Principles and Significance of Nitrogen Management for Blackberry Production

Abstract

Blackberry cultivation presents significant opportunities for fruit growers in subtropical regions, where nitrogen (N) is identified as a crucial macronutrient for optimal production. Given the variability in climate and soil conditions, determining the ideal N fertilizer amount can be complex. Effective blackberry cultivation requires careful attention to the principles of nutrient stewardship, including the selection of appropriate N sources, application rates, timing, and placement. Recommended N rates generally range from 25–45 kg/ha in the first year and 45–70 kg/ha in subsequent years, with adjustments based on plant type and regional conditions. The choice of fertilizer, particularly NH₄⁺, is beneficial for blackberry plants, which thrive in acidic soils and show improved biomass and chlorophyll levels with this form of N. Research on N-cycling reveals its importance in supporting new plant growth, such as primocane development. However, improper N management, either excessive or insufficient, can negatively impact flower bud production and, consequently, fruit setting and yield. By using databases such as Google Scholar, Scopus, and Web of Science, this review synthesizes existing research on the role of N in blackberry cultivation, emphasizing the importance of precise fertilization practices tailored to regional climate and soil conditions. By highlighting variations in recommended N amounts and underscoring the principles of nutrient stewardship, this review aims to guide growers in achieving sustainable and high-quality blackberry production.

1. Introduction

Overview of Blackberry Production

The blackberry (Rubus fruticosus L.) is an indigenous fruit crop to North America and Europe, belonging to the Rubus genus within the Rosaceae family [1]. The cultivated blackberries originate from perennial plants with biennial canes, which are the main parental species. These plants generate vegetative canes termed primocanes in the first year, which are then referred to as floricanes following a period of dormancy. The floricanes undergo flowering, fruiting, and subsequent death, while fresh vegetative primocanes are in the process of growing. Primocane-fruiting varieties have been recently created. Blackberries often exhibit greater size and vitality compared to raspberries. Cultivated varieties of blackberries have either a prostrate (trailing) or highly upright (erect) growth pattern, with canes reaching heights of up to 5 m [2].

Renowned for their nutritional richness, blackberries boast high levels of sugars, various organic acids, vitamins, and phenolics, particularly anthocyanins and ellagic acid. They are lauded for their health benefits, exhibiting the greatest antioxidant activity among berries such as red raspberries, strawberries, cherries, and blueberries [3]. Blackberries are also rich in phenols and anthocyanins, contributing to their anti-inflammatory, antibacterial, and anti-aging properties. Regular consumption of blackberries strengthens the immune system, aids in body weight control, and reduces the risk of neurological disorders and certain cancers [4,5].

A global assessment of blackberry production conducted in 2005 [6] revealed a significant expansion in the production area, with approximately 20,035 hectares of blackberries being planted and commercially grown worldwide. This represented a substantial 45% increase compared to the production area in 1995. Wild blackberries also made a considerable contribution to global output, with 8000 hectares and 13,460 metric tons harvested in 2004. In 2005, Europe had a total of 7692 hectares of blackberry cultivation for commercial purposes. Serbia had the greatest blackberry acreage in Europe, accounting for 69% of the total with 5300 hectares. It was also the world’s leading producer. Serbia’s output reached 25,000 tons, ranking it fourth globally [6]. The Agricultural Marketing Service of the USDA played a pivotal role in providing data on shipments and prices, enabling the monitoring of trends in the fresh blackberry market in the United States. The analysis focused on the growing number of blackberry shipments from both domestic and foreign production locations, as well as pricing trends in key terminal markets across the country. The data revealed a significant increase in newly harvested blackberry exports, with a staggering 530% rise observed between 2000 and 2008. The USDA reported a steady rise in blackberry output from 26,140 tons in 2012 to 29,180 tons in 2016, indicating a continued increase in consumer demand for blackberries [7].

According to Business Research Insights, the global blackberry market size is projected to reach $1834.72 million (USD) by 2027, underscoring the growing popularity and economic significance of blackberries on a global scale [8]. Despite their short shelf life due to rapid respiration and delicate skin, blackberries are versatile and commonly processed into various products such as jam, juice, yogurt, fruit wine, and pastries [9]. Quality is paramount for both consumers and food processors, driving growers to prioritize traits like firmness, size, flavor, and nutritional content. Efficient cultivation practices, including fertilization management, play a crucial role in achieving these desired attributes [10].

