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Article

An Evaluation of the Effect of Fertilizer Rate on Tree Growth and the Detection of Nutrient Stress in Different Irrigation Systems

by
Krishna Neupane
,
Anthony Witcher
and
Fulya Baysal-Gurel
*
Department of Agricultural Sciences and Engineering, College of Agriculture, Tennessee State University, Otis L. Floyd Nursery Research Center, 472 Cadillac Lane, McMinnville, TN 37209, USA
*
Author to whom correspondence should be addressed.
Horticulturae 2024, 10(7), 767; https://doi.org/10.3390/horticulturae10070767
Submission received: 19 June 2024 / Revised: 15 July 2024 / Accepted: 15 July 2024 / Published: 19 July 2024
(This article belongs to the Special Issue Studies on Biotic and Abiotic Stress Responses of Horticultural Plants)
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Abstract

:
Early season monitoring of nutrient stress is important in red maple (Acer rubrum L.) and flowering dogwood (Cornus florida L.) to optimize management practices and ensure healthy crop production in containers. Two different irrigation systems (drip and overhead irrigation) were used in this study. Two rates (low and high) of controlled-release fertilizer were used with no fertilizer as a control treatment. Data were recorded for plant height, stem diameter, substrate pH and electrical conductivity (EC), chlorophyll content, normalized difference vegetation index (NDVI), visual observation of plant quality, and leaf nutrient content. The results of this study showed that the increase in plant height and stem diameter was greater among the fertilized maple tree, whereas no differences were observed in the flowering dogwoods for an increase in plant height. NDVI was greater for drip irrigation for both fertilizer rates in both red maples and flowering dogwoods. A positive correlation of 73% to 83% was observed for red maples and 79% to 83% was observed for flowering dogwoods between handheld NDVI and unmanned aerial vehicle-mounted NDVI sensors. In red maple, a high fertilizer rate resulted in greater substrate pH, whereas in flowering dogwood, no differences were observed. Varied responses were observed among the treatments for nutrient content; however, both rates of fertilizer application were sufficient for both tree species. Drip-irrigated red maples had higher nitrogen and phosphorous content, whereas nitrogen content was higher in both irrigation systems in flowering dogwoods. This study provides useful insights into understanding the effect of nutrient stress on tree growth and the application of sensing technology for the monitoring and early detection of nutrient stress in container-grown nursery crops.

1. Introduction

The monitoring of the nutrient status provides an overall condition of health and vigor and is considered an important aspect of crop production. Several researchers and governing bodies have defined plant nutrients and fertilizers differently. Dobermann et al. [1] defined plant nutrients as an important element, the absence of which might restrict plants from completing their lifecycle, whereas Mee et al. [2] highlighted the importance of plant nutrients as essential components for growth and vitality. Kirkby [3] published an article stating that fourteen essential nutrients are required for plants to complete their life cycle. Fertilizers, on the other hand, are defined as important additional substances that help plants increase their growth, maintain their health, and improve productivity. Both inadequate and excessive amounts of nutrients available to the plants are considered nutrient stress or fertilizer stress. Insufficient fertilizer application causes a deficiency of nutrients in the soil, while excessive application becomes detrimental to plants, soil, water, and the environment [4]. Fertilizer stress could result in restricted root and shoot growth, chlorosis, early defoliation of leaves, and decreased biomass [2,5,6]. Thus, it is important to manage the application of fertilizer, providing only the required amount without over-exceeding or restraining nutrient status in plants.
The total amount of nutrients present in the soil may not always be available to the plants. The availability of nutrients to plants depend on several factors, such as soil moisture content, soil temperature, pH, the physical conditions of the soil, and other toxic elements or salts [7]. Li et al. [8] reported that the movement of nutrients from the soil to roots and roots to the above-ground part is greatly influenced by adequate water content. Similarly, Rose et al. [9,10] described that adequate water is required for nutrient uptake in container-grown crops. Low soil water potential causes a restriction in the flow of nutrients to the roots through diffusion and mass flow [9,11]. Additionally, an adequate supply of nutrients to the plants helps them to withstand adverse environmental conditions [6,12,13,14]. A study conducted by Novair et al. [15] showed that the application of organic amendments like azo-compost and rice straw plays a vital role in increasing nutrient (phosphorus) uptake by plants in low-irrigation conditions. An increase in phosphorus uptake was recorded by Samadi et al. [16] while using biofertilizers in corn fields in drought conditions.
Irrigation systems play a vital role, particularly in terms of maintaining soil water status. They also play a huge factor in the growth of plants. Irrigation systems and fertilizers affect plant growth [17]. Diverse types of irrigation systems have been developed to irrigate containerized crops. Each system has its own pros and cons. Overhead irrigation systems supply water while also washing off the dust and fungal debris attached to the foliage of plants and reducing the temperature of the plants [18]. However, overhead irrigation uses excess water to ensure complete coverage of the production area [19]. On the other hand, drip irrigation reduces leaching and evaporation loss and allows efficient management of the water [20]. A study conducted by Rowe et al. [21] found that overhead-irrigated Sedum album L. and S. floriferum L. had greater plant height and plant health, while a study conducted by Davies et al. [22] found no significant differences between overhead irrigation and drip irrigation in terms of the vegetative growth of Cornus alba L. and Lonicera periclymenum L. On the other hand, Yang et al. [20] reported that drip irrigation is a clear winner among other irrigation systems as it is efficient in terms of water productivity.
Additionally, several approaches have been identified for monitoring the biotic and abiotic stress of plants. Remote sensing has been widely used by scientists and researchers to identify and monitor crop stress. Chowhan and Chakraborty [23] explained remote sensing as a dynamic technique that records stress in the crop through the changes in electromagnetic radiation and quantifies the amount of stress. Mee, Siva, and Ahmad [2] explained that specific wavelength measurements could help to identify different types of plant stresses. Cadet and Samson [24] and Gulzar et al. [25] used the fluorescence emission technique to detect nitrogen, potassium, and phosphorus deficiency in sunflowers. An in situ real-time monitoring system was used to monitor nitrate and phosphorous uptake in plants [26]. Mahajan et al. [27] used hyperspectral reflectance techniques, applying eight different vegetative indices, to monitor nitrogen, phosphorus, sulfur, and potassium in wheat. Additionally, remote sensing was used to evaluate the performance of irrigation systems and their management [28]. This shows that the application of remote sensing and UAVs can have an enormous impact on the monitoring of water status and fertilizer stress in woody ornamental production systems.
Red maple and flowering dogwoods are important ornamental landscape crops. However, their water requirements are different. Red maples are moderately tolerant to flooding [29], but flowering dogwoods are intolerant to flooding [30]. Most commercial ornamental nursery growers use overhead sprinklers (small containers) or micro-irrigation systems (large containers) to supply adequate water to the crops [31]. However, only 20–40% of the water from overhead irrigation is retained in the containers [32]. The objective of this study was to evaluate the response of two tree species in a container production system while using different fertilizer rates in two irrigation systems. Also, the objective was to monitor which irrigation system performs well in different fertility conditions to improve the production of these tree species. Similarly, this study was also designed to monitor nutrient stress through aerial imagery and sensing technology. This study will help with understanding the irrigation system, water use efficiency, fertilizer requirements, and optimization of important ornamental crops like red maple and flowering dogwoods.

