Effect of Calcium Cyanamide as an Alternative Nitrogen Source on Growth, Yield, and Nitrogen Use Efficiency of Short-Day Onion

Abstract

Effective nitrogen (N) management in agriculture is vital to optimize crop growth and yield while minimizing environmental impact. Conventional nitrogen (N) sources, such as urea, have limitations in promoting growth and reducing N leaching. A two-year field experiment was carried out to investigate the effects of calcium cyanamide (CaCN₂) as a slow-release N source on short-day onion growth, yield, and N use efficiency (NUE). Six types of N sources were administered: (i) an initial application of 80 kg ha⁻¹ N in the form of CaCN₂ before planting; (ii) an initial application of 80 kg ha⁻¹ N in the form of CaCN₂ before planting, followed by a topdressing of 50 kg ha⁻¹ N in the form of limestone ammonium nitrate (LAN); (iii) an initial application of 80 kg ha⁻¹ N in the form of CaCN₂ before planting, followed by a topdressing of 50 kg ha⁻¹ N in the form of urea; (iv) an initial application of 80 kg ha⁻¹ N in the form of LAN before planting, followed by a topdressing of 50 kg ha⁻¹ N in the form of LAN; (v) an initial application of 80 kg ha⁻¹ N in the form of urea before planting, followed by a topdressing of 50 kg ha⁻¹ N in the form of urea; and (vi) control (0 kg ha⁻¹ N). Preplant CaCN₂ (80 kg ha⁻¹ N) outperformed the standard fertilizers used in onion as an N source (urea and LAN) by improving growth and yield, and reducing N leaching. Preplant CaCN₂ topdressed with either LAN or urea led to a significant increase in plant growth and total yield compared to using LAN or urea alone. The application of CaCN₂, followed by topdressing with either LAN or urea, decreased onion bolting by 1.6% and 1.83%, respectively, compared to the control. The study suggests that applying LAN or urea as a topdressing to preplant CaCN₂ enhances N utilization efficiency, leading to increased onion bulb yield and quality while reducing N leaching. This approach can help mitigate farm-level environmental pollution and provide valuable insights for improving onion production and sustainable agriculture practices in South Africa.

1. Introduction

Onion (Allium cepa L.) is one of the major vegetable crops in South Africa [1], serving both culinary and culinary purposes [2], and elevating the flavors of dishes like stews, soups, and salads [3,4]. Onions, like other plants, require essential nutrients, with nitrogen (N), phosphorus (P), and potassium (K) being crucial for their growth [5,6]. Most crops, including onions, often lack these nutrients. Nitrogen plays a pivotal role as an essential nutrient in crop production, frequently serving as a limiting factor for yield [7] and it is primarily absorbed in the form of inorganic compounds (NH₄⁺, NO₂⁻, and NO₃⁻) [7]. In the cultivation of onion, which is characterized by a sparse and shallow root system, N fertilizer efficiency tends to be suboptimal, while the potential for nitrate leaching losses remains notably high [4].

Nitrogen fertilizer plays a key role in global food production, providing half of the total N [8,9]. However, its efficiency in agriculture is low (10–50%) due to substantial losses through various processes, polluting groundwater and the atmosphere [10,11]. With limited arable land and increasing food demand, improving fertilizer N efficiency is essential. To achieve this, understanding the sources, forms, and pathways of N loss, along with controlling factors, is crucial. This knowledge is vital for developing strategies to minimize N loss and enhance N use efficiency (NUE) [12].

Research on sweet Spanish onion [13] found a significant increase in bulb yield (8.9% to 26.4%) with higher N fertilizer application. However, this also increased nitrate leaching. Lowering N application reduced yield but maintained consistent leaching. Nitrate accumulation in plants depends on fertilizer type and soil properties [14,15,16]. Selecting the right N fertilizer is vital for onion yield, influencing leaching and overall crop parameters.

Urea and LAN are common N fertilizers in onion production, but their low efficiency affects yields and the environment [15,17,18]. Agriculture contributes to 50% of the global ammonia emissions, posing health risks [19]. Innovative N fertilizers are needed for higher efficiency, productivity, and environmental sustainability [20,21]. Slow-release fertilizers have emerged to counter N leaching issues, and rectify the inefficiencies seen in traditional onion production fertilizers. Several researchers have observed that increased onion bulb yields are linked to the application of slow-release fertilizers [22,23]. Perlka (calcium cyanamide) has been identified as a potential N slow-release fertilizer for minimizing N leaching losses [24]. Unlike conventional methods, adopting slow-release N fertilizers reduces environmental impact while effectively boosting onion yields [25]. Generally, commercial onion growers plant onions by direct seeding, which requires frequent overhead irrigation to keep the topsoil moist and allow seed emergence. Normally, this practice results in the leaching of the preplant N fertilizer as it is a mobile element. Hence, as an alternative approach, employing N slow-release fertilizers like calcium cyanamide could potentially reduce N leaching and enhance onion yield. However, its agronomic performance and effectiveness for agronomic crops have not been well established.