Blackberry plants have a very modest fertilizer need in comparison to several other perennial fruit crops like blueberries and raspberries. Understanding the yearly buildup of nutrients and the time periods when they are rapidly absorbed eases more effective control of fertilization strategies. The annual buildup of N in the aboveground plant varied between 37 and 44 kg/ha for blackberry [11]. The effect of N fertilizer rate on blackberry yield is influenced by factors such as the specific types and cultivars of blackberries, the growing areas, and perhaps the soil type [12,13,14,15]. For plants that bear fruit every two years, applying N in one season can have an influence on both the current production of the mature canes, mostly by increasing the size of the fruit, and the output of the following season, by affecting the growth of new canes and the formation of flower buds [16]. Applying N to annual-fruiting varieties can enhance the number of primocanes, accelerate development and flowering, and ultimately boost the yield [15,17].

2. Nitrogen vs. Blackberry Production

2.1. Nitrogen as an Essential Nutrient for Blackberry

The unique development pattern of blackberry plants, with nutrient accumulation in primocanes and roots followed by redistribution during bud break and fruit production is primarily facilitated by the vascular system of the plant, specifically through the phloem, complicates nutrient management [18]. Nitrogen fertilizer significantly impacts fruiting laterals’ growth and fruit development throughout the year, as well as the creation of new primocanes and leaves during non-productive periods. Accumulated N during off-years facilitates early growth of floricanes, fruiting laterals, and subsequent fruit production in the following year [19]. Nitrogen rate recommendations differ depending on the blackberry cultivar, with starting rates typically between 34 and 56 kg/ha, regardless of the growth habits of the cultivar. Trailing cultivars respond well to nitrogen rates between 56 and 78 kg/ha, while upright cultivars do well with rates of 56 to 90 kg/ha starting in the second year [20].

Nitrogen stands as a cornerstone of life, surpassing all other elements in terms of its necessity for most plants, second only to carbon, oxygen, and hydrogen. Its presence often acts as a limiting factor in both natural and managed ecosystems [21,22]. In conditions of N deficiency, blackberry plants exhibit symptoms such as chlorosis (yellowing of leaves) and reduced growth rates. Older leaves are more affected, as N reallocates to active sinks like younger leaves and reproductive organs [23]. Severe N deficiency inhibits growth and leads to wilting, while moderate deficiency impedes development, reducing crop yields compared to those obtained with sufficient N. Furthermore, insufficient N accelerates aging and ripening processes [23], while excess N can reduce yield, worsen quality, and even pose hazards to animals. The complex interplay of these factors complicates determining the optimal quantity of N fertilizer to complement existing soil N content [24].

2.2. Nitrogen Fertilization

Nitrogen is essential for blackberry production, and crucial for plant growth and development [25]. About 60% of the total N in leaves is found in proteins, key components of the photosynthetic system, with chloroplasts in mesophyll cells containing up to 75% of this N, directly influencing photosynthetic capacity [26,27,28]. Adequate N availability supports greater photosynthetic rates, vigorous vegetative growth, and healthy green foliage by enhancing chlorophyll synthesis and optimizing Rubisco function [29,30].

Despite blackberry crops’ relatively modest nutrient requirements compared to other perennial fruit crops, understanding nutrient accumulation dynamics and peak demand periods is crucial for devising effective fertilization strategies [31]. This becomes particularly relevant in scenarios where accelerated growth is observed, necessitating tailored fertilization approaches [32]. Harmonizing the fertilization plan with overall crop management practices is imperative, and N fertilization has been shown to positively influence blackberry plant development and fruit production under varying environmental conditions [32]. Strategic N fertilizer application significantly boosts photosynthesis in blackberry plants, thereby enhancing fruit yield and elevating concentrations of beneficial compounds like anthocyanins, polyphenols, and ellagic acid [33].

