to reach bulb maturity. The result is also in line with the findings of Islam et al. (1999) who reported that application of 180 kg N ha-1 prolonged the growing period of onions. The delay in maturity due to nitrogen fertilizer application might be attributed to the prolonged canopy growth in response to higher nitrogen doses to maintain physiological activity for an extended period and thereby continuing photosynthesis (Brewster, 1994; Marschner, 1995).

Table 2. Effect of applied N and K2SO4 rates on days to maturity, leaf length, leaf number and plant height of Onion.

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.

Number of leaves per plant
Application of nitrogen highly significantly (P<0.01) increased the number of leaves per plant. The highest numbers of leaves (12) were obtained from plants that received 200 kg N ha-1. However, the smallest numbers of leaves per plant (7) was observed from plants in the control treatment (0 level of nitrogen). Plants supplied with 50, 100, 150 and 200 kg N ha-1 produced 15, 30, 44 and 58% more number of leaves, respectively, than plants that were not supplied with nitrogen at all (Table 2). These results are in accord with those of Ghaffoor et al. (2003), Yadav et al. (2003) and Mozumder et al. (2007) who reported that application of 150 kg N ha-1 significantly increased number of leaves per plant. Similar results were also reported by Syed et al. (2000), Abdulsalam and Hamaiel (2004) and Nasreen et al. (2007) who observed that the maximum number of onion leaves per plant in plots supplied with nitrogen fertilization in the range of 100-125 kg N ha-1. The results are also corroborated by those of Islam et al. (1999) who reported that maximum number of leaves was recorded for plants grown with the supply of 160 kg N ha-1. The increment in vegetative growth due to application of nitrogen may be attributed to the pronounced role of nitrogen in plant metabolism as nitrogen is a constituent of proteins, enzymes, hormones, vitamins, alkaloids and chlorophyll, which may have led to an increment in plant metabolism and vegetative growth expressed as number of leaves per plant (Kumar et al., 1998).
The leaf number per plant varied significantly (P<0.01) due to different doses of potassium sulphate application. Increasing K2SO4 from 0 to 692 kg ha-1 showed consistent increment of number of leaves per plant. Thus, compared to plants that were not supplied with potassium sulphate, plants that were treated with potassium sulphate at the rates of 346 and 692 kg K2SO4 ha-1 produced 12 and 26% more number of leaves (Table 2). The results of the experiment agree with those of Yadav et al. (2003), Abd El-Al et al. (2005), Mozumder et al. (2007) and Islam et al. (2008) who reported that application of potassium sulphate in the range of 300-365 kg K2SO4 ha-1 significantly increased the number of leaves per plant. The result is also in line with the findings of El-Bassiony (2006), EL-Desuki et al.(2006b), Islam et al. (2007) and Aisha and Taalab (2008) who indicated that the highest number of leaves was recorded at 533, 714, 667 and 714 kg K2SO4 ha-1. Generally, the increase in plant growth parameters caused by high rates of potassium might be due to its fundamental role as potassium for enhanced metabolism and plant growth, which further stimulates uptake and utilization efficiency of other nutrients from the soil (Yadav et. al., 2005; Aisha et al., 2007).

Leaf length
Highly significant (P<0.01) difference in leaf length were observed due to increase in the application rates of N. Increasing N from 0 to 200 kg N ha-1 showed consistent increment of leaf length. The highest leaf length (56.43 cm) was observed at 200 kg N ha-1, while the lowest (41.76 cm) at control. It is also clear that leaf length were increased by 10, 19, 29 and 35% due to application of 50, 100, 150 and 200 kg N ha-1 compared to the control, respectively. On the contrary, there was no significant difference (P>0.05) among the treatments in leaf length with application of potassium sulphate (Table 2).
These results are in harmony with those obtained by Kumar et al. (1998), Singh and Chaure (1999), Ghaffoor et al. (2003) and Jilani et al. (2004) who observed that the maximum leaf length was recorded at application of nitrogen in the range of 150-160 kg N ha-1, while the minimum was recorded at control level of N. Similar observations were also obtained by Muhammad (2004) who reported that application 200 kg N ha-1 enhanced leaf length. This increment of leaf length by applied N in part could be due to major function of N contributing to increasing number and size of leaves and also gives dark color to the leaves (Marschner,1995; Gupta and Sharma, 2000).

