The 4R Nutrient Stewardship framework is based on applying fertilizer from the right source, at the right rate, at the right time, and in the right place. The 4R's provide a comprehensive guide for optimizing nutrient management in vegetable crops such as tomatoes, bell peppers, broccoli, and cabbage. Implementing these principles effectively enhances efficient use of nutrients, boosts crop yield and quality, and mitigates environmental impacts. This publication is targeted to vegetable growers in North Carolina and Virginia, although the information could be applied to other states in the Southeast. The publication delves into each fertilization principle, describing advanced, research-backed applications specific to key vegetable crops and exploring integrated implementation.

The synergistic implementation of the 4R principles creates a holistic nutrient-management strategy greater than the sum of its parts. For example, in tomato production, the 4R's constitute applying fertilizer from the right source (calcium nitrate), at the right time (in split doses based on growth stages, such as flowering and fruit set), at the right rate (ensuring that the correct amount is available), and at the right place (delivery via a drip irrigation system). This integrated approach enhances nutrient use efficiency and crop yield and reduces environmental impacts such as nitrate leaching and greenhouse gas emissions. By embracing the comprehensive application of the 4R Nutrient Stewardship framework, vegetable growers can achieve sustainable intensification of production, balancing economic viability with environmental stewardship. This approach ensures the long-term health of agricultural ecosystems and the communities that depend on them.

The appropriate nutrient source is crucial for aligning with crop-specific requirements and soil characteristics. For example, in tomato cultivation, calcium nitrate is often preferred over ammonium-based nitrogen fertilizers to prevent blossom-end rot (a disorder sometimes linked to calcium deficiency). Also, research has indicated that using calcium nitrate can enhance fruit firmness and shelf life when combined with adequate irrigation. In broccoli and cabbage, sulfur-containing fertilizers like ammonium sulfate are often beneficial, as sulfur is essential for synthesizing glucosinolates, compounds that contribute to flavor and have health-promoting properties. Adequate sulfur nutrition enhances these compounds, improving both taste and nutritional value. In addition, incorporating organic amendments such as compost can improve soil structure and microbial activity, improving nutrient availability and uptake. In some instances, specific formulation of fertilizers can delay nutrient release, incorporate additives that reduce nitrification and volatilization, and exist in states (such as liquid, solid, or gas) that determine the application method. This flexibility in fertilizer forms (for example, granular sulfur-coated fertilizers) could be beneficial in different production systems, such as those affected by heavy rain and prone to nutrient losses.

Why the Right Source Matters

Choosing the right fertilizer source is a critical first step in nutrient management for vegetable production. Each fertilizer source has unique properties that determine its suitability based on soil conditions, crop requirements, and environmental considerations. Fertilizers are classified based on their nutrient composition and form. They can be single-nutrient sources, such as urea (46% nitrogen) or triple superphosphate (46% P₂O₅), or multinutrient blends that provide a balanced supply of essential nutrients. The choice of source should align with the crop's nutrient demands, soil properties, and application methods.

Sources of Critical Nutrients in Key Vegetable Crops

1. Tomatoes benefit from nitrate-based nitrogen sources, such as calcium nitrate or potassium nitrate, as these promote steady nitrogen availability and reduce the potential risk of ammonium toxicity in excessively cold soils (ammonium-based fertilizers are not usually a problem when the growing medium is warm, however, because nitrifying bacteria naturally found in growing media will convert ammonium to nitrate). In acidic and clay soils often common in North Carolina, lime-based fertilizers such as dolomitic lime can also contribute calcium and magnesium while raising soil pH.
2. Bell peppers are highly responsive to balanced fertilization. Controlled-release fertilizers or sources that combine nitrogen, phosphorus, and potassium in a single granule are an excellent option for reducing nutrient losses to the environment and ensuring steady nutrient availability. For example, urea-formaldehyde (a controlled-release fertilizer) provides a slow-release nitrogen source, ideal for extended production cycles. When micronutrient deficiencies are detected, products like chelated iron or zinc can be applied to meet specific needs.
3. Broccoli requires sulfur-containing fertilizers, such as ammonium sulfate, gypsum, or elemental sulfur. For nitrogen management, ammonium sulfate provides both nitrogen and sulfur, which is especially beneficial in soils with low organic matter. Preplant applications of superphosphate can meet phosphorus requirements, improving early root development.
4. Cabbage requires sufficient boron and calcium to prevent disorders like hollow stem and tip burn. Fertilizer sources such as borax or Solubor provide supplemental boron. Calcium nitrate, a preferred nitrogen source for cabbage, also provides calcium in a readily available form. For potassium needs, potassium sulfate is an excellent chloride-free option, ensuring high-quality head formation without increasing soil salinity.

