Nitrogen Fertilizer Programs for Organic Flue-Cured Tobacco (Nicotiana Tabacum L.) Seedling Production

Introduction

Greenhouse production of tobacco transplants using the float system is widely employed because of the low labor costs, greater control of environmental conditions, and uniform plant growth (13). This system utilizes expanded polystyrene trays containing open-ended, inverted pyramidal cells filled with soilless media that float on shallow water in black plastic-lined beds (7). Nutrients in the form of a complete fertilizer can be either mixed directly in the float system water or injected as a solution through the use of an injector system. Proper fertilizer application is critical for ensuring adequate growth and transplant health. Among the elements essential for the commercial production of tobacco, none has as pronounced an effect nor requires the degree of attention in fertility practices as nitrogen (N) (9). At least 60% of the applied N should be from a multinutrient fertilizer, such as a 20–10–20 (N–P₂O₅–K₂O), to ensure sufficient amounts of phosphorus (P) and potassium (K) (7); the remainder of N needs can be met through the use of sodium nitrate (16–0–0).

Nitrogen can exist in many different forms, which can impact the availability for plant uptake. The organic form of N must be converted to ammonium through the process of mineralization in order to be plant available. Other forms of N include urea and nitrate; however, the former cannot be assimilated directly by plants and therefore must be converted to ammonium. If present in high enough concentrations, urea can cause plant toxicity because accumulation of nitrite as urea and ammonium is converted to nitrate (12). According to Fisher and Vann (1), N levels in the float water solution should consistently measure 125 mg · L⁻¹. Higher levels can produce tender, succulent seedlings that are more susceptible to diseases and more prone to fertilizer salts injury (1).

The number of hectares of U.S. Department of Agriculture certified organic tobacco continues to rise in the United States; organic tobacco was harvested from 2,704 ha in 2015 and 4,405 ha in 2016 (14). For organic flue-cured tobacco growers, managing fertility can be challenging because of the many different factors that impact nutrient release and availability with these organic sources. In general, organic fertilizers do not have a high concentration of N in a plant-available form and are typically high in phosphorus concentration and low in potassium. Conversely, conventionally grown tobacco is fertilized with synthetic, inorganic fertilizers that are available for plant uptake, or become quickly available through hydrolysis reactions (6). One option available to organic flue-cured tobacco producers is the use of dry organic fertilizers such as fishery waste, feather meal, and seabird guano, all of which have a high N content (>10% of dry weight) and relatively rapid N mineralization (5). In contrast to manure-based compost, dry organic fertilizers mineralize 60%–80% of N within 4–8 weeks after application to agricultural soils (2,4–6).

Seabird guano is a natural deposit of the excrement of birds living along ocean coasts and was a historically important fertilizer source prior to the development of the Haber-Bosch process (3). One benefit to the use of guano is its relatively rapid rate of nitrification in soil compared to other organic fertilizers (11). Mineralization is essential to produce plant-available N, but in anaerobic environments, such as a greenhouse float system, denitrification can occur. How guano behaves in a float system, and the resultant transplants produced when using guano, are unknown. As such, the objective of the following study was to compare the effects of synthetic versus organic N sources on the nutrient status of float system water and seeding media, as well as transplant health.

Materials and Methods

This study was conducted from June 2016 to January 2017 in a tobacco research greenhouse at the Central Crops Research Station in Clayton, NC. Two experimental runs were conducted during this time; Run 1 lasted from June 4, 2016 to July 23, 2016, and Run 2 from November 9, 2017 to January 13, 2017. During each run of the experiment, seedlings were clipped five times in order to maximize stem-diameter expansion. After the fifth mowing, the study was terminated. Because of reduced day length experienced during Run 2, the five mowing intervals took longer to achieve. A conventional, complete fertilizer source (Conv; SQM Ultrasol Premium 16–5–15 [6% ammoniacal nitrogen, 10% nitrate nitrogen, 5% P₂O₅, 15% K₂O, 3.0% Mg, 4.0% S, 0.017% B, 0.01% chelated Cu, 0.05% chelated Mn, 0.10% chelated Fe, 0.001% Mo, 0.015% chelated Zn]) was used as the conventional control. Additional fertilizer treatments included combinations of pelletized Peruvian seabird guano (Sunleaves®; 12–11–2), sodium nitrate (SQM Allganic® Nitrogen; 16–0–0), and potassium sulfate (SQM Allganic® Potassium; 0–0–50). Potassium sulfate was added to the organic fertilizers to achieve a similar nitrogen:potassium ratio as the Conv treatment. A total of four fertilizer treatments were used in the study: the conventional, complete fertilizer (Conv), seabird guano plus potassium sulfate (SG), sodium nitrate plus potassium sulfate (SN), and a combination of seabird guano and sodium nitrate plus potassium sulfate (SN_SG).