This fertilizer has the remarkable ability to activate beneficial soil organisms, enhancing soil fertility. Additionally, it provides calcium in a readily available form to the plant, promoting healthy and robust cell walls [25]. Despite these advantages, there have been limited studies on the impact of calcium cyanamide on onion agronomic performance. This study aims to fill this knowledge gap by investigating the effects of calcium cyanamide, acting as a slow-release N fertilizer, on onion yield, quality, and NUE, in comparison to the standard N sources of urea and limestone ammonium nitrate (LAN).

This research explores the potential use of calcium cyanamide as a slow-release N fertilizer for onion cultivation, offering a novel solution to the inefficiencies and environmental concerns associated with traditional N sources. Slow-release calcium cyanamide (CaCN₂) is hypothesized to boost onion yield, enhance quality, and increase NUE compared to traditional fertilizers like urea and LAN, due to its ability to reduce leaching and having positive effects on soil and plants.

2. Material and Methods

2.1. Description of the Study Area

A field experiment was conducted from March 2019 to November 2019 under a sprinkler irrigation system at the Hygrotech Experimental Farm, Dewagensdrift, 60 km from Pretoria (25.4580° S, 28.6411° E, altitude 1214 m), and repeated in 2020.

Approximately 600 mm of irrigation was applied in 2019. In the first week, plants were irrigated with 25 mm and thereafter with 35 mm per week, while 620 mm of irrigation was applied during the 2020 growing season, which was around 35 to 40 mm per week. Irrigation was stopped when the leaves of about 50% of the plants started to fall off; the plants were pulled up, shaken to remove the soil, and laid out to cure by hanging in sacks under a shelter. During the two growing seasons, an annual average of 700 mm of rainfall was received (Figure 1). For this study, the crop water requirement (ETc) throughout the growing season was calculated using the crop coefficient (Kc) and the potential evapotranspiration of the study area, expressed as ETc = Kc × ETo. The irrigation schedule was determined based on climatic, soil, and crop data, utilizing the FAO-model CROPWAT 8.0.

2.2. Experimental Design and Treatment Combinations

The land was prepared by ploughing to a soil depth of 25–30 cm, followed by disking and rotovating the soil. Before planting, the experimental plots were harrowed to a fine tilth and leveled well. The optimal nutritional needs of onions have been documented as 80–100 kg ha⁻¹ N, 80–100 kg ha⁻¹ P, 150–170 kg ha⁻¹ K, and 20–40 kg ha⁻¹ S [24]. Short-day onion variety ‘Texas Grano’ was used as a test crop in response to six treatments, which included three N sources of fertilizer, 130 kg ha⁻¹ N from limestone ammonium nitrate (LAN, 28 N%), 80 kg ha⁻¹ N from urea (46 N%), 80 kg ha⁻¹ N from calcium cyanamide (CaCN₂, 19.8 N%) + 50 kg ha⁻¹ N from LAN, 80 kg ha⁻¹ N from CaCN₂ + 50 kg ha⁻¹ N from urea), and a control (without N fertilizer), using a randomized complete block design with four replicates. The fertilizer application method was by broadcasting fertilizer on beds. Plants received the same amount of N at 130 kg ha⁻¹ except CaCN₂ applied at 80 kg ha⁻¹ N. Onion seeds were sown directly into sandy loam soil. The planting was performed in plots; each measured 6 m². Within each plot, the area was divided into five single rows. Once the seedlings emerged, they were thinned out to achieve an intra-row spacing of 7.5 cm, with each row containing 40 plants. To ensure proper growth, the recommended inter-row spacing of 20 cm was maintained for all plots. Between each plot, a distance of 1 m was kept, while the blocks were separated by 1.5 m. Preplant fertilizer was applied based on soil analysis, i.e., 2000 kg ha⁻¹ calcitic lime (except for CaCN₂ treatments), 300 kg ha⁻¹ superphosphate (10.5% P), 150 kg ha⁻¹ potassium sulphate (22.4% K and 18.4% S), and 150 kg ha⁻¹ magnesium sulphate (10% Mg and 13% S) were broadcast, with the addition of fertilizer according to N source treatment, i.e., LAN (28% N, 285.7 kg ha⁻¹) and urea (46% N, 173.9 kg ha⁻¹). Perlka (CaCN₂, 19.8% N) was applied at 80 kg N ha⁻¹ by broadcasting on the plot surface and then incorporated into the soil at a depth of 10 cm (Perlka; AlzChem, Trostberg, Germany). Calcium cyanamide was watered and kept moist for 8 days before seeding. Topdressing with LAN and urea was performed at 35.71 and 22 kg ha⁻¹ per week, respectively (5 applications from week 4–8) after seeding according to the treatments; LAN, urea, CaCN₂ + LAN, CaCN₂ + urea, excluding preplant CaCN₂, and 0 N source (control) fertilizer were applied 5 times on a weekly basis, from week 4–8 after emergence, on LAN and urea. Potassium sulphate was applied at 50 kg ha⁻¹ per week from week 6–10 after sowing (5 applications) for all treatments, irrespective of N source.

