HYDROPONIC TOMATO GROWING

HISTORY

Hydroponics as a potential commercial plant growing technique was understudy by researchers in the late 1920s.   The word “hydroponics” first appeared in print in a scientific journal article in 1937, authored by Dr. W.F. Gericke.   He combined two Greek words, “hydro” meaning water and “ponos” meaning labor, the combination becoming “working water.”   Although the public fascination with this proposed method of growing plants was high at the time, its commercial application did not follow.   Possibly the economic depression in the United States and then the starting of WWII were mitigating factors.   However, the U.S. Army put hydroponics to good use during WWII in outposts, mainly in the Pacific, in order to provide, mainly fresh lettuce and tomatoes, to troops operating in these areas of combat.

Following WWII, hydroponically-grown tomatoes were commercially produced in southern Florida and on some Caribbean Islands; however, these ventures did not succeed financially and soon disappeared.   It was not until the late 1980s when hydroponics began to be used for the production of tomatoes as well as others crops in enclosed shelters and greenhouses.   The objective was to produce tomatoes in the off season when field-grown fruit was unavailable.   The initial hydroponic greenhouses devoted to the production of off-season tomatoes were in The Netherlands, followed by similar types of greenhouses in England.   Today, hydroponic tomato greenhouses are found in other countries, the largest number in Canada, the United States, Mexico, and Spain.

HYDROPONIC GROWING TECHNIQUES

There are basically three hydroponic growing systems that have been or are being used to grow tomatoes commercially. Initially, the ebb-and-flow method (or modifications of the technique) was the method in use from the late 1930s into the 1950s. In the mid-1970s, Allan Cooper introduced his nutrient film technique (NFT) that substantially changed the basic concept of hydroponic growing.   The system is relatively inexpensive to install and maintain, and is quite precise in its control of the nutrient-root environment.

With the introduction of drip irrigation combined with fertilizer injector systems, placement of water and/or a nutrient solution at the base of the tomato plant on a regulated basis became possible. With this type of water-nutrient solution delivery system, rockwool slabs and perlite in either bags or buckets as the rooting media have come into wide use.

All of the commonly used hydroponic techniques have flaws that have to be dealt with.   The “ideal” hydroponic growing system has yet to be developed, although initially the NFT method was thought to be the one that would come closest to being “ideal”.

EBB-AND-FLOW (FLOOD-AND-DRAIN)

The ebb-and-flow system (sometimes known as flood-and-drain) consists of a water-tight growing bed (containing either gravel or/and sand) and a nutrient solution supply tank. The nutrient solution is pumped periodically from the supply tank into the growing bed, flooding it for a short period of time (5-10 min).   By placing the nutrient solution supply tank below the growing bed, the nutrient solution is then allowed to drain back into the supply tank. The system was widely used by the U.S. Army during World War 11 to produce vegetables, mainly lettuce and tomato, for troops operating in the Pacific, followed by its commercial application in Florida and in some semi- and tropical regions.   However, the ebb-and-flow system is little used today other than in hobby-type hydroponic growing systems. The method is very inefficient in its use of water and plant nutrient elements. Repeated use of the nutrient solution can lead to disease and nutrient element imbalances.   Therefore after each use, the nutrient solution needs to be reconstituted, filtered, and sterilized. In addition, periodic replacement of the nutrient solution and then the growing medium is required, both procedures being wasteful and expensive.

An unique application of the ebb-and-flow technique in which a tomato plant is rooted in a large rockwool block placed on a table periodically flooded with nutrient solution and the fruit harvested from only the first truss is being tested with some success.

NUTRIENT FILM TECHNIQUE (NFT)

A rockwool cube in which a young tomato plant has been germinated is set in a sloping trough of flowing nutrient solution. The trough usually consists of a plastic sheet that is pulled up over the cube, enclosing it in a pyramid-shaped trough. The slope of the trough and the rate of nutrient solution flow down the trough can have a significant effect on the plant depending on their position in the trough, whether at the head or foot. As roots fill the trough, the flow of nutrient solution down the sloping trough becomes restricted, with the flow going either over the top of the root mass or down the sides of the plastic trough rather than through the root mass. The center of the root mass can then become anaerobic and roots will begin to die from lack of sufficient oxygen.   The NFT system is inefficient in its use of water and plant nutrient elements, and with recirculation of the nutrient solution, disease and nutritional problems can easily occur.   Altering the design of the trough to a “W” configuration significantly changed the potential for root clogging of the channel, but the technique still presented problems in maintaining a suitable root environment for best plant growth and development.

The NFT method initially attracted considerable attention. However, when put into commercial use, the technique was found to have significant flaws that impacted its long-term use.