Table 1 provides a comprehensive list of common synthetic fertilizers available on the market, along with their typical macronutrient compositions.

Organic Soil Amendments

Organic amendments, such as compost or poultry litter, provide a sustainable alternative or complement to synthetic fertilizers. In addition, these amendments improve soil structure, add organic matter, and provide a slow-release nutrient supply. Recent developments in controlled-environment agriculture and automation have opened new possibilities for optimizing the timing and efficiency of organic inputs, especially when integrated with precision monitoring systems. In organic systems, blood meal, bone meal, or fish emulsion can provide nitrogen, phosphorus, and micronutrients. Farmers should ensure that organic amendments are tested for nutrient concentrations to match crop needs. Organic nutrient sources frequently release nutrients at a rate that may not necessarily align with crop demands, necessitating careful irrigation management (maintaining balanced moisture in the top layer of soil) and regular foliar analysis by a plant diagnostics lab.

Recommendations for Farmers

• Soil testing. Conduct soil tests before planting to determine baseline nutrient levels and pH. Use the test results to select fertilizer products that will address specific deficiencies.
• Plant tissue (foliar) analysis. Regular foliar testing during the growing season helps identify emerging nutrient deficiencies, guiding the use of specialized products like chelated micronutrients.
• Crop-specific needs. Match fertilizer sources to the crop’s unique nutrient demands and growth stages. For example, calcium nitrate is ideal for tomatoes during fruiting, while ammonium sulfate suits broccoli’s vegetative growth phase.
• Application compatibility. Ensure that fertilizer sources are compatible with the planned application method. For example, liquid fertilizers are well suited for fertigation systems, while granular options work better in broadcast or side-dress applications. Test the pH of water and fertigation additives to ensure compatibility within the growing system.
• Environmental considerations. Choose source material that minimizes environmental risks such as nitrate leaching or phosphorus runoff. Controlled-release formulations or stabilized nitrogen products are excellent options for reducing nutrient losses. Use irrigation practices that prevent nutrient leaching, such as integration of soil moisture sensors or crop irrigation models (for example, potential Evapotranspiration [ETo] x crop coefficient [Kc] = Crop Evapotranspiration [ETc]) for estimating irrigation requirements.

Determining the precise amount of nutrients to apply will meet crop demands without risk of excess losses that could lead to environmental contamination. Generally, the optimal nutrient rates are determined through soil testing before the crop is established, while in-season adjustments are fine-tuned using plant foliar analysis. Advanced tools like crop vegetation indexes and foliar analysis enable real-time monitoring of plant nutrient status and precise adjustments in fertilization. Incorporating real-time sensor networks and predictive algorithms can further refine nutrient delivery strategies, tailoring inputs to spatial and temporal field variability. For example, in bell pepper production, split applications of nitrogen, guided by plant demand assessments, can enhance fruit yield and quality while reducing nitrate leaching. Utilizing decision-support systems that integrate weather data, soil tests, and crop models can help you further refine nutrient application rates, ensuring that they are tailored to specific growing conditions. Determining and applying nutrients at the correct rate is essential for maximizing crop yield and quality while minimizing nutrient losses and environmental impacts.

Why the Right Rate Matters

Applying too little fertilizer results in nutrient deficiencies that reduce crop yield and quality. On the other hand, the overapplication of fertilizer increases production costs and contributes to environmental problems like nutrient runoff and groundwater contamination. Using the correct rate ensures that crops receive only the nutrients they need for optimal growth.

