Introduction
Onion (Allium cepa L.) belongs to the family Amaryllidaceae, it is one of the most important vegetable crops. It has a medicinal value, contains carbohydrates, protein, vitamin A, thiamine, riboflavin, niacin, and ascorbic acid [1]. Increasing crop yields is significantly aided by appropriate crop management. The primary causes of the low productivity in vegetable production are the lack of technical knowledge and skill among farmers and development agents [2]. The environment and crop variety may influence the optimal level of agronomic practices like plant population density and fertilizer rate [3]. To optimize onion productivity a full package of information is required for specific growing system. The ideal plant population density makes efficient use of the land resources while, avoiding intense competition between plants for growth factors like light, water, and nutrients. If the plants were grown at the ideal density, they could produce more seeds and have better control over bulbs that were too big or too small. Therefore, controlling plant spacing is a useful method for controlling bulb size, shape, and yield [4-6]. Inadequate fertilization and the cultivation of unsuitable varieties in an area’s agroclimatic conditions are to blame for the low onion production. Fertilization is very important technique in the agricultural system to increase the yield and the productivity. Nitrogen is a significant element for increasing bulb size and yield in onions and plays an important role in achieving the highest possible yield. It is an important element for many important substances such protein and nucleic acids [7-10]. An attempt has been made by several workers to find out the optimum plant spacing and nitrogen fertilizer for onion plants to maximize total yield with the best quality and improve storability, especially under old land conditions. The plants grown at a wider spacing received more nutrients, light, and moisture around each plant compared to plants at a closer spacing. This is probably the cause of the better performance and yield of individual onions grown at a wider spacing. Also, these plants with wider spacing produced the highest percentage of multiplier bulbs, which were not better for storing or meeting consumer demand [11-13]. Whereas the plants grown under the closest spacing gave the maximum total yield of onions due to the presence of more plants, resulting in the highest total yield. But the size of the bulbs under the closest spacing was so small that they were not suitable for marketing due to consumers’ choices [14]. Therefore, the aim of this study was to estimate the response of onion fresh weight, nutrients uptake, nitrogen use efficiency, yield, and bulb quality to different nitrogen fertilization levels and plant density.
Experimental
Field Experiment
A field experiment was carried out during the two successive winter seasons of 2020/2021 and 2021/2022 on the private farm at Diarb Negm District, Sharkia Governorate, Egypt, to study the effect of mineral nitrogen levels and plant density on dry weight, yield, and its components, as well as bulb quality of onions onion (cv. Ahmar Tanawy).
This experiment included 12 treatments, which were combinations between four levels of mineral nitrogen (0, 192, 240, and 288 kg N/ha), and three plant densities (4, 5, and 6 rows/ridge equal 33.33, 41.67, and 50 plants/m2, respectively). These treatments were arranged in a split-plot design with three replications. Nitrogen levels were randomly arranged in the main plots, and plant densities were randomly distributed in the subplots. Nitrogen levels were in the form of ammonium sulphate (20.6% N) and added as a soil application (three portions) every month, beginning one month after transplanting. Seeds of onion were sown in the nursery on November 5th and 10th in the 2020/2021 and 2021/2022 seasons, respectively. Onion transplants were transplanted on December 25th and 28th in the first and second seasons, respectively, at 10 cm apart. All experimental units had an area of 21.6 m2, and they contained three ridges with a length of 6 m and a width of 120 cm. One ridge was used for the samples to measure vegetative growth, and the other two ridges were used for yield determination. Plant densities and the number of plants per m2 and hectare are presented in Table 2.
Table 1.
The physical and chemical properties of the experimental soil in 2020/2021 and 2021/2022 seasons.
Table 2.
