From the Field-Agronomy Notes: Vol. 2, Num. 3
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February 23rd, 2017
As the winter of 2017 continues to be practically nonexistent across North Carolina, growers continue to ramp up their efforts in the greenhouse. Given how favorable the forecast has been over the past 10+ days (and how favorable the extended forecast appears to be), it wouldn’t be surprising if trays seeded on or just after Valentine’s Day are already starting to germinate. With rapid and [hopefully] uniform germination, it’s now time to think about nutrient management. What follows are a few of the considerations that growers should be making.
Fertilizer Source Selection:
Producers should consider complete fertilizer sources that have a lower concentration of phosphorus (P) than nitrogen (N) and potassium (K). Most commonly these sources are 2:1:2 or 3:1:3 ratios, examples of both would be materials like 20-10-20 or 16-5-16, respectively. Homogenized fertilizer sources such as these will provide sufficient N and K while limiting the quantity of applied P. Excess P has been reported to produce seedlings that are leggy (tall) and spindly (small stem diameter).
In addition to N-P-K content, pay close attention to secondary macro and micronutrients. Nutrients such as calcium, magnesium, sulfur, and boron can test low in greenhouse source water; therefore, their inclusion in fertilizer sources might be warranted. If fertilizer sources are chosen that do not contain sufficient quantity of secondary macro and micronutrients, then additional fertilizer sources might be needed during the greenhouse season. For example, some fertilizer sources do not contain enough calcium (Ca) to sustain seedling growth and deficiencies become apparent (Figure 1). In this situation, 3.51 oz 15.5-0-0/100 gallons of float water is recommended to add 50 ppm Ca to the float bed. Other examples include magnesium and sulfur, which can be addressed through applications of Epsom salt (3.5 to 4.0 oz/100 gallons of float water).
Regardless of how these nutrient issues are addressed, it is sometimes best to wait until deficiencies are observed before making supplemental fertilizer applications. Excess rates of these nutrients can be extremely toxic to seedlings (primarily boron), can increase the cost of production, or may not even be needed. Of course, the exception to this rule is if you have a history of nutrient deficiency due to source water and fertilizer selection. In this case a preventative application of the required nutrient just might be in order. This also touches on the benefits of having greenhouse source water analyzed every season!
Organic options aren’t as cut and dry. To our knowledge, there are no organic N-P-K fertilizers available to tobacco producers that contain the balanced nutritional content found in conventional fertilizer sources. What this means is that organic fertilizer sources typically contain too little or too much P relative to N and K, very little K relative to N, are slow release materials, and might not have sufficient quantities of secondary macro and micronutrients in certain situations. This also means that organic producers might have to make their own blends to satisfy seedling needs.
Research conducted here in the Crop and Soil Sciences Department has shown that a blend of Peruvian Seabird Guano (12-11-2), Allganic Sodium Nitrate (16-0-0), and Allganic Potassium Sulfate (0-0-52) can produce transplants as successfully as conventional 16-5-16. In this system, Seabird Guano was used as a means to provide phosphorus to the system at 40 ppm. This also provides roughly 45 ppm N. The remaining N was delivered through 16-0-0 at a rate of 80 ppm for a total of 125 ppm N in the first fertilizer application. In addition, 125 ppm K was delivered through 0-0-52.
The goal of this program was to supply some N and all P from the Seabird Guano, nitrate nitrogen from 16-0-0, and K and S from 0-0-52. Table 1 contains the application rates of each material. Fertilizer application in this study occurred nine days after seeding and then again 2-4 weeks later, depending upon seedling growth rates. Nutritional deficiencies were not observed in this study, and seedling growth/usability was exceptionally good when compared to the conventional fertility program.
The take home message is that organic nutrient sources can be used successfully, but will require a little more thought and micromanagement than conventional sources. When using these nutrient sources, producers should attempt to dissolve them in warm water prior to application. Organic producers are also encouraged to collect diagnostic float bed solution samples on at least a weekly basis to accurately monitor nutrient levels.
Table 1. Organic Fertilizer Sources and Application Rates
|Targeted Nutrient Rate
Peruvian Seabird Guano (12-11-2)
|4.85 oz./100 gal
|40 ppm P, ≈45 ppm N
|Allganic Sodium Nitrate (16-0-0)
|6.67 oz./100 gal
80 ppm N
|Allganic Potassium Sulfate (0-0-52)
|3.21 oz./100 gal
125 ppm K, 43 ppm S
|*Fertilizer sources selected based upon current grower use and OMRI listing
**Before using ANY fertilizer in the greenhouse, be sure that it is approved for use by your organic certifier and the groups contracting to buy your tobacco.
The first fertilizer application should occur 7 to 10 days after seeding. The recommendation to wait at least seven days before adding fertilizer stems from the injury potential offered by fertilizer salts. We encourage growers to wait a minimum of seven days to allow for some plant growth, which allows seedlings to outpace the influx of fertilizer salts into the media.
Of course, the other piece of this puzzle is that it is assumed growers are applying the correct rate of fertilizers. Excessive fertilizer rates (>150 ppm N) can induce soluble salts injury even when some seedling growth has occurred. The final piece of information is to wait no longer than 10 days to add fertilizer under the current growing conditions. Delaying fertilizer application beyond 10 days can result in under-supplying nutrients to the plant and can result in uneven seedling growth due to nutrient depletion in the media prior to the wicking of the nutrient solution form the float bed (Figure 2). The second fertilizer application should occur three to four weeks after the first application. Table 2 contains fertilizer application options. Regardless of when fertilizer is applied, producers should adequately circulate the float water solution to ensure uniform nutrient distribution. This point is extremely critical when Seabird Guano is used, as it can be difficult to dissolve after application.
|Table 2. Suggested Fertilizer Application Rates
|150 ppm N
|100 ppm N
|125 ppm N
|125 ppm N
|100 ppm N
|150 ppm N
Chapter 4 of the 2017 Flue-Cured Tobacco Guide contains more information for successful transplant production. In addition, the North Carolina Department of Agriculture and Consumer Services-Agronomic Division has produced bulletins (solution analysis and sample collection) that are excellent guidelines for nutrient management. All are excellent resources for general information.