2. Materials and Methods

2.1. Experimental Site, Planting Materials, and Design of Study

This study was conducted at the Otis L. Floyd Nursery Research Center in McMinnville, TN, during the summer of 2022. Red maple ‘Sun Valley’ and flowering dogwood ‘Cherokee Princess’ bare root liners were purchased from a commercial nursery and were transplanted (February 2022) into 11.4 L (3-gallon) nursery containers (C1200; Nursery Supplies, Chambersburg, PA, USA) filled with a standard pine bark substrate (1.6 cm screened; Morton’s Horticultural Products, Inc., McMinnville, TN, USA). The transplanted trees were moved to an outdoor gravel pad in the last week of April. Trees appearing similar in shoot size and health conditions visually were selected for this study. Twenty individual trees were arranged randomly into four replications (block of five individuals per replication) for each treatment in a split-plot design. This study was conducted from 9 May to 10 August 2022.

2.2. Fertilizer Rate

Three fertilizer treatments included control (no fertilizer), low rate of fertilizer, and high rate of fertilizer. Controlled-release fertilizer (Florikan 17-7-8 NPK total, 6 months @ 80 °F; Florikan LLC, Sarasota, FL, USA) (composition: total N: 17%; P: 7%; K: 8%; S: 4%; Mg: 1.20%; Fe: 0.20%; Mn: 0.06%; Mo: 0.002%) was applied as a top-dress to each container on 9 May 2022 in different rates for each tree species. The low rates for flowering dogwood and red maple were 18 and 35 g per container, respectively. High rates were 55 and 70 g per container for flowering dogwood and red maple, respectively (manufacturer’s recommendation). Lower rates were used for flowering dogwood due to sensitivity to highly soluble salt levels [33].

2.3. Irrigation System

Two distinct types of irrigation systems (drip and overhead) were used. Drip irrigation was provided three times daily (11 a.m., 12 p.m., and 1 p.m.) using a dribble ring (15.2 cm diameter; Dramm Corp., Manitowoc, WI, USA) fitted with a pressure-compensating emitter (4 L h1; Netafim USA, Fresno, CA, USA). Irrigation application volume for each treatment was adjusted monthly based on replacement of daily water use/loss from the previous 24 h period. A subset of containers (4 replications) was irrigated (hand-watered) to saturation, allowed to drain for 1 h, then weighed. After 24 h, the same containers were weighed, and the difference in weight was calculated and converted to ml, which was then applied daily (equally divided among the three irrigation times). Overhead irrigation was provided once daily using multi-stream nozzles (MP3000 Rotator; Hunter Industries, San Marcos, CA, USA). Overhead irrigation application volume was adjusted periodically to maintain a leaching fraction of 10–30%. A subset of containers (4 replications) was nested inside 9.5 L plastic buckets so there was no gap between the top rim of the bucket and the container sidewall to measure irrigation leach volume of containers. A separate set of empty buckets (4 replications) was also placed between plants to measure irrigation application volume. One hour after the daily irrigation, irrigation application rate and leaching volume were recorded, and leaching fraction was calculated: (leach volume ÷ application volume) × 100 = leaching fraction. Each irrigation system was designated in a separate gravel pad controlled by a single solenoid.