At the edges of each plot, soil bunds were made to prevent the movement of nutrients across plot blocks.

2.3. Soil Sample Collection and Analysis

The onion plants possess a shallow and minimally branched root system, primarily concentrated in the top 30 cm of soil, as documented in references [9,27]. To assess the soil properties before seed planting, samples were collected from 18 different points at depths of 0–15 cm and 15–30 cm. These samples were analyzed for both chemical and physical properties, following the methods outlined in the soil and plant laboratory manuals [28]. The results of these analyses are summarized in

Table 1. Physical and chemical properties of the soil at the experimental site.

Parameter	Value
Physical property
Sand %	54.10
Silt %	41.30
Clay %	4.60
Chemical property
pH	5.5
Calcium (%)	69.23
Magnesium (%)	20.95
Potassium (%)	8.48
Phosphorus (mg kg−1)	47
Sodium (%)	1.34
Ammonium nitrogen (mg kg−1)	0.29
Nitrate nitrogen (mg kg−1)	0.50

.

2.4. Study Data Collection

Plant growth parameters were assessed biweekly, commencing 30 days post-emergence and continuing until harvest. Characteristics of the bulbs were documented upon reaching physiological maturity (106 days after seeding). Plant height was gauged using a measuring tape, measuring from the soil surface to the tip of the longest mature leaf. Leaf count per plant was recorded at maturity. Leaf length was measured from the base to the tip of the longest leaf, using a measuring tape. Leaf diameter denoted the widest point on the longest leaf, while neck thickness was gauged at the narrowest section of the bulb using vernier calipers. Bulb length and diameter referred to the height and average width of the central portion of the mature bulb, respectively, as measured with vernier calipers at harvest. To determine the average bulb weight, 15 bulbs were collectively weighed after air-drying in sacks for 14 days post-harvest. Total dry biomass was recorded as the weight of the bulb and aboveground parts at maturity after drying in an oven at 70 °C until a constant weight was attained. In this study, the harvest index was defined as the ratio of dry bulb weight to the total dry biomass yield per plant. The data presented here are the average of 15 randomly selected plants from each experimental block. The percentage of plants showing bolting per plot indicated the number of plants with flower stalks relative to the total plant count. The days to physiological maturity denoted the actual number of days from seed sowing to the point when over 80% of the plants in a plot displayed yellow leaves. Total bulb yield refers to the weight of matured bulbs per plot, converted into a per-hectare basis and expressed in metric tons. Marketable bulb yield was determined after excluding bulbs smaller than 3 cm in diameter, as well as those that were rotten, split, thick-necked, or discolored. The percentage of split bulbs was calculated by dividing the number of split bulbs per plot by the total number of normal bulbs per plot.

2.5. Leaf Chlorophyll Content

Leaf chlorophyll content was measured using a hand-held SPAD-502 chlorophyll meter (Spectrum Technologies, Inc., Aurora, IL, USA). Chlorophyll was measured in the middle of the healthy young third or fourth leaf of the plant.