Recommendations for Farmers

• Soil testing. Conducting soil tests before planting provides the foundation for determining the correct nutrient rate. Soil tests measure nutrient availability and pH, providing baseline data for fertilization. For example, in open fields, determinate tomatoes typically require 120 to 180 pounds of potash (K₂O) per acre over the growing season (90 to 120 days). However, soil tests can identify whether preplant potash levels need adjustment or if supplemental phosphorus and potassium are required. Note that most soil testing labs focus on measuring more stable nutrients (like phosphorus, potassium, calcium, and magnesium) and pH levels. Nitrogen recommendations, on the other hand, are typically based on crop-specific needs, expected yield, and regional guidelines, rather than on soil testing. Consult your local Cooperative Extension center for specific recommendations tailored to your soil type, crop variety, and growing conditions.
• Plant tissue (foliar) analysis. Regular foliar testing during the growing season provides real-time feedback on plant nutrient status. This feedback is particularly useful in bell pepper cultivation because nitrogen and potassium uptake vary significantly between vegetative and fruiting stages. Foliar analysis can detect hidden deficiencies, allowing corrective applications to maintain optimal growth.
• Crop-specific needs. Understanding crop-specific nutrient requirements is critical. In broccoli, for example, nitrogen application rates of 150 to 200 pounds per acre are generally recommended, with half applied at planting and the rest split during the vegetative growth phase. In cabbage, consistent nitrogen supply during heading is essential to avoid hollow stems and poor head formation.
• Yield goals. Fertilizer rates should align with realistic yield expectations, meaning an achievable crop yield estimate based on local conditions such as soil type, weather patterns, management practices, and historical performance rather than ideal or maximum potential yields. Overestimating yield potential can lead to excessive nutrient applications. For example, a high-yielding tomato variety (>25 tons per acre) might justify a higher phosphate rate, but this need should be verified through soil and foliar tests to optimize nutrient use and prevent waste.
• Environmental considerations. The "right rate" principle balances maximizing economic returns with protecting the environment. Applying the correct amount of nutrients reduces input costs while avoiding runoff and groundwater contamination. Weather and soil conditions affect nutrient availability and uptake. Sandy soils common in parts of North Carolina and Virginia have lower nutrient-holding capacity, necessitating split applications to avoid leaching. In addition, prolonged rainfall may wash nutrients out of the root zone, requiring adjustments to planned rates.

Table 2 describes the recommended nitrogen, phosphorus, and potassium rates for common consumer crops in North Carolina and Virginia, based on the 2025 Southeastern U.S. Vegetable Crop Handbook.

Table 2. Recommended Fertilizer Rates for Common Consumer Crops (Pounds per Acre)

¹Grown in open-field plasticulture systems. ↲

²Apply nitrogen in split applications. For example, apply half at planting and the remainder during the growing season, based on needs of the crop grown. Note that nitrogen is not usually included in standard soil tests because it is highly mobile and variable in the soil, making it unreliable to measure at a single point in time. ↲

³Base applications on soil test results. If soil tests indicate high levels, you may reduce or omit these nutrients. ↲

Timing nutrient applications to coincide with critical growth stages maximizes uptake and minimizes losses. In tomatoes, for example, peak nitrogen demand occurs during the flowering and fruit-set stages. Applying nitrogen fertilizers just before these stages ensures that nutrients are readily available when the plant's demand is highest, promoting optimal fruit development. For broccoli and cabbage, phosphorus is crucial during early root development. Applying phosphorus fertilizers at planting can enhance root growth and establishment, improving plant vigor. Using controlled-release fertilizers or fertigation techniques allow for a steady supply of nutrients, aligning with the crop's growth dynamics and reducing the risk of leaching or volatilization. Applying nutrients at the right time is critical to effective nutrient management in vegetable production. The "right time" principle ensures that nutrients are available to crops precisely when needed, thus optimizing plant growth, enhancing yields, and minimizing environmental losses.

Why the Right Time Matters

Timing nutrient applications to align with crop nutrient uptake patterns ensures maximum efficiency. Applications made too early can lead to nitrogen losses through leaching or volatilization, while late applications may fail to meet the crop’s physiological needs. Proper timing reduces waste, protects the environment, and enhances yield and economic returns by ensuring that nutrients are available during critical growth stages.