The utilization of N fertilizers in fruit crops has raised concerns regarding increased susceptibility to physical damage, as excessive N levels can stimulate excessive vegetative growth, potentially compromising fruit development and resulting in a softer texture [34,35,36]. Studies have demonstrated that increasing the N application rate can lead to notable improvements in fruit quality attributes, such as pH, soluble solids, sugar content, and anthocyanin concentration in blackberry fruits [37]. However, while high N dosages may increase biomass and fruit yield in the initial year, fruit quality may suffer in subsequent years [38]. The postharvest quality of blackberries is intricately linked to N application rates and fertilizer sources, with research indicating that increasing N supply from 60 kg/ha to 100 kg/ha can elevate fruit sugar content and pH, particularly when combined with high potassium (K) levels, thereby enhancing antioxidant content [37]. Meanwhile, research by Alleyne and Clark [20] showed that while increasing N fertilizer rates did not affect sugars or titratable acidity (TA), it significantly increased pH in ‘Arapaho’ blackberries.

This literature review addresses the question: How do N fertilization practices influence blackberry yield, fruit quality, and sustainability? The review underscores the importance of adhering to principles such as selecting the appropriate source, rate, timing, and placement of N fertilizer to optimize blackberry productivity. Additionally, it examines the preference of blackberry plants for specific N forms, particularly NH₄⁺, and the impact of N application at different growth stages on biomass, chlorophyll levels, and overall plant development. It also analyzes the effects of N on critical physiological processes like chlorophyll concentration and photosynthetic activity.

2.3. The Four R Principles vs. Nitrogen Management

The four Rs of nutrient stewardship serve as a practical framework emphasizing precision and accuracy in nutrient management practices that help keep all nutrients applied in the root zone. These principles entail utilizing

• the right source, applying it at
• the right rate, timing the application at
• the right time, and locating it in
• the right placement [39].

While these principles are widely recognized, some regions may overlook the inclusion of irrigation. Integrating irrigation into these principles is essential for maintaining nutrient control in commercial crop production [40,41]. The choice of nutrient source, application rate, timing, and placement are fundamental components of effective nutrient management. Their significant impact lies in their ability to enhance sustainability by reducing pollution through nitrate leaching, minimizing N loss from ammonia volatilization, mitigating the effects of global warming by reducing soil greenhouse gas emissions, and maintaining plant health and crop yield [42].

2.3.1. Selecting the Right N Source

Choosing the appropriate N fertilizer source is pivotal in optimizing blackberry cultivation. Factors such as cost-effectiveness and the specific nutritional requirements of the plants must be carefully considered [43]. It is advisable to opt for either solution-grade fertilizer or noncoated, water-soluble fertilizer to ensure efficient nutrient delivery [44]. Previous research indicates that blackberry plants exhibit enhanced biomass and chlorophyll levels, along with stronger antioxidant systems when supplied with NH₄⁺ [45,46]. This preference for NH₄⁺ suggests its importance in blackberry nutrition. In contrast, the application of NO₃⁻-N treatment has been observed to significantly reduce the total soluble solids (TSS) content of blackberry fruits, while NH₄⁺-N treatment stimulates TSS accumulation [10].

Furthermore, studies have shown that blackberry fruits exhibit optimal size and firmness when subjected to NH₄⁺-N or urea, as opposed to NO₃⁻-N alone [10]. Among commonly used fertilizers, including ammonium nitrate, urea, and ammonium sulfate, the latter is preferred for blackberry cultivation in Brazil due to the crop’s substantial sulfur requirement [47,48]. However, it is essential to note that prolonged use of ammonium sulfate as an N source may lead to significant reductions in soil pH and an increase in the concentration of Al³⁺ ions [49]. Therefore, regular monitoring of soil pH and Al³⁺ concentration is imperative when utilizing ammonium sulfate to ensure optimal growing conditions for blackberries and avoiding soil acidification.

2.3.2. Determining the Right N Rate

Selecting the appropriate rate of N application is essential for maximizing blackberry production while minimizing potential adverse effects on plant growth and fruit quality. This decision relies on extensive research spanning various factors such as location, crop types, varieties, and specific crop seasons [43]. The optimal fertilization rate refers to the precise amount of fertilizer applied during a single occurrence throughout the growing season [43]. Assessing the current soil nutrient status is crucial in determining the need for supplemental nutrients, as some nutrients may already be naturally present in the soil.