2.6. Measurement of Nitrogen

The Kjeldahl method was employed to ascertain the N levels within the plant samples. N uptake was computed by multiplying the N content (expressed as a percentage) by the corresponding yield and then dividing the result by 100. During the vegetative stage of the plant, leaf N content was assessed using a N meter test. Nitrogen uptake efficiency (NUPE) was measured by the following formula [29]:

$$NUPE=\frac{\left(plant dry weight\times population\times leaf N\right)}{Total N applied}$$

(1)

Nitrogen utilization efficiency (NUTE) was measured by the following formula [29].

$$NUTE=\frac{Harvested yield}{plant dry weight\times population\times leaf N}$$

(2)

Nitrogen Use Efficiency (NUE) was measured by the following formula [29]:

$$NUE=NUPE\times NUTE$$

(3)

2.7. Nitrogen Leaching

A wetting front detector (WFD) named FullStop was buried in the soil to collect water samples for measuring N leaching within the rootzone (Figure 2). The FullStop consists of a specifically designed funnel, a filter, and a float mechanism buried 25 cm deep in the soil. When there is rainfall or irrigation, water percolates downward through the root zone, saturating the soil. Any excess water filters out, passing through a filter and collecting in a reservoir. This collected water triggers a float mechanism, causing an indicator flag to move above the soil surface. The remaining water is drawn out by capillarity, and a sample of the soil solution is extracted using a syringe. Both the soil and water samples were examined for nitrogen content (NH₄-N and NO₃-N) using a continuous-flow nitrogen analyzer (SKALAR, San Plus System, Breda, The Netherlands).

2.8. Statistical Analysis

The study was organized following a Randomized Complete Block Design (RCBD) and was replicated over a span of two years. The data were subjected to separate analysis of variance (ANOVA) for each year, adhering to the experimental design. This analysis was carried out using the GLM (General Linear Model) procedure in SAS software (Version 9.4; SAS Institute Inc., Cary, NC, USA). The observations from both years were also combined in a single ANOVA [31]. To facilitate combined analyses, the uniformity of experimental error variances was assessed using the F-test [32]. In cases where the error variances varied, ANOVA was conducted using inverse variance weighting. Residuals were scrutinized for deviations from normality, and outliers causing skewness were eliminated. Fisher’s least significant difference (LSD) was computed at a 5% significance level to compare means for significant effects [33]. A significance threshold of 5% was applied to all tests.

3. Results and Discussion

CaCN₂ had a substantial impact on chlorophyll content. When CaCN₂ was applied as a preplant fertilizer along with topdressing of LAN or urea, it significantly increased leaf chlorophyll content compared to LAN or urea used alone, and the control treatment (Figure 3). This boost in chlorophyll content is attributed to the availability of N, a crucial factor in chlorophyll synthesis [34]. CaCN₂, which releases N slowly, benefited from soil moisture and microbial activity, providing a continuous supply of N for sustained chlorophyll production. Nonetheless, CaCN₂ (with only preplant application of 80 kg ha⁻¹ N) outperformed standard LAN or urea N sources (130 kg ha⁻¹ N) regarding leaf chlorophyll content.

3.1. Bolting, Physiological Maturity, and Split Bulbs

The study found that different N sources significantly influenced bolting and the occurrence of split bulbs in onion plants. Specifically, the type of N source had a notable impact on bolting (p = 0.0001) and split bulbs (p = 0.0001) without affecting the physiological maturity of the plants. There were no significant interactions between N source and year for days to maturity and bolting percentage, except for split bulbs. The control treatment had the highest bolting incidence (4.34%) compared to treatments with specific N sources (Figure 4). CaCN₂ (80 kg ha⁻¹ N) application or topdressing with 50 kg ha⁻¹ N, along with LAN or urea, significantly reduced bolting percentage compared to LAN (3.96%) and urea (3.74%) used alone. This effect was attributed to CaCN₂’s slow-release properties, providing a continuous N supply and extending the vegetative growth period of onion plants [35,36]. This effect not only delayed flowering [37] but also minimized leaching as compared to urea and LAN, ensuring sustained availability of nutrients for the plants throughout their growth cycle. Limited N availability in the control treatment promoted bolting, indicating that sufficient N supply is crucial to suppress premature flowering in onion plants. Bolting in onion is normally triggered in reaction to limited N supply [37], and stimulates the emergence of flowers before bulbs are adequately grown to suppress crop flower initiation.

Different N sources had a significant effect on the number of split onion bulbs (p < 0.0001,

Table 2. Summary of analysis of variance for the effects on nitrogen source and year (2019 and 2020 growing seasons) on onion growth and development.

Growth Parameter	Nitrogen Source (NS)	Year (Y)	NS × Y
Days to maturity	ns	ns	ns
Bolting%	***	ns	ns
Leaf length	**	*	ns
Leaf diameter	ns	ns	ns
Split bulb	***	*	*

ns, *, **, *** Nonsignificant or significant at 5% (p < 0.05), 1% (p < 0.01), or 0.1% (p < 0.001).

