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Article

Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes

by
Fabrício E. Rodrigues
1,
Adalton M. Fernandes
1,2,*,
Arthur V. Oliveira
1,
Pablo F. Vargas
2,3,
Emerson F. C. Souza
4,
Politon T. P. Guedes
1,
Ricardo T. Figueiredo
1 and
Ítala T. Guimarães
1
1
College of Agricultural Sciences, São Paulo State University (UNESP), Botucatu 18610-034, São Paulo, Brazil
2
Center for Tropical Roots and Starches (CERAT), São Paulo State University (UNESP), Botucatu 18610-034, São Paulo, Brazil
3
Faculty of Agricultural Sciences of Vale do Ribeira (FCAVR), São Paulo State University (UNESP), Registro 11900-000, São Paulo, Brazil
4
Department of Soil, Water, and Climate, University of Minnesota, St. Paul Campus, Falcon Heights, MN 55108, USA
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(12), 2167; https://doi.org/10.3390/agriculture14122167
Submission received: 30 October 2024 / Revised: 13 November 2024 / Accepted: 26 November 2024 / Published: 28 November 2024
(This article belongs to the Section Crop Production)
Browse Figures
Versions Notes

Abstract

:
Balanced nitrogen (N) supply is essential for high root yield in sweet potatoes (Ipomoea potatoes [L.] Lam.). A portable chlorophyll meter can support N fertilization management. Here, we determined the appropriate N sufficiency index (NSI) for sweet potato leaves to achieve the best leaf N status, plant growth, N uptake and removal, and storage root yield and quality. Experiments were conducted at three sites (Braúna, São Manuel, and Regente Feijó) in São Paulo, Brazil, using a randomized block design with four replicates. Treatments included a control (without N application), conventional N fertilization (50 kg ha−1), reference N fertilization (150 kg ha−1), and NSI-based N fertilization (NSI: 90% or 95%, based on the chlorophyll meter readings). Plant response to N fertilization was low, with no N deficiency observed in the conventional and chlorophyll meter-managed treatments. NSI < 90% was better than NSI < 95% for N top-dressing management, reducing N application rates by 44–66%, depending on the site. In contrast, NSI < 95% increased the N application rate without any yield benefit. Thus, monitoring N fertilization using a portable chlorophyll meter with 90% NSI can reduce N fertilization rates without negatively impacting the sweet potato root yield.

1. Introduction

Sweet potato (Ipomoea batatas [L.] Lam.) is native to Central and South America and has significant economic value due to its high energy production per unit area [1]. Its storage roots are rich in carbohydrates, making it an important energy source for human food and animal feed, and it is also used to make flour, starch, dried chips, juice, bread, noodles, snacks, and pectin [2,3]. Brazil is the largest producer of sweet potatoes in South America, with an annual production of 847 thousand tons [4]. Sweet potato cultivation has steadily increased in Brazil, with the cultivated area expanding by 46% from 2010 to 2023, reaching 61 thousand hectares [5]. Although sweet potatoes can grow in low-fertility soils, balanced nutrient fertilization management is essential to maximize storage root yield [6]. Nitrogen (N) is one of the nutrients most absorbed by sweet potatoes [6,7,8,9], playing a crucial role in plant development as a component of amino acids, proteins, enzymes, and chlorophyll [10]. Specifically, sweet potato N recovery can reach amounts ranging from 215 to 350 kg·N·ha−1 [11], or approximately 5.1 kg of N per ton of storage roots [6].
Despite high N uptake [6,11], studies suggest that high N application rates do not always increase root yield [7,9,12,13]. In sandy soils with low amounts of organic matter and minimal N fertilization, sweet potatoes still show high N uptake [11], indicating that their N recovery is not solely dependent on soil N levels. The ability of sweet potato plants to associate with endophytic diazotrophic bacteria to fix atmospheric N2 [14,15] may contribute to their high N accumulation, even under low mineral N availability, which often reduces their response to additional N application. Optimal N rates for sweet potatoes are relatively low [7,9,12,13,16]; however, proper N fertilization remains critical for high root yields. Insufficient or excessive N during the early crop stages can inhibit cambium activity, reducing the formation of adventitious roots and the storage root number [17], while an adequate N supply after root differentiation is necessary for storage root growth [17].
A portable chlorophyll meter offers a practical method to monitor the N status of sweet potato plants, and it has been widely used in other crops [18,19,20,21,22]. This device provides an indirect measurement of relative chlorophyll concentration in plant leaves, expressed as the soil–plant analysis development (SPAD) index [23], which correlates positively with the leaf N concentration and crop yield [24,25,26,27,28,29]. Using SPAD as a tool for monitoring plant N levels may improve N management in sweet potatoes, enhancing root yield by optimizing N supply. Due to their ease of use and portability, portable chlorophyll meters are especially beneficial for medium- to small-scale farmers and technical advisors working with multiple farms [29].
To normalize the SPAD readings, the N sufficiency index (NSI) is calculated by dividing the SPAD readings from plants receiving in-season fertilizer, based on SPAD monitoring, by the SPAD readings from a reference area with non-limiting N [28,30]. The reference area typically receives about twice the recommended N rate to ensure maximum chlorophyll concentration in the leaves [19,27,28,29]. Nitrogen sufficiency index values of 90 or 95% are often used to determine the optimal timing for N side dressing application in crops such as maize, beans, and potatoes [27,28,29,30]. Nitrogen fertilization is performed when NSI in the target area falls below 90 or 95% of the reference area [28,29,30,31], a strategy that prevents unnecessary N application and improves N use efficiency [28,31].
Research has demonstrated a correlation between the SPAD index and chlorophyll content in sweet potato leaves [32]. While the impact of N supply on the SPAD index values has been explored [33,34,35], the optimal NSI values for managing N fertilization in sweet potatoes remain unknown. By contrast, other crops, such as potatoes (Solanum tuberosum L.), have been examined for this relationship, although the chlorophyll meter is sometimes insufficient for detecting late-season N deficiency in time for fertilization, even with a NSI of 95% [29,36]. In leguminous crops like common beans, a NSI of 90% is often more effective than that of 95% for N management [27,28]. It still unclear, however, whether a NSI of 95% is more effective for avoiding unnecessary N application in sweet potato crops [28,30]. In maize cultivation, the NSI of 95% is more suitable for managing N fertilization [31,37,38], and in common beans, a N application rate of 1.0–1.5 kg ha−1 N per 0.1 unit below a NSI of 95% has been recommended to adjust N fertilization [30]. Therefore, using a portable chlorophyll meter to establish specific guidelines for top-dressing N fertilization in sweet potatoes can lead to practical N fertilizer savings for sweet potato growers.
In this study, we aimed to determine the appropriate NSI for sweet potato leaves to optimize leaf N status, plant growth, and N uptake and removal, as well as storage root yield and quality.