However, nitrogen is mobile in soils, so a soil test for this nutrient offers only a brief snapshot of the soil’s nutrient levels. The results can be rapidly altered by rain or irrigation [50]. It is important to strike a balance between providing adequate N for plant growth and avoiding excessive application rates that can lead to negative impacts. For blackberry cultivation, research has shown that exceeding an N delivery rate of 40 g per plant during critical phases of development, such as flowering and fruiting, can result in adverse effects. These effects include a reduction in the number of buds per plant, individual fruit weight, and the transverse and vertical diameters of fruits, ultimately leading to a decline in overall yield [33]. Therefore, careful consideration of N application rates is necessary to optimize blackberry production while maintaining fruit quality and yield. The recommended N rate (

Table 1. Recommended N rate based on the year of production and different places in the United States.

Type of Plant	Recommended Range (N) Based on Year of Production		Recommended Range (N) Based on Region
	First Year (kg/ha)	Subsequent Year (kg/ha)	Recommendation (kg/ha)	Place/Region
Erect	25–45	45–70	None	Arkansas
Semi-erect	25–45	45–70	123	Kentucky
Trailing	25–45	45–60	67	Oregon

) is based on the production year of blackberries and the locations in the United States [13,32,51,52].

In a study conducted on semi-erect ‘Hull’ blackberry plants in Kentucky, manipulation of N application rates yielded insightful results. By varying N rates at 0, 41, and 123 kg per hectare, researchers observed a consistent and proportional increase in both individual fruit weight and overall yield with greater quantities of N fertilizer [13]. Furthermore, optimizing N rates within the range of 20 to 40 g per plant resulted in favorable outcomes. This included greater levels of soluble solids and anthocyanin, reduced acidity, and improved fruit quality, sweetness, and taste [33].

2.3.3. Timing the Right N Application

The timing of N application is crucial for maximizing crop productivity while minimizing nutrient loss. Robertson and Vitousek [53] emphasized the importance of matching N availability with crop demand to prevent excess N loss. It is recommended to apply N fertilizers cautiously to avoid adverse effects on plant roots or excessive foliage growth at the expense of fruit buds [54]. In blackberry production, optimal N application timing is closely linked to critical growth stages. Research on the ‘Shuofeng’ blackberry demonstrated that applying 20–40 g of ammonium sulfate per plant at three key stages—early bud burst, flowering, and fruit development—resulted in optimal yield and fruit quality [33]. These stages are pivotal for achieving greater yields in blackberry production.

Dividing N applications is more efficient in the Rubus genus. Typically, the first application is done in March, followed by another in May [55]. However, Naraguma [51] found that while split applications increased N concentrations in ‘Arapaho’ blackberry leaves, no further benefits were observed. Regarding timing, N should also be applied post-harvest and in spring [56]. The timing of the annual N fertilizer application should align with the onset of spring, just before significant plant growth begins, to effectively support both fruit development and the establishment of primocanes [49].

Post-harvest fertilizer application, typically during trimming, aims to stimulate primocane growth, resulting in sturdier stems capable of supporting larger fruit yields in the following season. Stronger stems correlate with larger fruit sizes in blackberries, highlighting the importance of post-harvest N application for future productivity [57,58].

2.3.4. Ensuring the Right Placement of N Application

It is imperative to deliver nutrients where plants can easily access them to optimize nutrient efficiency. Typically, the best location for nutrient application is within the root zone or slightly ahead of the growing root system, maximizing absorption potential [43]. For blackberry cultivation, nitrogen is commonly applied in a granular form directly within the row during the initial planting stage within the growing root system [48]. Alternatively, it can be applied at different growth stages using methods such as fertigation or foliar fertilization [48]. While foliar fertilization is an option, it often fails to yield desirable results due to the plant’s significant nutritional demands [32].

3. Understanding the Mechanism of Nitrogen Dynamics

3.1. Nitrogen Acquisition by Blackberry Plants

Nitrogen, a vital macronutrient, serves a multitude of functions and is often a limiting factor in crop production [23]. In perennial plant communities, the physiological processes involving N include its acquisition from the soil, translocation within the plant, assimilation (integration of N into organic molecules), storage, and remobilization when needed [59,60,61]. These processes exert significant influence on plant growth, development, and overall productivity across various agricultural and ecological contexts. Therefore, comprehending and effectively managing N dynamics are paramount for optimizing nutrient use efficiency and maintaining the health of ecosystems.

