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Article

Impact of Polymer-Coated Controlled-Release Fertilizer on Maize Growth, Production, and Soil Nitrate in Sandy Soils

by
Morgan Morrow
1,
Vivek Sharma
2,*,
Rakesh K. Singh
2,
Jonathan Adam Watson
2,
Gabriel Maltais-Landry
3 and
Robert Conway Hochmuth
4
1
Florida Department of Agriculture and Consumer Services, Office of Agricultural Water Policy, 729 E. Wade Street, Trenton, FL 32693, USA
2
Agricultural and Biological Engineering Department, University of Florida, 1741 Museum Rd., Gainesville, FL 32611, USA
3
Department of Soil, Water, and Ecosystem Sciences, University of Florida, 2181 McCarty Hall A, P.O. Box 110290, Gainesville, FL 32611, USA
4
North Florida Research and Education Center—Suwannee Valley, Institute of Food and Agricultural Sciences, University of Florida, 7850 County Road 136, Live Oak, FL 32060, USA
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(2), 455; https://doi.org/10.3390/agronomy15020455
Submission received: 8 January 2025 / Revised: 8 February 2025 / Accepted: 11 February 2025 / Published: 13 February 2025
(This article belongs to the Special Issue Conventional and Alternative Fertilization of Crops)
Browse Figures
Figure 1
<p>Graphical weather data depicting lines of maximum (red), minimum (blue), and average (grey) temperatures along with bars of total daily rainfall (orange) throughout 2022 and 2023 maize growing seasons.</p> "> Figure 2
<p>Crop health and growth parameters including (<b>A</b>) plant height, (<b>B</b>) leaf area index, (<b>C</b>) leaf tissue nitrogen, and (<b>D</b>) aboveground biomass (AGB) for 2022 and 2023 maize growing seasons across fertilizer treatments.</p> "> Figure 3
<p>Soil nitrate-nitrogen (NO<sub>3</sub>-N, mg kg<sup>−1</sup>) at various soil profile depths including (<b>A</b>) 0–30 cm, (<b>B</b>) 30–60 cm, (<b>C</b>) 60–90 cm, and (<b>D</b>) 90–120 cm for the 2022 and 2023 maize growing seasons across fertilizer treatments.</p> "> Figure 4
<p>The mean soil nitrate-nitrogen (NO<sub>3</sub>-N) within the 60–120 cm soil profile for 2022 and 2023 maize growing seasons; this figure shows the color gradient of the growth stages, with the youngest stage (V6) being the darkest purple and progressively getting lighter until the R5 growth stage.</p> "> Figure 5
<p>Violin plot graph of grain yield under different nitrogen fertilizer treatments for 2022 and 2023 maize growing seasons. Treatments with same letters within each year are not significantly different at <span class="html-italic">p</span> &lt; 0.05.</p> "> Figure 6
<p>A correlation matrix illustrating the relationships between climatic factors, soil moisture, nitrate-nitrogen (NO<sub>3</sub>-N) concentrations at different depths, and plant growth attributes at the vegetative (V12) and reproductive (R3 and R5) growth stages during the 2022 and 2023 maize growing seasons. The variables include soil moisture (VWC) at 30 cm, 60 cm, and 90 cm depths (VWC_30, VWC_60, VWC_90, respectively); NO<sub>3</sub>-N concentrations at three soil depths (NO<sub>3</sub>-N_30: 0–30 cm, NO<sub>3</sub>-N_60: 30–60 cm, NO<sub>3</sub>-N_90: 60–90 cm); and plant growth parameters including plant height (Height) and the leaf area index (LAI). Positive correlations are shown in red, while negative correlations are represented in blue, with the intensity of the color corresponding to the strength of the correlation.</p> "> Figure 7
<p>A Principal Component Analysis (PCA) biplot showing the relationships between soil nitrate-nitrogen concentrations (NO<sub>3</sub>_30, NO<sub>3</sub>_60, NO<sub>3</sub>_90), soil moisture (VWC_30, VWC_60, VWC_90), cumulative rainfall (RAIN_sum, RAIN_cum), and crop performance metrics (LAI, height, yield) during the 2022 (red) and 2023 (blue) maize growing seasons.</p> ">
Review Reports Versions Notes

Abstract

:
Polymer-coated controlled-release fertilizers’ (CRFs) unique nutrient release mechanism has the potential to mitigate the leaching of mobile soil nutrients, such as nitrate-nitrogen (NO3-N). The study aimed to evaluate the capacity of a polymer-coated CRFs to maintain maize (Zea mays L.) crop growth/health indicators and production goals, while reducing NO3-N leaching risks compared to conventional (CONV) fertilizers in North Florida. Four CRF rates (168, 224, 280, 336 kg N ha−1) were assessed against a no nitrogen (N) application and the current University of Florida Institute for Food and Agricultural Sciences (UF/IFAS) recommended CONV (269 kg N ha−1) fertilizer rate. All CRF treatments, even the lowest CRF rate (168 kg N ha−1), produced yields, leaf tissue N concentrations, plant heights, aboveground biomasses (AGB), and leaf area index (LAI) significantly (p < 0.05) greater than or similar to the CONV fertilizer treatment. Additionally, in 2022, the CONV fertilizer treatment resulted in increases in late-season movement of soil NO3-N into highly leachable areas of the soil profile (60–120 cm), while none of the CRF treatments did. However, back-to-back leaching rainfall (>76.2 mm over three days) events in the 2023 growing season masked any trends as NO3-N was likely completely flushed from the system. The results of this two-year study suggest that polymer-coated CRFs can achieve desirable crop growth, crop health, and production goals, while also having the potential to reduce the late-season leaching potential of NO3-N; however, more research is needed to fully capture and quantify the movement of NO3-N through the soil profile. Correlation and Principal Component Analysis (PCA) revealed that CRF performance was significantly influenced by environmental factors such as rainfall and temperature. In 2022, temperature-driven nitrogen release aligned with crop uptake, supporting higher yields and minimizing NO3-N movement. In 2023, however, rainfall-driven variability led to an increase in NO3-N leaching and masked the benefits of CRF treatments. These analyses provided critical insights into the relationships between environmental factors and CRF performance, emphasizing the importance of adaptive fertilizer management under varying climatic conditions.

