Abstract: The experiment was conducted to investigate the effect of different levels of nitrogen and potassium on yield contributing bulb traits of onion cv BARI Peaj-1. All the bulb traits in the experiment were influenced significantly by the individual treatment of different doses of nitrogen and potassium except dry matter content for nitrogen and showed a positive linear relationship. In the experiment, fresh weight of bulb, bulb dry weight, bulb diameter and yield per plot gave almost two times higher performance (35.29 g, 3.21g, 3.98 cm and 1.68 kg, respectively) by the treatment of 100 kg N ha-1 than that of no nitrogen application (18.93 g, 1.48 g, 2.02 cm and 0.95 kg , respectively). The performance of the doses of potassium (0 kg K2O ha-1 to 120 kg K2O ha-1) on bulb characters and yield of onion was less than that of nitrogen. There was no interraction between nitrogen and potassium in the influence of the expression of the characters. Among the combinations, the treatment 100 kg N with 120 kg K2O ha-1 was found to be the best from overall considerations.
Introduction
Onion (Allium cepa L.) is one of the most important bulb and spice crops of the world (Jones and Mann, 1963) belonging to the family Alliaceae. In Bangladesh, it is mainly used as spice crop in all Bangladeshi curries and thus have a large scale of requirements. The annual requirement of onion in Bangladesh is about 450 thousand tons (Rahim, 1992), where as the total production is only 134 thousand tons. As well, it has an average yield of only 4.026 t ha-1 (BBS, 2001), which is very low as compared to that of many other countries (FAO, 1999). As the cultivable land area is limited in Bangladesh, it is not possible to extend the land under onion cultivation. So, emphasis must be given to increase the productivity of this crop on yield context, which can be made possible with the efficient use of fertilizers namely nitrogen and potassium. Judicious application of fertilizers may enhance bulb yield significantly. Among the yield promoting factors, application of proper doses of nitrogen and potassium is of great importance. Nitrogen plays the most important role for the vegetative growth of the crop, which ultimately helps in increasing bulb size and total yield (Singh and Kumar, 1969; Rai, 1981). Delvin (1966) stated that, nitrogen imparts greenness to plants by enhancing chlorophyll synthesis and induces more photosynthate production per unit photosynthetic area. Potassium helps in root development and increases the photosynthetic efficiency of leaves. Potassium exerts a balancing role on the effect of both nitrogen and phosphorus, consequently, it is especially important in a multi nutrient fertilizer application (Brady, 1995). Keeping view of the above situation, the present study was undertaken to determine the optimum level of nitrogen and potassium affecting on different bulb characters to get maximum yield of onion cv. BARI Peaj-1
Materials and Methods
The experiment was conducted at the Horticulture Farm of Bangladesh Agricultural University, Mymensingh, during the period from November 2000 to August 2001 to investigate the effect of different levels of nitrogen and potassium on the bulb characters of onion variety BARI Peaj-1. The crop was raised during November 2000 to April 2001. The experiment consisted of two factors, namely, nitrogen (factor -A) and potassium (factor-B). The different levels of nitrogen were I. 0 kg N ha-1 (N0) ii. 50 kg N ha-1 (N50) iii. 75 kg N ha-1 (N75) iv. 100 kg N ha-1 (N100) and that of potassium were I. 0 kg K2O ha-1 (K0) ii. 40 kg K2O ha-1 (K40) iii. 80 kg K2O ha-1 (K80) iv. 120 kg K2O ha1 (K120). The two-factor experiment consisting of 16 treatment combinations was laid out in the Randomized Complete Block Design (RCBD) with 3 replications. Each plot received a treatment combination of the experiment. The size of a unit plot was 1.5 m x 1m with a 75 cm spacing between the replications or blocks, 50 cm between the adjacent unit plots and border was 50 cm. The selected land for seedbeds was well prepared before sowing. The seeds were sown in the seedbeds on 1 November 2000. Then the seeds were covered with light soil and Heptachlor 40WP was dusted over the seedbeds to avoid the attack of ants. Germination was completed within seven days. All the intercultural operations were done as and when necessary. In addition with the treatments consisting of nitrogen and potassium, TSP was applied at the rate of 180 kg ha-1. Well decomposed cowdung was applied at the rate of 15 t ha-1 just after opening of the land. The entire amount of TSP was applied at final land preparation. Urea and MP were applied in two equal splits as top dressing at 20 and 50 days after transplanting. Healthy and disease free uniform sized 55 days old seedlings were transplanted in the main field on 25 December 2000. Pesticides like Dursban 20 EC at the rate of 7.41 L ha-1 and Ridomil MZ at the rate of 2 g L-1 were applied to protect plants from mole cricket insect and purple leaf blotch disease, respectively. The flowering stalks were broken whenever appeared in plants. The crop was harvested on April 10, 2001 when the plants showed the sign of maturity by drying out most of the leaves and by bending of necks of the bulbs. Care was taken to avoid injury of onion bulbs during harvesting. Data were recorded on fresh weight of bulb per plant, dry weight of bulb per plant, diameter of bulb per plant, dry matter content of bulb, splitting of bulb per plot and yield of bulb (t ha-1). The calculated data on various parameters under study were statistically analyzed (using statistical package programme). The means for all the treatments were calculated and analyses of variances for all the characters were performed by F test. The significance of the differences among the pairs of treatment means were evaluated by the Least Significance Difference (LSD) test for the interpretation the results.