To provide adequate growing space and to separate treatments, 12 minibeds were constructed inside of the greenhouse with each minibed designed to hold six 288-cell expanded polystyrene (EPS) trays in 185.33 L of solution. The study design was a randomized complete block design with three blocks. Within each block the fertilizer treatments were randomly assigned to one mini bed within which six EPS trays were placed. Each tray had the center 100 cells marked for data collection. Trays were seeded with organic soilless peat-based media with a pH of 4.56 (Carolina Choice All Natural Mix; Carolina Soil Company) on June 4, 2016 (Run 1) and November 9, 2016 (Run 2) with variety K 326 and then floated in the designated bed. All beds were filled with water from a deep well to a depth of 12.7 cm. All fertilizers were applied directly into the solution of each individual bed 9 and 21 days after seeding (DAS). The SG treatment was pulverized to pass through a 1-mm sieve in order to increase surface area and solubility. All components of each fertility treatment were mixed with in hot tap water (50°C) for 2 min until completely dissolved. Once cooled, the fertility treatments were applied to the appropriate mini float bed. This process was repeated for the second fertilizer application.

Nitrogen, phosphorus (P), and potassium (K) nutrient rates targeted 125, 40, and 125 mg · L⁻¹, respectively, for the Conv, and SN_SG treatments, which required 0.78 g · L⁻¹ (16−5−16) and 0.36 g · L⁻¹ (12–11–2) + 0.51 g · L⁻¹ (16–0–0) + 0.24 g · L⁻¹ (0–0–52) of material added to the solution, respectively. Target concentrations for the SG treatment were 125 mg · L⁻¹ of N and K which resulted in 115 mg · L⁻¹ of P and required 1.04 g · L⁻¹ (12−11−2) + 0.24 g · L⁻¹ (0–0–52) of material added to the solution. Finally, the SN target concentrations were 125 mg · L⁻¹ of N and K; however, P was not included in this treatment and required 0.78 g · L⁻¹ (16–0–0) +0.24 g · L⁻¹ (0–0–52) in material added to the float solution.

Throughout the study, N (in the form of urea, ammonium, and nitrate), pH, dissolved oxygen, and bicarbonate of the float system nutrient solution was quantified for all fertilizer treatments. Samples were taken every 5 d, starting at 10 DAS and ending 45 DAS in Run 1. In Run 2, samples were once again taken every 5 d, starting on 10 DAS and ending 65 DAS. At each sample collection date, float system water depth was elevated to maintain the 12.7-cm depth established at the beginning of the study. Solution samples were collected in a sterile hard-plastic bottle and immediately transferred to North Carolina Department of Agriculture and Consumer Services (NCDA&CS) Agronomic Division for analysis. Analysis methodology followed those established by NCDA&CS protocol (10). Media samples were collected 20 DAS and at the end of each run from outside the center 100 cells that were marked off and used for data collection. Fifteen media samples were collected from each tray for a total of 45 samples per treatment. A second media sampling was conducted at the conclusion of each run. Media samples were immediately transferred to NCDA&CS Agronomic Division for analysis. Finally, 10 whole-plant samples were collected from each tray, detached from the soilless media, and then measured for stem height and diameter. Stem height was measured from the base of the stem to the apical meristem, and stem diameter was taken from the first node above the media line. At the end of each run, the percent of usable transplants within the 100-cell barrier was calculated for each tray. Usable transplants were defined as transplants that would be healthy enough to survive field conditions.

Results

Float System Solution.