1. Introduction

Maize (Zea mays L.) is one of the highest yielding and widely cultivated crops globally due to its adaptation to various environmental conditions [1]. It contributes significantly to human and animal consumption, bioenergy, and a multitude of industrial products [2]. The necessity of nitrogen (N) fertilizer to meet maize production demands is clear, but excess N inputs can not only lead to N losses to the environment but also waste the producer’s financial resources [3,4,5]. Some of the negative environmental impacts brought on by the excess nutrients are eutrophication, hypoxia, harmful algal blooms, and acidification, which can deteriorate the quality of ecotourism, drinking water, and agricultural resources provided by these important water bodies [6,7].
Current conventional (CONV) forms of N fertilizer used in maize cropping systems are generally in liquid or granular forms that are immediately available for crop uptake when applied, meaning these fertilizers are also vulnerable to losses. Several fertilizer best management practices (such as 4R principles) are in place to reduce nutrient losses and carbon footprint and to increase N retention in the soil. Apart from that, current research efforts are focusing on the development and adequate use of urease inhibitors [8] and enhanced-efficiency fertilizers, such as slow-release fertilizers (SRFs), controlled-release fertilizers (CRFs), and their blends [9]. Polymer-coated CRFs are considered a possible solution to these N loss issues in maize cropping systems [9,10,11]. Polymer-coated CRFs are different from CONV fertilizers because they are granular nutrients covered in a semi-permeable coating that shields nutrients from immediate loss to the environment via leaching and volatilization as well as helping to hold the nutrients in the soil profile [12,13]. The mobile nutrients are protected within pellets and slowly released out of the porous polymer coating via diffusion, matching the nutrient demand of the crop [13,14]. The main drivers that influence the timing and amount of nutrients released from the polymer-coated CRF are coating thickness and temperature. There is an inverse relationship where hotter temperatures and thinner coatings tend to release a higher nutrient mass over a given amount of time, and colder temperatures and thicker coatings tend to release a lower nutrient mass over the same period [14,15].
By having more control over how these nutrients are released, the ability to synchronize nutrient release with crop uptake demand can be refined and improved with CRFs. This can, in turn, limit the loss/waste of N and reduce environmental impacts [16,17,18,19,20,21,22]. In Shoji et al.’s study [22], yields were greater, and nitrate-nitrogen (NO3-N) leaching was lower in CRF treatments compared to CONV fertilizer treatments. A meta-analysis conducted by Zhang et al. [10] reviewed 120 studies and indicated that maize managed under CRFs had higher nutrient-use efficiency (NUE) by roughly 24% and reduced nitrous oxide emissions, ammonia volatilization, and NO3-N losses by roughly 24%, 39%, and 27%, respectively. In a three-year study, Sun et al. [23] observed that CRFs added at a rate of 210 kg of N ha−1 maintained yields and promoted N uptake by an average of 16.7% compared to conventional urea in Northeast China, which further improved NUE by 21.1%. Similarly, Yao et al. [9] compared polymer-coated CRFs mixed with CONV fertilizers at a 1:1 ratio at rates of 90, 180, 270, and 360 kg N ha−1 to solely CONV fertilizers applied at the same rates; yields were either not statistically different or higher in the CRF-incorporated treatments, while NO3-N leaching was consistently reduced in all CRF treatments. Another study highlighted that the larger particle size of the pelleted polymer-coated CRFs helped to hold nutrients in the upper soil profile which aided in reducing the downward particle transport and leaching risk/potential of that N, while also improving yields [24]. Li et al. [25] investigated the integrated effect of irrigation and CRFs on maize growth and development. This study discovered that under water stress, the limited moisture content restricted the CRFs’ N release capacity which in turn impacted crop N availability. This further reduced the efficiency of soil nutrient and water absorption by roots and reduced normal plant growth and development, thereby reducing dry matter and N uptake.
While CRFs are beneficial in optimizing crop yield and reducing nutrient leaching simultaneously, widespread adoption is limited due to higher costs. Noellsch et al. [20] highlight that CRFs have the potential to enhance NUE, but a cost–benefit analysis of substituting CRFs with CONV fertilizers should be considered since CRFs can be more expensive than CONV fertilizers. However, depending on how a producer typically manages a field, the higher costs associated with CRFs may be offset by lower costs in other areas of farm management. With CRFs only needing to be applied once at the beginning of the growing season, producers could save on labor and production costs that are typically incurred from the multiple fertilizer applications associated with CONV fertilizers [26]. Zhang et al. [27] demonstrated that a single application of CRFs saved half of the labor cost, which balanced the higher cost of the CRF product. Similarly, Shivay et al. [28] reported a higher yield return and benefit–cost ratio when using CRFs compared to the non-coated prilled urea, which offset the higher cost of the CRF product. Yao et al. [9] reported that a single application of a 1:1 ratio blend of urea and polymer-coated urea resulted in 75% savings in labor costs and 24–62% savings in agricultural input costs, even with the blended treatment costing 13–14% more than a CONV fertilizer treatment. Therefore, it is essential for producers to consider all components of the production operation, not just fertilizer costs, to properly determine the economic trade-offs of incorporating CRFs.
Despite previous studies highlighting the benefits of CRFs, little is known about the efficacy of polymer-coated CRFs in well-draining sandy soils coupled with a shallow water table. This is critically important in the Suwannee River Basin (SRB) of North Florida, which has the highest density of large artesian springs in the world, with approximately 300 springs feeding the Suwannee River. The basin’s nutrient-poor soils make N inputs necessary to achieve desired crop production goals, but the region’s humid subtropical climate with high amounts of yearly rainfall, coupled with coarse-textured well-draining soils with low water and nutrient holding capacity, make it vulnerable to nutrient leaching, specifically mobile nutrients such as NO3-N [29,30,31]. With CRFs’ unique ability to protect mobile nutrients and manipulate nutrient release timing with a single field application, it is hypothesized that CRFs will be able to reduce NO3-N leaching and achieve crop quality/health indicators and yields like those of CONV fertilizers. Therefore, the objectives of this study were to measure the effects of a polymer-coated CRF on maize (i) crop health/growth and production goals and (ii) NO3-N leaching potential in the SRB in North Florida.

2. Materials and Methods

2.1. Site Characteristics

This study was conducted over two growing seasons (2022–2023) at the University of Florida Institute for Food and Agricultural Sciences (UF/IFAS) North Florida Research and Education Center—Suwannee Valley (NFREC-SV) in Live Oak, FL, USA (30°18′18″ N 82°53′55.536″ W). The research station is located 50.3 m above sea level and the climate at the site is classified as humid subtropical with the 30-year normal annual average temperature and total rainfall being 20.4 °C and 1360 mm, respectively [32]. The soils at the study site are generally classified as fine sand with each field site mapped as Hurricane, Albany, and Chipley soils, 0 to 3 percent slopes in 2023, and Blanton–Foxworth–Alpin complex, 0 to 5 percent slopes in 2022 (SSURGO: Soil Survey Geographic database). The Hurricane series is composed of very deep sandy, siliceous, thermic oxyaquic Alorthod, somewhat poorly drained, rapidly permeable soils, while the Blanton series comprises loamy, silicous, semiactive thermic Grossarenic Paleudults, deep and moderately well drained soils. Additional pre-planting soil characteristics for each growing season are highlighted in Table 1.

2.2. Experimental Design, Treatments, and Management Practices

The experimental design of the trial was a Randomized Complete Block Design (RCBD) [33], with six fertilizer rate treatments replicated four times. The plots were 6.1 m wide and 15.2 m long, with 6.1 m alleys to minimize the potential for cross contamination between treatments. Each plot consisted of eight rows spaced 76.2 cm apart. In this study, six N fertilizer treatments were investigated, including a control treatment of no N (0 kg N ha−1), four CRF treatments (168, 224, 280, 336 kg N ha−1), and a conventional (CONV) fertilizer treatment of 269 kg N ha−1 (the current UF/IFAS-recommended N rate of fertilizer for irrigated maize; [34]). The CRF under investigation was POLYON® CRF (43-0-0) (Harrell’s LLC, Lakeland, FL, USA) which is a polyurethane polymer-coated urea fertilizer that was banded two inches on both sides of the seeds at a depth of two inches at the time of planting. The CRF (43-0-0) product has a longevity period of 14 weeks with an average release rate of 19.2 kg N ha−1 per week (Harrell’s, personnel communication). A starter liquid (23-9-0) fertilizer application (33.6 kg N ha−1; 13.1 kg P ha−1) was applied using the First Product (First Product Inc., Tifton, GA, USA) double disc opener at 2 in deep and 2 in to the side of the row at planting, across all treatments. The starter fertilizer was applied to ensure a full and even plant stand. A detailed description of N treatments investigated in this research is provided in Table 2.
All CRF treatments were fertilized at planting, while the CONV treatment was fertilized both at planting and at different growth stages with liquid UAN (32-0-0) fertilizer applications (Table 2). These additional CONV fertilizer applications were carried out throughout the growing season using a Miller High Boy (CNH Industrial America LLC, Oak Brooks, IL, USA) rig equipped with a drop nozzle Y-drop system of delivery. Application amounts of this CONV fertilizer varied to coincide with the variable maize growth stage N needs as outlined by Mylavarapu et al. [34]. Additional agronomic management information for each growing season is shown in Table 3.
Phosphorus and potassium (K) fertilizers were applied based on the soil analysis results and equally applied across all treatments. Phosphorous fertilizer in the form of superphosphate (0-20-0; 78.5 kg P2O5 ha−1) was applied shortly after planting in both years. For both years, K fertilizer was broadcasted in two granular applications: a first application as potassium chloride (KCl, 0-0-60, 195.7 kg K2O ha−1) and a second application as potassium sulfate blended with other nutrients (0-0-37-12.9SO3-6.4MgO; 195.7 kg K2O ha−1; 68 kg SO3 ha−1).