). A higher percentage of split bulbs occurred in the 2020 growing season compared to 2019, with LAN and urea leading to the highest (Figure 5). The lowest percentage of split bulbs was found in the control treatment in both years (1.297% in 2020 and 0.875% in 2019). LAN fertilizer resulted in a significantly higher number of split bulbs compared to other N sources, while the topdressing of LAN and urea on CaCN₂ (preplant) during the 2019 season reduced split bulbs by 20% and 24%, respectively. However, no significant differences were found between LAN and urea in either the 2019 or 2020 growing season. This could be associated with the fact that onions can be adversely affected by rapid and excessive growth resulting from the immediate availability of N provided by fast-release fertilizers like urea and LAN, which leads to the splitting of onion bulbs instead of a single intact bulb. The findings align with the notion that immediate access to N, facilitated by the application of fast-release fertilizers, leads to rapid and excessive growth in onions [37,38]. This accelerated growth has been identified as a cause of split bulbs in onion plants.

3.2. Leaf Length and Diameter

Nitrogen source significantly increased leaf length (p < 0.001) in onions, but without having any significant effect on leaf diameter (Table 2). CaCN₂ + LAN produced the longest leaves in 2020 and 2019 growing seasons and the shortest leaf length was achieved with the control treatment in both year 2019 and 2020 (40.81 and 41.25 cm, respectively). The year or the growing season did not have a significant impact on leaf length (Figure 6). Adding LAN or urea as topdressing to preplant CaCN₂ application increased the leaf length of onions by 1.35 cm and 1.55 cm, respectively. The improved leaf length of onions from CaCN₂ might be attributable to its positive effect on leaf chlorophyll. CaCN₂ provides plant-available N and releases calcium into the soil as it breaks down [38]. Calcium is an important nutrient for plant growth and can help improve root and shoot development, as well as overall plant health [39]. Adequate N availability during the growing period is the main constituent of proteins, resulting in longer leaves and increased carbohydrates [37].

3.3. Plant Height and Leaves per Plant

Table 3. Summary of analysis of variance for the effects of nitrogen source and year (2019 and 2020 growing seasons) on onion vegetative growth.

Growth Parameter	Nitrogen Source (NS)	Year (Y)	NS × Y
Plant height	*	ns	ns
Number of leaves per plant	***	ns	ns
Bulb weight	***	ns	ns
Bulb length	ns	ns	ns
Bulb diameter	*	ns	*
Neck thickness	*	ns	ns

ns, *, *** Nonsignificant or significant at 5% (p < 0.05), or 0.1% (p < 0.001).

indicates that there were no noteworthy interactions between the N source and the year in terms of plant height and the number of leaves. However, the N source significantly influenced both plant height (p < 0.0223) and the number of leaves per plant (p < 0.0001). CaCN₂ + LAN, CaCN₂ + urea, and CaCN₂ produced significantly taller onion plants, followed by LAN then urea, and the fewest leaves were observed in the control treatment (Figure 7). Topdressing either LAN or urea on preplant CaCN₂ produced significantly taller and more onion leaves per plant compared to other treatments. CaCN₂ (56.16 cm) outperformed LAN (54.83 cm) and urea (54.02 cm), and the smallest plant height was found with the control treatment (49.27 cm). No significant distinctions were observed between CaCN₂ + urea, CaCN₂ + LAN, and CaCN₂.

The highest number of leaves per plant was recorded under CaCN₂ + urea (9.23 leaves per plant), followed by CaCN₂ + LAN (9.08 leaves per plant), and the fewest was observed in the control treatment (6.78 leaves per plant) (Figure 8). No significant distinctions were observed between CaCN₂ + urea and CaCN₂ + LAN, as well as between urea and LAN. In some studies, certain N sources have been shown to promote greater plant height and leaf number compared to others. The use of poultry manure as an N source resulted in taller onion plants and more leaves compared to urea fertilizer [40]. Another study [41] found that the use of N-fixing bacteria as an N source resulted in taller onion plants and more leaves compared to ammonium nitrate fertilizer. The tallest onion plants following the application of CaCN₂ may be attributed to the enhanced availability of slow-release N, which promotes more leaf growth, resulting in higher photosynthetic assimilates and, consequently, more dry matter accumulation. The importance of N in many essential cell mechanisms has been noted [42,43]. N holds a crucial position in plant tissues, contributing significantly to robust plant growth—a characteristic unmatched by any other element [43]. These observations align with earlier studies that suggest a prolonged presence of N in the soil often correlates with improved growth parameters in onion plants [41].