2. Materials and Methods

2.1. Site, Climate, and Soil

Three experiments were conducted within São Paulo state, Brazil: one at a commercial sweet potato farm in Braúna (21°32′ S; 50°13′ W; 441 m altitude), one at the Experimental Farm of the College of Agricultural Science (FCA) of São Paulo State University (UNESP) in São Manuel (22°46′ S; 48°34′ W; 740 m altitude), and one at a commercial sweet potato farm in Regente Feijó (22°17′ S; 51°14′ W; 457 m altitude). The region has a Cwa climate (tropical, with dry winters and hot, rainy summers), according to the Köppen classification system. Rainfall and temperature were measured throughout the experimental period (Figure 1). The soil was classified as a sandy-textured Oxisol, and soil samples (0–20 cm depth) were collected from each experimental area before planting. The soil chemical properties are presented in Table 1, with analysis performed as described by van Raij et al. [39].

2.2. Experimental Design and Treatment

All experiments were conducted in a randomized block design, with five treatments and four replicates. The treatments consisted of a zero-N control, conventional N fertilization (50 kg ha−1), reference N fertilization (150 kg ha−1), and NSI-based N fertilization (90 or 95%, based on the chlorophyll meter readings).
Ammonium nitrate (33% N) was used as the N source. The conventional treatment received the recommended N application rate for sweet potato in São Paulo State [40], as presented in Table 2. The reference plot received three times the recommended N rate (150 kg ha−1; Table 2), following recommendations of other studies to achieve the maximum chlorophyll concentration in the leaves [19,27,28,29].
The chlorophyll meter-managed treatments received 17 kg ha−1 N at planting and 1.0 kg ha−1 N per 0.10 unit below the NSI of 90% or 95%. Each plot had four 4 m long rows of plants, spaced with 1.30 m between rows and 0.30 m between plants. The two central rows were considered for evaluation, discarding 0.5 m end areas.
The first chlorophyll meter readings were taken 15 d after planting (DAP) and continued at weekly intervals until 85 DAP. Readings were taken using the most recent fully expanded leaves of 15 plants in the plots, avoiding the leaf veins. After the readings, leaves were collected for the total N content analysis. Then, NSI was calculated using the readings from the plot managed via a chlorophyll meter (chlorophyll meter plot), as well as those from the reference plot (without N limitation), as follows:
NSI (%) = (chlorophyll meter plot reading/reference plot reading) × 100
In the chlorophyll meter-managed treatments, top-dressing N fertilization was applied on the same day as the readings occurred when the NSI was below 90% or 95%.

2.3. Sweet Potato Planting and Management

The experiment was implemented on 31 August 2021 in Braúna; 19 November 2021 in São Manuel; and 13 January 2022 in Regente Feijó. Before planting, soil samplings and analysis were conducted as described by van Raij et al. [39] (Table 1). Conventional soil tillage, included heavy harrowing, plowing, and light harrowing, was performed, followed by the formation of 0.30 m high hills.
Planting fertilization was based on soil analysis and the recommendation of Lorenzi et al. [40]. Except for the control, all treatments received 17 kg ha−1 N in the planting furrow (Table 2). Phosphate fertilization in the planting furrow was performed with 54, 35, and 31 kg ha−1 P, while potassium rates were 34, 14, and 28 kg ha−1 K in Braúna, São Manuel, and Regente Feijó, respectively. The fertilizers used were N-P2O5-K2O 4-30-10 in Braúna, a mix of N-P2O5-K2O 20-5-20 with simple superphosphate in São Manuel, and a mix of ammonium nitrate, single superphosphate, and potassium chloride in Regente Feijó. For the control treatment, only a mix of single superphosphate and KCl was used.
The 0.30 m long stem cuttings of the Canadense cultivar were planted at a depth of 10–12 cm at the top of the hills, with a 0.30 m plant spacing. Top-dressing N fertilization was manually performed in a continuous fillet over the hills and next to the plants, according to each treatment (Table 2). At 36 and 85 DAP, K was applied in the same way as the N application, using equal quantities in each application to achieve a total rate of 101 kg ha−1 (planting plus top-dressing) in all areas.
In all experiments, irrigation with approximately 30 mm of water was performed only in the first two weeks after planting to ensure the proper rooting of the stem cuttings. Weed, pest, and disease control were performed as required. The sweet potato crop was harvested on 7 February 2022 in Braúna (160 DAP), 12 April 2022 in São Manuel (144 DAP), and 16 June 2022 in Regente Feijó (185 DAP).