2.4. Data Collection

2.4.1. Plant Growth Data

Changes in growth (plant height and stem diameter) were recorded as plant growth data. Plant height was recorded as total length from the substrate surface to the top growing point of the plant. Stem diameter was measured 5 cm above the base of the tree using a digital caliper (Mitutoyo Corp., Kanagawa, Japan). Plant height and stem diameter were recorded (twenty plants per treatment) at the beginning of this study and monthly for 3 months (June, July, August).

2.4.2. Normalized Difference Vegetation Index (NDVI)

NDVI values were collected from a handheld Field Scout CM 1000 NDVI Meter (Spectrum Technologies Inc., Aurora, IL, USA) and an unmanned aerial vehicle (UAV) (DJI Mavic 2; DJI Technology Co., Ltd., Shenzhen, Guangdong, China) mounted with Sentera single-sensor NDVI camera (Sentera Inc., Saint Paul, MN, USA). The UAV was flown monthly at a height of 20 m in bright, sunny conditions between 9 a.m. and 2 p.m. The aerial images obtained from UAV were processed to obtain NDVI values using commercial Sentera Field Agent software [Version v5.20.0].

2.4.3. Relative Chlorophyll

Relative chlorophyll content was recorded as Soil–Plant Analytical Development (SPAD) values monthly using a 502-leaf chlorophyll meter (Spectrum Technologies Inc., Aurora, IL, USA). The SPAD meter was calibrated each time while collecting data for different treatments to reduce the chances of error [34]. Data were recorded for every plant on a bright, sunny day between 9 a.m. and 2 p.m.

2.4.4. Visual Observation

Plants were visually rated for leaf chlorosis (monthly) to document the severity of any symptoms that could have resulted from nutrient deficiency. Ratings were ranked on a scale of 1–5 (1 = 1–20%; 2 = 21–40%; 3 = 41–60%; 4 = 61–80%; 5 = 81–100%) for the percentage of foliage showing chlorotic symptoms. Data were recorded for every tree used in this study.

2.4.5. Substrate Chemical Properties

Substrate pH and electrical conductivity (EC) were recorded monthly from leachate samples collected using the pour-through method [35]. Briefly, containers were irrigated just to saturation and allowed to drain for 1 h; then, 295 mL water was slowly applied to the substrate surface, and leachate was collected after 5 min and then brought to the lab for analysis. Leachate samples (four replications) were collected from a single plant from each replication and treatment. Leachates were analyzed, as their study helps to understand the salinity and nutrient uptake by the plants [36].

2.4.6. Leaf Tissue Analysis

At the end of this study, leaf samples were collected from representative plants of each treatment (n = 3) to analyze the nutrient content in the leaves. The samples were collected, dried in a forced-air oven at 50 °C, and then sent to a commercial laboratory (Waypoint Analytical Corp., Memphis, TN, USA) for analysis. Leaf tissue was analyzed for nitrogen (N), phosphorus (P), and potassium (K) content. Total nitrogen was analyzed using the automated combustion method (Total Nitrogen Analyzer: Leco, Carl Erba, Elementar, and PerkinElmer with resistance furnace with thermal conductivity detector). Total phosphorus and potassium were analyzed using the microwave digestion/dissolution closed vessel method.

2.5. Statistical Analysis

Data were analyzed using SAS 9.4. Variables were analyzed using two-factor analysis with fertilizer rates and irrigation systems as factors. Plant growth data (height and stem diameter), NDVI (handheld meter), leaf greenness, leaf nutrient content, and substrate pH and EC were analyzed using general linear model, and means were separated using Fisher’s LSD test (Proc GLM). Visual observation ratings were analyzed using General Linear Mixed Model (GLMM) using normal distribution. Data with 100% and 0% rankings were converted to 0.99 and 0.01 to meet the assumptions of Proc GLIMMIX. Brown–Forsythe t-test ANOVA was used for data with unequal variances. Data for each month (June, July, and August) were analyzed separately.

3. Results

Among the treatments, significant differences were observed in the increase in plant height, stem diameter, NDVI, chlorophyll content, visual observation of plant quality, pH and EC, and leaf nutrient contents. A strong and positive correlation was observed between the UAV-mounted NDVI and the handheld NDVI.

