Basics of Tobacco Plug Growing

Please Note: Basics of Tobacco Plug Growing was written in 1985 for growers who were growing their direct seed tobacco plants in black plastic trays using over head irrigation. The direct seed float system was not in existence until 1989. Read “History of Transplant Systems” if interested in how the direct seed float system came about. We have had many requests for this publication and decided to place it on our site. The information in this publication is relevant to the float system as well.

 

Basics of Tobacco Plug Growing©1985

By Ray DeBruhl, Agronomist

 

Single cell plant production in a controlled environment is mechanizing the last phase of tobacco production. Plug production provides many advantages not found in conventional plant bed production methods. Mechanization has made the process of producing tobacco more efficient with the exception of the transplant producing phase of the process. Now plug production or single cell plant production opens the door to mechanizing this last but most important part of growing tobacco.

Transplants produced in plug growing systems are more uniform in growth and in age than bare root plants. This results in a more uniform crop with less premature flowering and more uniform growth. This uniform growth simplifies topping, sucker control, harvesting and management of the crop. Couple these advantages with the fact that we can partially mechanize the transplanting of the crop, you can reduce your labor costs considerably.

There are several important considerations a grower needs to think about before embarking on a plug growing program. Transplant Systems has made significant progress in the past few years in plug production of tobacco plants. It is the purpose of this growing manual to discuss the cultural requirements needed for growing your tobacco plants in a plug production system.

Germination Culture

Uniform germination is one of the many obstacles facing the greenhouse tobacco plant grower, Your success or failure in achieving good uniform germination depends on maintaining optimum levels of temperature and moisture.

There are three variables that successful seed germination is dependent upon and they are, soil temperature, moisture, and light. While lighting is not necessary for germination, it is essential, it is essential for the emergence of the radical and hypocotyls which together is essential for subsequent seedling growth.

Good germination is directly related to moisture applied to the seed. Too little moisture will give uneven germination and can result in desiccation of the seed. Too much moisture on the other hand displaces the oxygen in the soil mix and also leads to death of the seed. The initial wetting should wet the plug cell completely through. Proof of complete “wet through” will be water drops on the bottom of the tray.

Remember that the seed is on top of the mix and little soil around it to actually supply it with constant moisture. It is for this reason that several passes with the irrigator are needed during the day to maintain the proper moisture around the seed.

During the germination period it is necessary to operate the irrigator on a faster speed and irrigate more often. Remember the moisture needed for germination is being supplied more from the irrigator than from the soil.

Soil temperature is very important for germination. Ideally we would like to maintain 72 degrees soil temperature. However this is not always possible with our overhead heating systems. This is not to say that tobacco seed will not germinate at temperatures under 72 degrees. You will realize germination at much lower temperatures. However, if we achieve a constant 72 degrees soil temperature, we would benefit by a rapid uniform germination over a shorter period of time. This of course would result in having more uniform plants and a lower fuel bill. Heating the soil by heating the air overhead means we have to run high ambient air temperatures. Bottom heat such as root zone heating can provide the proper soil temperature. Some growers have reported 30% overall energy savings with root zone heating versus overhead heating.

With the research conducted by Rutgers University, Transplant Systems has developed a bottom heat system that is very effective in achieving rapid and uniform germination of tobacco plants. Germination will begin in 3 to 4 days with bottom heat. With overhead heat germination begins on the seventh or eight day.

Favorable soil temperature are easier to maintain if irrigation water is tempered to 75 to 80 degrees.

A practice that has been proven to help germination is to place a “Reemay” type material over the trays once you have wet them through. The temperature under the Reemay will be 4-6 degrees warmer than the temperature directly above it. The reason for this is that the Reemay reduces the amount of moisture evaporation, which has a cooling effect on the soil if left unchecked. Remove the cover by the tenth day. The stems will elongate if left on longer. If a seed bed cover is used, it is best to use one without a hem, as the hem creates dry spots. Also, if you are using a cover be sure your irrigations are penetrating the cover.