2.3. Irrigation Management and Weather

Volumetric soil moisture measurements were collected using Meter Group’s (Meter Group, Inc., Pullman, WA, USA) ZL6 loggers combined with TEROS 12 soil moisture sensors (SMSs) installed in every plot for three out of the four replications and placed at depths of 30, 60, and 90 cm. These SMSs allowed for soil water content to be monitored throughout the growing seasons. Total soil water (mm) was calculated by multiplying the sensor measured soil moisture with an equivalent depth of water in a soil layer. Irrigation was applied using a Valley® (Valmont Industries, Inc., Omaha, NE, USA) 8000 Series Center Pivot irrigation system. Irrigation scheduling was based on the 269 kg N ha−1 CONV fertilizer treatment. The active soil profile was managed to maintain the soil water content between 40–45% and 90% of the total available soil water to prevent water stress and drainage. In the 2022 and 2023 growing seasons, a total of 16 and 17 irrigation events occurred, respectively, where plots received a maximum of 12.7 mm of irrigation water per event. Total irrigation of 175 mm and 171 mm was applied in the 2022 and 2023 crop growing season, respectively.
Daily weather data were obtained through the Florida Automated Weather Network (FAWN) Live Oak weather station located at the UF/IFAS NFREC-SV [35]. Daily weather data included daily maximum and minimum air temperature (Tmax and Tmin), daily maximum and minimum relative humidity (RHmax and RHmin), wind speed at 3 m height (u3), incoming solar radiation (Rs), and precipitation (P).

2.4. Crop Health and Growth Sampling

Non-destructive plant sampling included plant height and leaf area index (LAI). Plant heights were determined using a surveyor’s pole that was placed on the ground next to the base of three randomly selected plants in each plot and measuring the highest point the plants naturally reached. LAI measurements were taken using a Li-Cor® (LI-COR, Inc., Lincoln, NE, USA) LAI-2000 Plant Canopy Analyzer. Since plant heights and LAI were non-destructive, the collection of data could be conducted in the middle two rows of each plot.
Destructive plant sampling included aboveground biomass (AGB) and leaf tissue sampling. AGB was obtained by cutting either one (2023) or two (2022) plants randomly from each plot at the base of the maize stalk, while leaf tissue sampling included collecting the most recently matured leaves of 10–15 randomly selected plants from each plot. For each of these destructive samplings, the middle two rows were avoided as these rows were later used to determine grain yield. After collection, destructive samples were left in a 60 °C walk-in dryer to dry. Once a constant weight was reached, the dry weights of each sample were recorded and then samples were sent to Waters Agricultural Laboratories, Inc. (Waters Agricultural Laboratories, Inc., Camilla, GA, USA) for N analysis.

2.5. Quantifying Maize Production

A subsection of the middle two rows of each plot was created by cutting down 3.81 m on both sides of the middle two rows. This subsection was created to minimize the influence of edge effect variables on harvested crops. The remaining 7.6 m by 1.5 m subsection of the center of the middle two rows was harvested at the end of each growing season using a Kincaid® (Kincaid Equipment Manufacturing Corporation, Haven, KS, USA) 8-XP Single Plot Combine. The combine collected total grain weights and percent moisture information from each plot’s subsection, which were converted to a 15.5% moisture content when final maize yields were converted to kg ha−1 [36].

2.6. Soil Sampling

Multi-level soil sampling from three out of the four replicates was collected throughout the growing season. A gas-powered auger with a 5.08 cm wide and 15.24 cm long head was used to collect the multi-level soil samples. Soil samples were taken at depths of 0–30, 30–60, and 60–90 cm, with an additional 90–120 cm depth added later in the growing season to account for deeper rooting depths. Each soil level was knocked out of the auger head using a rubber mallet and into one of four buckets corresponding to each respective depth [37]. Once inside the bucket, the soil was mixed to ensure a representative sample was taken for each level. Soil samples from each depth were then transferred to paper bags and sent to Waters Agricultural Laboratories, Inc. (Waters Agricultural Laboratories, Inc., Camilla, GA, USA), where they were ground and analyzed for NO3-N using KCl extraction and colorimetry with the Cadmium Reduction method with Flow Injection (Waters Agricultural Laboratories, Inc., personal communication, 4 November 2022) [38].

2.7. Statistical Analysis

Data analysis was performed in R version 4.2.3 [39]. For each crop growth and health variable (i.e., AGB, leaf tissue N, plant height, LAI, and yield), separate linear mixed effects models were fit using the nlme package, v. 3.1-167 [40]. Different sampling times for all variables were treated as separate growth stages, except for yield, which was assessed once per year. In all cases, blocks were treated as random. Fixed effects included treatment, the growth stage (or year) at which the measurement was taken, and the interaction. Model assumptions were verified via diagnostic plots of the residuals using the ggResidpanel package, v. 0.3.0 and, where necessary, models were revised to include heterogenous variances among different treatments and/or growth stages [41]. Estimation of the marginal means, as well as the performance of F-tests, contrasts, and pairwise comparisons, were performed using the emmeans package, v. 1.10.7 with testing performed at the 0.05 significance level [42].
Soil NO3-N was modeled using generalized linear mixed models using the nlme package [43,44,45]. The fixed effects again included treatment, growth stage, and their interaction. Random effects for this model included not only block, but the block x treatment and block x stage interactions, with the latter two incorporated to account for the repeated measures aspect of the study [46].
To further explore the impact of environmental variables on NO3-N dynamics and maize yield, a correlation matrix analysis was conducted on selected variables, including rainfall; soil moisture at 30, 60, and 90 cm depths; NO3-N concentrations at 0–30, 30–60, and 60–90 cm depths; and growth parameters such as plant height, leaf area index (LAI), aboveground biomass (AGB), and yield. The correlation matrix was generated and visualized using the corrplot package, v. 0.95 [47], providing insights into relationships between climate, soil moisture, and plant growth variables.
Principal Component Analysis (PCA) was subsequently applied to assess inter-relationships between these variables and to identify year-specific patterns. PCA was conducted using the factoextra package, v. 1.0.7 and the prcomp function [48]. Analyses were performed separately for each year (2022 and 2023) and combined to examine inter-annual variability, with the year included as a categorical factor. The PCA biplot highlighted the differential influence of environmental variables on NO3-N retention and leaching potential, particularly under varying rainfall conditions.

3. Results and Discussion

3.1. Weather Conditions

Average maximum and minimum temperatures at a 2 m height during the 2022 and 2023 growing seasons ranged from 19.6 °C to 31.4 °C and 19.0 °C to 30.2 °C with cumulative precipitation for each growing season ranging between 474 mm and 654 mm, respectively (Figure 1). Additionally, according to historical FAWN data, the average total rainfall, minimum temperature, and maximum temperature for the outlined maize growing seasons from 2003 to 2021 at the Live Oak station were 644 mm, 18.5 °C, and 30.1 °C, respectively. The 2022 growing season was drier and hotter than both the average of the past 19 years and the 2023 growing season. Meanwhile, the 2023 growing season’s temperatures and total rainfall were close to the 19-year average. Additionally, there were back-to-back leaching rainfall events between 85 and 100 DAP (R3 and R4 growth stage) in the 2023 growing season. The Florida Department of Agriculture and Consumer Services (FDACS) defines a leaching rainfall event as a rainfall event in which 76.2 mm of rainfall occurs over three days or 101.6 mm occurs over seven days [49]. During these leaching events in 2023, both outlined definitions were achieved, with the first event accumulating 81 mm in three days or 111 mm in seven days and the second rainfall event amassing 156 mm in three days or 175 mm in seven days.