The primary forms of N uptake by plants include ammonium (NH₄⁺) and nitrate (NO₃⁻), facilitated by specialized transporters [62,63]. However, different plants exhibit distinct preferences for these N forms, influenced by factors such as plant species, growth stage, and environmental conditions like temperature, precipitation, and light [64,65]. Typically, there is limited absorption of fertilizer during plant dormancy, but uptake increases as new shoots and leaves emerge in spring [60]. Interestingly, only 32% of the fertilizer supplied to highbush blueberry bushes during dormancy is effectively utilized by the plant throughout the growing season [66].

In general, nitrate (NO_₃⁻) N may offer greater advantages during periods of drought, whereas ammonium (NH_₄⁺) N serves as the predominant N source assimilated in soils subjected to flooding, freezing, or acidity [63]. Ammonium N is favored by most plants as their primary N source due to its ease of assimilation, leading to reduced metabolic expenditures during its absorption [62]. Several studies have explored the N cycling patterns in blackberries and raspberries. For instance, some studies [19,67,68] have examined N cycling in blackberries, while investigations on raspberries have been carried out by [69,70] utilizing isotopic labeling with ¹⁵N. It highlights the specific research employing isotopic labeling techniques to understand how different N forms affect these plants under varying conditions.

Nitrogen from newly applied fertilizer primarily supports the development of new plant parts in blackberry cultivars like ‘Chester Thornless’ and ‘Arapaho’, including primocanes, primocane leaves, and fruits. This N allocation remains present in these plant tissues until the later stages of the growth season [67,68]. Although some N moves within the primocanes during later growth seasons, a large part of the N stored in the primocanes is reused by the floricanes in the following growing season [67]. Additionally, a small amount of N accumulated in the primocanes during winter is mobilized to initiate fresh primocane development in the spring. Therefore, the development of primocanes relies heavily on newly applied N fertilizer [19,67,68,69].

Mohadjer et al. [19] investigated the ‘Kotata’ trailing blackberry using a biennial production system, following the framework proposed by Strik and Finn [71]. They observed a limited quantity of N in the primocanes of “on-year” plants, characterized by the presence of both fruit-bearing primocanes and floricanes. Approximately 20% of the N obtained from fertilizer was detected in the produce of the subsequent productive year when ¹⁵N was applied during the dormancy period (primocanes alone), but this percentage may vary due to unique soil and climatic conditions, such as those found in Florida. This suggests that reserves acquired during the non-productive year play a vital role in fueling growth during the productive year. Nitrogen deposited in the crown and root tissues was utilized throughout the annual growth cycle to promote the growth of fruit-bearing branches and produce. Therefore, N fertilizer is required during both the “on-year”, when it stimulates the growth of fruiting branches and fruit, and the “off-year”, when it promotes the establishment of new primocanes. A study found that N recovery was significantly greater in primocanes and their leaves compared to other tissues in both years. However, in 1995, roots and floricanes had lower N levels compared to primocanes and their leaves, indicating that new growth relied more on newly applied fertilizer rather than accumulated reserves [68]. The N fertilizer recovery in blackberry, expressed as a percentage of the given ¹⁵N, ranged from 32% to 45% [19,68], while red raspberry plants exhibited a recovery range between 24% and 37%, with specific percentages varying depending on the timing of fertilizer application [69]. However, focusing solely on the recovered N may not provide a comprehensive understanding of the plant’s total N consumption, as N fertilizer uptake can influence the intake and use of alternative N sources in the plant, including stored N and soil N [72].

3.2. Nitrogen Accumulation by Blackberry Plants

Blackberry plants display relatively low dry weight and N accumulation per hectare compared to other perennial crops, attributed to wider row spacing (typically 3 m) and smaller plant size, coupled with lower yields [19,48,69]. Trailing blackberry plants in fields absorb approximately 45% of the provided fertilizer, with peak N absorption occurring in August [19]. By late June, just before fruit harvest, fertilizer N distribution is as follows: 39% in the fruit, 37% in the laterals, 19% in the primocanes, 3% in the floricanes, and 2% in the crowns [48].