3.1. Plant Height

For red maple, significant differences were observed among the treatments for fertilizer rate (June: p ≤ 0.0001, July: p ≤ 0.0001, August: p ≤ 0.0001) and irrigation systems (June: p = 0.0109, July: p = 0.0006, August: p ≤ 0.0001) throughout the study duration. However, significant interactions between fertilizer rates and irrigation systems were observed in June and July but not in August (June: p = 0.0028, July: p = 0.0026, August: p = 0.0597). In June and July, both rates of fertilizers in both irrigation systems led to a greater increase in plant height compared to the no-fertilizer controls. In August, both rates of fertilizer in overhead irrigation and the low rate of fertilizer in drip irrigation led to greater increases in plant height compared to the no-fertilizer controls and the high rate of fertilizer in drip irrigation (Figure 1).
For flowering dogwood, no significant differences were observed among the treatments for fertilizer rate (June: p = 0.2740, July: 0.1138; August: p = 0.9517), irrigation system (June: p = 0.0546, July: 0.1136, August: p = 0.8086), and interaction between fertilizer rates and irrigation systems (June: p = 0.7511, July: 0.5374, August: p = 0.6723) in terms of an increase in plant height throughout the study duration. In June, the low rate of fertilizer in drip irrigation led to a greater increase in plant height, but no significant differences were observed among the treatments within each irrigation system. In July, the high rate of fertilizer in overhead irrigation led to a significantly greater increase in plant height compared to both no-fertilizer controls, whereas no significant differences were observed among the treatments in August (Figure 1).

3.2. Stem Diameter

In red maple, significant differences in fertilizer rate were observed throughout the study period (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). For the irrigation system, significant differences were observed in July and August but not in June (June: p = 0.1612; July: p ≤ 0.0001; August: p = 0.0050). The interaction between the fertilizer rate and the irrigation system was significant in June and July (June: p = 0.0066; July: p ≤ 0.0001; August: p = 0.1520). Throughout the study duration, both rates of fertilizers applied on both irrigation systems led to a greater increase in stem diameter compared to the no-fertilizer controls. Controls of both irrigation systems had statistically similar increases in stem diameter (Figure 2).
In flowering dogwood, significant differences were only observed in June for the irrigation system (June: p = 0.0205; July: p = 0.1267; August: p = 0.2428) and in July and August for the fertilizer rate (June: p = 0.4277; July: p = 0.0007; August: p = 0.0112). No interactions between fertilizer rates and irrigation systems were observed (June: p = 0.4903; July: p = 0.3460; August: p = 0.4875). In June, drip irrigation led to a greater stem diameter compared to overhead irrigation in the no-fertilizer control, but there were no differences using the two other fertilizer rates. In July, all treatments had greater stem diameters compared to those that underwent drip irrigation with no-fertilizer controls. In August, fertilizer that underwent a low rate of drip irrigation led to greater stem diameter compared to the no-fertilizer and high fertilizer rates (regardless of irrigation system) (Figure 2).

3.3. NDVI (Handheld Meter)

For red maple, significant differences were observed in fertilizer rates throughout the study period (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). For irrigation systems, statistical differences were observed among the treatments in July and August (June: p = 0.1740; July: p ≤ 0.0001; August: p = 0.0244). Interactions between the fertilizer rate and the irrigation system were observed in July only (June: p = 0.2120; July: p = 0.0422; August: p = 0.2839). Red maples fertilized using both low and high fertilizer rates, regardless of irrigation system, had greater NDVI compared to both controls. Controls in both irrigation systems had statistically similar and significantly lower NDVI compared to others in June and August, while in July, the control of overhead irrigation was significantly higher than that of drip irrigation (Figure 3).
Among flowering dogwoods, significant differences were observed for fertilizer rates throughout the study period (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p = 0.0099); however, statistical differences were only observed in June and August for irrigation system (June: p ≤ 0.0001; July: p = 0.2699; August: p = 0.0120). Interactions between fertilizer rates and irrigation systems occurred in June (June: p = 0.0346; July: p = 0.2250; August: p = 0.1009). In June, drip-irrigated flowering dogwoods with a high rate of fertilizer had the greatest NDVI (Figure 3). In July, both rates of fertilizer for overhead-irrigated plants and the high fertilizer rate for drip-irrigated plants led to greater NDVI compared to both the no-fertilizer controls. In August, both rates of fertilizer in overhead irrigation led to greater NDVI compared to all other treatments except for the low fertilizer rate for drip irrigation.
A positive correlation was observed between the UAV-mounted NDVI sensor and the handheld NDVI sensor. In red maples, 77%, 73%, and 83% correlations were observed in June, July, and August, respectively (June: r = 0.77, p ≤ 0.0001; July: r = 0.73, p ≤ 0.0001; August: r = 0.83, p ≤ 0.0001; Figure 4). In flowering dogwood, 79%, 88%, and 83% correlations were observed in June, July, and August (June: r = 0.79, p ≤ 0.0001; July: r = 0.88, p ≤ 0.0001; August: r = 0.83, p ≤ 0.0001; Figure 4). An NDVI mosaic of red maples and flowering dogwoods treated with different rates of fertilizers is shown in Figure 5.

