North Carolina tobacco growers have produced tobacco transplants in greenhouses since the early 1990s. The controlled environment of the greenhouse allows for a wider window of seeding dates and increases seed germination, leading to high plant uniformity when compared to outdoor plant beds. Management is also generally easier and labor costs are much lower. This publication discusses the sources and water chemistry essential to economically producing uniform, high-quality transplants.

Water quality is an important aspect of greenhouse transplant production. Regardless of the source, all water contains some impurities, typically consisting of elements and chemical compounds such as sodium, chloride, calcium, magnesium, boron, and bicarbonate. Some of these substances (boron, for example) can benefit plant growth when present in small quantities. Others, such as sodium and chloride, can be detrimental in large quantities. Careful selection of the water source, evaluation of its chemical components through water analysis, and corrective measures when necessary are essential to successful transplant production.

An on-site well is the most desirable water source. Water from shallow wells is generally of acceptable quality; water from deep wells may be of poor quality in certain areas of the state. Municipal water that is treated and filtered varies in usability for transplant production, depending on the area of the state and the source of the water. Prior to selecting a municipal water source, producers are encouraged to check with municipalities to ensure that chloride (Cl^-) content is not an issue.

Surface water is much more likely than well water to contain excessive levels of iron (Fe), which can lead to iron toxicity in seedlings. Surface water is also more likely to contain pathogens that cause tobacco diseases; black shank has been reported on seedlings when pond and river water are used. Surface water may also contain harmful herbicides that have entered the water in surface runoff from nearby agricultural fields. For these reasons, avoid using surface water, if possible.

Transplant production can be successful using water containing a wide range of chemistry if a sample is analyzed and corrective action is taken before seeding. The North Carolina Department of Agriculture & Consumer Services (NCDA&CS) analyzes water samples at a moderate cost; for more information on water analysis and fees, visit the Agronomic Services web page. Forms and information on collecting and submitting samples are available from county Cooperative Extension centers and the NCDA&CS website. When the analysis is completed, you will receive a detailed report with a recommendation on the suitability of the water for transplant production and any corrective action that may be necessary.

A 16-ounce water sample from each potential water source is needed for analysis. A clean, plastic soft-drink bottle with a screw-on cap makes an excellent sample container. Allow the water to run for several minutes before collecting the sample (long enough to clear standing water from the pipes). Afterward, rinse the bottle several times using the water to be tested. Do not wash bottle with soap. For recently constructed wells, let the water run at least 30 minutes to flush the system of impurities from new piping and water purification treatments that might lead to an incorrect analysis.

Table 1 lists desirable ranges for several important components of greenhouse water. The following sections discuss some common water quality problems and the steps to correct them.

Table 1. Acceptable ranges for solution analysis parameters in the production of flue-cured tobacco transplants.†

Water in North Carolina may be high in carbonates that are usually associated with bicarbonate salts (HCO₃^-). Coggins 1993 indicated that excessive levels of bicarbonate are very detrimental to seedling growth, as shown in Table 2. Seedlings produced with water containing bicarbonate at concentrations greater than about 2 milliequivalents per liter (meq/L) or 100 parts per million (ppm) CaCO₃ are stunted and yellow, and have small, brown root systems. Their leaves are often cupped downward. While HCO₃^- is not directly toxic to seedlings, high levels raise the pH of the growing media, which has an impact on nutrient availability. High-bicarbonate water is made suitable for transplant production by adding acid to neutralize some of the bicarbonate. The most common product used for bicarbonate neutralization is sulfuric acid (H₂SO₄), which provides some sulfur for plant use.

Alkalinity and pH adjustment

Water is acidic, neutral, or basic (alkaline), depending on its chemical composition. The pH scale ranges from 0 to 14. A pH of 7 is neutral; pH below 7 and above 7 are acidic and basic/alkaline, respectively.

Alkalinity indicates water's tendency to neutralize acids or resist a drop in pH when acid is added. Carbonates (CO₃^2-), bicarbonates (HCO₃^-), and hydroxides (OH^-) are the major contributors of alkalinity. Alkalinity is expressed in terms of calcium carbonate (CaCO₃) equivalency in ppm or total carbonates (TC) as a concentration in meq/L. Alkalinity in ppm of CaCO₃ can be converted to TC by this formula:

TC (meq/L) = Alkalinity (ppm CaCO₃) × 0.02

If enough acid is added to water, the pH is reduced as a result of the neutralization of CO₃^2-, bicarbonate, and OH^-.