The plant densities and number of plants per m2 and hectare.
| Plant densities | Wide (cm) | Plant spacing (cm) | Area/ plant (m2) | Number of plants per | ||
|---|---|---|---|---|---|---|
| ridge | row | m2 | ha | |||
| 4 rows / ridge | 120 | 30 | 10 | 0.03 | 33.33 | 333300 |
| 5 rows / ridge | 120 | 24 | 10 | 0.024 | 41.67 | 417700 |
| 6 rows / ridge | 120 | 20 | 10 | 0.02 | 50 | 500000 |
Phosphorus and potassium were added at a rate of 144 and 204 kg/ha in the form of calcium superphosphate (16-18% P2O5) and potassium sulphate (48-52% K2O), respectively. All the amounts of phosphorus fertilizer and one fourth of the amounts of K mineral fertilizer were added during soil preparation. The K fertilizer dose was divided into three portions and added to the soil every month, the first dose was added 30 days after transplanting.
Sampling and Measurements
Plant growth: Ten plants from each plot were randomly taken at 100 days after transplanting, and the following data were recorded: Fresh weight of different parts of the onion, i.e., bulb and leaves, as well as total fresh weight (bulb + leaves) per plant.
Nitrogen, phosphorus, and potassium total uptake: The contents of nitrogen, phosphorus, and potassium were assayed in the dry matter from 5 random samples of bulbs and leaves at 100 days after planting in both seasons. The samples were finely ground and wet digested for N, P, and K determinations according to the methods advocated by Jackson [15] and Olsen et al. [16], respectively. N, P, and K uptake in bulbs and leaves and their total uptake were calculated.
Total yield and yield components percentage: At the proper maturity stage of the bulbs, bulbs from each plot were harvested and graded into four categories according to specifications laid down by the Ministry of Economic Affairs for onion exportation (1963) as follows: Grade 1: Bulbs with a diameter above 5.5 cm; grade 2: bulbs with a diameter between 4.5 and 5.5 cm; grade 3: bulbs with a diameter between 3.5 and 4.4 cm; and grade 4: bulbs with a diameter less than 3.5 cm. Each grade was weighed separately on the same day, and the following data was recorded: Total yield (grade 1+ grade 2 + grade 3 + grade 4) tonne/ha.
Nitrogen use efficiency (NUE): It was determined by dividing the bulb yield/ ha by the nitrogen quantity/ ha and expressed as kg bulb/kg N.
Bulbs quality: At harvest time, five bulbs were randomly taken from each treatment and oven dried at 70ºC until constant weight, and the chemical constituents of onion bulbs during the two seasons were determined as follows: Total carbohydrate (%): It was determined calorimetrically as outlined by DuBois et al. [17], and ascorbic acid (Vitamin C) was assayed in juice (mg/100 g FW) using 2,6 dichlorophenol indophenol dye [18].
Statistical analysis: Collected data were subjected to proper statistical analysis of variance and the differences among treatments were compared using Duncan’s multiple range test [19].
Results
Fresh Weight of Leaves/Plant, Fresh Weight of Bulb, and Fresh Weight of Plant
The effect of nitrogen levels, plant density and their interaction on fresh weight of leaves/plant (g), fresh weight of bulb (g), and fresh weight of plant (g) at 100 days from transplanting of onion plants during 2020/2021 and 2021/2022 seasons are presented in Table 3. At 100 days after transplanting, the fresh weight of the leaves per plant, the fresh weight of the bulb, and the fresh weight of the plant were significantly affected by the nitrogen fertilizer levels (192, 240, and 288 Kg/ha). From 192 to 240 and 288 kg N/ha, all the examined growth characteristics rose significantly, and the variations were noticeable throughout both growing seasons. In both growing seasons, the best results were seen 100 days after transplanting when the greatest nitrogen fertilizer application (288 kg N/ha) was used. The low plant density (4 rows/ridge) produced the highest growth parameters at 100 days from transplanting in both seasons, followed by 5 rows/ridge and 6 rows/ridge. Plant densities of up to 6 rows per ridge at 100 days in both seasons resulted in a decrease in the fresh weight of the leaves per plant, the fresh weight of the bulb, and the fresh weight of the plant (Table 2). Maximum values for leaf, bulb, and total fresh weight per plant were found at 100 DAT in both seasons when nitrogen levels were 288 kg N/ha and plant density was 4 rows/ridge (Table 3).
