Watermelon production in the Suwannee Valley commonly occurs on sandy soils that are highly susceptible to nitrogen leaching, increasing the risk of nutrient losses and groundwater contamination. Efficient nitrogen management is therefore critical to sustain yield and fruit quality while minimizing losses. Controlled-release fertilizers (CRF) provide an opportunity to better align nitrogen availability with watermelon uptake patterns, potentially enhancing nitrogen use efficiency. This publication evaluated the use of controlled-release nitrogen in watermelon systems and outlines management considerations to optimize fertilizer performance under conditions present in the Suwannee Valley. This information is intended to support watermelon growers, Extension agents, crop consultants, and researchers working to improve nitrogen management in Florida’s sandy soils.
Introduction
Florida is the largest producer of watermelon in the United States, accounting for 25.5% of total domestic production (USDA-NASS 2026). The Suwannee Valley region of north Florida is one of the largest production regions within the state, with over one-third of the state’s watermelon acreage. The region’s well-drained sandy soils are highly favorable for watermelon growth but can also pose challenges for nutrient and water retention. As these soils are prone to leaching, careful nutrient and irrigation management is essential. For more than 25 years, growers in the Suwannee Valley have been leaders in adopting Best Management Practices (BMPs) to improve efficiency and reduce environmental impact. In the last 60 years, research regarding watermelon fertilization has influenced growers’ practices. Early research focused on the impacts of various fertilization practices and specific nutrient rates on fruit yield and quality. In recent years, the scope of research has been expanded to include environmental impacts such as nitrogen use efficiency.
Drip irrigation is currently the primary irrigation and fertigation method used in the Suwannee Valley. Drip irrigation, when used in conjunction with plastic mulch, allows for more precise applications compared to older methods such as overhead irrigation. This increased precision reduces water usage by up to 50%, which is particularly beneficial in sandy soils where water retention is naturally low. Despite these advances, fertilizer management remains a critical challenge. Granular fertilizers, often applied before bedding and planting to supply a large share of seasonal nitrogen needs, are relatively inexpensive but highly soluble, thus posing a high leaching risk. The main consideration in nitrogen management of watermelon grown using plastic mulch and drip irrigation is the high risk of leaching soluble bed fertilizer during the first few weeks of the season. The risk is high early in the season because plant root systems are small, soluble nitrogen is available to leach, and over-irrigation and leaching rain events can move high amounts of nitrogen past the root system and deeper into the soil profile.
Overview of Controlled-Release Fertilizer
Controlled-release fertilizers (CRFs) are coated with a semipermeable polymer that is designed to allow nutrients to be gradually released into the soil, at rates aligned with the crop’s growth stages (Figure 1). These fertilizers can deliver macronutrients such as nitrogen, phosphorus, and potassium, as well as micronutrients. The rate of nutrient release is dictated by the temperature of the soil where the CRF is placed, and sufficient soil moisture is required to activate the soluble fertilizer within the coating. Commercial CRFs are designed to slow nutrient release to better align with crop growth stages and management practices throughout a given season.
Credit: Taite Miller and Sydney Williams, UF/IFAS.
One of the primary advantages of CRF is its ability to reduce nutrient losses compared to conventional soluble fertilizers. Conventional soluble fertilizer programs often result in significant inefficiencies, with up to 40% of applied nitrogen lost through leaching or volatilization (Timilsena et al. 2015). CRF helps mitigate those losses and increase nitrogen use efficiency by aligning nutrient release with the crop’s growth curve. In regions like the Suwannee Valley of north Florida, CRF products are most often formulated to control the release of nitrogen, as nitrogen leaching is the predominant nutrient management concern in the deep sandy soils common to the region.
Full-Season Versus Partial-Season Programs
The use of CRF in watermelon production can follow either a full-season or partial-season program. In a full-season program, a single application of CRF is incorporated into the soil only in bedded areas during bed formation. The soil is then covered with plastic before planting. The CRF is formulated to steadily release nutrients over the 100-to-120-day growing period. This approach reduces the need for multiple fertilizer applications, lowering labor demands and operational costs for growers. While CRF is designed to minimize nutrient losses through leaching, especially early in the season, effective irrigation management remains critical to ensure the fertilizer is not lost, lasts throughout the season, and aligns with crop uptake. CRFs are more expensive than conventional sources, making it essential to weigh additional cost-saving measures when using them.
A partial-season CRF program combines CRF, applied in-bed in the same manner as a full-season program, with supplemental, in-season fertigation treatments through the drip irrigation systems. This method provides growers with more flexibility and control over nutrient management. When paired with petiole-sap testing or other leaf tissue testing methods, the partial-season CRF program allows for more precise adjustments to nitrogen applications, supporting both crop performance and nitrogen reduction goals. Partial-season CRF programs are often considered the more popular and effective option for watermelon production because they are adaptable and can balance consistent nutrient supply with in-season adjustments. This is particularly true for sandy soils, where nutrient losses are a primary concern, and in considering total fertilizer program costs.