3.4. Bulb Weight

Nitrogen source had a significant effect (p < 0.0001) on average onion bulb weight, while the interaction effect with year (NS × Year) was not significant (Table 3). The application of CaCN₂ in combination with topdressing either LAN or urea produced significantly greater onion bulb weight compared with other treatments (Figure 9). CaCN₂ (143.21 g) demonstrated superior performance compared to both LAN (140.5 g) and urea (140 g), with no significant difference observed between LAN and urea, while the control treatment yielded the lowest results (114.82 g). Improved onion bulb weight in response to slow-release CaCN₂ fertilizer or CaCN₂ in combination with topdressing either LAN or urea could be attributed to an increase in leaf chlorophyll, plant height, number of leaves produced per plant, and leaf length, which might have increased assimilate production and allocation to the onion bulbs.

3.5. Bulb Length and Diameter

Neither N source nor year, nor their interactions, had a significant influence on bulb length (Table 3). These findings contradict those previously reported [41], where N fertilization was found to significantly (p < 0.05) increase onion bulb length. The interaction between N source and year significantly influenced bulb diameter (Table 3). The largest onion bulb diameters were obtained with CaCN₂ + urea and CaCN₂ + LAN in both years (2019 and 2020), as well as from CaCN₂ in 2020, followed by LAN and urea, which were not different from each other, and the lowest was from the control treatment (Figure 10). Improved onion bulb size was reported to result from N application [27]. The slow-release CaCN₂ acts as a gradual N fertilizer and ensures a steady N supply to plants over an extended period, thus reducing the risk associated with overfertilization and excessive vegetative growth [35]. Unlike traditional soluble N fertilizers, it also exhibits the ability to minimize nitrate leaching into groundwater. N fertilizers like urea and LAN dissolve more quickly in water and become available to plants shortly after application, which can lead to leaching below the root zone [34]. The results show that topdressing with LAN or urea with preplant application of CaCN₂ will provide a continuous N supply, potentially benefiting the growth of onion bulbs.

3.6. Neck Thickness

The data in Table 3 indicate that N source had a significant influence on the development of neck thickness. However, the interaction between N source x year did not significantly influence the development of neck thickness. The greatest neck thickness was achieved under CaCN₂ + urea (1.44287 cm), CaCN₂ + LAN (1.43617 cm), and the least neck thickness was achieved under the 0 N control (1.30922 cm) (Figure 11). The efficiency of N utilization can vary among different sources. CaCN₂ + urea treatment may have resulted in higher NUE, meaning that a greater proportion of the applied N was effectively taken up and used by the plants for growth, including neck thickness.

These results are in clear agreement with the research findings presented by other authors [44], who reported that N application increases the number of thick-necked onion bulbs. The current study results are in contrast to the findings presented by the authors of [4], who disputed those claims and stated that neck-thickness is a physiological parameter that is influenced by seasons, sites, and cultivars, and not by fertility.

3.7. Yield Parameters

Nitrogen source had a significant effect on total bulb yield, marketable bulb yield, total dry mass, dry bulb yield, and harvest index (

Table 4. Summary of analysis of variance for the effect of nitrogen source and year on onion yield parameters.

Yield Parameter	Year (Y)	Nitrogen Source (NS)	Y x NS
Total bulb yield	ns	*	*
Marketable bulb yield	*	***	ns
Total dry mass	ns	**	*
Dry bulb yield	ns	*	*
Harvest index	ns	**	ns

ns, *, **, ***Nonsignificant or significant at 5% (p < 0.05), 1% (p < 0.01), or 0.1% (p < 0.001).

). The interaction effect between N source and year was significant for total bulb yield, total dry mass, and dry bulb yield. However, the interaction effect between N source and year did not significantly influence marketable bulb yield and harvest index, although year had an influence on marketable bulb yield.

3.8. Total Bulb Yield

Highest total bulb yield was achieved under CaCN₂ + urea (39.57 t/ha⁻¹) and CaCN_{2 +} LAN (39.05 t ha⁻¹), followed by CaCN₂ (37.75 t ha⁻¹), urea (36.29 t ha⁻¹), and then LAN (36.01 t ha⁻¹), while the lowest was recorded in the control treatment (16.61 t ha⁻¹) (Figure 12). There were no significant differences between the application of CaCN₂ + urea and CaCN₂ + LAN, as well as between LAN and urea, for total bulb yield. Increased bulb yield in response to N availability was previously reported [44,45].