2.4. Plant Sampling and Analyses

The SPAD index was measured using a digital chlorophyll meter (model SPAD-502 Plus, Konica Minolta Sensing Americas Inc., Ramsey, NJ, USA), calibrated according to the manufacturer’s recommendations. Readings were taken in the morning (8:00–10:00 a.m.) at 15, 22, 29, 36, 43, 50, 57, 64, 71, 78, and 85 DAP. Measurements were taken in plants without signs of nutrient disorders, pests, or diseases. During the readings, the plants were shielded to prevent sunlight interference, and readings were carefully taken to avoid the central vein of the leaves. The most recent fully expanded leaves were selected, with one reading taken per leaf on 15 different plants per plot, totaling 15 readings per plot. The leaves were then collected, washed, placed in paper bags, and dried in a forced-air oven at 65 °C for 72 h. The dried material was ground, and the total N concentration was determined using the micro-Kjeldahl method [41].
At the final harvest, the shoots and storage roots of all plants were collected from two 2 m long rows in the central plots. The storage roots were brushed, classified, counted, and weighed to determine the total, marketable, and non-marketable storage root yields. The mean storage root weight was calculated as the ratio of the total storage root weight to the total number of storage roots per plot. The marketable storage roots were smooth, elongated, and uniform in shape, weighing 80–800 g [42]. Deformed storage roots or those weighing <80 g or >800 g were considered non-marketable. The total storage root yield was the sum of the marketable and non-marketable storage root yields, calculated using the storage root weight and plant population data. Shoots were weighed, sampled, and washed. Samples from the shoots and storage roots (of all sizes) of the plants were washed, sliced, and dried in an oven at 65 °C for 96 h to determine the dry matter (DM) content. The dried samples were ground in a Wiley mill with a 1 mm sieve, and N concentration was determined using the micro-Kjeldahl method [41]. Then, N concentration was multiplied by the amount of DM accumulated in each plant part, and the amount of N taken up by the crop was calculated as the sum of the amounts of N accumulated in different plant parts. Nitrogen removal was defined as the amount of N accumulated in the storage roots.
The starch content was determined in the dried samples of the storage roots. Initially, after agitation at 90 °C in the presence of the alpha-amylase enzyme, the samples were cooled to 50 °C, and amyloglucosidase was added, followed by agitation for 2 h. Then, the sugar content was quantified using the Somogyi method, adapted from Nelson [43]. The sugar content was converted to starch content by multiplying the results by a factor of 0.9 [44]. The starch content was further converted to fresh matter content, and the starch yield was calculated by multiplying the starch content with the total fresh storage root yield.

2.5. Data Analyses

Data from each site were subjected to analysis of variance. The means of the treatments were compared using the least significant difference test at 5% probability using SISVAR software (version 5.8; UFLA, Lavras, Brazil) [45]. Regression analysis of the relationship between N uptake by sweet potatoes and amount of N applied was performed using SigmaPlot software (version 14.0; Systat Software, Inc., San Jose, CA, USA).

3. Results

3.1. SPAD Index and N Status Monitoring in Leaves

In Braúna, no significant differences in the SPAD index were observed during the initial evaluations (15, 22, 36, 57, and 64 DAP). However, at 50 and 71–85 DAP, the reference treatment exhibited a higher SPAD index than did the control and NSI < 90% treatments (Figure 2). In São Manuel, the reference treatment resulted in a higher SPAD index than did the control and chlorophyll meter-managed treatments (NSI < 90% and NSI < 95%, respectively) between 36–71 and 43–64 DAP. In the final evaluation, only the conventional treatment displayed a lower SPAD index than did the reference. In Regente Feijó, at 15 and 29 DAP, the SPAD index in the control was lower than that in the reference treatment, and at 36 DAP, the SPAD index values in both the control and NSI < 90% treatments were lower than in the reference. In other evaluations, the SPAD index values did not differ among the treatments.
In Braúna and Regente Feijó, the NSI remained > 90% in all treatments, so the NSI < 90% treatment did not receive a top-dressing N application (Figure 2; Table 2). In Braúna and Regente Feijó, NSI in the NSI < 95% treatment decreased below 95% only at 29 and 42 DAP, respectively. In São Manuel, the NSI in the NSI < 90% treatment was below 90% only at 64 DAP, while in the NSI < 95% treatment, it fell below 95% at 36, 43, and 64 DAP.
Leaf N concentrations were not affected by the treatments until 36 DAP in Braúna and 29 DAP in Regente Feijó (Figure 2). In Braúna, between 71 and 85 DAP, leaf N concentrations in the reference treatment were higher than in the other treatments, while in Regente Feijó, the reference treatment surpassed the other treatments at 78 DAP. The control treatment exhibited lower N concentrations at 36, 78, and 85 DAP. In São Manuel, the leaf N concentrations in the reference treatment were higher than in the control between 15 and 43 DAP, with similar concentrations across all treatments between 57 and 64 DAP. Between 71 to 85 DAP, the NSI < 95% treatment showed higher leaf N concentrations than did the control and conventional treatments.