Germination Medium

Selecting a suitable germination medium is very important in successful plug growing. While plug cells the size of 162’s do not present as much a problem as 512’s still there are considerations to be met. Problems can arise from moisture content, aeration, ph, soluble salts and nutrient levels. For this reason a desirable soilless medium should have a high buffer capacity, meaning the mix should resist abrupt changes in ph, have a high cation exchange capacity, a high water holding capacity, and a broad particle size distribution to insure proper drainage.

Most commercial mixes usually contain a nutrient charge and it is important to test the mix to determine its nutrient content. The best ph range at the start of the germination period is 5.0 to 6.0.

The biggest differences between different brands of mix will be in air porosity. A mix that has good particle distribution will have good air porosity in the media. A mix that does not have a good variation in particle size may result in poor germination and poor quality seedlings. If the physical properties of a mix are such that they are similar in size, they will likely compact when you water thus preventing sufficient drainage. The germination will be poor due to the lack of oxygen in the water saturated medium.

Water Quality

 

A major factor in successful production of greenhouse tobacco plants is the relationship between plant nutrition and water quality. The most important step in setting up and properly managing a fertility program is the analysis of your water quality. The reason for this is because the presence and concentration of soluble elemental ions can have a dramatic impact on plant growth. Because the soil media does not contain any soil, the proper management of calcium and magnesium is of major concern.

In a soilless media the calcium and magnesium admendments can become depleted, especially where the levels of calcium and magnesium in the water is low. A leading cause of calcium and magnesium deficiencies in a soilless medium is irrigation water that is too pure. Pure water is characterized by having low soluble salt levels.

GUIDELINES FOR IRRIGATION WATER

PARAMETERS

LEVELS IN PURE H2O

ACCEPTABLE RANGE

HIGH LEVELS

*SOLUBLE SALTS

UNDER 25

0.3 – 1.0

OVER 1.3

**CALCIUM

UNDER 20

40 – 75

OVER 100

**MAGNESIUM

UNDER 15

30 – 50

OVER 50

*Measured in millimhos per centimeter

       ** Measured in parts per million

Pure water is more of a problem in long term crops (greater than 3 months) but can be a problem in shorter crops such as direct seeded tobacco plants.

Often times visual deficiency symptoms are not observed but there may be a lack of strong vigorous plant growth. Sometimes the soil mix gets blamed for a nutritional problem, when in fact the problem lies elsewhere.

The use of soilless mixes has been a major horticultural development in recent years. Their development has simplified nutritional management and has virtually eliminated plant pathogens in the potting media.

Soilless mixes differ in their ability to provide calcium and magnesium because of the quality of their components and liming source. Different water sources also vary in their ability to supply supplemental calcium and magnesium. The only way to assess the nutritional contribution of your water is to have it analyzed.

CALCIUM MANAGEMENT

If the calcium level is less than 25 ppm (parts per million) it is often necessary to supplement feed with a calcium containing fertilizer. If calcium is needed it is desirable to use calcium nitrate since the calcium would be readily available. Do not add calcium fertilizers to stock solutions containing phosphorus or Epsom salts (magnesium sulfate) as precipitates may form.

High calcium levels, those above 100 are a good sign of high alkalinity levels. This often occurs where there is limestone bed rock which consists of calcium carbonate. More on this later.

MAGNESIUM MANAGEMENT

Low magnesium levels can also be troublesome. If magnesium levels drop below 15 ppm it is necessary to supplement feed with magnesium sulfate (Epsom salt). It is desirable to have the magnesium level of our water between 30 to 50 ppm. Do not add Epsom salts to water soluble fertilizers containing calcium.

Generally speaking if your magnesium levels are low adding Epsom salts at the rate of 15 ppm on a constant feed basis is sufficient. Do not exceed 50 to 60 ppm for constant feed. While low levels cause more problems than high levels, high levels can cause problems in enhancing the deficiencies of other elements such as potassium and calcium.

CALCIUM AND MAGNESIUM BALANCE

A good fertility program should not only supply an adequate amount of essential nutrients but they must also be in balance. Some nutrients compete against each other for uptake. If one element is much more available than another a deficiency in the less available element may occur.