3.4. Relative Chlorophyll Content

In red maple, for relative chlorophyll content, significant differences occurred during July and August but not in June for fertilizer rates and the interaction between the fertilizer rate and the irrigation system (fertilizer rate: June: p = 0.3010; July: p ≤ 0.0001; August: p ≤ 0.0001; interaction: June: p = 0.1424; July: p = 0.0085; August: p = 0.0192). For the irrigation system, significant differences were only observed in August (June: p = 0.4277; July: p = 0.9410; August: p = 0.0408). In June, the high fertilizer rate in drip irrigation led to greater relative chlorophyll content compared to the low fertilizer rate in overhead irrigation and the no-fertilizer drip irrigation. In July and August, both fertilizer rates, regardless of irrigation system, led to greater relative chlorophyll content compared to both no-fertilizer controls (Figure 6).
In flowering dogwood, significant differences were observed for fertilizer rates in June, July, and August (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). For irrigation systems (June: p ≤ 0.0001; July: p = 0.5668; August: p ≤ 0.0001) and interactions between fertilizer rates and irrigation systems (June: p = 0.0031; July: p = 0.2067; August: p ≤ 0.0106), significant differences were observed in June and August only. In June, the high fertilizer rates in both irrigation systems and the low fertilizer rate in the overhead irrigation led to greater relative chlorophyll content compared to both no-fertilizer controls (Figure 6). In July and August, both fertilizer rates in both irrigation systems led to greater relative chlorophyll content compared to both no-fertilizer controls.

3.5. Visual Observation

Controls in both irrigation systems and both tree species showed significantly higher chlorotic symptoms compared to the other treatments. In red maple, significant differences were observed in the fertilizer rates throughout the study period (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). For the irrigation system, significant differences were observed in July and August (June: p = 0.1640; July: p = 0.0066; August: p = 0.0011). The interaction between fertilizer rates and irrigation systems was only observed in August (June: p = 0.1452; July: p = 0.8452; August: p = 0.0003; Figure 7). Throughout the study period, the no-fertilizer controls in both irrigation systems had the greatest chlorotic symptoms among all other treatments. Within the no-fertilizer controls, drip irrigation led to greater chlorotic symptoms than overhead irrigation in July and August.
In flowering dogwood, significant differences were observed in fertilizer rate throughout the study period (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). For the irrigation system (June: p = 0.0027; July: p = 0.0119; August: p = 0.8077) and the interaction between fertilizer rates and irrigation systems (June: p = 0.0434; July: p = 0.4740; August: p ≤ 0.0001; Figure 7), significant differences were observed in June and July and June and August, respectively. Throughout this study, both no-fertilizer controls had greater chlorotic symptoms compared to all other treatments.

3.6. Substrate pH and EC

In red maple, significant differences were observed for fertilizer rates throughout the study period (June: p = 0.0361; July: p = 0.0173; August: p = 0.0015), whereas no significant differences were observed for irrigation systems (June: p = 0.5749; July: p = 0.8637; August: p = 0.8311) or the interaction between fertilizer rates and irrigation systems (June: p = 0.7895; July: p = 0.7817; August: p = 0.3177). In June, substrate pH was greater for low fertilizer rate (both irrigation systems) and the no-fertilizer drip irrigation compared to the high fertilizer rate in overhead irrigation (Figure 8). In July and August, substrate pH decreased as the fertilizer rate increased regardless of the irrigation system. For substrate EC, significant differences were observed for fertilizer rates throughout the study period (June: p ≤ 0.0001; July: p = 0.0052; August: p = 0.0079). For irrigation systems (June: p ≤ 0.0001; July: p = 0.5049; August: p = 0.0159) and the interaction between fertilizer rates and irrigation systems (June: p ≤ 0.0001; July: p = 0.2978; August: p = 0.0358), significant differences were observed in June and August. Overall, substrate EC increased with fertilizer rate, and substrate EC tended to be numerically greater for the drip-irrigated plants. In June, substrate EC was greater for drip irrigation involving low and high fertilizer rates compared to overhead irrigation (Figure 9). In August, substrate EC was greatest for the high fertilizer rate in drip irrigation compared to all other treatments.
In flowering dogwood, no significant differences in substrate pH were observed for fertilizer rates (June: p = 0.0526; July: p = 0.3315; August: p = 0.0851), irrigation systems (June: p = 0.3199; July: p = 0.0695; August: p = 0.8459), or interactions between fertilizer rates and irrigation systems (June: p = 0.2909; July: p = 0.1861; August: p = 0.1003). For substrate EC, significant differences were observed for fertilizer rates in June, July, and August (June: p ≤ 0.0001; July: p ≤ 0.0001; August: p ≤ 0.0001). Significant differences were only observed for irrigation systems (June: p = 0.2649; July: p = 0.0061; August: p = 0.9789) and interactions between fertilizer rates and irrigation systems in July (June: p = 0.2758; July: p = 0.0076; August: p = 0.9783). Throughout this study, substrate EC was greater for the high fertilizer rate (both irrigation systems) compared to all other treatments (Figure 9). The irrigation system only influenced substrate EC in July, when EC was the greatest for the high-fertilizer drip irrigation.