3.2. Crop Growth and Health Parameters

As N plays an important role in promoting crop growth at the cellular level, limited N availability in the soil can be directly linked to N deficiencies [50,51,52]. With leaf N content driving photosynthetic processes, N deficiency can lead to stunted plants due to reduced leaf expansion/leaf-area development, which in turn can make a positive feedback loop of growth and health hindrances [50,53,54,55,56]. For all parameters under investigation, the no-N treatment was consistently below all fertilized treatments in the 2022 and 2023 growing seasons, except for a few instances early in the growing season where no difference was observed (Figure 2). Although capacity for N uptake is smaller early in the growing season when crops are not as large, the lack of differences early in the growing season between the no-N treatment and fertilized treatments is likely due to the residual soil NO3-N (Table 1) masking the effects of N deficiency in the no-N fertilizer treatment by providing some of the necessary nutritional needs to the crops in those early growth stages [57,58,59,60] (Supplement Materials, Figures S1–S6). This is supported by previous studies, which observed that when maize was grown in unconstrained N conditions, the potential for AGB production was higher than in restricted conditions, whereas plant height and leaf area were negatively impacted by reduced N rates [61,62,63]. A study by Amissah et al. [64] also points to the possibility that the lower nutrient requirement for maize at earlier growth stages is another reason that variable N rates only show marginal differences between crop health and growth parameters at these early stages.
There is a general trend of increasing CRF rates leading to increasing crop growth and health parameters in both the 2022 and 2023 growing seasons. No significant difference in growth parameters was observed between the CRF and CONV treatments during the vegetative growth stages (V8–V12) in both years. The no-N treatment’s plant height, LAI, leaf tissue nitrogen, and aboveground biomass were significantly lower than both the CONV fertilizer treatment and CRF treatments starting from the vegetative growth stages (Supplement Materials, Figures S1–S8). As the maize crops start to develop into the reproductive stage (R3), the separation between the no-N treatment and all other fertilizer treatments becomes greater (Supplement Materials, Figures S1–S8). The LAI values of the CRF336, CRF280, CRF224, CRF168, and CONV296 treatments were 31%, 29%, 27%, 35%, and 23% higher than those of the no-N treatment in 2023 at R3, respectively. Similarly, the aboveground biomass at harvest for the CRF336, CRF280, CRF224, CRF168, and CONV296 treatments was 34%, 34%, 46%, 36%, and 31% higher than that of the no-N treatment in 2023, respectively. However, no significant difference in growth parameters was observed between CRF treatments.
For both years, the CONV fertilizer treatment had similar or lesser values for crop health and growth parameters when compared against all CRF rates, even at the lowest CRF rate (168 kg N ha−1), which applied less N than the CONV fertilizer treatment (269 kg N ha−1). The plant height and LAI for the CONV treatment were 5.3% and 10.3% lower in 2022 and 4.1% and 9.9% lower in 2023 than the CRF treatments (combined data for all treatments) at the R3 growth stage (Figure 2; Supplement Materials, Figures S1–S8). With the CONV fertilizer treatment not following the positive relationship between maize crop growth and health parameters and N rates, it indicates that the CRF continuous release mechanism likely allowed for greater N availability to the crop than the CONV fertilizer treatment [65,66]. This highlights the ability of the CRF treatments to adequately provide enough N to the crop so as to not stunt or negatively impact crop growth and health parameters even at CRF rates below the CONV fertilizer treatment rate. A study conducted by Strachan et al. [67] found that higher N rates led to faster leaf expansion and N uptake, suggesting that the separation between the CRF and CONV fertilizer treatments may be linked to the split-fertilization nature of the CONV fertilizer treatment not being able to supply N as continuously as the CRF treatment, especially since low N levels in the soil can lead to lower leaf N concentrations [68]. This is likely why across both growing seasons, similarly to other studies, all CRF treatments were able to achieve similar or greater crop growth and health parameters than the CONV fertilizer treatment [69,70].
Overall, the results in 2022 and 2023 were consistent with only minor variations between years due to differences in the climatic conditions between the years. A study conducted by [58] found that well-distributed rainfall enhances maize uptake of N. In this study, the 2022 growing season had longer periods between rainfall events with more sporadic events having larger inputs than in 2023. The 2023 growing season had more rainfall events with lower rainfall inputs at a time before the two major leaching events later in the season. Furthermore, with CRF N release influenced by temperature and 2022 having a warmer average temperature, it is possible that more N was released over a shorter time in 2022, allowing for lower N rates to appear competitive with higher N rates [13,14,15]. The more consistent water input and nutrient release in 2023 which allowed for more ideal N uptake conditions may explain why there was more separation and expected trends between the CRF rates in the 2023 growing season than in the 2022 growing season.

3.3. Soil Nitrate-Nitrogen (NO3-N)

Soil samples taken at pre-planting revealed that the 2022 field averaged approximately four times the amount of residual NO3-N in the soil profile than the 2023 field (Table 1 & Figure 3). The large amount of residual soil NO3-N throughout the soil profile in 2022 can serve as a proxy for greater general soil fertility in the 2022 soil relative to the 2023 soil. This is supported by the fact that the 2022 field was only recently cleared and put into production starting around 2015, while the 2023 field has been in production for several decades. Additionally, the higher clay content, and therefore higher cation exchange capacity, in 2022 allows the positively charged forms of N, such as ammonium (NH4+), to be held in the soil profile, resulting in more opportunities to retain residual N.
Differing fertilizer application amounts and release mechanisms influenced the dynamics of available soil NO3-N and its movement in the soil profile (0–120 cm) throughout the 2022 and 2023 growing seasons (Figure 3). The 0–30 cm layer held most of the soil NO3-N, which was expected as (i) CRF pellets and CONV fertilizer are applied in this soil layer, and (ii) desired as roots can readily take up NO3-N at this shallow depth. There was large variability and spiking among CRF treatments in this 0–30 cm range, but little spiking at depths below 30 cm, illustrating that the pelleted nature of CRFs likely allowed nutrients to be held in the soil profile. However, movement of the NO3-N to deeper depths was seen in the CONV fertilizer treatments in 2022. For example, a large spike of NO3-N is apparent in the CONV fertilizer treatment in the 0–30 cm range at roughly 90 DAP (Figure 3A), and as time progresses, NO3-N is seen spiking at the 30–60 cm range (Figure 3B), then the 60–90 cm range (Figure 3C), and a fraction is eventually seen in the 90–120 cm range (Figure 3D), corroborating the vulnerability of CONV fertilizers to downward movement through the soil profile. The presentation of significant differences in soil NO3-N between treatments at various growth stages across growing seasons is provided in the Supplemental Materials, Figures S9 and S10.
Since the leaching potential of NO3-N is of particular interest in this study, soil NO3-N levels at the 60–90 and 90–120 cm ranges over the course of both growing seasons were investigated (Figure 4). Soil NO3-N found at 60–120 cm was classified as having a high potential for leaching in this study because even though a maize rooting system can reach depths of up to 150–180 cm, the bulk of the maize rooting zone resides within the top 75 cm of the soil profile. Thus, soil NO3-N found beyond this depth is not likely to be taken up by the crop and is therefore at a high risk of being leached [12,71,72]. The combined soil NO3-N content from 60–120 cm was obtained by considering the bulk density, volume of soil, mass of soil, and nutrient concentration of each 60–90 cm and 90–120 cm soil layer at each growth stage. We assumed a uniform bulk density (i.e., 1.50 g cm−3) as soil at these depths is likely undisturbed by tillage and fairly stable in composition. Additionally, it is likely that the difference between the samples’ bulk densities would fall within the natural variability of the measurements, meaning it would likely be negligible from the 60–90 cm range to the 90–120 cm range. A detailed description of the combined soil NO3-N calculation is provided in the Supplementary Material (Figure S12).
The POLYON® polymer-coated CRF used in this study is manufactured to release around 70% of the nutrients at 70 to 80 days after being applied to the field, which in this study, was around the R1/R2 growth stages ([15]; Harell’s, personal communication, 23 June 2023). With less N being released later in the season and most corn N uptake occurring at the R3/R4 growth stages, it is anticipated that NO3-N moving into the high leaching zone (60–120 cm) would decrease [73,74]. All CRF treatments were consistent with this, as the level of soil NO3-N in the 60–120 cm soil profile at the R4 and R5 growth stages was lower than each respective treatment’s R3 growth stage soil NO3-N across both growing seasons (Figure 4). In contrast, the soil NO3-N of the CONV fertilizer treatment in these highly vulnerable leaching zones increased later in the growing season in 2022. In fact, the highest amount of soil NO3-N (3.1 mg N kg−1; 27.0 kg N ha−1) seen within the 60–120 cm range for the CONV fertilizer treatment occurred at the R4 growth stage, and the highest R5-growth-stage soil NO3-N level across all treatments and years (2.1 mg N kg−1; 18.3 kg N ha−1) occurred during the CONV fertilizer treatment.
Increased rainfall later in the season (post-100-DAP) is expected to induce the movement of NO3-N into these high leaching potential zones of the soil profile [3,75]. A study conducted by Gehl et al. [76] found that in loamy sand textured soils, such as those seen in the 2022 growing season, 25.4 mm of water input can move NO3-N by 15 to 20 cm. The soil texture and high levels of residual soil NO3-N throughout the soil profile are likely linked to later leaching events in the CONV fertilizer treatment in the 2022 growing season. Even though the last liquid UAN application in the CONV fertilizer treatment occurred around the R1/R2 growth stage, spiking of NO3-N was expected as nitrification of ammonium (NH4+) is higher in moist, warm, and well-aerated soils, and 40% of the total precipitation (190.2 mm) in 2022 occurred post-100-DAP (between the R3 and R4 growth stages) [77]. Yet, the movement of this NO3-N into these vulnerable regions of the soil profile is only seen in the CONV fertilizer treatment. The increasing trend in NO3-N is only seen in the CONV fertilizer treatment. This is due to the fact that the CRF not only protected the N within the pellets from being moved through the soil profile but likely protected earlier season N from being held by the clay soils, which in turn limited later season nitrification. This agrees with other studies reporting that CRFs’ protective coatings and timed release of N allow for reductions in the leaching potential of NO3-N when compared against CONV fertilizers [9,10,23,24,78].
In the 2023 growing season, it is not immediately apparent that the CRF mitigated late-season leaching events when compared to CONV fertilizers. Unlike the 2022 growing season, there is a decreasing trend of the CONV fertilizer treatment’s soil NO3-N within the 60–120 cm range after the R3 growth stage in 2023. However, this dip in the CONV fertilizer treatment may be deceiving. The decrease is likely due to the large majority of the CONV fertilizer’s soil NO3-N being completely flushed through the soil profile, below the deepest sampling depth. This is due to the lower water and nutrient holding capacity of the soils in the 2023 growing season as compared to the 2022 growing season and back-to-back leaching rainfall events in which 44% of total rainfall (285.5 mm) occurred within a 14-day span between the R3 and R4 soil samplings. This is supported by the significantly lower yields obtained in the CONV fertilizer treatment compared to three out of the four CRF treatments (CRF 224, 280, 336 kg ha−1), which was likely due to N deficiencies at later growth stages. Although 60% of N uptake is achieved around the VT growth stage, maize continues to take up N until as far as three weeks after the R1 growth stage [76,79]. A study conducted by Binder et al. [80] even highlighted that N applications as late as the R3 growth stage could have increasing effects on maize grain yield. Therefore, even though soil NO3-N trends in the 2023 growing season were similar between the CRF and CONV fertilizer treatments, it is possible that a large undetected leaching event may have occurred late in the growing season. Thus, the CONV fertilizer treatment’s soil NO3-N at these high leaching zones actually mirrored the trends seen in the 2022 growing season, where soil NO3-N in the 60–120 cm range increased after the R3 growth stage, but where large rainfall between the R3 and R4 soil samplings did not allow for the direct quantification of this NO3-N movement in the soil.