Growth characteristics such as plant height and bulb diameter influence onion yield [39,46,47,48]. The significant increase in data collected on vegetative growth parameters (leaf chlorophyll, plant height, leaf number and leaf length), and a positive increase in bulb diameter, contributed to the high total yield when plants received CaCN₂ + LAN, CaCN₂ + urea, followed by CaCN₂ alone compared to the standard N source fertilizers, urea and LAN. The high yield when either LAN or urea were topdressed can be attributed to the fact that both fertilizers are fast-release fertilizers. This characteristic makes the nutrients readily available to the onion plants as soon as they are applied [49]. However, a potential downside is that these fast-release fertilizers can also leach quickly, leading to nutrient loss. In contrast, CaCN₂ serves as a slow-release N fertilizer. It gradually releases N to the onion plants, ensuring a steady and consistent supply of nutrients. This slow-release N fertilizer offers several advantages over fast-release fertilizers, especially in terms of reducing nutrient leaching [24,25,39]. CaCN₂ provides continuous and controlled N [36] that enhances overall plant growth and high onion yields compared to standard N sources of urea and LAN, which have short-lived nutrient availability. Even though CaCN₂ was applied at 80 kg ha⁻¹ as preplant only, it outperformed standard N sources of LAN and urea applied at 130 kg ha⁻¹.

3.9. Marketable Bulb Yield

N source and year significantly affected onion marketable bulb yield (p ≤ 0.05) (Table 4). In both years, the highest total yield per ton was achieved under CaCN₂ + urea (35.5 and 32.07 t ha⁻¹), while the lowest was recorded under the control (13.42 and 13.87 t ha⁻¹) (Figure 13). Moreover, it should be considered that variations in temperature may have contributed to these fluctuations observed between the two study years, potentially impacting the onion crop yield. Temperature fluctuations can influence plant growth, development, and crop maturation, thus affecting yield outcomes. Therefore, while the nitrogen source clearly plays a pivotal role in onion bulb yield, the year-to-year variations in temperature should also be considered [47]. The highest total yield achieved under CaCN₂ + urea could be associated with the slow-release fertilizer effect of CaCN₂, which is less prone to N loss from leaching or volatilization [36], reducing the risk of nutrient deficiency in the onions. Similar findings that slow-release N fertilizer provided a consistent and sustained supply of N were reported, ensuring optimal onion growth over an extended period [48,50]. In contrast, fast-release fertilizers deliver N rapidly, but for a shorter duration.

3.10. Total Dry Mass, Dry Bulb Yield, and Harvest Index

N source significantly increased total dry mass (p < 0.001), bulb dry yield (p = 0.0338), and harvest index (p = 0.0203). Total dry mass was significantly greater with CaCN₂, CaCN₂ + LAN, and CaCN₂ + urea, followed by LAN and urea, while 0 N was the lowest (Figure 12). Plants grown in CaCN₂ + LAN and CaCN₂ + urea had a higher dry bulb yield, followed by CaCN₂, while urea and LAN had the lowest among the N sources, with the least recorded for 0 N (Figure 12). The harvest index displayed a notably higher value for both the CaCN₂ + LAN and CaCN₂ + urea treatments. Additionally, no significant differences were observed among the CaCN₂, LAN, and urea treatments for harvest index (Figure 14). This enhanced productivity may be attributed to more assimilated production and partitioning by plants with slow-release CaCN₂ fertilization as an N source. The ability of CaCN₂, as a slow-release fertilizer, to be available for a longer period during the plant growing period while promoting vigorous growth could be an added advantage [34,35,36]. Similar results were also reported in onion when using a slow-release fertilizer [46,51].

3.11. Nitrogen Uptake Efficiency, N Utilization Efficiency, and N Use Efficiency

Nitrogen significantly influenced the leaf N content of onions, as depicted in Figure 15. The source of N exerted a highly significant effect on N uptake efficiency (NUPE), N utilization efficiency (NUTE), and N use efficiency (NUE) (Figure 15). The interaction between year and source of N showed no significant impact on NUTE and NUE but was significant for NUPE. Among the treatments, urea resulted in the highest NUPE (1.405%), while CaCN₂ yielded the lowest value (1.23%), significantly different from other treatments (Figure 15). Topdressing with either LAN or urea to CaCN₂ did not significantly influence NUPE. However, there was no significant difference between LAN, CaCN₂ + LAN, and CaCN₂ + urea on the NUPE. The source of N also had a significant effect on NUTE, with CaCN₂ + urea leading to the highest value (0.81%) and LAN leading to the lowest (0.63%). Nevertheless, there was no significant difference between CaCN₂ + LAN and CaCN₂ + urea on NUTE. The source of N significantly increased NUE in onions, with CaCN₂ + urea achieving the highest NUE (0.973%) and LAN yielding the lowest (0.723%) (Figure 15).