3.2. Biomass Accumulation and N Uptake and Removal

In São Manuel and Regente Feijó, biomass accumulation in the plant shoot and whole plant was not affected by the treatments. However, in Braúna, the plant shoot biomass was higher in the reference than in the other treatments, which showed no significant differences among themselves (Table 3).
The biomass accumulation in the storage roots varied with the treatment type and site (Table 3). In Braúna, chlorophyll meter-managed treatments accumulated more biomass in the storage roots than did the reference treatment, but the root biomass accumulation was similar to that of the other treatments. In São Manuel, the conventional treatment displayed a higher storage root biomass than did the control treatment but did not differ from the other treatments. In Regente Feijó, a difference was observed between the NSI < 90% and reference treatments, with the lowest storage root biomass detected in the reference treatment.
The whole plant biomass was not affected by the treatments in Braúna and Regente Feijó. However, in São Manuel, the whole plant biomass in the conventional treatment was greater than that in the control treatment (Table 3).
In Regente Feijó, N uptake and removal by the sweet potato plants were not influenced by the treatments, with average values of 144 and 112 kg ha−1 N, respectively (Table 3). In Braúna, the reference treatment showed a higher N uptake compared to that of the NSI < 90% and control treatments, although N removal was not affected, ranging from 41 to 55 kg ha−1 N. In São Manuel, the highest N uptake was observed in the reference treatment, followed by the conventional and NSI < 95% treatments, with the lowest N uptake in the control treatment. In this location, N removal was higher in the reference treatment compared to that of the NSI < 90% and control treatments but did not differ significantly from the conventional and NSI < 95% treatments.

3.3. Yield Components and Storage Root and Starch Yields

In Braúna, chlorophyll meter-managed treatments resulted in a higher total number of storage roots per plant compared to that in the reference treatment, although the number did not differ significantly from those of the other treatments (Table 4). In São Manuel, the total number of storage roots per plant in the conventional treatment was similar to that in the chlorophyll meter-managed treatments but higher than in the reference and control treatments. The NSI < 90% treatment exhibited a higher total storage root number per plant than did the control, but this number did not significantly differ from the other treatments.
In Braúna, the NSI < 95% treatment resulted in a higher number of marketable storage roots per plant compared to those of the reference and control treatments (Table 4). In São Manuel, both the conventional and reference treatments had more marketable storage roots per plant than did the control treatment. In Regente Feijó, the number of marketable roots per plant in the NSI < 90% treatment was higher than in the reference treatment. Notably, the mean weight and starch content of the storage roots were unaffected by the treatments at any experimental site (Table 4).
In Regente Feijó, the total, marketable, and starch yields, as well as the DM content of the storage roots, were not influenced by the treatments (Table 4). In São Manuel, the marketable storage root yield and the storage root DM content were unaffected by the treatments; however, the total storage root and starch yields were influenced by N management. In this location, the storage root and starch yields in the reference treatment were higher than in the other treatments, although they did not differ from each other. In Braúna, both the total and marketable storage root yields and the root DM content were affected by N management. Here, the conventional treatment resulted in higher total storage root yield compared to that of the reference treatment, although it did not differ significantly from the other treatments. However, the marketable storage root yield in the NSI > 95% treatment was higher than that in the reference and control treatments. The storage roots in the conventional treatment exhibited a lower DM content than those in the NSI < 90% treatment.
The amount of N applied in each treatment was plotted against the sweet potato N uptake (Figure 3). In Regente Feijó, 144.4 kg ha−1 N were taken up by the plants, and N fertilization did not increase the N uptake by the sweet potato crop. In São Manuel, increasing N rates linearly increased sweet potato N uptake. In Braúna, however, sweet potato N uptake increased only up to an estimated N rate of 63.9 kg ha−1, at which point the crop reached an uptake of 151 kg N ha−1.