3.7. Nutrient Content

Leaf samples were analyzed to determine the leaf nutrient content at the end of this study. In red maple, significant differences in leaf N content were observed for the fertilizer rate but not for irrigation systems or the interaction between fertilizer rates and irrigation systems (fertilizer rate: p = 0.0003; irrigation system: p = 0.4465; interaction: p = 0.4018). For P content, significant differences were observed for fertilizer rates and irrigation systems but not the interaction (fertilizer rate: p = 0.0084; irrigation system: p = 0.0014; interaction: p = 0.0576). Similar to P content, K content had significant differences for fertilizer rates and irrigation systems but not for their interaction (fertilizer rate: p = 0.0170; irrigation system: p ≤ 0.0001; interaction: p = 0.0850). For N content, the high rate of fertilizer in both irrigation systems and the low rate of fertilizer in drip irrigation led to greater N content compared to both no-fertilizer controls (drip and overhead irrigation) (Figure 10). Leaf P content was greater in drip irrigation (low and high fertilizer rates) compared to the no-fertilizer controls of both irrigation systems. Additionally, no differences were observed for P content among the overhead irrigation treatments. All treatments involving drip irrigation led to greater K content compared to those involving overhead irrigation. However, no differences were observed between the no-fertilizer controls and each fertilizer rate within each irrigation system (Figure 10).
In flowering dogwood, significant differences were observed in leaf N content among the treatments for fertilizer rates (p ≤ 0.0001), irrigation systems (p = 0.0004), and their interactions (p = 0.0242). All treatments in both irrigation systems had higher leaf N content compared to the controls of both irrigation systems. For P content, significant differences were observed for fertilizer rates (p ≤ 0.0001) and irrigation systems (p = 0.0045) but not for their interactions (p = 0.0810). Both rates of fertilizer in the overhead irrigation system and the high rate of fertilizer in the drip irrigation system led to significantly higher P content compared to both controls. For K content, significant differences were observed among the treatments for the fertilizer rate (p = 0.0026) and the interactions between fertilizer rates and irrigation systems (p = 0.0463) but not for irrigation systems (p = 0.5003). Only overhead irrigation led to significantly higher potassium content compared to other treatments (Figure 10).