Table 3.
Effect of nitrogen levels, plant density and their interaction on fresh weight of leaves/plant (g), fresh weight of bulb (g), and fresh weight of plant (g) at 100 days from transplanting of onion plants during 2020/2021 and 2021/2022 seasons.
Table 4.
Effect of nitrogen levels, plant density and their interaction on N, P, and K-uptake (mg) of onion plants at 100 days from transplanting of onion plants during 2020/2021 and 2021/2022 seasons.
Table 5.
Effect of nitrogen levels, plant density and their interaction between them on total yield (ton/ha.) of onion plants during 2020/2021 and 2021/2022 seasons.
Table 6.
Effect of nitrogen levels, plant density, and their interaction between them on nitrogen use effectiveness (kg bulb per kg nitrogen) of onion plants during the 2020/2021 and 2021/2022 seasons under clay soil conditions.
N, P and K‐uptake of Onion Plants
The effect of nitrogen levels, plant density and their interaction on N, P, and K-uptake (mg) of onion plants at 100 days from transplanting of onion plants during 2020/2021 and 2021/2022 seasons are presented in Table 4. Data in Table 4 show the effect of nitrogen fertilizer levels on N, P, and K total uptake. Application of the highest level of nitrogen fertilizer at 288 kg/ha gave the highest values for total N, P, and K uptake by the onion plant at 100 days in both seasons. Planting onion plants with low plant density (4 rows per ridge) gave the plant’s highest values of total N, P, and K uptake at 100 days in both seasons. The interaction between the N rate at 288 kg /ha and plant density at 4 rows/ridge increased N, P, and K total uptake by plants at 100 days in both seasons (Table 4). In general, under low plant density (4 rows/ridge), all N levels (192, 244, and 288 kg/ha) as well as control increased total N, P, and K uptake by plants compared to high plant density (5 and 6 rows/ridge) with all N levels.
Table 7.
Effect of nitrogen levels, plant density, and their interaction between them on total carbohydrates (%) and vitamin C (mg/100 g FW) in bulbs at harvesting time of onion plants during the 2020/2021 and 2021/2022 seasons under clay soil conditions.
Total Yield
The effect of nitrogen levels, plant density and their interaction on total yield (ton/ha.) of onion plants during 2020/2021 and 2021/2022 seasons are presented in Table 5. Results in Table 4 showed that the total yield was significantly affected by nitrogen fertilizer levels. Application of 288 kg N/ha produced the highest total yield in both seasons. High plant density (6 rows per ridge) increased total yield in both seasons (Table 5). The interaction between N at 244 kg/ha and high plant density (6 rows/ridge) increased total yield/ha with no significant differences with the interaction between N at 288 kg/ha and plant density (5 and 6 rows/ridge), as shown in Table 5.
Nitrogen Use Efficiency (NUE)
Data in Table 6 illustrate that N at 244 kg /ha gave the highest NUE by onion plant (103.76 and 98.31 kg bulb/kg N), followed by N at 288 kg/ha (87.95 and 79.31 kg bulb/kg N) in the 1st and 2nd seasons, respectively. Nitrogen use efficiency increased with increasing plant density up to 6 rows per ridge (Table 6). This means that high plant density (6 rows per ridge) gave the highest NUE by onion plant (94.32 and 91.21 kg bulb per kg N) in the 1st and 2nd seasons, respectively. The interaction between N at 244 kg/ha and plant density at 6 rows/ridge increased NUE by onion plants (106.1 and 118.5 kg bulb/kg N), followed by the interaction between N at 244 kg/ha and plant density at 5 rows/ridge (103.5 and 89.89 kg bulb/kg N) in the 1st and 2nd seasons, respectively.