Timing of Nutrient Application in Key Vegetable Crops

1. Tomatoes have distinct nutrient demands at specific growth stages. Nitrogen is essential during early vegetative growth to support leaf and root development. As the plant transitions to flowering and fruiting, potassium becomes more critical to support fruit development and improve quality. Phosphorus is particularly important during early root establishment and fruit set. Fertilization schedules should align with these stages to ensure optimal nutrient availability.
2. Bell peppers require a steady supply of various nutrients throughout their growth cycle. Nitrogen demand peaks during early vegetative growth and fruit set and development. Potassium becomes crucial as the fruits develop, improving size, quality, and shelf life. Applying nutrients at the right time ensures consistent growth and high yields.
3. Broccoli requires nitrogen during its vegetative stage to drive rapid leaf and stem growth that supports head formation, while phosphorus and potassium play key roles early in the growing season for root development and stress tolerance. Late-season nutrient applications are less critical but may be needed if deficiencies arise.
4. Cabbage demands balanced nitrogen and potassium to support uniform head development throughout its vegetative phase. Nitrogen should be applied consistently, as excessive applications late in the season can lead to poor head firmness and quality issues. Potassium applications during head formation enhance crop quality and disease resistance.

Table 3 describes the recommended timing of nitrogen, phosphorus, and potassium applications for tomato, bell pepper, broccoli, and cabbage in North Carolina, based on the 2025 Southeastern U.S. Vegetable Crop Handbook.

Table 3. Fertilizer Application Schedule with Total Seasonal Rates

¹Includes phosphorus (P) and potassium (K) based on soil test recommendations. These nutrients are applied when planting to support root development and early growth. ↲

²The daily nitrogen (N) application rate is calculated to ensure a consistent supply of nutrients over the growing season. The range accounts for variability in growth rate and environmental factors. ↲

³The cumulative nitrogen applied through fertigation equals the crop's minimum recommended seasonal nitrogen. ↲

⁴In sandy soils, split application of K may be beneficial to avoid nutrient loss. ↲

Determining the Right Time

• Early application. Preplant fertilizer applications ensure that essential nutrients, such as phosphorus and potassium, are available for early root development. For example, applying phosphorus-rich fertilizers like triple superphosphate during bed preparation is critical for broccoli and cabbage, which require strong root systems to support vigorous growth.
• Split nutrient applications. Some crops benefit from split applications, which ensure a consistent supply of nutrients while minimizing losses. For example, in tomatoes, 30% to 50% of nitrogen is applied at planting or preplant, and the remaining amount is split during the growing season (daily, every couple of days, or weekly). Bell peppers benefit from split potassium applications during flowering and fruiting stages to sustain fruit development and quality.
• Fertigation. Delivering nutrients through irrigation systems allows for precise timing of nutrient applications. This method is especially effective in sandy soils prone to nutrient leaching. In cabbage production, for example, fertigation can deliver nitrogen in small, frequent doses, aligning with the crop’s uptake pattern and minimizing environmental losses.
• Utilizing technology. Adopting precision agriculture practices, such as variable-rate application, ensures that nutrients are applied where they are needed most. Such practices are greatly enhanced by data-driven control systems and field-deployable automation platforms. For example, mapping soil variability in a tomato field allows for tailored nutrient applications that address specific deficiencies, avoiding overapplication in areas with sufficient nutrient levels.
• Monitoring and adjusting. Regularly monitor plant growth and nutrient status through foliar analysis and visual assessments. Adjust fertilization rates as needed based on crop performance and environmental conditions. For example, supplemental potassium applications may be necessary in tomatoes during heavy fruiting if deficiencies are detected.
• Follow Extension guidelines. Rely on state-specific Extension recommendations for nutrient application rates and time, as these are based on local research and conditions.