4. Discussion

In this study, we observed that plant growth varied depending on plant species and irrigation systems. Red maples showed an increase in plant height, whereas no significant plant height increase was observed for flowering dogwoods. Similarly, stem diameter was also greater in fertilized red maples, irrespective of irrigation systems. Unlike plant height, flowering dogwoods had a greater increase in stem diameter in fertilized plants compared to non-fertilized plants with drip irrigation for June but not in July. A study conducted by Worrall et al. [37] found that container-grown woody ornamentals responded variedly to controlled-release fertilizers; however, their response to the fertilizer was significant, positive, and linear. Similarly, a positive response of fertilizer on plant health was observed by Clark and Zheng [38] when they compared the high and low rates of controlled-release fertilizer on containerized nursery crops. However, different research has found that flowering dogwoods have struggled to show significant growth in the first year of the growing season. A study conducted by Witte et al. [39] found flowering dogwoods did not respond to fertilizer in the first growing season. Similarly, a study conducted by Warren [40] found that nitrogen had no effect on the growth of flowering dogwood until 56 days, but they had increased stem diameter. Thus, Warren [40] suggested applying limited nitrogen for flowering dogwoods in the first year of transplantation as it impacts root development.
In our study, no-fertilizer controls had lower NDVI compared to the fertilizer-applied plants in both irrigation systems. Similar results were also recorded by Guan et al. [41] while assessing the NDVI of rice and wheat crops at different rates of fertilizer application. Edalat et al. [42] recorded higher NDVI for the high rate of fertilizer application while comparing conventional and reduced till systems for nitrogen management. An increased rate of fertilizer yielding higher NDVI was also found in Geraniums (Pelargonium sp. L.) by Dunn et al. [43] and Wang et al. [44,45].
However, our study found some inconsistencies in the NDVI values depending on the irrigation system. In flowering dogwood, in the drip irrigation system, a high rate of fertilizer application led to greater NDVI compared to the other treatments, but in overhead irrigation, both rates of fertilizer application were similar in June. In the drip irrigation system, the control and low rate of fertilizer had similar NDVI in July, whereas the control and high rate of fertilizer were statistically similar in August. This could be because of the effectiveness of drip irrigation during the initial phase of plant establishment compared to overhead irrigation. In a similar context, Fox [46] explained that drip irrigation was found effective in young transplants as they were applied to root zones. In addition, Kjelgren and Cleveland [47] and Gilman et al. [48] explained the effectiveness of drip irrigation in the establishment of young transplants. The researcher explained that young transplants have smaller and younger root systems and, thus, require lower water amounts but in greater frequency. The other reasons could be due to the constant rehumidification of the substrate by drip irrigation. The major benefit drip irrigation provides is maintaining precise control over the substrate moisture at the necessary interphase period [49]. Repetition of a similar volume of water throughout the day to constantly moisturize the substrate was found to ensure plant growth [50]. Also, repeated application of smaller volumes of water also reduces the leaching of mineral nutrients and surface runoff [51,52]. Interestingly, it was more prominent in flowering dogwoods and not in red maples. Red maples were able to grow taller and with more new roots in frequent irrigation compared to periodic irrigation [53]. These findings could be used to explain the variation in NDVI between red maples and flowering dogwoods in different irrigation systems. A strong and positive correlation was found between handheld NDVI and UAV-mounted NDVI sensors, similar to the findings of Neupane et al. [54].
We observed that relative chlorophyll content was significantly higher in both rates of fertilizer application compared to no-fertilizer controls in both tree species irrespective of irrigation system, which was expected since pine bark substrate does not contain mineral nutrients in concentrations that will support substantial plant growth. Chang and Robison [55] found that leaf chlorophyll content was positively correlated with nitrogen content in woody species. Higher chlorophyll content was also measured during foliar analysis by Demotes-Mainard et al. [56] and Cabrera [10] in Lagestroemia indica L. Similar to our findings, Freidenreich et al. [57] also observed no significant differences between the two rates of fertilizer in total chlorophyll content. The chlorophyll content observed from SPAD-502 was similar to those observed in the visual observation, as fertilizer-treated trees were healthy-looking and had significantly fewer chlorotic symptoms.
In our study, it was observed that overhead-irrigated maples had lower substrate pH after July, whereas flowering dogwoods with high fertilizer rates in overhead irrigation had lower pH compared to the no-fertilizer controls in June. This could be because of the excessive leaching due to overhead irrigation. Gamble and Daniels [58] and Daniels et al. [59] explained that intensive leaching caused the loss of bases, thus lowering the pH. Similarly, in our study, the high rate of fertilizer led to greater EC. Similar results were observed by Richards and Reed [60] while analyzing the growth response of New Guinea Impatiens using controlled-release fertilizers. Bayer et al. [61] also observed comparable results regarding the pH and EC of the leachates. They found that a high rate of fertilizer application and regulated irrigation led to higher EC.
While analyzing leaf tissue, we found that N and P content was greater in higher rates of fertilizer application for both red maples and flowering dogwoods. Potassium content was greater in drip-irrigated red maples yet contrastingly greater in overhead-irrigated flowering dogwood. This inconsistency in K content in different irrigation systems is explained by Neilsen et al. [62], who evaluated the effects of different irrigation systems in ‘Gala’ apples. Similarly, ShalekBriski et al. [63] also observed that irrigation systems impacted the foliar K content while using different irrigation systems.
There were a few unclear observations recorded in our study, such as variations in foliar concentration of nutrients in distinct species in the same irrigation system and variations in pH in different species. A study conducted by Wu et al. [64] explained that deficit irrigation results in decreased nitrogen uptake in alfalfa grass, while the other study conducted by Amankwaa-Yeboah et al. [65] suggested that deficit irrigation can be utilized for increasing nutrient uptake and crop water efficiency. This suggests that more detailed and in-depth studies are needed in the future to provide clear evidence of nutrient uptake in different irrigation systems. Similarly, inconsistencies in plant growth and NDVI of flowering dogwoods in different irrigation systems could not be addressed in our study. The results showed that not only NDVI but SPAD values of the flowering dogwoods have lowered with drip irrigation in the later month, highly suggesting that some other factors like water distribution and/or physiology of the flowering dogwoods could be in play for flowering dogwoods. These areas could be identified as a prospect for further research, which will help us better understand the effect of fertilizers and nutrients on different tree species in different irrigation systems.

5. Conclusions

As a low rate of fertilizer did not have significant differences among different variables compared to a high rate of fertilizers, we can conclude that the application of a low rate of fertilizer for ornamental tree production, such as red maples and flowering dogwoods, will be beneficial. Freidenreich et al. [57] also recommended the application of 20 g of Florikan while comparing 0, 10, 20, 30, 40, and 50 g for Justicia brandegeana (Brandegee) in the container production system. Irrigation systems could be used based on tree species and stage of stress, as drip irrigation was found effective in earlier days of plant establishment; however, overhead irrigation was more effective later in the summer. Similarly, studies conducted by Novair et al. [15] and Samadi et al. [16] also suggested that the addition of organic amendment and biofertilizers can be helpful for increasing nutrient uptake and overall plant health in low irrigation conditions. In this study, we also found a high correlation between handheld NDVI and UAV-mounted NDVI. The application of aerial imagery and sensor technology provides higher efficiency and economic benefits for ornamental growers [54]. The findings of this study provide beneficial insights into ornamental tree growers to optimize nutrient management, irrigation strategy, and the application of technology in a container-based production system.