Sulfuric acid is commonly used to neutralize alkalinity. It can be obtained as ordinary battery acid or as 93% reagent-grade acid.

Acid strength is measured in terms of its normality (N). Battery acid is 9.19N H₂SO₄. Reagent-grade 93% H₂SO₄ is much stronger: 34.7N. Both are frequently used to reduce transplant water acidity. One unit (1 meq/L) of acid neutralizes one unit (1 meq/L) of alkalinity.

Formulas are available to compute the amount of acid needed to neutralize a given level of alkalinity (TC) in a volume of water. The NCDA&CS recommends the following formulas to neutralize 80% of the CO₃^2-.

1. For alkalinity expressed as ppm or milligrams per liter (mg/L) of CaCO₃:

V = (0.204 × CaCO₃) ÷ N

where:
V = fl. oz. of acid to add to 100 gal. of water
N = normality of the acid
CaCO₃ = alkalinity expressed as ppm of CaCO₃

2. For alkalinity expressed as meq/L of TC:

V = 10.2 × TC ÷ N

where:
V = fl. oz. of acid to add to 100 gal. of water
N = normality of the acid
TC = TC concentration in meq/L

Example calculations:

1. The water source alkalinity is 300 ppm CaCO₃.

For neutralization with battery acid (9.19N):

V = (0.204 × 300) ÷ 9.19 = 6.7 fl. oz. battery acid/100 gal. water

For neutralization with 93% H₂SO₄ (34.7N):

V = (0.204 × 300) ÷ 34.7 = 1.8 fl. oz. 93% H₂SO₄/100 gal. water

2. The water source alkalinity is 8 meq/L of TC per liter.

For neutralization with battery acid (9.19N):

V = (10.2 x 8) ÷ 9.19 = 8.9 fl. oz. battery acid/100 gal. of water

For neutralization with 93% H₂SO₄ (34.7N):

V = (10.2 x 8) ÷ 34.7 = 2.4 fl. oz. 93% H₂SO₄/100 gal. water

Fertilization and alkalinity

Several fertilizers can provide the essential nutrients in float production systems. Good quality transplants are produced by using 150 ppm nitrogen (N) concentration made from 20-10-20 fertilizer in the float water 7 to 10 days after seeding, followed by an additional 100 ppm of N four weeks later. Plants respond best in alkaline water when at least 75% of the N is in nitrate (NO₃^-) form. Nitrogen can be lost through volatilization when a large proportion of the N is in urea or ammonium (NH₄⁺) form.

Some fertilizers (such as 20-10-20, 20-20-20, and 21-5-20) are acidic, whereas others are alkaline. When water is marginally alkaline (TC = 2 to 3 meq/L), an acid fertilizer can reduce pH in the nutrient solution. When water is moderately acid (pH 4.0 to 5.0), a basic fertilizer (such as 15-5-15) can be used to raise the pH.

When water is only marginally alkaline (with a TC = 2 to 3 meq/L), be careful not to over-acidify. Generally, either an acid treatment or an acid-forming fertilizer (not both) is required when alkalinity is marginally high. A solution sample should be tested a few hours after acid treatment or fertilization to confirm that pH and nutrient concentrations are within the desired ranges.

Tobacco seedlings in float systems are very tolerant of sodium (Na). Coggins 1993 reported that seedling growth was normal at Na concentrations up to 500 ppm. However, Na can alter physical properties of the medium and interfere with calcium (Ca) and magnesium (Mg) uptake if their concentrations are less than the minimum values shown in Table 1. Sodium salts can also accumulate to excessive levels in the root zone, particularly in the upper one third of the cell. A good indicator of the Na hazard of water is the sodium adsorption ratio (SAR), which is the proportion of Na to Ca plus Mg. The SAR should always be less than 4.0.

If Na levels are too high, adding Ca and Mg will provide increased competition with Na for plant uptake in both float and overhead-watered systems. When Na levels are high, the media in trays should be kept moist to limit root injury. Ca and Mg can be added to nutrient solutions by using fertilizer with formulas such as 15-5-15 or 16-4-16 or by furnishing a portion of the N using calcium nitrate and adding magnesium sulfate (Epsom salts) to the solution. Check the fertilizer bag label to verify the presence of Ca and Mg.