The calcium and magnesium ratio is very important in a successful fertility program. The optimum ratio of calcium to magnesium is 2:1. It is acceptable from 3:1 to 1:1.

If sodium exceeds the calcium and magnesium levels it can reduce the uptake of calcium and magnesium. Sodium levels above 50 ppm should be watched closely. Sodium is often used in city water systems.

A final word on calcium and magnesium levels, the move from field soil mixes of the past to the soilless organic mixes of today has caused the need to provide certain major and trace elements. The old field soil mixes of the past provided sufficient residual trace and major nutrients and growers did not need to be concerned about them. Growers today using modern soilless mixes must be concerned about providing trace elements in a soluble form as part of a complete soluble fertilization program.

Because of the reduced amount of limestone which is being used in modern organic mixes and the precipitation problems associated with formulating soluble fertilizers that contain both phosphorus and calcium or magnesium, low calcium and magnesium levels seem to be more prevalent.

WATER PH & ALKALINITY

Water that has low soluble salt and low calcium and magnesium will generally have low alkalinity levels also. Irrigating with water with these characteristics in a constant feed fertilization program can lower the growing media PH. This will increase the solubility of calcium and magnesium and cause them to leach out more readily thus further aggravating the problem of low calcium and magnesium levels.

The more common problem is with alkaline water, that is, water with a high pH.

Before we go any further let’s stop here and differentiate between ph and alkalinity. Alkalinity is a chemical factor which is in a manner, related to pH but in itself is a different parameter.

We should not confuse the term “alkalinity” with the term alkaline. Alkaline describes situations where pH levels are above 7.0. Just as acidic describes situations where pH levels are below 7.0. “Basic” would be a better term to describe alkaline levels or high pH levels. Water alkalinity is a measure of a water’s capacity to neutralize acids. It is expressed chemically as milligrams per liter of calcium carbonate equivalents (mg/L CaC03).

Allow me to stop here and relate something very important to you. You may think this manual has gone too deep and you have no interest in water alkalinity or milligrams per liter of calcium carbonate equivalents. Then let me assure you of this, the mere success or failure of your growing tobacco plants may hinge on whether your water is 70mg/L of CaCO3 or 200mg/L of CaCO3.

The alkalinity of water is important and is related to pH because alkalinity establishes the buffering capacity of the water. The alkalinity affects the ability to change the water pH by adding acids.

The most common problem with high pH water with high alkalinity levels is that it can cause the soil media to rise in pH very rapidly. Tobacco plants do not live well in high pH environments.

The relationship between pH and alkalinity is very important to growers who are trying to reduce the pH of their irrigation water. The higher the alkalinity of the irrigation water the more rapidly the soil media pH will rise. And of course it is this increase in the soil media pH that lead to nutritional imbalances and consequently poor plant growth.

Water pH can be lowered if too high by injecting either nitric, sulfuric, or phosphoric acid into the water. Sulfuric acid is the easiest of the three to find , as common battery acid contains 35% sulfuric acid.

If the pH of your water must be lowered it can easily be done by adding the appropriate amount of acid to your stock fertilizer tank.

SOIL PH

The pH of your soil mix has far reaching effects on the fertility of the plant. The soil is neutral at 7.0, alkaline if above and acidic if below. The pH of the soil controls the availability of all nutrients.

As the pH decreases; iron, manganese, zinc, and copper increase in availability, whereas phosphorus, calcium, magnesium and molybdenum lessens in availability. At pH levels, above 7.0, the availability of iron, manganese, zinc, copper and boron drop to deficiency levels.

The pH range of 6.2 to 6.8 provides the greatest average level of availability for all plant nutrients. This range is acceptable for growing tobacco plants.

PLUG NUTRITION

Most greenhouse fertilization recommendations are expressed in parts per million (ppm) values. Therefore it is necessary to have an understanding of this system of measuring fertilizer concentrations. The advantage to this method is that parts per million values enable us to identify a specific concentration of fertilizer source or analysis where as concentrations based on weight measurements will differ according to the analysis of the fertilizer source.

To determine the amount of fertilizer needed in 100 gallons of water for any given part per million the following equation is used.