Research Farm Trials at North Florida Research and Education Center—Suwannee Valley
Controlled-release fertilizer research at the UF/IFAS North Florida Research and Education Center—Suwannee Valley (NFREC-SV) near Live Oak, Florida, began in 2019 with two trials designed to test controlled-release fertilizer (CRF) against conventional soluble programs in watermelon production (Figure 2). In CRF technology, almost all nutrients can be coated. These trials focused primarily on coated nitrogen, as this is the nutrient of greatest environmental concern within the Suwannee Valley. The first trial compared a full-season CRF program (i.e., a single preplant CRF application) to a standard conventional fertilizer program, both targeting 150 lb per acre of nitrogen, including through fertigation. It found no total season yield differences, though CRF produced significantly more fruit at the first harvest. Both programs aligned with the UF/IFAS nutrient management recommendation of 150 lb per acre of total-season nitrogen. A second trial followed a partial-season program that combined CRF targeting 50 lb per acre of nitrogen with soluble sources in split application programs. Again, this program showed comparable yields and highlighted that nitrogen leaching was minimal across CRF treatments. In both trials, multi-depth soil sampling and measuring electrical conductivity levels through soil moisture sensors provided evidence that the polymer-coated nitrogen is less likely to leach. These initial findings demonstrated that CRF could match conventional fertilizer strategies in yield performance while addressing concerns about nutrient loss through leaching.
Credit: Sydney Williams, UF/IFAS.
Building on these results, a 2020 trial evaluated the interaction of irrigation regimes with nitrogen sources. Treatments compared soluble and CRF fertilizers, with one treatment using best management practice (BMP) irrigation rates and another treatment using double those irrigation rates. Once again, yields were statistically similar, and soil nitrate remained low below the root zone, reinforcing that CRF could maintain crop nutrition while reducing leaching risk. This trial further emphasized the importance of irrigation management alongside fertilizer source by showing that leaching could be minimized when the two BMPs were integrated.
In subsequent years, the focus shifted toward refining CRF strategies, particularly partial-season programs that combined preplant CRF with in-season fertigation. This approach consistently emerged as a very effective balance, reducing leaching losses while ensuring nutrient availability during peak crop demand. By 2023, a comprehensive trial compared conventional fertilizer, full-season CRF, partial-season CRF, and all-liquid fertigation programs (Figures 3 and 4). All programs produced statistically similar yields, but partial-season CRF and all-liquid fertigation treatments offered slight advantages in early-season fruit size while maintaining nutrient efficiency (Table 1). This trial demonstrated the importance of good irrigation management regardless of the fertilizer source and program.
Over the years, these research projects demonstrated that CRF could reliably reduce nutrient leaching without sacrificing yield. The consistency of these results at the NFREC-SV built confidence in technology and laid the foundation for expanding to larger-scale trials on commercial farms.
Credit: Bob Hochmuth, UF/IFAS.
Credit: Sydney Williams, UF/IFAS.
Table 1. Yield Data from 2023 Trial. Different lowercase letters indicate statistically significant differences among fertilizer treatments, while treatments sharing the same letter are not significantly different.
Commercial Farm Trials
The success of research trials at the NFREC-SV led to a series of commercial on-farm CRF demonstration projects beginning in 2020, comparing conventional and CRF strategies under both full- and partial-season programs. Nitrogen applications were monitored throughout the season using leaf tissue and petiole-sap testing, following the principles of the “4Rs” of nutrient stewardship: right source, rate, place, and time.
Results confirmed that CRF programs could match conventional yields while providing labor savings, improved nutrient use efficiency, and environmental benefits. Yields in the on-farm trials were comparable to those of conventional fertilizer programs, and yields across all programs were considered high. Full-season programs are more difficult to institute, mainly because a longer release rate provides less flexibility to adjust for extreme weather events or unusual growth patterns. As a result, cooperating farmers often chose a partial-season program with 50–75 lb per acre of nitrogen in the bed. This approach is generally more cost-effective and reduces the risk of leaching early in the season when losses are most likely. These growers seemed to prefer developing their own liquid fertigation program for the second half of the season. When using CRF, each grower could design a customized program that fit their practices and yield goals.
Conclusion
The multi-year on-farm CRF demonstrations in the Suwannee Valley highlighted several important insights. A critical factor in CRF performance is the alignment of nutrient release with irrigation management. Partial-season CRF programs provide growers with flexibility to adjust nutrient supply based on crop demand and plant nutrient status, thus mitigating leaching risk and potentially offering greater nitrogen use efficiency. The use of tools such as petiole-sap testing, leaf tissue analysis, and soil moisture sensors further enhances precise nutrient management. As CRF continues to evolve as an emerging technology, ongoing research and on-farm experience will refine best management strategies, allowing growers to adapt and optimize its use to fit their practice.
Credit: Bob Hochmuth, UF/IFAS.
Credit: Sydney Williams, UF/IFAS.
Acknowledgments
We thank this multi-year project’s partners: the Florida Watermelon Association, Florida Department of Agriculture and Consumer Services Office of Agricultural Water Policy, Suwannee River Water Management District, Harrell’s, Pursell Agri-Tech, Mayo Ag Services, BMP Logic, Clifton Seed Company, and all cooperating farms. We also thank the farm staff at the NFREC-SV and the commercial farms for implementing and executing these trials, from preparation to harvest.
References
Timilsena, Y. P., R. Adhikari, P. Casey, T. Muster, H. Gill, and B. Adhikari. 2015. “Enhanced Efficiency Fertilisers: A Review of Formulation and Nutrient Release Patterns.” Journal of the Science of Food and Agriculture 95 (6): 1131–1142. https://doi.org/10.1002/jsfa.6812
USDA-NASS (United States Department of Agriculture, National Agricultural Statistics Service). 2026. Vegetables 2025 Summary. https://esmis.nal.usda.gov/sites/default/release-files/795772/vegean26.pdf
Further Reading
UF/IFAS North Florida Research and Education Center—Suwannee Valley. 2025. “Weekly Watermelon Crop Update.” University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS). https://svaec.ifas.ufl.edu/crops/vegetables/watermelon/