The reason for the increased N use efficiency seen when topdressing urea to CaCN₂, in contrast to the blend of LAN and urea, is a result of the distinctive synergistic interplay between calcium cyanamide’s gradual N release and urea’s rapid N release, effectively enhancing the availability of nutrients for plants across an extended duration. As a result, this controlled and prolonged release mechanism can contribute to improved NUE. The impact of different N fertilization methods on onion yield was examined [52,53,54]. It was observed that slow-release N fertilizers exhibited higher NUE, leading to improved onion yields compared to fast-release fertilizers. The consistent and sustained supply of N from slow-release fertilizers positively influenced onion growth, resulting in higher crop productivity over an extended period. The interaction between year and source of N was only significant for NUPE, and no significant differences were observed for either NUTE or NUE. Nitrogen use efficiency tended to be greater when either LAN or urea was top-dressed to slow-release CaCN₂ compared to LAN and urea alone. This effect may be primarily attributed to a reduction in NUPE, as the proportion of LAN and urea increased, thereby minimizing losses by N leaching. When NUE is high, onion plants can effectively convert the available N into the essential compounds needed for bulb enlargement and maturation. This efficient utilization of N promotes the formation of larger and healthier onion bulbs [55,56,57]. The higher N use efficiency presented in Figure 15 aligns with the results observed on yield, indicating that adequate N supply during the bulb formation stage leads to larger and heavier bulbs. CaCN₂, as a slow-release fertilizer, possesses the ability to continuously provide sufficient N to onion plants. Additionally, top-dressing with urea offers even better advantages since it is readily available to the plant while the N from CaCN₂ is slowly released throughout the entire growing period.

3.12. Nitrogen Leaching

Soil N concentrations from WFDs (Figure 1) were considerably significant (p < 0.001) and influenced by different N sources (Figure 16). No N leaching was available on the WFD from the first seven days after planting, but leaching started to be available on the second week after planting. The highest N concentration from the WFD was collected from the fourth week on all treatments and the lowest was at week 2 (Figure 16). LAN registered the highest N leaching (40.75 mg L⁻¹ NO₃-N) and the lowest (4.81 mg L⁻¹ NO₃-N) was obtained when CaCN₂ was added to urea treatments at week 5 (Figure 16). All treatments showed a similar trend in soil solution N concentrations, which were mostly lower than the South African permissible drinking water standard of 44.5 mg L⁻¹ NO₃ (10 mg L⁻¹ NO₃-N) (DWAF, 1993). The addition of slow-release CaCN₂ fertilizer to both LAN and urea decreased N leaching by 44% and 51%, respectively (Figure 16).

Calcium cyanamide’s slow-release nature offers a remarkable advantage for onion plants, ensuring a consistent and gradual supply of N throughout their entire growth cycle [39]. This steady uptake of N optimizes the plant’s nutrient utilization, fostering robust growth and development. In contrast to conventional fast-release N fertilizers like LAN and urea, calcium cyanamide’s gradual N release significantly reduces the risk of N leaching into the soil and groundwater. This environmentally friendly characteristic minimizes water pollution and nutrient loss, contributing to sustainable agricultural practices [18,56,57]. The controlled and sustained release of N from CaCN₂ perfectly aligns with the onion plant’s changing nutrient demands during various growth stages. This delicate balance in nutrient availability supports essential physiological processes, ultimately enhancing overall plant health and maximizing yield potential [58,59].

4. Conclusions

A pre-plant application of 80 kg ha⁻¹ N from CaCN₂ resulted in better performance than standard fertilizers of urea and LAN applied at 130 kg ha⁻¹ N. This led to a cost-saving of 50 kg ha⁻¹ N (38.5% N) and reduced top-dressing expenses (labor and fuel) on sandy loam soil. However, when CaCN₂ was pre-applied (80 kg ha⁻¹ N) and then top-dressed with LAN (50 kg ha⁻¹ N) or urea (50 kg ha⁻¹ N), the highest yield was achieved compared to using urea and LAN as sole N sources. Calcium cyanamide demonstrated its potential as an alternative N source, enhancing growth, yield, and reducing N leaching in sandy loam soil-grown onions. Future studies should focus on optimizing slow-release nitrogen fertilizer application across various soil types. Additionally, further research is needed to determine the optimal application rates for urea and LAN as top-dressing following preplant CaCN₂ application. The study’s implications are significant for onion production practices. Incorporating preplant CaCN₂ application, followed by appropriate top-dressing, can substantially improve onion yield compared to traditional N sources like LAN and urea. This approach can also minimize N leaching, enhance N use efficiency, and contribute to sustainable agriculture by reducing environmental impact.