Plant height
Plant height significantly (P<0.01) increased in response to increasing N fertilizer application. The tallest plants (63.64 cm) were recorded from those supplied with nitrogen at the rate of 200 kg N ha-1. Minimum plant height (46.64 cm) was observed in the control. There was also significant increase in height of onion plants that received lower levels of N. The increase of plant height was 11, 25, 30 and 36% more in case using 50, 100, 150 and 200 kg N ha-1, respectively (Table 2). The present finding in accord with those of Kumar et al. (1998), Ghaffoor et al. (2003), Yadav et al. (2003) and Mozumder et al. (2007) who reported that application of 150 kg N ha-1 significantly increased plant height. Similar results were reported by Syed et al. (2000), Khan et al. (2001 ) and Nasreen et al. (2007) who observed that application of nitrogen in the range of 90-120 kg N ha-1 increased plant height significantly over the control. The increment in vegetative growth due to application of nitrogen might be attributed to the role of nitrogen in plant growth through cell division and elongation (Brady, 1985; Marschner, 1995).
Application of potassium sulphate significantly (P<0.01) increased plant height of onion. Plants that received potassium sulphate at the rates of 346 and 692 kg K2SO4 were taller than those that were grown without potassium sulphate supply by 5 and 10%, respectively (Table 2). This result conforms to the finding of Yadav et al. (2003) and Mozumder et al. (2007) who reported that application of potassium sulphate in the range of 313-375 kg K2SO4 significantly increased plant height. These results are similar also to those of El-Bassiony (2006), EL-Desuki et al. (2006b) and Islam et al. (2007) who indicated that potassium sulphate application in the range of 533-1217 kg K2SO4 ha-1 had a significant effect on plant height of onion. Generally, the increase in plant growth parameters caused by potassium fertilization might be due to its beneficial effect of such level on plant growth and its fundamental role in plant growth (EL-Desuki et al., 2006b).

3.3 Effect of Nitrogen and Potassium Sulphate on Bulb Yield of Onion
Total bulb yield
Nitrogen and potassium sulphate fertilization had highly significant (p<0.01) interaction effects on bulb yield. Accordingly, the treatment combinations of N at 100, 150 and 200 kg N ha-1 with potassium sulphate at 346 and 692 kg K2SO4 ha-1 resulted in increased bulb yield by 52, 53, 124, 112, 64 and 82%, respectively, over the control. Significantly higher total bulb yield (32.16 t ha-1) was recorded in the treatment combination of 150 kg N ha-1 and 346 kg K2SO4 ha-1 closely followed by 150 kg N ha-1 and 692 kg K2SO4 ha-1 with yield (30.48 t ha-1), while the lowest bulb yield (14.37 t ha-1) was obtained at the combination of 0 level of nitrogen and potassium sulphate (Table 3).

Table 3. Interaction effects of applied N and K2SO4 on marketable, unmarketable and total yield of Onion.

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.

The results of this experiment are consistent with the findings of Shanmugasundaram (2000)and Yadav et al. (2003) who indicated that application of 150:375 kg N:K2SO4 ha-1 gave the highest bulb yield. Similarly, FAO (2000) report indicated that application of 150:375 kg N:K2SO4 ha-1 on acid acrisols in Nigeria gave best yield in onion. This result was supported well by the findings of Mitrache and Burileanu (1984) and Akhtar et al. (2002) who reported that the highest yield was obtained at 150:417 kg N: K2SO4 ha-1.These results are further supported by those of Islam et al. (2007) and Yadav et al. (2007) who concluded that application of 150:396 kg N:K2SO4 ha-1 produced the maximum bulb yield. In this study, total bulb yield showed positive and significant (p<0.01) correlations with plant height (r=0.73**), number of leaves per plant (r=0.66**), leaf length (r=0.74**), bulb diameter(r=0.59**), mean bulb weight (r=0.91**) and marketable bulb yield (r=0.99**). These suggests that N fertilization rates improve the above ground plant growth, delay maturity, improve the physiological capacity of the crop to mobilize and translocate photosyntate to the organs of economic value and increasing both bulb number and individual bulb size, which in turn increase the bulb yield (Table 5). The positive N x K interaction expressed in total bulb yield might probably be attributed to a possible function of K in increasing nitrogen use efficiency. Research showed that, without K, N efficiency declined, whereas when all nutrients were applied together K efficiency increased steadily (FAO, 2000; Khan et al., 2002; Aisha et al., 2007).