The fertilizer treatment × DAS interaction was significant for all) and converse to those observed in nitrate. No ammonium was present in the SN treatment, but levels were consistently the highest in SG. Conv had significantly lower ammonium levels than SG, and SN_SG had levels that never reached above 50 mg · L⁻¹ in Run 1 and 60 mg · L⁻¹ in Run 2. Both ammonium and nitrate concentrations showed a spike after 20 DAS that is due to application of fertilizer treatments at 21 DAS. The Conv or SN treatments had no appreciable urea concentrations for Runs 1 and 2 (Figure 3). SG had the highest urea concentrations at the beginning of each run, but these levels dropped precipitously to approximately 0 mg · L⁻¹ by 30 DAS. In Run 2, these levels increased to 7 mg · L⁻¹, but once again fell back down close to 3 mg · L⁻¹. SN_SG had lower levels of urea than SG, but this trend reversed following fertilizer application at 21 DAS. Though levels did increase in SN_SG at 25 DAS, a similar decline was observed in this treatment as what was observed in SG.

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Bicarbonate levels were significantly higher in the SG treatment in Runs 1 and 2 compared to all other fertilizer treatments (Figs. 4 A and 4B). Both Conv and SN treatments maintained bicarbonate levels close to zero in both runs. The SN_SG treatment had bicarbonate levels intermediate to SG and SN/Conv; however, towards the end of both runs these bicarbonate levels in SN_SG reached close to zero.

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Float system solution pH in Run 1 remained between 7.5 and 6 for all treatments (Figure 4C). pH was significantly highest in the SG treatment and lowest in the Conv treatment. Similar trends were observed in Run 2 (Figure 4D); however, pH in the Conv treatment continued to drop close to 4 by the end of the study.

Dissolved oxygen levels were variable in both runs (Figure 5). Conv and SN maintained similar levels throughout Run 1, which were both higher than SG and SN_SG. Although the SN treatment behaved similarly in Runs 1 and 2, the Conv treatment dropped to significantly lower levels. Levels of dissolved oxygen in SN_SG and SG dropped close to 1 mg · L⁻¹ at 25 DAS in Run 1 and 40 DAS in Run 2, and remained at this low level for the remainder of both runs.

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Float System Media.

The fertility treatment × DAS interaction was significant for nitrate ammonium, and pH in Run 1 and nitrate and pH in Run 2 (Table 2). Media urea concentrations were not affected by the main effects of interaction in either run. A strong main effect of fertility treatment was observed on ammonium concentrations in Run 2.

Table 1. Results from a repeated-measures analysis of variance of the effect of fertility treatment and days after seeding on tobacco float system water.

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Table 2. Results from a repeated-measures analysis of variance of the effect of fertility treatment and days after seeding on tobacco float system tray media.

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Trends in media nitrate concentrations were consistent in both runs (Figures 6 A and 6B). No nitrate was found in the SG treatment, whereas SN had significantly higher concentrations in both runs. SN_SG and Conv nitrate levels were nearly identical in both runs of the study.

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A crossover interaction was observed in media ammonium concentrations in Run 1 (Figure 6C). At 20 DAS SG had the highest concentrations, SN_SG and Conv were similar and intermediate to Conv and SN. At 44 DAS ammonium levels increased in Conv, which were no different than the values observed in SG. Levels of ammonium dropped to zero in SN_SG, and were no different than those observed in SN. In Run 2 the main SG had the highest concentration of ammonium compared to all other fertility treatments (Figure 6D). Ammonium concentrations in Conv were nearly half that of SG, but not significantly different from SN_SG. SN had close to 0 mg · L⁻¹ of ammonium.

Crossover interactions were observed in Runs 1 and 2 for media pH (Figure 7). At 20 DAS no differences were observed in pH; however, pH increased significantly in SN_SG and SN and dropped in Conv and SG. In Runs 1 and 2 Conv and SG had low pH levels ranging from 5.0 to 5.3.

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Plant Performance.

Plant height and stem diameter were significantly affected by fertility treatment in Run 1 (Table 3). In Run 2 only stem diameter was affected by fertility treatment. No usable plants were produced using the SN treatment, whereas all other treatments resulted in comparable plant stands (average of 70.4% and 91.6% in Runs 1 and 2, respectively; Table 4). In Runs 1 and 2 the SG treatment resulted in the thinnest plants with SN_SG giving the thickest stem diameter. The Conv treatment resulted in the tallest plants and SG resulted in the shortest usable plants; the SN_SG treatment was intermediate to both.