Bulb Quality
Total carbohydrates and vitamin C in the bulb significantly increased with increasing N up to 288 kg N/ha (Table 7). Planting at low plant density (4 rows per ridge) significantly increased total carbohydrates and vitamin C in bulbs, except for vitamin C content in the 2nd season. Data in Table 6 show that the interaction between N at 288 kg/ha and planting at 4 rows/ridge increased total carbohydrates and vitamin C contents, except vitamin C content in the 2nd season.
Discussion
Onion is a very important crop in Egypt, it is highly sought after both domestically and abroad. After rice and citrus, it is third in this category [20]. Selecting the optimal level of mineral nitrogen and plant density are just two of the many agricultural practices that determine the productivity of onions. Nitrogen fertilizer is very important element for all living processes in enhancing the productivity in the plant [7]. It is the limiting nutrient and very important element in agricultural production. The excess of nitrogen hindered the bulbs from ripening properly and made them of poor storage quality, despite the fact that nitrogen is required for increasing bulb size and productivity [21, 22].
In the present study, at 100 days after transplanting, the fresh weight of the leaves per plant, the fresh weight of the bulb, and the fresh weight of the plant were significantly affected by the nitrogen fertilizer levels (192, 240, and 288 Kg/ha). The important role of nitrogen fertilizer was recorded in many plants such as onion [1], rice [23], sugar beet [24], wheat [25] and cowpea plants [26]. Nitrogen may play a significant role in increasing an onion’s vegetative growth by enhancing its color, vigor of the leaf canopy, and meristematic activity, which contributes to the increase in cell number and cell elongation, and ultimately, its overall size [27]. These results are in harmony with those recorded by Barrales et al. [28]. At 100 days after transplanting, the present study found that the lowest plant density (4 rows/ridge) yielded the best growth metrics in both growing seasons. This was followed by densities of 5 rows/ridge and 6 rows/ridge. Results improved with increased distance between plants, perhaps because there was less competition for resources such as water, nutrients, and light. Plant spacing within onion seedlings, as measured by El-Fakharany et al. [29].
The essential role of nitrogen fertilizer in the formation of proteins, protoplasm, and chlorophyll can be attributed to the observed gradual increase in N, P, and K uptake with increasing nitrogen fertilizer levels. This in turn can be attributed to the observed increase in cell size, leaf area, photosynthetic activity, and growth characters, consistent findings with those reported by [22,30]. Increasing nitrogen fertilizer rates up to 288 kg N/ha led to increase total yield because nitrogen stimulates plant growth, which in turn increased yield components and yield per unit area for bulbs [31]. High plant density (6 rows per ridge) increased total yield may be due to those six rows or ridges increasing yields in grades 2, 3, and 4. Khan et al. [32] found that 12 cm plant spacing with 100 kg N/ha gave the best results for size of bulb, weight of bulb, and bulb yield. Effect of nitrogen concentration, plant density and their interaction on N, P, and K-uptake using of the maximum level of nitrogen fertilizer at 288 kg/ha gave the highest values for N, P, and K total uptake by the onion plant the same way found amaranth [33]. Planting onion plants with low plant density gave the plant’s highest values of N, P, and K total previous study on onion showed that low density increase yield [34]. Carbohydrates and vitamin C in the bulb significantly increased with increasing N fertilizer. The vitamin C value obtained falls within range of vitamin C in onion. Using the low plant density pointedly increased total carbohydrates and vitamin C in bulbs [34].
Conclusions
Our findings indicated that, the interaction between nitrogen at 288 kg N/ha and plant density (4 rows/ ridge) gave the best results of fresh bulbs, total fresh weight/plant and percentages of the yield of grade 1, grade 2, and exportable yield from total yield. Also, total N, P, and K uptake by the plant, total carbohydrates, total sugars, and vitamin C contents were increased in bulbs. The interaction between N at 244 kg /ha and high plant density (6 rows/ridge) increased total yield per hectare and nitrogen use efficiency in onion plants (cv. Ahmar Tanawy).