4. Discussion

The SPAD index varies based on leaf position, water availability, genotype, growth stage, leaf age, temperature, light incidence, and time of year [46,47,48]. In this study, chlorophyll meter-based N status monitoring in sweet potatoes was primarily influenced by the evaluation period and experimental sites. In Braúna and São Manuel, the SPAD index increased earlier during crop cycle than in Regente Feijó. Studies have shown that SPAD values generally increase with plant age [27,28,29], although this trend does not hold true under all conditions [29].
In Regente Feijó and Braúna, the NSI < 90% management revealed no need for N top-dressing, while the NSI < 95% management indicated the need for top-dressing N fertilization at N rates equal to (53 kg ha−1) or lower than (19 kg ha−1), when compared with conventional rates (Table 2). Conversely, in São Manuel, NSI < 90% management reduced N application by 44% compared to conventional N fertilization, while NSI < 95% management required 66% more N (83 kg ha−1) than the conventional N fertilization treatment. Despite similar rainfall in the early season (85 DAP) and comparable soil organic matter levels (Table 2; São Manuel = 541 mm and 11.0 g dm−3 S.O.M.; Regente Feijó = 536 mm and 12.0 g dm−3 S.O.M.) in São Manuel and Regente Feijó, São Manuel required more N with chlorophyll meter monitoring, whereas the lower levels of early season rainfall (rainfall = 226 mm; Table 2) and soil organic matter in Braúna resulted in N demands equal to or less than conventional rates.
In Braúna and São Manuel, the NSI values in the control treatment, which did not receive any mineral N, remained above 90% throughout the monitoring period. In Regente Feijó, this was true between 35 and 85 DAP. While the chlorophyll meter indicated that sweet potatoes required minimal mineral N, it is possible that the plants benefited from alternative N sources, such as N2 fixation [14,15]. The chlorophyll meter appeared to effectively capture the N dynamics in sweet potatoes, as leaf N concentration analysis also indicated high N sufficiency. In legume crops like common beans, which do not fulfill all their N needs through atmospheric fixation, the chlorophyll meter has been efficient for monitoring N dynamics [27,28,30], with optimal NSI values and N application rates previously established [30].
In Brazil, the range of N concentrations adequate for sweet potato growth in the first fully expanded leaves at 60 DAP is 33–45 g kg−1 [40,49]. Leaf N concentrations in Braúna and Regente Feijó were generally above 33 g kg−1 throughout the monitoring period. In São Manuel, leaf N concentrations dropped below the 33 g kg−1 threshold a few times in some treatments, including the control, the NSI < 90%, and the conventional treatment (Figure 2). However, sweet potato plants did not show N deficiency early in the growing season, even in the control treatment. Other studies indicate that when legumes are grown before sweet potatoes in crop rotation, leaf N concentrations are sustained above the critical level of 33 g kg−1 [6,7].
Nitrogen application rates ranging from 0–150 kg ha−1 did not significantly affect the shoot or whole plant biomass, aligning with findings that excess N favors vegetative growth over root biomass [8,50,51,52,53]. In São Manuel and Regente Feijó, N-fertilized treatments did not yield shoot and biomass levels above those observed in the controls. In Braúna, the reference treatment with high N supply (150 kg ha−1) showed increased shoot biomass, without increased storage root biomass, corroborating findings that N rates > 95 kg ha−1 benefit shoot over root growth [8].
The whole plant biomass in Braúna and Regente Feijó remained similar across all treatments, while in São Manuel, chlorophyll meter-managed treatments aligned with the conventional and reference treatments, showing a low N response. Similarly, the storage root biomass in the meter-managed treatments was comparable to that of conventional treatments, even with lower (NSI < 90%) or higher (NSI < 95%) N application rates.
Sweet potato N uptake was not influenced by variable N rates. In Regente Feijó, the N uptake in the control was similar to that in the N-fertilized treatments, averaging 144 kg ha−1 N (Figure 3). More specifically, in Braúna and São Manuel, the N uptake in the chlorophyll meter-based treatments was similar to that observed in conventional treatments, with the reference treatment recovering 64–75% more N than that noted for sweet potatoes grown without a N supply. Despite receiving only one-third of the N applied in the reference treatment, N uptake in the NSI < 95% treatment was comparable to that in the reference treatment. Notably, plants did not take up more than 151 kg ha−1 N, even when application rates exceeded 64 kg ha−1 N (Figure 3). These results suggest that, although high temperatures and rainfall during the early stages of sweet potato growth contribute to soil N mineralization [54], it was also possibly impacted by atmospheric N2 fixation by the sweet potato plants [14,15]. Consistently, without mineral N application, sweet potato N uptake reached 56–142 kg N ha−1 in the present study. A previous study on N status monitoring in common beans (a legume crop) attributed the high seed yield in the control treatment to symbiotic N2 fixation and mineralization of N from prior crop residues [28]. Nitrogen removal did not differ significantly among treatments in two of the three areas studied. However, in São Manuel, the chlorophyll meter-managed treatment exhibited a N removal rate similar to that of the conventional N treatment.
Despite minimal variations in sweet potato growth and N uptake, the chlorophyll meter-managed treatments exhibited in a similar number of storage roots per plant, similar marketable storage root yield, and similar root quality (i.e., mean weight, DM, and starch content) compared to those of the conventional N treatment across the study areas. These findings evidence the low N response by sweet potatoes. Fernandes et al. [7] reported an optimal N rate of 49.6 kg ha–1 for the maximum relative root yield of sweet potatoes grown after legumes, while Ribeiro et al. [9] observed a 50 kg N ha–1 rate to be sufficient to achieve maximum fresh storage root yield in the rainy season, which is similar to the findings of this study. In São Manuel, the conventional treatment produced higher total storage root and starch yields than did the chlorophyll meter-managed treatments, suggesting that chlorophyll meter monitoring may be less practical for sweet potatoes intended for industrial use. In fresh market production, undersized or damaged storage roots are typically discarded [55]. However, in industrial production, all root sizes are utilized, which may require specific NSI adjustments to optimize starch production.
Managing N fertilization management with the chlorophyll meter did not reduce the sweet potato storage root yield but reduced the N fertilizer application rate by 44–66% using NSI < 90%. Studies on other crops have also highlighted the benefit of N fertilizer management when monitoring the plant N status with a portable chlorophyll meter [27,28], especially in areas exhibiting low responses to N fertilization [29]. Excessive N application to sandy soils can lead to increased N leaching [56] and nitrous oxide emissions, resulting from the nitrification and denitrification processes, which are also elevated by higher soil N concentrations and fertilizer application [57,58]. Therefore, optimizing N fertilization for sweet potato production using a chlorophyll meter could help reduce N losses through leaching and mitigate greenhouse gas emissions. Future research in various soil types and over extended periods could further validate the potential of chlorophyll meter-based N management in sweet potatoes to enhance fertilizer efficiency and provide environmental benefits.