3.4. Grain Yield

The control no-N treatment yielded more in 2022 than in 2023, with yields of 6.1 Mg ha−1 in 2022 but only 2.4 Mg ha−1 in 2023 (Table 4, Figure 5). A potential cause for these higher yields could be the soil texture differences between 2022 and 2023. A study conducted by [81] which investigated differences between soil types on maize grain yield found that no-N treatments in sandy soils averaged grain yields of 5.8 Mg ha−1 while averaging yields of 9.5 Mg ha−1 in clay soils. Similarly in this study, although both soils were characterized as sandy, larger amounts of clay observed in the 2022 growing season (nearly double those in 2023) resulting in higher residual NO3-N content were linked with higher yields as compared to the 2023 growing season (Table 1). With N playing a key role in crop growth and development, differences in maize grain yield were observed between a no-N treatment and fertilized treatments in both years [82,83,84]. On average, the grain yield of CRF168, CRF223, CRF280, CRF336, and CONV269 was 53%, 55%, 58%, 57% and 54% higher in 2022 and 77%, 81%, 82%, 83%, and 72% in 2023, respectively, than that of the no-N treatment (Table 4). The lower difference in 2022 might be due to the more optimal N uptake and N nutrient release observed in the 2022 growing season, as previously discussed for crop growth and health parameters. This is supported by the fact that all treatments, except for the CRF’s highest rate of 336 kg N ha−1, produced higher yields in the 2022 growing season than in the 2023 growing season (Table 4).
Mean grain yields for all CRF rates ranged from 12.9 Mg ha−1 to 14.5 Mg ha−1 and 10.9 Mg ha−1 to 14.3 Mg ha−1 for the 2022 and 2023 growing seasons, respectively, while the CONV fertilizer treatment obtained mean yields of 13.1 Mg ha−1 and 8.6 Mg ha−1 for the 2022 and 2023 growing seasons, respectively (Table 4). In 2022, all CRF rates were not statistically different from the CONV fertilizer treatment. However, for the 2023 growing season, the CRF rates of 224, 280, and 336 kg N ha−1 obtained significantly greater yields than the CONV fertilizer treatment. The CRF rate of 168 kg N ha−1 remained not statistically different from the CONV fertilizer treatment in 2023, but significantly lower than the CRF rates of 280 and 336 kg N ha−1. Thus, over the course of the study, all CRF rates produced similar or greater grain yields than the CONV fertilizer treatment, indicating that the CRF was able to supply the crops with sufficient amounts of N to produce yields comparable to those of the current CONV fertilizer treatment, even at lower N rates. Similarly, in [16], a polymer-coated CRF had no significant yield differences to CONV fertilized treatments, even when the CRF rates were only 70% of the CONV fertilizer rates. Several studies have even found that CRF is able to produce significantly greater yields when applied at lower rates to CONV fertilizer treatments [18,25,85], which is also seen in this study between the rate of CRF 224 kg N ha−1 and the CONV 269 kg N ha−1 treatment in the 2023 growing season. In Maharjan et al.’s study [11], when CONV fertilizer was applied as a split application multiple times throughout the growing season, it resulted in higher yields than the single application of CRF. This highlights that when CONV fertilizer treatments are applied using best management practices, such as repeated applications, they can compete with CRFs. Even though in this study, the CONV fertilizer treatment implemented the BMP of repeated applications, the CONV treatment yields were either significantly lower or similar to those of the CRF fertilizer treatment even when the CRF rates were lower (Table 4), further emphasizing the potential of CRFs in sandy well-draining soils.

3.5. Nitrogen Release Dynamics and Multivariate Analysis of CRF Performance

The release of nutrients from the polymer-coated CRF is a very complex process and it exhibits interannual variation and is interrelated to many factors such as soil and crop management practices and environmental factors (which also vary substantially between growing seasons). To further analyze the dynamics of NO3-N in the soil profile and maize yield under varying environmental conditions, we employed both correlation analysis and Principal Component Analysis (PCA). These additional analyses enabled us to separate environmental impacts from treatment-specific effects, assess the consistency of CRFs across varied climatic conditions (relatively drier in 2022 and wetter in 2023), and explore practical implications for CRF applications.

3.5.1. Correlation Analysis: Relationships Between Soil NO3-N, Soil Moisture, Crop Performance Indicators, and Environmental Variables