Results and Discussion
Different levels of nitrogen and potassium individually exhibited significant variations in respect of all the bulb traits under study except the percentage of dry matter content of bulb (Table 1). For the production of dry matter contents in the bulb, the variations were non-significant in case of nitrogen but significant for potassium. But the interraction between the factors (N and K2O) was non-significant for those characters.
Influence of Nitrogen on Bulb Traits of Onion
The study of the effect of nitrogen on bulb characters (Table
2) revealed that there was a positive linear relationship between the expression
of the traits and the doses of nitrogen. Increased levels of nitrogen increased
the fresh weight of bulbs. Similar results were also observed in case of all
other traits. Maximum weight of bulb per plant was (35.29 g) at fresh condition
were obtained with the application of 100 kg N ha-1, which showed
significant different results from all other treatments. For the traits-dry
weight of bulb, bulb diameter and yield, the highest dose of nitrogen (100 kg
ha-1) also exhibited best performance in comparison to all other
treatments. On an average, the highest nitrogen levels, which is perhaps to
be optimum, produced bulbs of 3.21 g dry weight, 3.9 cm diameter and 1.68 kg
yield per plot.
The bulb weight (fresh) was higher with the application of nitrogen dose, may be, due to higher synthesis of carbohydrates in the leaf and their translocation to the bulb, which comparatively helped in the increased weight, diameter, yield of onion bulb. The results were also in agreement with the findings of Vachhani and Patel (1993) for bulb weight, Maier et al. (1990) for bulb diameter and Amin et al. (1995) for yield.
The maximum dry matter content of the bulb (%) was obtained from the highest nitrogen dose. But statistically similar results were also observed from the treatments of 75 kg N ha-1 and 50 kg N ha-1. Treatment without nitrogen gave lowest drymatter content (8.36%). Thus, for this trait, 75 kg N ha-1 and 100 kg N ha-1 would be considered as the highest performer. From the economic point of view, the optimum dose of nitrogen for getting best performance in dry matter content of bulb is 75 kg N ha-1. Contrary to this, Maier et al. (1990) reported that dry matter of bulbs was not affected by nitrogen.
Though the highest dose of nitrogen gave maximum splitting of bulb (25.3%), but it also produced statistically similar results with the treatment of 75 kg N ha-1. So, 75 kg N ha-1 may the optimum dose for getting better splitting, as found in the experiment. The above result indicated that the higher levels of nitrogen caused more vegetative growth along with tillering that resulted in higher percentage of bulbs splitting.
Influence of Potassium on Bulb Traits of Onion
Doses of potassium showed a positive linear relationship for all the traits
under study (Table 3). Though the highest dose of potassium
(120 kg K2O ha-1) exhibited best bulb trait performance
among the treatments, but 80-kg K2O ha-1 was found to
be optimum, in case of bulb dry weight and percentage of dry matter content.
The treatment of 120 kg K2O ha-1 produced on an average
32.58 g fresh weight of bulb, 2.92 g bulb dry weight, 3.76 cm bulb diameter,
9.75% dry matter content, 26.75% splitting of bulbs and yield of 1.53 kg plot-1.
The minimum results were obtained from the application of 0 kg K2O
ha-1 for these characters. Similar results were obtained by Jasa
and Robtkova (1965) forbulb weight, Chroboczek (1936) for bulb size, Singh and
Dhankhar (1989) for dry matter content and Rahman et al. (1976) for yield.