On average, over a two-year period, 37% of freshly accumulated N is associated with harvested fruit, while 58% is recovered from pruning, and 5% is allocated to crowns for the next development cycle. During the off-year, there is a 44% increase in N buildup [19]. Most N acquired from fertilizer is distributed to primocanes, primocane leaves, and fruit. Negligible transfer of accumulated N from fertilizer occurred in 1989, and N stored in floricanes ceased to function as a source of stored N after senescence. Although roots retain most residual fertilizer N after two seasons, this accounts for only 15% of the initially supplied N [67].

Plants receiving larger N doses exhibit increased N absorption, leading to greater tissue N levels [69]. Mature fruit from fertilized plants show variable N content, ranging from 1.4% to 1.7% in red raspberries, 1.4% to 1.6% in ‘Kotata’ trailing blackberry, 1.5% to 1.6% in ‘Arapaho’ erect blackberry, and 0.9% to 1.1% in ‘Black Diamond’ and ‘Marion’ trailing blackberry [20,69,73]. Elevated N content in fruit is attributed to increased N fertilization rates [69]. These findings focus on greater N fertilization boosting N levels in blackberry and raspberry fruit, but the response varies by cultivar. While increased N can improve nutrient content, it is crucial to tailor fertilization to avoid potential drawbacks like reduced fruit quality.

4. Nitrogen vs. Blackberry Berry Yield and Quality

4.1. Blackberry Berry Yield

The correlation observed between the nutritional status of plants and fruit yield across two growth cycles underscores the critical role of N as a fundamental component in overall production [18,32,58,74]. A significant relationship was identified between the N and K levels in leaves and fruit production, suggesting that the nutritional status of blackberry plants may influence variations in fruit yield [75].

Appropriate N fertilization resulted in a considerable increase in chlorophyll levels, maintaining them at greater levels throughout the growth phase and substantially enhancing photosynthetic capacity, thereby extending the period of photosynthetic activity. Conversely, excessive N led to prolonged vegetative growth, depleting significant nutrient reserves, and hindering flower bud development. Consequently, reduced flower bud production resulted in a decline in fruit set rate and overall yield [76].

An experiment demonstrated that a high N treatment of 212 kg/ha significantly increased yield compared to a low N treatment of 53 kg/ha, particularly due to the greater fruit quantity and size [38]. The notable increase in overall harvest can be attributed to the recurring two-year cycle typical of blackberries bearing fruit on second-year canes. These findings align with previous research [43,62,63], indicating that N fertilizer inputs primarily promote new growth, especially primocanes.

Studies on semi-erect ‘Hull’ blackberry plants in Kentucky and erect ‘Arapaho’ blackberry plants in Arkansas showed varying effects of N fertilizer rates. For ‘Hull’ blackberry plants, increasing N rates led to a linear increase in both single-fruit weight and overall yield [13]. However, N fertilizer did not affect the yield of erect ‘Arapaho’ blackberry plants in Arkansas [68], suggesting a likelihood of increased yield in the second year.

4.2. Blackberry Berry Quality

Based on the findings of Strik and Vance [48,77], it has been demonstrated that N acquired in the early stages of the season is primarily utilized to promote the growth of new plant parts, including primocanes, primocane leaves, and fruits. As the growing season progresses, plants continue to absorb N and store it in primocanes, roots, and crowns for the winter [51,67,77]. The quantity of N fertilizer applied has significant effects on the physical attributes of ‘Shuofeng’ blackberry fruits, notably influencing their firmness, transverse diameter, and vertical diameter [33]. Although Strik and Vance [48] do not provide specific data on yield, they offer definitive information on how the N application rate affects the N content in the fruit, which subsequently impacts its nutritional value (Figure 1). Alleyne and Clark [20] noted that greater N rates resulted in increased N levels and pH in berries, but had no significant impact on other quality factors, such as the concentration of soluble solids, titratable acidity, sugar–acid ratio, and total solids. In a similar vein, research on the ‘Thornfree’ blackberry revealed that N fertilizer did not significantly affect the fruit’s pH level, total acidity (TA) content, or soluble solids level [78]. Conversely, different findings were observed in the ‘Loch Ness’ blackberry, where the application of varying amounts of N and K fertilizers notably affected the acidity and juice pH of the berries [37]. A recent study further supports this by showing that applying N at the recommended rate of 20–40 g per plant led to greater levels of soluble solids and anthocyanin contents while reducing acid content, which enhanced fruit quality, sweetness, and taste [33].