The correlation analysis provided foundational insights into the relationships between environmental factors, soil NO3-N levels, and crop performance metrics across the growing seasons. To fully understand the dynamics of NO3-N and its interaction with crop performance and environmental indicators, correlation analysis was performed by combining the data from all of the treatments at different plant growth stages, mainly the vegetative growth stage V12 and reproductive growth stages R3 and R5. The correlation analysis revealed distinct relationships across the 2022 and 2023 growing seasons, with variations primarily driven by differences in weather patterns, which further dictate the release of CRF nitrogen. For both years, the crop growth indicators, such as the leaf area index (LAI) and plant height were strongly correlated with each other and with the yield, demonstrating that the CRF’s availability supported vegetative growth and the critical role of nitrogen uptake in supporting crop productivity (Figure 6).
During the early vegetative stage in 2022, NO3-N availability was effectively managed within the root-accessible zones under drier conditions, significantly aided by controlled-release fertilizers (CRFs). Correlation analysis from this year shows moderate positive relationships between shallow NO3-N levels (NO3-N_30) and essential plant growth parameters, such as plant height (r = 0.44) and the leaf area index (LAI, r = 0.28). These relationships indicate that CRFs effectively maintained NO3-N availability, which is crucial under limited water conditions. Additionally, a strong positive correlation between NO3-N_30 and soil moisture at 30 cm (VWC_30, r = 0.71) suggests that minimal rainfall contributed significantly to preserving NO3-N within the root zone, enhancing early crop growth—a phenomenon consistent with findings by Blanco-Canqui and Claassen [86], who observed similar NO3-N retention patterns under constrained water conditions, facilitating early plant vigor. Conversely, in 2023, increased precipitation altered NO3-N dynamics during the early vegetative stage. The correlation between NO3-N_30 and plant height weakened (r = 0.30), indicating reduced NO3-N availability at shallower depths due to leaching. The correlation between NO3-N_60 and the LAI (r = 0.60) strengthened, suggesting downward NO3-N translocation driven by rainfall, with mid depths becoming more critical for nutrient uptake. Additionally, the slight positive correlation between NO3-N_90 and VWC_90 (r = 0.35) indicates NO3-N movement to deeper soil layers, rendering them less accessible to young plants.
By the mid-reproductive stage (R3), plants had achieved much of their structural growth, with NO3-N availability playing a critical role in supporting biomass accumulation and reproductive processes. In 2022, the moderate correlation between NO3-N_60 and plant height (r = 0.18) suggests that NO3-N retained at mid-soil depths (60 cm) contributed positively to plant growth. The strong correlation between NO3-N_60 and VWC_60 (r = 0.62) indicates efficient NO3-N retention at mid depths, supporting sustained nutrient uptake during this phase. Deeper NO3-N levels (NO3-N_90) showed a high correlation with deep soil moisture (VWC_90, r = 0.82), reflecting minimal leaching and suggesting that NO3-N remained present but less accessible to the root zone. In 2023, wetter conditions disrupted NO3-N retention patterns. The correlation between NO3-N_60 and VWC_60 dropped to r = 0.34, indicating reduced NO3-N retention and increased leaching. Additionally, the correlation between NO3-N_90 and VWC_90 (r = 0.02) suggests that substantial NO3-N was leached beyond the root-accessible zone, rendering it unavailable to plants. This pattern aligns with studies conducted by Wu et al. [87], who reported similar NO3-N displacement trends under heavy rainfall conditions. The contrasting patterns across the years underscore the role of environmental conditions in NO3-N retention at mid and deeper soil layers. While 2022 supported consistent NO3-N availability across depths, 2023 demonstrated increased NO3-N losses due to heavy rainfall, reducing mid-depth retention and decreasing NO3-N accessibility to plants.
By the late reproductive stage (R5), NO3-N availability becomes increasingly critical for grain filling and yield formation. In the 2022 growing season, strong positive correlations were observed between shallow NO3-N levels (NO3-N_30) and grain yield (r = 0.49), emphasizing the importance of NO3-N retention within the root zone for grain filling. This observation aligns with findings from Ergo et al. [88], who noted similar NO3-N-driven yield responses under dry conditions. Moderate correlations between mid-depth NO3-N levels (NO3-N_60) and soil moisture (VWC_30: r = 0.78) further confirmed the stability of NO3-N availability under drier conditions. In 2023, excessive rainfall disrupted NO3-N utilization during the grain-filling stage. The correlations between NO3-N_30 and yield (r = 0.73) and NO3-N_60 and yield (r = 0.77) suggest that NO3-Ns were available but less effectively utilized for grain production. Instead, the negative correlation between NO3-N_90 and VWC_90 (r = −0.60) highlights substantial NO3-N leaching beyond the root zone, limiting its contribution to grain yield. These observations align with Blanco-Canqui and Claassen [86], who documented significant NO3-N losses under wetter conditions, reducing nutrient use efficiency and impacting final yields. The comparative analysis highlights that 2022’s drier conditions facilitated NO3-N retention across shallow and mid depths, supporting efficient grain filling and higher yields. Conversely, in 2023, despite mid-depth NO3-N availability, extensive NO3-N leaching at deeper layers limited nutrient availability for grain filling, contributing to reduced yield outcomes.

3.5.2. Comparative Insights Across Growth Stages and Seasons

The correlation analysis across the V12, R3, and R5 stages underscores the influence of environmental variability on NO3-N-soil moisture dynamics and crop productivity. In 2022, drier conditions promoted efficient NO3-N retention at root-accessible depths, minimizing leaching and enhancing nutrient use efficiency. This pattern was reflected in higher correlations between NO3-N levels and plant growth metrics across all stages. In contrast, 2023’s wetter conditions disrupted NO3-N retention, causing leaching at mid and deeper soil layers, and reducing overall nutrient efficiency. Despite NO3-N availability at mid depths, deeper leaching compromised yield potential. These findings emphasize the critical importance of synchronized water and nutrient management strategies to optimize crop productivity under varying environmental conditions. CRFs demonstrated their effectiveness under drier conditions but faced challenges in wetter years, highlighting the need for adaptive management practices tailored to seasonal rainfall variability.

3.5.3. Growth Stage PCA: Variance Drivers and NO3-N Dynamics

The Principal Component Analysis (PCA) provided an integrated view of the relationships between NO3-N availability, soil moisture, plant growth parameters, and weather variables across the contrasting growing seasons of 2022 and 2023. The first two principal components (PC1 and PC2) explained 57.81% of the total variance, effectively summarizing the key factors driving crop performance and nutrient dynamics under varying environmental conditions (Figure 7).
PC1, explaining 32.02% of the total variance, was primarily associated with plant growth parameters (the LAI, height), grain yield, and NO3-N levels at shallow (NO3-N_30) and mid depths (NO3-N_60). Strong contributions from the LAI (0.36), height (0.36), and yield (0.38) underscore the central role of root-zone NO3-N availability in supporting crop growth and yield outcomes, particularly under the more stable conditions observed in 2022. Additionally, VWC_60 (0.34) also contributed significantly to PC1, emphasizing the role of mid-soil moisture in retaining NO3-Ns for crop uptake during critical growth phases. PC2, accounting for 25.79% of the variance, was heavily influenced by weather-related factors (RAIN_cum, VWC_90) and deep NO3-N levels (NO3-N_90). Variables such as RAIN_cum (−0.40) and VWC_90 (−0.10) highlight the significant role of rainfall in redistributing NO3-N within the soil profile, particularly in 2023, when increased precipitation facilitated leaching below root-accessible depths. The alignment of NO3-N_90 with PC2 reflects the reduced availability of deeper NO3-N for crop uptake, aligning with the lower yield outcomes observed in this wetter year.
The PCA also highlighted year-specific differences in NO3-N dynamics and crop responses. In the 2022 growing season, data points formed a more compact cluster, suggesting less variability between treatments under the stable weather conditions of that year. The alignment of NO3-N levels with growth parameters and yield in this drier year emphasizes that retained NO3-N in the root zone was crucial for meeting crop nitrogen demands. This allows lower CRF rates (168 kg N ha−1) to achieve yields comparable to higher rates and outperform the CONV fertilizer treatment. Conversely, the 2023 growing season showed greater variability between treatments, likely influenced by the effects of cumulative rainfall and associated NO3-N leaching. This dynamic, evidenced by the stronger correlation of deeper NO3-N levels with PC2, points to the challenges of nutrient management in wetter conditions, where leaching can significantly impact nutrient availability. Similar observations were made by Crista et al. [89], who noted the challenges of managing nutrient leaching under high rainfall conditions. In conclusion, the PCA results reinforce the importance of understanding and managing NO3-N dynamics within soil profiles to sustain crop performance. They highlight the need for adaptive nutrient management strategies that consider both soil moisture conditions and weather forecasts, emphasizing the importance of controlled-release fertilizers (CRFs) and other technologies to mitigate NO3-N leaching and ensure consistent crop performance under varying weather conditions. These strategies are essential for maintaining agricultural productivity in the face of climate variability, as highlighted by the extended findings and recommendations of Crista et al. [89].