As an essential element of photosynthesis potattium plays a vital role in increased starch formation, which resulted in increased weight and bulb diameter. As well, the increased dry matter accumulation in the bulb may be attributed to more synthesis and translocation of photosynthates from the leaf to the bulb and also due to the availability of more nutrients from the soil.
| Table 1: | Analysis of variance of the data on six bulb characters of onion as influenced by different doses of nitrogen and potassium |
| * significant at 5% level, ** significant at 1% level | |
| Table 2: | Effect of nitrogen on six bulb characters of onion* |
| * Measured at 5% level | |
| Table 3: | Effect of potassium on six bulb characters of onion |
Similarly higher rate of photosynthesis due to higher doses of potassium enhanced the vegetative growth and accumulated more food, which perhaps encouraged the rate of splitting of bulbs and increased yield.
Combined Effect of Nitrogen and Potassium on Bulb Traits of Onion
Significant combined effect of nitrogen and potassium showed the maximum
fresh weight of bulb per plant (40.50 g) from the treatment combination of 100
kg N ha-1 and 120 kg K2O ha-1 (Table
4). Statistically similar results were exhibited by the treatment conbination
of 100 kg N/ha and 80 kg K2O ha-1. While, the minimum fresh weight
of bulbs/plant (15.33g) was recorded in 0 kg N/ha and 0 kg K2O ha-1
treatment combination. The results indicated that the fresh weight of bulb was
higher with the treatment containing higher doses of nitrogen and potassium.
Singh and Dhankhar (1988) reported similar role of nitrogen and potassium on
fresh weight of onion bulb.
With the increase in nitrogen and potassium doses, dry weight of bulbs was increased (Table 4) and the maximum dry weight of bulb per plant (3.90 g) was observed from the treatment combination of 100 kg N with 120 kg K2O ha-1, which showed statistically similarity with the treatments of 100 kg N with 80 kg K2O ha-1 and 75 kg N with 120 kg K2O. Though, the minimum dry weight of bulb (1.15 g) was found in the lowest dose of nitrogen (0 kg N ha-1) with the lowest dose of potassium (0 kg K2O ha-1), but statistically similar results were observed in all combination of K2O with the lowest dose of nitrogen. The highest bulb diameter per plant (4.63 cm) was noticed from the treatment combination of 100 kg N and 120 kg K2O ha-1 and the lowest (2.28 cm) was found in 0 kg N and 0 kg K2O ha-1 treatment combinations.
| Table 4: | Combined effect of nitrogen and potassium on six bulb characters of onion* |
| * measured at 5% level of significance | |
Singh and Dhankhar (1988) reported that a combination of 120 kg N and 40 kg K2O ha-1 gave the highest bulb diameter. The highest percentage of dry matter (9.99%) was observed from 100 kg N and 120 kg K2O ha-1. Statistically similar results were found in all highest dose combinations of N and K2O. It was surprisingly found that most of the dose combinations gave statistically similar results for bulb dry matter percentage though there was an increasing trend of dry matter percentage with the increased doses of N and K2O. The lowest percentage of dry matter (7.50%) was showed at 0 kg N and 0 kg K2O ha-1 treatment combination (Table 4). Higher dry matter production enhance the uptake of other nutrient elements like N, P, K, Ca and Mg (Hedge, 1988) and thus takes part in higher yield.
The highest percentage of split bulb was recorded from the treatment combination of 100 kg N ha-1 with 120 kg K2O ha-1 having 30% splitting, which was statistically similar with the treatment combination of 75 kg N and 120 kg K2O ha-1. Whereas, the lowest percentage of split bulb was found at 0 kg N with 0 kg K2O ha-1 treatment combination having 14% splitting. The above result indicated that the higher level of potassium resulted in the maximum splitting of bulb. The treatment combination of 100 kg N and 120 kg K2O ha-1 gave the highest yield (13 t ha-1), as it showed significant different results than other treatments. While the lowest yield (5.13 t ha-1) was achieved from 0 kg N and 0 kg K2O ha-1 treatments. Similar result was also obtained by Rizk (1997) who reported that increased N P K S increased bulb yield.
From the above discussion, it was found that doses of N and K2O have significant influence on bulb traits of onion showing a positive linear relationship up to the highest level. Highest dose of nitogen and potassium gave three times or above bulb weight in comparison with no fertilizer application. While in other cases, it was almost double except for dry matter content where little variation was observed due to the differences in doses of nitrogen and potassium. Though the higher doses of both fertilizers gave maximum performance, but better role of nitrogen than potassium was found in most of the traits except bulb dry matter content.