Recommendations for Farmers

• Flexible nitrogen application timing. Adjust daily nitrogen rates based on crop development, weather conditions, and irrigation system efficiency.
• Plant tissue (foliar) analysis. Use foliar analysis to ensure that the nutrient supply aligns with crop needs throughout the season.
• Best management practices. Incorporate measures to minimize nutrient loss, such as applying nitrogen in small, frequent doses and avoiding excessive irrigation.
• Foliar applications. Applying nutrients to foliage is an excellent strategy for quickly addressing mid-season nutrient deficiencies. The secondary nutrient calcium (for tomatoes and peppers) or boron (for cabbage) can be applied as foliar sprays during critical growth stages to prevent physiological disorders and improve crop quality. Be sure to follow adequate procedures for applying foliar sprays. Refer to Foliar Analysis for Bell Pepper Production in North Carolina: A Guide for Growers for more information.
• Monitor weather. Weather considerations are critical in determining the timing of fertilization. Monitor weather conditions closely, as heavy rains can wash nutrients out of the root zone. In such circumstances, supplemental applications may be needed to replenish lost nutrients. Delaying fertilization after heavy rainfall can prevent further losses and ensure that nutrients are available when crops resume active growth.
• Consider controlled-release fertilizers. Though they are more expensive, controlled-release fertilizers provide a steady supply of nutrients over time, reducing the need for frequent applications. For broccoli, controlled-release nitrogen fertilizers can supply nutrients consistently through the vegetative and early reproductive stages, aligning with the crop’s uptake pattern.

Placing nutrients where plants can easily access them, such as in the root zone or on foliage, is particularly critical in high-value vegetable crops like tomatoes, bell peppers, broccoli, and cabbage, which require precise nutrient management to achieve optimal yields and quality. Research has shown that precision placement methods can significantly improve nutrient uptake and crop yield. Integrating geospatial data can support optimal placement strategies by identifying variable availability of nutrients in the root zone. Banding phosphorus fertilizers below the seed zone in broccoli and cabbage has been demonstrated to improve early root development and nutrient uptake. In bell peppers and tomatoes, fertigation through drip irrigation systems delivers nutrients directly to the root zone, optimizing use efficiency and minimizing runoff. In addition, foliar applications of micronutrients, such as zinc and manganese, can quickly correct deficiencies during critical growth stages, ensuring optimal plant health and productivity.

Why the Right Place Matters

Nutrient placement directly affects nutrient availability, plant uptake, and environmental losses. Poor placement can make nutrients inaccessible to plant roots, leading to deficiencies, wasted inputs, and potential environmental harm, such as runoff or leaching. Strategically placing nutrients ensures that they are absorbed efficiently by the crop while reducing the risk of contamination to surrounding ecosystems.

Placement of Nutrients in Key Vegetable Crops

1. Banding fertilizers. Banding fertilizers is an effective strategy to enhance nutrient uptake. It involves placing phosphorus and potassium in bands 2 to 4 inches to the side and below the seed or transplant row. This method ensures that nutrients are close to the root zone, promoting early root establishment and vigorous growth. For example, in tomatoes, banding phosphorus improves fruit set and yield, particularly in soils with low phosphorus availability. In bell peppers, banding phosphorus helps establish strong roots and enhances early plant development, which is crucial for high fruit production.
2. Fertigation. Delivering nutrients via drip irrigation systems allows precise delivery of water-soluble fertilizers directly to the root zone. This method ensures that nutrients are available when demand is highest, optimizing uptake and minimizing waste. In tomatoes, for example, fertigation is commonly used during flowering and fruiting stages to supply nitrogen and potassium efficiently. In bell peppers, precise fertigation supports nutrient demands during critical growth phases, improving fruit quality and yield.
3. Plasticulture. Plasticulture systems use plastic mulch with drip irrigation, which confines liquid fertilizers to the root zone, reducing nutrient loss through runoff and maximizing efficiency. In tomatoes, for example, liquid fertilizers applied through drip lines in plasticulture systems promote sustained nutrient availability during fruit development. In bell peppers, this approach minimizes leaching while maintaining consistent nutrient levels, particularly in areas prone to heavy rainfall. Plasticulture systems are especially useful in sandy soils.
4. Foliar applications. Applying nutrients such as calcium and magnesium to foliage can quickly address deficiencies during critical growth stages. Though not a delivery method for primary nutrients, foliar application is an effective supplementary approach for rapidly correcting deficiencies of secondary macronutrients. For example, foliar calcium sprays prevent blossom-end rot during fruiting of tomatoes. In bell peppers, foliar magnesium sprays address deficiencies that may appear during peak fruiting periods.
5. Preplant incorporation. Applying fertilizers and organic amendments to the soil during bed preparation ensures even distribution of nutrients in the root zone. This principle is applicable across crops. In broccoli, in particular, incorporating phosphorus and potassium during bed preparation supports early root development and sustained nutrient availability. In heavy-feeding crops like cabbage, preplant incorporation of fertilizers provides even distribution, ensuring consistent growth. For transplants, placing a starter fertilizer solution in the transplant hole ensures that nutrients are immediately accessible to the young root system, promoting vigorous early growth.
6. Precision fertilizer application. Using precision spreaders to apply granular fertilizers ensures uniform nutrient distribution across the field, reducing variability. Targeted side-dress applications further optimize nutrient uptake during critical growth stages. In broccoli, for example, side-dressing with nitrogen during vegetative growth ensures that nutrients are accessible to expanding root systems. In cabbage, precision application of nitrogen supports the high nutrient demand during head formation.
7. Controlled-release fertilizers. Controlled-released fertilizers provide a steady supply of nutrients over an extended period, minimizing leaching and improving efficiency. In cabbage, for example, controlled-release nitrogen supports both vegetative and head development stages, reducing the need for frequent applications. Controlled-release fertilizers are particularly effective in sandy soils.