5. Blackberry Fertilization Guided

Effective N fertilization is crucial for optimizing the yield and quality of blackberries (

Table 2. Summary of nitrogen fertilization in blackberry cultivation.

Plant Type and Different Stages	Recommended Dose (kg/ha N)	Timing of Application	Effect on Yield	Effect on Quality	Study/Reference
General N application	60–100	Pre-bloom	Elevated fruit sugar content, increased pH	Enhanced antioxidant content when combined with high K levels	[37]
Trailing blackberries	45–60	N/A	Higher N rates improve yield	Balanced with K for better antioxidant content	[32]
Flowering/fruiting	>40 g/plant	Flowering and fruiting	Reduced bud count, individual fruit weight, and fruit dimensions	Decline in overall yield due to adverse effects on fruit development	[33]
Semi-erect ‘Hull’ blackberry	123	N/A	Proportional increase in fruit weight and yield with increased N	Improved fruit quality, greater soluble solids, anthocyanin, reduced acidity, enhanced sweetness and taste	[13,33]
Erect ‘Ouachita’ blackberry	212 vs. 53	N/A	Significantly increased yield	Larger fruit quantity and size	[38]

). However, the application rates and timing must be carefully managed to avoid negative effects on plant growth and fruit quality. In the first year of production, it is recommended to apply 25–45 kg/ha N, focusing on developing a strong root system and vegetative growth. In subsequent years, this rate should be increased to 45–70 kg/ha N, depending on plant type and regional conditions. For example, erect blackberries in Arkansas should receive 25–45 kg/ha N in the first year, increasing to 45–70 kg/ha N in subsequent years. Semi-erect blackberries in Kentucky may require 25–45 kg/ha N initially, with potential increases up to 123 kg/ha N depending on specific conditions. Trailing blackberries in Oregon should receive 25–45 kg/ha N in the first year and 45–60 kg/ha N in subsequent years (Table 2).

During critical developmental phases such as flowering and fruiting, it is important to avoid exceeding 40 g of N per plant, as this can lead to a reduction in buds, fruit weight, and overall yield. Research indicates that N application rates between 60–100 kg/ha can improve fruit sugar content, pH, and antioxidant levels, especially when combined with adequate potassium (K) levels (Table 2). Additionally, controlled-release fertilizers can be beneficial for maintaining steady N availability, particularly in soils with varying properties. In conclusion, careful management of N fertilization is essential for achieving high yields and maintaining the quality of blackberries. By following these guidelines and adjusting based on local soil conditions and plant responses, growers can optimize their production practices. Continued research and field trials are recommended to further refine these recommendations.

6. Conclusions

Blackberry cultivation in subtropical regions presents a significant opportunity for fruit growers, with N fertilization playing a crucial role in optimizing plant growth and fruit quality. The complexity of determining the optimal N fertilizer amount stems from varying climate and soil conditions. Nutrient stewardship is essential, focusing on the correct source, rate, timing, and placement of N fertilizer. NH₄⁺ form of N has been shown to positively impact biomass and chlorophyll levels, enhancing productivity and fruit quality. Adequate N levels improve chlorophyll concentration and photosynthetic activity, which are vital for plant health and yield. However, both excessive and insufficient N can negatively affect flower bud production and fruit setting rates, leading to variability in yields.

The intricate dynamics of N acquisition, uptake, accumulation, and their effects on blackberry yield and fruit quality underscore a complex interplay of factors. Variations in N uptake preferences among plant species, maturation stages, and environmental conditions emphasize the importance of tailored fertilization programs. Understanding the timing of N absorption, influenced by dormancy and growth phases, is essential for meeting seasonal nutrient demands.

Previous research has shown that optimal N levels lead to increased berry production and improved quality. While N is indispensable for blackberry quality and yield, optimal application rates considering various forms and environmental conditions are essential to minimize adverse effects and achieve desired outcomes. Advancing our understanding of N dynamics in blackberry plants will ultimately contribute to more sustainable and productive agricultural practices.