5. Conclusions

In this study, chlorophyll meter-based nitrogen (N) management demonstrated potential for optimizing N application in sweet potato cultivation across diverse sites. The SPAD index and N demand were site-specific, influenced by environmental factors, highlighting the importance of localized N management. Nitrogen sufficiency index-based thresholds (NSI < 90%) reduced N application rates by 44–66%, without compromising yield or root quality, offering economic and environmental benefits. However, the limited response to N across the treatments suggests that sweet potatoes have low N demands, potentially benefiting from soil N mineralization and possible atmospheric N2 fixation. Chlorophyll meter monitoring appears effective for managing N in sweet potato cultivation, though it may be less suitable for crops grown for industrial starch production, which showed higher N requirements.

Author Contributions

Conceptualization, F.E.R., A.M.F., and A.V.O.; methodology, F.E.R., A.V.O., P.T.P.G., R.T.F., and Í.T.G.; software, F.E.R. and A.V.O.; validation, A.M.F.; formal analysis, F.E.R.; investigation, F.E.R., A.V.O., P.T.P.G., R.T.F., and Í.T.G.; resources, A.M.F.; data curation, A.M.F., P.F.V., and E.F.C.S.; writing—original draft preparation, F.E.R., A.M.F., and A.V.O.; writing—review and editing, A.M.F., P.F.V., and E.F.C.S.; visualization, P.F.V. and E.F.C.S.; supervision, A.M.F.; project administration, A.M.F.; funding acquisition, A.M.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the São Paulo Research Foundation (FAPESP; grant numbers 2021/05039-9 and 2021/12973–0).

Data Availability Statement

Data are available upon request.