Marketable bulb yield
The interaction of applied N and K2SO4 resulted in a significant (p<0.01) difference on marketable yield. Thus, the treatment combinations of N at 50,100, 150 and 200 kg N ha-1 with potassium sulphate at 346 and 692 kg K2SO4 ha-1 resulted in increased marketable bulb yields by 23, 32, 74, 75, 170, 155, 99 and 123%, respectively, over the control. Significantly higher marketable yield (29.69 t ha-1) was recorded in the treatment combination of 150 kg N ha-1 and 346 kg K2SO4 ha-1, while the lowest bulb yield (11.00 t ha-1) was obtained at the combined application of 0 nitrogen and 0 potassium sulphate (Table 3). These trends of results are very much similar with the findings of Shanmugasundaram (2000) and Yadav et al. (2003) who concluded that application of 150: 375 kg N:K produced maximum marketable yield. These results are in line with the findings of Muhammad (2004) who reported that higher marketable yield was obtained at 200:375 kg N:K2SO4 ha-1. These results are also consistent with those of Mozumder et al. (2007) who showed that application of 150:208 kg N:K2SO4 ha-1 had significant effect on marketable yield. Moreover, marketable bulb yield was positively and strongly associated with marketable bulb number and mean bulb weight (r=0.72** and 0.92**, respectively) indicating that the treatments increased marketable bulb yield by increasing both bulb number and individual bulb size (Table 5). The increment in marketable yield due to application of nitrogen and potassium sulphate could be attributed to the increment in vegetative growth and rising photosynthesis production, which is associated with increment in bulb size and single bulb weight (Khan et al., 2002; Nasreen et al., 2007).

Unmarketable bulb yield
Interactive effects of nitrogen and potassium sulphate rates on unmarketable yield was of significant differences. Accordingly, the treatment combinations of N at 50,100, 150 and 200 kg N ha-1 with potassium sulphate at 346 and 692 kg K2SO4 ha-1 resulted in a decreased unmarketable bulb yield by 29, 45, 20, 20, 26, 29, 52 and 51%, respectively, over the control. Significantly, higher unmarketable yield (3.37 t ha-1) was recorded in the control while the minimum unmarketable bulb yield (1.62 t ha-1) was recorded in the treatment combination of 200 kg N ha-1 and 346 kg K2SO4 with unmarketable yield of 1.66 t ha-1 closely followed by 200 kg N ha-1 (Table 3).
The result agrees with that of Muhammad (2004) who reported that application of 200:375 kg N:K2SO4 ha-1 produced the minimum unmarketable yield. The same results were reported by AL-Moshileh (2001) who observed that application of 150:357 kg N:K2SO4 reduced the unmarketable bulb yield as compared with the control. Comparable results were obtained by Syed et al. (2000) and Ghaffoor et al. (2003) who indicated that control gave significant maximum unmarketable yield, while minimum unmarketable yields were associated with high rates of N and potassium sulphate. In addition, unmarketable bulb yield was positively associated with strongly negatively associated with mean bulb weight (r=-0.37) indicating that the treatments increased unmarketable bulb yield when individual bulb size decreased (Table 5). Generally, maximum unmarketable yields were recorded in unfertilized plots, which may be ascribed mainly to nitrogen and potassium deficiency and sub-optimal growth of the onion plants. This may have also resulted in plants that were weaker and prone to disease and other biotic and abiotic stresses, resulting in lower weight of bulbs (Khan et al.,2002).

3.4 Effect of Nitrogen and Potassium Sulphate on Yield Components of Onion
Bulb diameter
The analysis of variance for bulb diameter revealed highly significant (P<0.01) response to N fertilizer. Therefore, in response to increasing the level of nitrogen from 0 to 50, 100, 150 and 200 kg N ha-1, bulb diameter increased by 8, 16, 33 and 24%, respectively. The highest bulb diameter (6.08 cm) occurred at 150 kg N ha-1, while the minimum (4.58 cm) was recorded at control treatment (Table 4).
These results agree with those of Jilani et al. (2004), Muhammad (2004), Islam et al. (2007) and Mozumder et al. (2007) who reported that nitrogen application in the range of 120-200 kg N ha–1 was best for the maximum bulb diameter of onion. In this study, bulb diameter showed positive and highly significant correlation with plant height (r= 0.58), number of leaves (r= 0.65**) and leaf length (r= 0.58**) (Table 5). This study seems to indicate that application of nitrogen contributed towards the bulb diameter increment probably as a consequence of interception of more photosynthetically active radiation, efficient radiation use, more dry matter accumulation and partitioning to bulbs. Thereby increased diameter of bulbs (Nasreen et al., 2007). Generally, the increase in bulb diameter in response to increased nitrogen fertilization might be ascribed to the fundamental role of nitrogen plays in growth and expansion of bulbs, which may have also stimulated uptake and efficient utilization of other nutrients from soil (Jone and Mann, 1963; Bohloo et al., 1992).

Table 4. Response of applied N and K2SO4 rates on bulb diameter and mean bulb weight of Onion.

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.