Excessive Na (and Cl^-) concentrations have also been found in tobacco media analyzed following tray sanitation with bleach-containing (sodium hypochlorite) solutions. In most of these situations, trays were not properly rinsed following sodium hypochlorite Cl^- exposure. Avoid Cl^- solutions for tray sanitation—use steam when possible.

High levels of Cl^- can cause root damage in tobacco. Unlike with Na, adding Ca or Mg will not mitigate the toxic effects of Cl^-. Chloride may occur in North Carolina source waters through saltwater intrusion or storm deposition into aquifers and ponds, primarily in the coastal plain. Cl^- may also be introduced into municipal water as a sanitation measure. Chloride concentrations in North Carolina source waters are typically less than 10 ppm but have been measured as high as 500 ppm. Levels greater than 70 ppm have the potential to burn roots of seedlings. At levels above 100 ppm, it is advisable to dilute with another water source or use an alternate water source.

An additional source of Cl^- in the float system is tray sanitation products. These products are problematic because their residue on tray surfaces contains a high concentration of Cl^-. Once wet, Cl^- moves from the surface of the tray into the media, where it often accumulates in the root zone. Seedling leaves with excessive Cl^- uptake often have a rubbery appearance and high moisture content, and they bruise easily when lightly pressed between your fingers (Figure 1).

Figure 1. Water-soaked bruising on leaves with high chloride concentration.

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Figure 1. Water-soaked bruising on leaves with high chloride concentration.

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Boron (B) deficiency causes bud distortion and death. It has been observed on flue-cured and burley seedlings grown in float systems in the piedmont and mountains. In all cases, the source water did not contain B and the seedlings were held for an extended period in the greenhouse at very low fertility levels because field conditions were not suitable for transplanting.

If an analysis of the source water indicates inadequate B, use a fertilizer containing a trace level of this element in the nutrient solution. If a fertilizer with B is unavailable, adding no more than 0.25 oz. of Borax or 0.125 oz. of Solubor per 100 gal. of nutrient solution is adequate to prevent a deficiency. Excessive B can be extremely toxic to transplants; therefore, a deficiency must be confirmed prior to supplemental application. See NC State Extension publication AG-439-54, Cold Injury and Boron Deficiency in Tobacco Seedlings, for more information about B nutrition.

Calcium is commonly found in most groundwater sources used for greenhouse production in North Carolina; however, concentrations will vary among growing regions. Most tobacco greenhouse media also contain a reasonable quantity of Ca because of added gypsum (calcium sulfate). When Ca content of source water and growing media is moderate to high, transplant needs are often satisfied. However, if the source water Ca content is low, you should treat float beds with fertilizer that contains Ca. Common greenhouse fertilizers, such as 16-5-16 and 20-10-20, do not contain Ca; therefore, gypsum should be added to the float water prior to floating trays at a rate of 5 oz./100 gal. of float water.

It is not uncommon to observe Ca deficiency late in the greenhouse season. Deficiency is characterized by deformed bud leaves with a “pinched” leaf tip that may be slightly chlorotic (yellow) to necrotic (brown/black) (Figure 2). In most cases, Ca reserves in the float water and in the growing media have been exhausted because of rapid growth and large plant size, or transplants have simply been in the greenhouse too long. It is also important to recognize that Ca is absorbed by the root tip, so uptake late in the seedling production season also depends on root mass. Factors that limit root growth, such as Pythium and black root rot, will also limit Ca uptake. If you are within about one week of transplanting, don’t apply fertilizer. Alternatively, if you are within a few weeks of transplanting, applying gypsum or greenhouse grade calcium nitrate (15.5-0-0) is appropriate. Note that 3.5 oz. of 15.5-0-0 per 100 gal. of float water will provide about 50 ppm Ca and 40 ppm N (as nitrate).

Figure 2. Calcium deficiency in older tobacco seedlings, as characterized by slightly recessed buds, rolled/pinched leaf tips on younger leaves, and slight necrosis of marginal tissue.

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Figure 2. Calcium deficiency in older tobacco seedlings, as characterized by slightly recessed buds, rolled/pinched leaf tips on younger leaves, and slight necrosis of marginal tissue.

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Coggins, T. E. 1993. Effect of Sodium and Bicarbonate on Tobacco Seedling Production in the Greenhouse Float System. Master of Science thesis. North Carolina State University, Raleigh, NC p. 129.

NCDA&CS. 2013. Solution Analysis for Tobacco Transplant Float Beds. Raleigh, NC: North Carolina Department of Agriculture & Consumer Services–Agronomic Division.