4. Conclusions

In this two-year study, polymer-coated controlled-release fertilizers (CRFs) provided crops with sufficient nitrogen (N) to support plant health, growth, and yields comparable to or exceeding those achieved with conventional (CONV) fertilizers in the sandy soils and wet climate of the Suwannee River Basin (SRB) of North Florida. This consistency was observed even amidst seasonal variations, such as the increased residual soil NO3-N, higher clay content, and greater soil N fertility noted in the 2022 growing season, which influenced the differentiation between treatments. All of the CRF rates investigated, including the lowest rate of 168 kg N ha−1, yielded a similar or greater plant height, leaf area index (LAI), leaf tissue N, aboveground biomass (AGB), and grain yield compared to the CONV treatment (269 kg N ha−1) across multiple growth stages in both growing seasons.
Additionally, correlation and PCA analyses highlighted distinct environmental drivers across years, with higher temperature and optimum rainfall conditions playing a dominant role in nitrogen release and crop performance in 2022, and intense rainfall driving NO3-N leaching and reducing crop productivity in 2023. This underscores the importance of adaptive management strategies, such as split applications or slower-release formulations, to sustain nitrogen availability. These findings emphasize the role of soil moisture and temperature in mediating CRF performance and the potential of CRFs to mitigate NO3-N losses under variable climatic conditions. Future studies should incorporate CRF placement, water quality sampling, and predictive modeling to comprehensively quantify NO3-N leaching events. These approaches would provide greater clarity on the environmental benefits of polymer-coated CRFs, particularly in terms of their role in mitigating NO3-N leaching in the SRB of North Florida. This study’s insights reinforce CRFs as a promising, climate-adaptive fertilizer strategy that aligns productivity goals with environmental stewardship.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15020455/s1, Figure S1: Mean plant height, including statistical analysis, at various maize growth stages during the 2022 growing season; Figure S2: Mean plant height, including statistical analysis, at various maize growth stages during the 2023 growing season; Figure S3: Mean leaf area index, including statistical analysis, at various maize growth stages during the 2022 growing season; Figure S4: Mean leaf area index, including statistical analysis, at various maize growth stages during the 2023 growing season; Figure S5: Mean leaf tissue nitrogen, including statistical analysis, at various maize growth stages during the 2022 growing season; Figure S6: Mean leaf tissue nitrogen, including statistical analysis, at various maize growth stages during the 2023 growing season; Figure S7: Mean aboveground biomass, including statistical analysis, at various maize growth stages during the 2022 growing season; Figure S8: Mean aboveground biomass, including statistical analysis, at various maize growth stages during the 2023 growing season; Figure S9: Mean soil nitrate-nitrogen, including statistical analysis, at various maize growth stages during the 2022 growing season; Figure S10: Mean soil nitrate-nitrogen, including statistical analysis, at various maize growth stages during the 2023 growing season; Figure S11: Distribution of rainfall and irrigation in the 2022 and 2023 maize growing season; Figure S12: Handwritten calculation to calculate the combined (60–120 cm) NO3-N content from soil depths 60–90 and 90–120; Table S1: Irrigation amount and timing for 2022 and 2023 maize growing season.

Author Contributions

Conceptualization, V.S. and R.C.H.; methodology, M.M., V.S. and R.C.H.; software, M.M., V.S. and R.K.S.; validation, M.M., V.S., J.A.W., G.M.-L. and R.C.H.; formal analysis, M.M., R.K.S. and V.S.; investigation, M.M., V.S., J.A.W., G.M.-L. and R.C.H.; resources, V.S.; data curation, M.M., V.S., and R.K.S.; writing—original draft preparation, M.M., V.S. and R.K.S.; writing—review and editing, V.S., J.A.W., G.M.-L. and R.C.H.; visualization, M.M., V.S. and R.K.S.; supervision, V.S., J.A.W., G.M.-L. and R.C.H.; project administration, V.S. and R.C.H.; funding acquisition, V.S. and R.C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Florida Department of Agriculture and Consumer Services (FDACS), Office of Agricultural Water Policy, Award ID: AWD10700 and the USDA National Institute Food and Agriculture (PI’s Hatch Project No. FLA-ABE-006033).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Acknowledgments

The authors of this publication would like to acknowledge and thank all of the personnel at the UF/IFAS NREC-SV for their contribution and support in helping to ensure the success of this project. We would also like to thank the UF ABE Precision Water lab members for assisting in data collection for this study, including Uday Bhanu Prakash Vaddevolu, Susanta Das, Varshitha Prasanna, and Bibek Acharya.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

AIC, Akaike’s Information Criteria; AN, ammonium nitrate; AS, ammonium sulfate; AGB, aboveground biomass; BMP, best management practice; CI, confidence interval; CONV, conventional; CRF, controlled-release fertilizer; DAP, Days After Planting; DSSAT; Decision Support System for Agrotechnology Transfer; FAWN, Florida Automated Weather Network; FDACS, Florida Department of Agriculture and Consumer Services; FDEP, Florida Department of Environmental Protection; FL, Florida; LAI, leaf area index; N, nitrogen; NFREC-SV, North Florida Research and Education Center—Suwannee Valley; NO3-N, nitrate-nitrogen; RCBD, Randomized Complete Block Design; SMS, soil moisture sensors; SRB, Suwannee River Basin; UAN, Urea Ammonium Nitrate; UF/IFAS, University of Florida Institute of Food and Agricultural Sciences.