Acknowledgments

The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES, Finance Code 001) for providing a scholarship to the first author, the São Paulo Research Foundation (FAPESP) for supporting this research (grant numbers 2021/05039-9 and 2021/12973–0), and the National Council for Scientific and Technological Development (CNPq) for providing an award for excellence in research to the second author. We would also like to thank the sweet potato growers (Danilo Martins Trindade and João Carlos Navarro) who provided access to their fields for this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 02167 g001
Figure 1. Daily rainfall (bars) and maximum (red lines) and minimum (blue lines) air temperatures during sweet potato cultivation at Braúna, São Manuel, and Regente Feijó. Planting and harvesting periods of the sweet potato crops are also shown.
Figure 1. Daily rainfall (bars) and maximum (red lines) and minimum (blue lines) air temperatures during sweet potato cultivation at Braúna, São Manuel, and Regente Feijó. Planting and harvesting periods of the sweet potato crops are also shown.
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Figure 2. Soil–plant analysis development (SPAD) readings: (a–c) nitrogen sufficiency index (NSI), (d–i) leaf N concentrations in sweet potato leaves in response to N fertilization management in (a,d,g) Braúna, (b,e,h) São Manuel, and (c,f,i) Regente Feijó. For SPAD readings and leaf N concentrations, vertical bars indicate the least significant difference (LSD) at p ≤ 0.05, determined via the LSD test. All N treatments are described in Table 2.
Figure 2. Soil–plant analysis development (SPAD) readings: (a–c) nitrogen sufficiency index (NSI), (d–i) leaf N concentrations in sweet potato leaves in response to N fertilization management in (a,d,g) Braúna, (b,e,h) São Manuel, and (c,f,i) Regente Feijó. For SPAD readings and leaf N concentrations, vertical bars indicate the least significant difference (LSD) at p ≤ 0.05, determined via the LSD test. All N treatments are described in Table 2.
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Figure 3. Relationships between applied N rate and total N uptake by sweet potatoes in Braúna, São Manuel, and Regente Feijó. Symbols indicate the means of each N management treatment. All N treatments are described in Table 2. **: p < 0.01.
Figure 3. Relationships between applied N rate and total N uptake by sweet potatoes in Braúna, São Manuel, and Regente Feijó. Symbols indicate the means of each N management treatment. All N treatments are described in Table 2. **: p < 0.01.
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Table 1. Soil chemical properties at the experimental sites at a depth of 0–0.20 m before sweet potato planting. (Average of four replicates.)
Table 1. Soil chemical properties at the experimental sites at a depth of 0–0.20 m before sweet potato planting. (Average of four replicates.)
Soil Properties 1Sites
BraúnaSão ManuelRegente Feijó
pH (CaCl2)5.05.45.1
SOM (g dm−3)6.311.012.0
P resin (mg dm−3)7.29.09.0
K (mmolc dm−3)1.21.81.5
Ca (mmolc dm –3)18.421.012.0
Mg (mmolc dm−3)8.19.04.0
H + Al (mmolc dm−3)13.319.016.0
CEC (mmolc dm−3)40.951.033.0
Base saturation (%)67.662.052.0
S (mg dm−3)4.52.09.0
B (mg dm−3)0.40.30.1
Cu (mg dm−3)0.51.30.6
Fe (mg dm−3)9.07.015.0
Mn (mg dm−3)4.75.36.2
Zn (mg dm−3)0.21.20.3
1 SOM, soil organic matter; CEC, cation exchange capacity.
Table 2. Nitrogen (N) time and rate of application in different treatments.
Table 2. Nitrogen (N) time and rate of application in different treatments.
N ManagementTreatments 1
ControlReferenceConventionalNSI < 90NSI < 95
Braúna
Application time-Planting, 15, 29, 57 DAPPlanting, 29 DAPPlantingPlanting, 29 DAP
N rate per each time017, 43, 45, 4517, 331717, 36
Total N rate0150501753
São Manuel
Application time-Planting, 15, 29, 57 DAPPlanting, 29 DAPPlanting, 64 DAPPlanting, 36, 43, 64 DAP
N rate per each time017, 43, 45, 4517, 3317, 1117, 16, 44, 6
Total N rate0150502883
Regente Feijó
Application time-Planting, 15, 29, 57 DAPPlanting, 29 DAPPlantingPlanting, 42 DAP
N rate per each time017, 43, 45, 4517, 331717, 2
Total N rate0150501719
1 Control, no N fertilization; Reference, non-limiting N area with 17 kg ha−1 N applied at planting, plus 43 kg ha−1 N at 15 d after planting (DAP), plus 45 kg ha−1 N at 29 DAP, plus 45 kg ha−1 N at 57 DAP; Conventional, 17 kg ha−1 N at planting, plus 33 kg ha−1 N at 29 DAP; NSI < 90% or NSI < 95%, 17 kg N ha−1 at planting, plus 1.0 kg ha−1 N per 0.10 unit of SPAD, when the soil–plant analysis development (SPAD) readings indicate NSI < 90% or NSI < 95%, respectively.
Table 3. Biomass yield and N uptake and removal in sweet potatoes as affected by N fertilization management using a portable chlorophyll meter (mean ± standard error).
Table 3. Biomass yield and N uptake and removal in sweet potatoes as affected by N fertilization management using a portable chlorophyll meter (mean ± standard error).
SiteControl 1ReferenceConventionalNSIF
Probability
<90<95
Shoot biomass (Mg ha−1)
Braúna4.0 ± 0.61 b 14.8 ± 0.76 a4.1 ± 0.72 b3.7 ± 0.39 b4.0 ± 0.48 b0.030
São Manuel1.9 ± 0.62 a2.1 ± 0.27 a1.9 ± 0.45 a2.2 ± 0.72 a1.8 ± 0.47 aNS 2
Regente Feijó2.7 ± 0.66 a3.1 ± 0.65 a3.0 ± 0.40 a2.8 ± 0.50 a3.1 ± 0.73 aNS
Storage root biomass (Mg ha−1)