Table 3. Results from an analysis of variance of the effect of fertility treatment on tobacco transplant height, stem diameter, and percent usability.

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Table 4. Percent of usable plants, stem diameter, and stem height of tobacco as affected by float system solution fertility program in Runs 1 and 2.

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Discussion

To maximize the quality and quantity of their end product, growers must start with healthy and vigorous tobacco transplants; this principle holds true in both organic and conventional production systems. The objective of this study was to compare an organic source of fertility on the production of tobacco transplants and the effects it has on float system solution and media. We found that seabird guano alone (SG) or in combination with sodium nitrate (SN_SG) can produce healthy, usable transplants comparable to a synthetic conventional fertilizer (Conv; Table 4). Although SG tended to produce transplants that were slightly shorter and narrower than Conv, the addition of sodium nitrate to SG led to transplants that were similar in size to those produced using Conv (Table 4). We were unable to produce any usable transplants using SN alone (Table 4). This is most likely because of the fertility treatment being completely devoid of phosphorus and other essential nutrients. This may be remedied through application of additional products such as rock phosphate or bone meal.

Although the transplants produced from SG and SN_SG were comparable physiologically to Conv, the effects of fertility treatment on float system solution and media differed. Bicarbonate concentrations were highest in fertility treatments containing seabird guano; however, when guano quantities were reduced and replaced with sodium nitrate (SN_SG treatment) there was a drop in the solution bicarbonate concentration (Figures 4A and 4B). These differences are due to a reduction in microbial activity associated with the mineralization and nitrification of organic N sources, as SN_SG had only 0.36 g · L⁻¹ of guano compared to 1.04 g · L⁻¹ in SG. Bicarbonate can affect nutrient uptake by increasing solution pH; however, we did not observe any dramatic increase in pH in solution treated with SG or SN_SG associated with these higher bicarbonate concentrations (Figures 4C and 4D). Although pH values were highest in these fertility treatments, levels never reached higher than 7.8 and consistently decreased in both runs to pH levels of around 6.5, which is close to the recommended pH of 6.2 (1). These results may be due to the high ammonium concentrations released by seabird guano (Figure 2) as well as the net increase in H⁺ ions during the mineralization and nitrification of organic N sources.

Fertility treatments that included seabird guano led to extremely low levels of dissolved oxygen (Figure 5). These observations are likely because of increased microbial activity, as the more complex forms of N found in organic fertilizers require microbial degradation for mineralization to occur (4). When oxygen levels become too low, denitrification may occur and some N may leave the system. As such, when utilizing an organic source of nitrogen, it may be beneficial to add aeration with the use of an air stone to maintain higher dissolved oxygen concentrations.

Higher urea concentrations were associated with SG and SN_SG (Figure 3), as these are the only two that contained an organic nitrogen source. Urease is a nickel-dependent enzyme commonly found in plants and microorganisms that catalyzes the conversion of urea to ammonia and carbon dioxide (8,15). Once released, CO₂ can bind to H⁺ ions and form bicarbonate. Furthermore, microbial respiration, which releases CO₂, likely added to the increase in bicarbonate seen in SG and SN_SG (Figures 4A and 4B). This would also explain the drop in dissolved oxygen observed in these two treatments, as the increase in microbial population likely depleted the dissolved oxygen during the mineralization and nitrification processes (Figure 5).

Trends observed in the soilless media nutrient status followed those of the float system solution. One slight difference was that soilless media pH values were consistently lower than the solution; pH never rose above 7.0 in the soilless media regardless of run (Figure 7). This may be caused by additional buffering capacity of the soilless media and the general lower pH of the product. This may also explain why the higher solution bicarbonate levels associated with seabird guano did not affect transplant health.

Conclusion

The organic fertility treatments used in this study were effective in establishing tobacco transplants of comparable health as those treated with a conventional fertilizer. When used as the sole source of N, seabird guano can introduce potentially unwanted high levels of bicarbonate, increased pH, and lower dissolved oxygen because of microbial activity. Reducing the amount of seabird guano applied and adding sodium nitrate to reach appropriate levels of N can help to mitigate these seabird guano–associated byproducts. Furthermore, other organic approved sources of P are available, such as bone meal, and may fit well in an organic float bed system; however, further research is required.