Recommendations for Farmers

• Crop-specific needs. Each crop has specific nutrient uptake patterns. Ensure that placement methods align with these requirements, such as banding phosphorus for broccoli or providing potassium during tomato fruiting.
• Drip irrigation and fertigation. Delivering nutrients directly to the root zone through drip irrigation ensures maximum efficiency. Fertigation allows for precise timing and placement, reducing waste and environmental impact.
• Combining methods for efficiency. Use a combination of preplant incorporation, side-dressing, and fertigation to optimize nutrient availability throughout the growing season.
• Utilizing technology. Incorporate precision agriculture methods such as GPS-guided spreaders, soil sensors, and soil variability maps. Soil variability maps, for example, can guide nutrient placement, ensuring that fertilizers are applied only where needed. This approach is especially useful in fields with varying soil fertility levels, reducing overapplication in high-fertility zones.

• Monitoring and adjustment. Regularly monitor crop growth and soil nutrient levels. Adjust placement techniques as needed to address emerging deficiencies.
• Strip-tillage systems. In conservation tillage systems, applying fertilizers in narrow tilled strips directly beneath the planting row concentrates nutrients that the crop can access while maintaining soil structure and organic matter in the surrounding areas.

Crop vegetation indexes: Calculated values (for example, Normalized Difference Vegetation Index [NDVI]) derived from spectral data to assess crop health, vigor, and biomass. ↲

Data-driven control systems: Automated systems that use collected data to make real-time decisions and adjust equipment or operations accordingly. ↲

Decision-support systems: Tools or platforms that analyze data and provide actionable recommendations to help users make informed decisions; often used in agriculture for optimizing inputs like irrigation, fertilization, and pest control. ↲

Field-deployable automation platforms: Mobile or stationary systems designed to operate autonomously in the field for tasks like planting, monitoring, or harvesting. ↲

Geospatial data: Information that includes a geographic location, such as coordinates, boundaries, or addresses, and is used to map, analyze, and interpret spatial patterns and relationships in the real world. ↲

Plant demand assessments: The evaluation of a plant’s specific needs for resources like water, nutrients, or light at a given growth stage, based on environmental conditions and physiological indicators. ↲

Predictive algorithms: Mathematical models that use historical and real-time data to forecast future outcomes, such as crop yield or disease risk. ↲

Real-time sensor networks: Interconnected sensors that continuously collect and transmit data instantly for immediate analysis or decision-making. ↲

Soil variability maps: Spatial representations that show differences in soil properties, such as texture, pH, organic matter, or nutrient levels, across a field, helping to guide site-specific management decisions in agriculture. ↲

Variable-rate application: The practice of adjusting the amount of inputs (such as fertilizer, water, or pesticides) applied to different areas of a field based on site-specific data to optimize efficiency and crop performance. ↲