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Figure 1. Graphical weather data depicting lines of maximum (red), minimum (blue), and average (grey) temperatures along with bars of total daily rainfall (orange) throughout 2022 and 2023 maize growing seasons.
Figure 1. Graphical weather data depicting lines of maximum (red), minimum (blue), and average (grey) temperatures along with bars of total daily rainfall (orange) throughout 2022 and 2023 maize growing seasons.
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Figure 2. Crop health and growth parameters including (A) plant height, (B) leaf area index, (C) leaf tissue nitrogen, and (D) aboveground biomass (AGB) for 2022 and 2023 maize growing seasons across fertilizer treatments.
Figure 2. Crop health and growth parameters including (A) plant height, (B) leaf area index, (C) leaf tissue nitrogen, and (D) aboveground biomass (AGB) for 2022 and 2023 maize growing seasons across fertilizer treatments.
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Figure 3. Soil nitrate-nitrogen (NO3-N, mg kg−1) at various soil profile depths including (A) 0–30 cm, (B) 30–60 cm, (C) 60–90 cm, and (D) 90–120 cm for the 2022 and 2023 maize growing seasons across fertilizer treatments.
Figure 3. Soil nitrate-nitrogen (NO3-N, mg kg−1) at various soil profile depths including (A) 0–30 cm, (B) 30–60 cm, (C) 60–90 cm, and (D) 90–120 cm for the 2022 and 2023 maize growing seasons across fertilizer treatments.
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Figure 4. The mean soil nitrate-nitrogen (NO3-N) within the 60–120 cm soil profile for 2022 and 2023 maize growing seasons; this figure shows the color gradient of the growth stages, with the youngest stage (V6) being the darkest purple and progressively getting lighter until the R5 growth stage.
Figure 4. The mean soil nitrate-nitrogen (NO3-N) within the 60–120 cm soil profile for 2022 and 2023 maize growing seasons; this figure shows the color gradient of the growth stages, with the youngest stage (V6) being the darkest purple and progressively getting lighter until the R5 growth stage.
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Figure 5. Violin plot graph of grain yield under different nitrogen fertilizer treatments for 2022 and 2023 maize growing seasons. Treatments with same letters within each year are not significantly different at p < 0.05.
Figure 5. Violin plot graph of grain yield under different nitrogen fertilizer treatments for 2022 and 2023 maize growing seasons. Treatments with same letters within each year are not significantly different at p < 0.05.
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Figure 6. A correlation matrix illustrating the relationships between climatic factors, soil moisture, nitrate-nitrogen (NO3-N) concentrations at different depths, and plant growth attributes at the vegetative (V12) and reproductive (R3 and R5) growth stages during the 2022 and 2023 maize growing seasons. The variables include soil moisture (VWC) at 30 cm, 60 cm, and 90 cm depths (VWC_30, VWC_60, VWC_90, respectively); NO3-N concentrations at three soil depths (NO3-N_30: 0–30 cm, NO3-N_60: 30–60 cm, NO3-N_90: 60–90 cm); and plant growth parameters including plant height (Height) and the leaf area index (LAI). Positive correlations are shown in red, while negative correlations are represented in blue, with the intensity of the color corresponding to the strength of the correlation.
Figure 6. A correlation matrix illustrating the relationships between climatic factors, soil moisture, nitrate-nitrogen (NO3-N) concentrations at different depths, and plant growth attributes at the vegetative (V12) and reproductive (R3 and R5) growth stages during the 2022 and 2023 maize growing seasons. The variables include soil moisture (VWC) at 30 cm, 60 cm, and 90 cm depths (VWC_30, VWC_60, VWC_90, respectively); NO3-N concentrations at three soil depths (NO3-N_30: 0–30 cm, NO3-N_60: 30–60 cm, NO3-N_90: 60–90 cm); and plant growth parameters including plant height (Height) and the leaf area index (LAI). Positive correlations are shown in red, while negative correlations are represented in blue, with the intensity of the color corresponding to the strength of the correlation.
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Figure 7. A Principal Component Analysis (PCA) biplot showing the relationships between soil nitrate-nitrogen concentrations (NO3_30, NO3_60, NO3_90), soil moisture (VWC_30, VWC_60, VWC_90), cumulative rainfall (RAIN_sum, RAIN_cum), and crop performance metrics (LAI, height, yield) during the 2022 (red) and 2023 (blue) maize growing seasons.
Figure 7. A Principal Component Analysis (PCA) biplot showing the relationships between soil nitrate-nitrogen concentrations (NO3_30, NO3_60, NO3_90), soil moisture (VWC_30, VWC_60, VWC_90), cumulative rainfall (RAIN_sum, RAIN_cum), and crop performance metrics (LAI, height, yield) during the 2022 (red) and 2023 (blue) maize growing seasons.
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Table 1. Field soil conditions for 2022 and 2023 maize growing seasons.
Table 1. Field soil conditions for 2022 and 2023 maize growing seasons.
Year Soil TextureResidual Soil
NO3-N
Soil SeriesSandSiltClay0–30 cm30–60 cm60–90 cm
— — — — — % — — — — —— — — — — mg kg−1 — — — —
2022Blanton–Foxworth–Alpin complex88.31.610.16.26.57.4
2023Hurricane, Albany, and Chipley soils92.42.25.43.80.60.5
Table 2. Description of different nitrogen treatments investigated in this study.
Table 2. Description of different nitrogen treatments investigated in this study.
Treatment Total N applicationFertilizer TypeSourceDescription
10Control-No N fertilizer added
2168CRFHarrell’s, LLC POLYON® CRF (43-0-0)Total of 168 kg N ha−1—this includes 34 kg N ha−1 liquid fertilizer as a starter fertilizer and the remaining 134 kg N ha−1 of CRF added at planting (all upfront—no fertilizer added during season)
3224CRFHarrell’s, LLC POLYON® CRF (43-0-0)Total of 224 kg N ha−1—this includes 34 kg N ha−1 liquid fertilizer as a starter fertilizer and the remaining 190 kg N ha−1 of CRF added at planting (all upfront—no fertilizer added during season)
4280CRFHarrell’s, LLC POLYON® CRF (43-0-0)Total of 280 kg N ha−1—this includes 34 kg N ha−1 liquid fertilizer as a starter fertilizer and the remaining 246 kg N ha−1 of CRF added at planting (all upfront—no fertilizer added during season)
5336CRFHarrell’s, LLC POLYON® CRF (43-0-0)Total of 336 kg N ha−1—this includes 34 kg N ha−1 liquid fertilizer as a starter fertilizer and the remaining 302 kg N ha−1 of CRF added at planting (all upfront—no fertilizer added during season)
6269Conventional *UAN (32-0-0)Total of 269 kg N ha−1—this includes 34 kg N ha−1 liquid fertilizer as a starter fertilizer and the remaining 235 kg N ha−1 applied throughout the growing season based on growth stage needs (V4, V8, V10, V12, V14, VT, and R1-R2)
* In-season remaining conventional N application of 235 kg N ha−1 was applied at different maize growth stages as: V4: 23.5 kg N ha−1; V8: 35 kg N ha−1; V10: 47 kg N ha−1; V12: 59 kg N ha−1; V14: 23.5 kg N ha−1; VT: 23.5 kg N ha−1; and R1–R2: 23.5 kg N ha−1.
Table 3. Crop management practices for 2022 and 2023 maize growing seasons.
Table 3. Crop management practices for 2022 and 2023 maize growing seasons.
YearSeed
Variety		Seeding RatePlanting DateHarvest DateHerbicideFungicideInsecticide
Seeds ha−1 a NameAmount
(L ha−1)		a NameAmount
(L ha−1)		a NameAmount (L ha−1)
2022Pioneer ‘1870 YHR’80,30925 March19 AugustDual II Magnum1.75Headline AMP0.73Counter6.05
Atrazine4.68
Roundup1.61
2023Pioneer ‘1870 YHR’79,07420 March17 AugustDual II Magnum1.75Headline AMP0.73Counter6.05
Atrazine4.68
Roundup1.61
a Dual II Magnum is S-metolachlor (C15H22ClNO2); atrazine is atrazine (C8H14ClN5); roundup is glyphosate (C3H8NO5P); headline AMP is pyraclostrobin and metconazole (C19H18ClN3O4 and C17H22ClN3O); counter is terbufos (C9H21O2PS3).
Table 4. The plant height, leaf area index (LAI), leaf tissue nitrogen, aboveground biomass (AGB), and yield data * for the 2022 and 2023 maize growing seasons. Treatments with the same letters within the same column and year are not significantly different at p < 0.05.
Table 4. The plant height, leaf area index (LAI), leaf tissue nitrogen, aboveground biomass (AGB), and yield data * for the 2022 and 2023 maize growing seasons. Treatments with the same letters within the same column and year are not significantly different at p < 0.05.
YearTreatment* Plant Height* Leaf Area Index* Leaf Tissue Nitrogen£ Aboveground Biomass£ Yield
—m——m2 m−2——%————— Mg ha−1 ———
2022No Nitrogen2.26 a2.79 a2.13 a15.1 a6.1 a
CRF 1682.92 c4.04 bc2.59 b23.7 b12.9 b
CRF 2242.91 c3.92 bc2.52 ab28.8 cd13.6 b
CRF 2802.95 c3.86 bc2.91 b31.0 d14.5 b
CRF 3362.98 c4.27 c2.68 b29.9 d14.2 b
CONV 2692.69 b3.60 b2.78 b25.6 bc13.1 b
2023No Nitrogen1.92 a2.38 a1.03 a14.9 a2.4 a
CRF 1682.69 b3.89 b2.14 b20.7 ab10.9 bc
CRF 2242.75 b3.77 b2.43 b23.9 b12.9 cd
CRF 2802.75 b3.94 b2.53 b24.1 b14.1 d
CRF 3362.72 b4.11 b3.13 c23.8 b14.3 d
CONV 2692.61 b3.54 b2.37 b20.6 ab8.6 b
* values are the mean values at the R3 growth stage; the values are assessed at this growth stage because the final CONV fertilizer application is applied at the R1/R2 growth stage. £ values are the mean values at the end of the growing season.
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MDPI and ACS Style

Morrow, M.; Sharma, V.; Singh, R.K.; Watson, J.A.; Maltais-Landry, G.; Hochmuth, R.C. Impact of Polymer-Coated Controlled-Release Fertilizer on Maize Growth, Production, and Soil Nitrate in Sandy Soils. Agronomy 2025, 15, 455. https://doi.org/10.3390/agronomy15020455

AMA Style

Morrow M, Sharma V, Singh RK, Watson JA, Maltais-Landry G, Hochmuth RC. Impact of Polymer-Coated Controlled-Release Fertilizer on Maize Growth, Production, and Soil Nitrate in Sandy Soils. Agronomy. 2025; 15(2):455. https://doi.org/10.3390/agronomy15020455

Chicago/Turabian Style

Morrow, Morgan, Vivek Sharma, Rakesh K. Singh, Jonathan Adam Watson, Gabriel Maltais-Landry, and Robert Conway Hochmuth. 2025. "Impact of Polymer-Coated Controlled-Release Fertilizer on Maize Growth, Production, and Soil Nitrate in Sandy Soils" Agronomy 15, no. 2: 455. https://doi.org/10.3390/agronomy15020455

APA Style

Morrow, M., Sharma, V., Singh, R. K., Watson, J. A., Maltais-Landry, G., & Hochmuth, R. C. (2025). Impact of Polymer-Coated Controlled-Release Fertilizer on Maize Growth, Production, and Soil Nitrate in Sandy Soils. Agronomy, 15(2), 455. https://doi.org/10.3390/agronomy15020455

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