(desired ppm/75)/decimal fraction of the desired nutrient in the fertilizer=oz of fertilizer per 100 gal of water

In the equation we must divide the desired ppm of nitrogen by 75 and then divide the resulting answer by the decimal fraction of nitrogen contained in the fertilizer grade being used.

This formula is very helpful to growers who are mixing their fertilizer in large tanks of water and then irrigating with this reservoir of water. However a far better system of fertilizing your tobacco plants is with an injector system. This system mixes precise volumes of concentrated fertilizer solutions and water together.

With the injector system you would first select the injector ratio you will use. Referring to an injection table, let’s assume we are going to use a ratio of 1:100. We will also assume that we would like to fertilizer at the rate of 100 ppm with a 20-10-20 fertilizer. An injector table shows that for an injector ratio of 1:100 and using a 20% nitrogen analysis fertilizer, we would use a value of 6.75 oz of fertilizer per gallon of water in the concentrate tank.

If we assume we are using a 30 gallon stock fertilizer tank, then you would multiply 6.75 oz of 20-10-20 x 30 gal of water to get 200 oz or 12.5 lbs of 20-10-20 fertilizer.

If you dissolve 12.5 pounds of 20-10-20 in your 30 gallon stock tank and set your injector on 1:100, you would then be fertilizing at the rate of 120 parts per million (ppm).

The injector system devised by Transplant Systems for its customer to use in growing their tobacco plants is very simple, but first we must make some basic assumptions. First, we must assume that our water is good quality, that is the pH is 5.5 – 6.5 and that the soluble salt level of Ca and Mg is adequate.

If you need to supplement feed with a fertilizer containing calcium it will be necessary to add another 30 gallon stock fertilizer tank. You cannot mix calcium containing fertilizers with sulfates or phosphorus fertilizers in a concentrated fertilizer solutions.

CHOOSING A NITROGEN SOURCE

In plug production the fertilizer source is very important in regard to the nitrogen source. Plants in general utilize nitrate nitrogen more readily than ammoniacal forms. Ammonium toxicity can occur where a continuous supply of ammonium nitrogen has been applied.

Ammonium toxicity can result in the constant use of a fertilizer high in ammoniacal nitrogren such as 20-20-20. Our use of soilles mixes compounds the problem because the soilless mixes lack the beneficial bacteria needed to convert ammonia ion to nitrate. Low light and cold temperature aggravate the problem also. Cold temperature limit the activity of the nitrifying bacteria.

A 20-20-20 fertilizer label would read:

Total Nitrogen (N)………………………………20%

  • 5.61% Nitrate Nitrogen
  • 3.96 % Ammoniacal Nitrogen
  • 10.42% Urea Nitrogen

Available Phosphoric Acid (P2O5)…………20%

Soluble Potash (K2O)…………………………..20%

We are interested in the forms of nitrogen in the fertilizer. Plants can use nitrate nitrogen very readily. The ammoniacal and urea forms present problems. Urea is simply 2 ammonium ions attached to a carbon and oxygen and are broken to 2 ammonium ions. Therfore theammonical and urea percentages have to be added together to get the total ammonium nitrogen (3.96+10.43=14.39%) in the fertilizers. It is easy to see that the 14.39 % of the total 20% is in the ammoniacal form.

A SOILl mix  has the necessary bacteria to convert the ammonium to nitrate but SOILLESS mixes do not.

Plants grown in soilless mixes using nitrate fertilizers will have fewer problems. The level of nitrate fertilizer should read 20-10-20:

Total Nitrogen (N)……………………20%

  • 12% Nitrate Nitrogen
  • 8% Ammonium Nitrogen

Available Phosphoric Acid………..10%

Souluble Potash (K2O)………………20%

Here you have 12% of the nitrogen in the nitrate form and only 8% in the ammonium form. This fertilizer is most desireable for plug growing.

In summary growers seeking to grow their tobacco plants in a plug system should strive to follow all the basic requirements fore plug growing. Your success in greenhouse tobacco plant production remains dependent on paying attention to the details of growing as outlined in this publication.

More Content to Come………