Author Contributions

Conceptualization, F.B.-G. and A.W.; methodology, F.B.-G., A.W. and K.N.; software, K.N.; validation, F.B.-G., A.W. and K.N.; formal analysis, A.W. and K.N.; writing—original draft preparation, K.N.; writing—review and editing, K.N., A.W. and F.B.-G.; visualization, K.N.; supervision, F.B.-G. and A.W.; project administration, F.B.-G.; funding acquisition, F.B.-G. and A.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Institute of Food and Agriculture, United States Department of Agriculture Capacity Building grant, under award number 2019-38821-29062.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We acknowledge Grayson Lane Delay for his contribution to data collection for this project.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Average increase in plant height (cm) for containerized red maples (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. The increase in height in June was calculated as the difference between height in June and initial height of the plant. Different letters above the error bar (standard error) represent significant differences in increase in plant height (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 1. Average increase in plant height (cm) for containerized red maples (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. The increase in height in June was calculated as the difference between height in June and initial height of the plant. Different letters above the error bar (standard error) represent significant differences in increase in plant height (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 2. Average increase in stem diameter (cm) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. The increase in stem diameter in June was calculated as the differences in height in June and initial stem diameter of the plant. Different letters above the error bar (standard error) represent significant differences in increase in stem diameter (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 2. Average increase in stem diameter (cm) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. The increase in stem diameter in June was calculated as the differences in height in June and initial stem diameter of the plant. Different letters above the error bar (standard error) represent significant differences in increase in stem diameter (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 3. Normalized Difference Vegetative Index (NDVI) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in NDVI (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 3. Normalized Difference Vegetative Index (NDVI) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in NDVI (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 4. Pearson’s correlation of the mean NDVI values between a handheld NDVI meter and a Sentera NDVI sensor mounted in a UAV for containerized red maple [June (A), July (B), August (C)] and flowering dogwoods [June (D), July (E), August (F)] treated with different fertilizer rates in overhead and drip irrigation systems.
Figure 4. Pearson’s correlation of the mean NDVI values between a handheld NDVI meter and a Sentera NDVI sensor mounted in a UAV for containerized red maple [June (A), July (B), August (C)] and flowering dogwoods [June (D), July (E), August (F)] treated with different fertilizer rates in overhead and drip irrigation systems.
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Figure 5. Layout of this study (top) [Trt 1: control; Trt 2: low rate of fertilizer; Trt 3: high rate of fertilizer] and Normalized Difference Vegetative Index (NDVI) mosaics combined (bottom right) for containerized red maple and flowering dogwoods treated with different fertilizer rates over three months for overhead [June (A), July (B), August (C)] and drip [June (D), July (E), August (F)] irrigation systems.
Figure 5. Layout of this study (top) [Trt 1: control; Trt 2: low rate of fertilizer; Trt 3: high rate of fertilizer] and Normalized Difference Vegetative Index (NDVI) mosaics combined (bottom right) for containerized red maple and flowering dogwoods treated with different fertilizer rates over three months for overhead [June (A), July (B), August (C)] and drip [June (D), July (E), August (F)] irrigation systems.
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Figure 6. Average SPAD value for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in leaf greenness (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 6. Average SPAD value for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in leaf greenness (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 7. Average percentage of foliage showing chlorotic symptoms (visual observation) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in chlorotic symptoms (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 7. Average percentage of foliage showing chlorotic symptoms (visual observation) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in chlorotic symptoms (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 8. Average substrate pH for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average pH (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 8. Average substrate pH for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average pH (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Figure 9. Average substrate electrical conductivity (EC) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average EC (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
Figure 9. Average substrate electrical conductivity (EC) for containerized red maple (left) and flowering dogwoods (right) over three months using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average EC (within each month) among the treatments (α = 0.05, Fisher’s LSD test).
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Horticulturae 10 00767 g010
Figure 10. Average nutrient content [nitrogen (N), phosphorus (P), and potassium (K)] expressed in percentage of the nutrient content in the leaf tissue at the end of the study period for containerized red maple (left) and flowering dogwoods (right) using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average N, P, and K content among the treatments (α = 0.05, Fisher’s LSD test).
Figure 10. Average nutrient content [nitrogen (N), phosphorus (P), and potassium (K)] expressed in percentage of the nutrient content in the leaf tissue at the end of the study period for containerized red maple (left) and flowering dogwoods (right) using different fertilizer rates in overhead and drip irrigation systems. Different letters above the error bar (standard error) represent significant differences in average N, P, and K content among the treatments (α = 0.05, Fisher’s LSD test).
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MDPI and ACS Style

Neupane, K.; Witcher, A.; Baysal-Gurel, F. An Evaluation of the Effect of Fertilizer Rate on Tree Growth and the Detection of Nutrient Stress in Different Irrigation Systems. Horticulturae 2024, 10, 767. https://doi.org/10.3390/horticulturae10070767

AMA Style

Neupane K, Witcher A, Baysal-Gurel F. An Evaluation of the Effect of Fertilizer Rate on Tree Growth and the Detection of Nutrient Stress in Different Irrigation Systems. Horticulturae. 2024; 10(7):767. https://doi.org/10.3390/horticulturae10070767

Chicago/Turabian Style

Neupane, Krishna, Anthony Witcher, and Fulya Baysal-Gurel. 2024. "An Evaluation of the Effect of Fertilizer Rate on Tree Growth and the Detection of Nutrient Stress in Different Irrigation Systems" Horticulturae 10, no. 7: 767. https://doi.org/10.3390/horticulturae10070767

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