Similarly, application of potassium sulphate fertilizer resulted in a significant (p<0.01) increase in the bulb diameter compared to the control. Thus, plants treated with 346 and 692 kg K2SO4 ha-1 had 10 and 26% more bulb diameter, respectively, than plants treated with no potassium sulphate. The highest bulb diameter (6.00 cm) occurred at the highest level of potassium sulphate, while the minimum (4.75 cm) observed at 0 kg K2SO4 ha-1 (Table 4).
The result was in line with those of El-Bassiony (2006), EL-Desuki et al. (2006a), Aisha et al. (2007), and Aisha and Taalab (2008) who reported that highest bulb diameter was observed at 535, 715, 596 and 715 kg K2SO4 ha-1 (cm), respectively. This stimulating effect of potassium on bulb diameter may be due to the role of potassium on production of enzymes activity and enhancing the translocation of assimilate and protein synthesis (Khan et al., 2002).

Mean bulb weight
Mean bulb weight also appeared to be highly significantly (p<0.01) affected due to the effect of different rates of applied N fertilizer. Therefore, in response to increasing the level of nitrogen from 0 to 50, 100, 150 and 200 kg N ha-1, mean bulb weight increased by 26, 58, 105 and 85%. The highest mean bulb weight (53.96 g) occurred at 150 kg N ha-1, while the minimum (26.26 g) was recorded at control treatment. However, Potassium sulphate did not affect mean bulb weight (Table 4).
The result on mean bulb weight agrees with that of Vachhani and Patel (1993), Singh and Chaure (1999) and Mozumder et al. (2007) who found that mean bulb weights of onion siginificantly increased with the increase in N fertilizer up to 150 kg N ha-1. The results are also comparable with the finding of several workers including Kashi and Frodi (1998), Islamet al. (2007) and Nasreen et al. (2007) who mentioned that application of 120 kg N ha-1 produced the highest mean bulb weight of onion. In this study, mean bulb weight showed positive and highly significant correlation with plant height (r=0.79**), number of leaves (r=0.70**) and leaf length (r=0.82**) (Table 5). This study seems to indicate that nitrogen fertilization contributed towards the mean bulb weight increment probably due to effect of nitrogen on increase leaf size and assimilate partition to the bulbs, Thereby increased weight of bulbs (Marschner, 1995). The increase in mean bulb weight of bulbs with the supply of N could be due to growth that is more luxuriant, more foliage and leaf area and higher supply of photosynthates, which helped in producing bigger bulbs (Reddy and Reddi, 2002).

Table 5. Simple correlation coefficients among different agronomic parameters

* and ** correlation significant at p<0.05 and p<0.01, respectively. PH=plant height, NLPP=number of leaves per plant, LL= leaf length, BD= bulb diameter, DM=days to maturity, MBW=mean bulb weight, MBY = marketable bulb yield, UMBY = unmarketable bulb yield, TBY= total bulb yield

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 has to be achieved during the production stage, coupled with appropriate post-harvest handling practices.
Therefore, the present study was conducted to investigate the effects of nitrogen and potassium sulphate on the yield and yield component 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, Wello North-East Ethiopia on a sandy clay loam soil. Results of the experiment showed that N fertilization significantly affects number of leaves per plant, leaf length, plant height, bulb diameter, days to maturity, mean bulb weight, unmarketable yield, marketable yield and total bulb yield. Similarly, K2SO4 fertilization significantly affected all the above parameters that were influenced by N except, days to maturity, mean bulb weight, marketable, unmarketable and total bulb yield. In all parameters, significantly highest mean value was recorded at 150 kg N ha-1 except unmarketable bulb yield that was recorded at 200 kg N ha-1 while minimum value were recorded at control. Similarly, significantly highest mean value was recorded at 692 kg K2SO4 ha-1. Moreover, application of N and K on the other hand significantly affected marketable bulb yield, unmarketable bulb yield and total bulb yield. Significantly higher total bulb yield (32.16 t ha-1) and marketable yield (29.69 t ha-1) was recorded in the treatment combination of 150 kg N ha and 346 kg K2SO4 ha-1, while the lowest bulb yield (14.37 t ha-1) and marketable yield (11.00 t ha-1) was recorded in the control. Similarly, significant higher unmarketable yield (3.37 t ha-1) was recorded in the control, while the minimum unmarketable bulb yield (1.62 t ha-1) was recorded in the treatment combination of 200 kg N ha-1 and 346 kg K2SO4 ha-1 closely followed by 200 kg N ha-1 and 692 kg K2SO4 ha-1 with unmarketable yield (1.66 t ha-1). In conclusion, the result of this study has shown combined application of 150 kg N ha-1 and 346 kg K2SO4 ha-1 is a good compromise for yield of onion bulbs. Moreover, the limited response of yield and yield components to applied K in this study should not preclude further research especially dealing with plant tissue K analysis on major soil types. 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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