Braúna8.5 ± 0.96 ab7.2 ± 1.24 b8.5 ± 0.98 ab9.2 ± 0.48 a9.4 ± 1.07 a0.050
São Manuel7.2 ± 1.15 b8.2 ± 1.03 ab9.5 ± 1.25 a7.9 ± 0.74 ab8.3 ± 1.42 ab0.050
Regente Feijó10.5 ± 0.96 ab9.6 ± 1.27 b10.2 ± 1.01 ab11.0 ± 0.83 a10.0 ± 0.76 ab0.050
Whole plant biomass (Mg ha−1)
Braúna12.5 ± 1.02 a12.0 ± 1.15 a12.5 ± 1.16 a12.8 ± 0.42 a13.3 ± 1.14 aNS
São Manuel9.1 ± 1.23 b10.3 ± 1.02 ab11.4 ± 1.32 a10.1 ± 0.72 ab10.1 ± 1.42 ab0.050
Regente Feijó13.5 ± 0.81 a12.8 ± 1.16 a12.9 ± 1.0 a13.8 ± 0.79 a13.1 ± 0.55 aNS
N uptake (kg ha−1)
Braúna92 ± 3.6 c151 ± 3.6 a129 ± 5.0 ab118 ± 2.0 bc148 ± 3.9 a0.002
São Manuel56 ± 2.8 c98 ± 4.1 a77 ± 2.5 b64 ± 3.2 bc81 ± 3.4 b0.002
Regente Feijó142 ± 4.0 a136 ± 3.7 a143 ± 4.4 a151 ± 3.5 a149 ± 5.8 aNS
N removal (kg ha−1)
Braúna41 ± 2.5 a54 ± 3.0 a55 ± 3.7 a46 ± 1.8 a54 ± 2.7 aNS
São Manuel29 ± 1.6 c68 ± 4.0 a53 ± 2.9 ab38 ± 2.7 bc53 ± 3.4 ab0.003
Regente Feijó108 ± 4.6 a105 ± 4.3 a112 ± 4.2 a119 ± 3.3 a115 ± 5.3 aNS
Values followed by the same letter in the same row indicate no significant differences at p < 0.05, determined via the least significant difference (LSD) test for N management within each site. 1 All treatments are described in Table 2. 2 NS: not significant.
Table 4. Storage root number per plant, mean weight, root yield, dry matter and starch contents, and starch yield of sweet potato as affected by N fertilization management using a portable chlorophyll meter (mean ± standard error).
Table 4. Storage root number per plant, mean weight, root yield, dry matter and starch contents, and starch yield of sweet potato as affected by N fertilization management using a portable chlorophyll meter (mean ± standard error).
SiteControl 1ReferenceConventialNSIF
Probability
<90<95
Total storage root number (no. pl−1)
Braúna7.4 ± 1.01 ab7.2 ± 0.80 b8.1 ± 0.60 ab8.3 ± 0.64 a8.3 ± 0.80 a0.050
São Manuel6.3 ± 0.78 b6.6 ± 0.52 b7.5 ± 0.51 a6.7 ± 0.80 ab7.0 ± 0.73 ab0.046
Regente Feijó11.1 ± 0.90 b11.9 ± 0.86 ab11.3 ± 0.92 ab12.5 ± 0.51 a11.5 ± 1.23 ab0.048
Marketable storage root number (no. pl−1)
Braúna3.2 ± 0.86 b3.0 ± 0.82 b3.4 ± 0.77 ab3.5 ± 0.68 ab3.8 ± 0.76 a0.049
São Manuel1.6 ± 0.50 b2.1 ± 0.60 a2.2 ± 0.44 a2.0 ± 0.76 ab1.8 ± 0.43 ab0.047
Regente Feijó4.1 ± 0.88 ab3.5 ± 0.94 b4.2 ± 0.83 ab4.6 ± 0.53 a4.5 ± 1.09 ab0.045
Storage root mean weight (g root−1)
Braúna252 ± 5.9 a236 ± 5.5 a251 ± 6.0 a246 ± 4.6 a243 ± 5.0 aNS 2
São Manuel204 ± 4.6 a215 ± 5.9 a220 ± 5.0 a212 ± 3.1 a204 ± 5.7 aNS
Regente Feijó188 ± 4.5 a172 ± 4.3 a186 ± 4.5 a274 ± 2.5 a177 ± 4.7 aNS
Total storage root yield (Mg ha−1)
Braúna38 ± 1.42 ab35 ± 2.19 b42 ± 2.65 a42 ± 1.50 ab42 ± 1.67 ab0.050
São Manuel27 ± 2.26 b30 ± 2.11 b35 ± 2.10 a30 ± 1.31 b30 ± 1.92 b0.050
Regente Feijó41.6 ± 1.84 a42.2 ± 2.46 a41.1 ± 1.41 a43.7 ± 1.54 a41.2 ± 1.62 aNS
Marketable storage root yield (Mg ha−1)
Braúna22 ± 0.75 b23 ± 1.88 b27 ± 2.30 ab26 ± 2.51 ab28 ± 1.05 a0.047
São Manuel13 ± 1.74 a15 ± 1.40 a16 ± 1.62 a15 ± 1.70 a12 ± 1.87 aNS
Regente Feijó26 ± 1.67 a25 ± 2.12 a26 ± 1.14 a28 ± 1.39 a28 ± 1.88 aNS
Storage root DM content (% fresh weight)
Braúna22.1 ± 1.29 ab20.5 ± 1.21 ab20.1 ± 1.34 b22.5 ± 0.66 a21.8 ± 1.25 ab0.050
São Manuel26.2 ± 0.43 a27.1 ± 1.16 a27.4 ± 1.37 a26.8 ± 1.78 a27.4 ± 1.23 aNS
Regente Feijó25.3 ± 0.62 a23.4 ± 1.47 a24.2 ± 1.32 a25.3 ± 0.76 a24.3 ± 1.10 aNS
Storage root starch content (% fresh weight)
Braúna13.2 ± 1.21 a13.1 ± 1.22 a13.1 ± 1.18 a14.5 ± 1.01 a13.8 ± 1.59 aNS
São Manuel19.6 ± 0.67 a19.3 ± 1.01 a20.7 ± 1.23 a20.0 ± 1.57 a19.6 ± 1.08 aNS
Regente Feijó18.8 ± 1.19 a17.5 ± 1.64 a18.4 ± 0.93 a19.4 ± 1.16 a18.3 ± 1.52 aNS
Starch yield (Mg ha−1)
Braúna5.1 ± 0.72 a4.6 ± 0.91 a5.6 ± 1.03 a6.1 ± 0.87 a5.8 ± 1.19 aNS
São Manuel5.4 ± 0.97 b5.8 ± 0.96 b7.3 ± 0.79 a6.0 ± 0.73 b5.9 ± 0.92 b0.035
Regente Feijó7.8 ± 0.62 a7.2 ± 1.10 a7.8 ± 0.7 a8.5 ± 0.90 a7.6 ± 1.00 aNS
Values followed by the same letter in the same row indicate no significant differences at p < 0.05, determined via the LSD test for N management within each site. 1 All treatments are described in Table 2. 2 NS: not significant.
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Rodrigues, F.E.; Fernandes, A.M.; Oliveira, A.V.; Vargas, P.F.; Souza, E.F.C.; Guedes, P.T.P.; Figueiredo, R.T.; Guimarães, Í.T. Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes. Agriculture 2024, 14, 2167. https://doi.org/10.3390/agriculture14122167

AMA Style

Rodrigues FE, Fernandes AM, Oliveira AV, Vargas PF, Souza EFC, Guedes PTP, Figueiredo RT, Guimarães ÍT. Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes. Agriculture. 2024; 14(12):2167. https://doi.org/10.3390/agriculture14122167

Chicago/Turabian Style

Rodrigues, Fabrício E., Adalton M. Fernandes, Arthur V. Oliveira, Pablo F. Vargas, Emerson F. C. Souza, Politon T. P. Guedes, Ricardo T. Figueiredo, and Ítala T. Guimarães. 2024. "Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes" Agriculture 14, no. 12: 2167. https://doi.org/10.3390/agriculture14122167

APA Style

Rodrigues, F. E., Fernandes, A. M., Oliveira, A. V., Vargas, P. F., Souza, E. F. C., Guedes, P. T. P., Figueiredo, R. T., & Guimarães, Í. T. (2024). Application of a Portable Chlorophyll Meter to Assess the Nitrogen Sufficiency Index and Nitrogen Requirements in Sweet Potatoes. Agriculture, 14(12), 2167. https://doi.org/10.3390/agriculture14122167

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