Rockwool (an inert fibrous material produced from a mixture of volcanic rock, limestone, and coke melted at 1500 to 2000˚C extruded as fine fibers and pressed into loosely woven sheets) has excellent water-holding and aeration characteristics, making it a very desirable rooting medium. A typical rockwool slab (0.3 x 0.8 x 3.6 meters) is encased in plastic. Small holes or cuts are made along the base of the plastic casing that allows excess nutrient solution to flow from the slab while keeping a small depth of nutrient solution in the bottom of the plastic casing.

This method of growing is as follows: a tomato seed is germinated in a small rockwool cube; and when the tomato seedling has initiated true leaves, the cube is placed into a larger rockwool block. When the plant roots are about to emerge from the base of the block, the rockwool block is placed on an opening in the rockwool slab.   The nutrient solution or water is delivered at the base of the plant in the rockwool block with sufficient volume so that the solution will flow into the rockwool slab. The management of the growing system in terms of nutrient solution composition, and frequency and amount delivered to the plant is based on environmental factors, such as temperature and light conditions, and plant status, such as size and stage of growth.

The nutrient solution that accumulates in the slab must be periodically monitored for its electrical conductivity (EC), and when reaching a certain level, the slab is leached with “pure” water to remove accumulated salts, with the leaching water being applied through the drip irrigation system. Therefore, an environmentally acceptable means of disposal of the effluent from the slab is needed. A rockwool slab can be reused several times and then discarded. In the Colorado greenhouses, the rockwool slabs are replaced on a schedule of 16-18 months. Currently for the hydroponic growing of tomato, rockwool is the preferred rooting medium worldwide.

PERLITE BAG DRIP IRRIGATION

Perlite (a natural substance crushed and heated to 1000˚C forming a white, lightweight aggregate with a closed cellular structure) is placed in a plastic bag of about the same dimensions as a rockwool slab. Small holes or cuts are made along the base of the plastic bag that allow excess nutrient solution to flow from the bag while keeping a small depth of nutrient solution in the bottom of the bag.  

A tomato seed is germinated in either a rockwool or germination cube; and when the tomato plant has true leaves, the cube is placed into either a larger rockwool block or a cup containing either perlite or rockwool. When the roots are about to emerge from the base of the block or cup, the plant is placed into an opening in the perlite bag.

The nutrient solution or water is delivered to the base of the plant in the rockwool block or cup by means of a drip irrigation system. The composition of the nutrient solution and its schedule for delivery are based on environmental conditions and plant growth stage as described for the rockwool slab method. The nutrient solution in the perlite bag is monitored for its EC, and when the EC reaches a certain level, the perlite bag is leached with pure water applied through the drip irrigation system. Therefore, an environmentally acceptable means of disposal of the effluent from the perlite bags is needed. The perlite in the bag can be used to produce two crops and then must be discarded.

Today, perlite-containing BATO buckets are replacing the perlite-containing bag. There is a siphon in its base of the bucket that fits into a PVC drain line, allowing easy collection of nutrient solution or water overflow.   The same cultural and management procedures used with the perlite bag system is used with BATO buckets.        

In this hydroponic growing system, the plant roots are suspended in a continuously aerated nutrient solution.   However, the standing-aerated system is not suitable for the large-scale commercial production of plants, being primarily used for plant nutrition studies since the composition of the nutrient solution can be easily manipulated.

In an aeroponic system, the plant roots are suspended in a fine mist of nutrient solution that is applied on a continuous or intermittent basis. The aeroponic technique is not suitable for the growing of tomato plants.

Of the 16 essential elements required by plants, 6 are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) known as the major elements.   Another 7 are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn), known as the micronutrients.   All the major elements and micronutrients must be present at specific concentrations in the nutrient solution in order to sustain normal growth. The composition of a nutrient solution can be adjusted based on the method of growing, stage of plant growth, current environmental conditions, and frequency of nutrient solution delivery. Most hydroponic nutrient solution formulations are based on Hoagland's and Aron's (1950 California Experiment Station Circular 347) original formula as given below:

Hoagland's and Aron's Nutrient Solution Formulas

For Solution No. l : 0.5% iron ammonium citrate.   To use: 1 mg/L
For Solution No. 2: 0.5% iron chelate (FeDPTA). To use: 2 mL/L                              

The nutrient solution when initially made should be analyzed by a competent laboratory to ensure that all the elements are present at their desired concentration. If an injection system is being used to dispense a nutrient solution concentrate, the composition of the nutrient solution being delivered through the drip irrigation system should also be monitored (assayed) periodically for the same purpose of ensuring proper concentration is being delivered.

The optimum and acceptable range in element composition is:

Major and Micronutrient Ionic Forms and Normal Concentration Ranges in a Nutrient Solution

aThe form depends on the pH of the nutrient solution.

bIt is being increasingly suggested that boron exists in the nutrient solution as molecular H3BO3 .

In both the rockwool slab drip irrigation and perlite bag/bucket drip irrigation systems, the solution in the slab, bag or bucket must be monitored due to the accumulation of salts that occurs with time in both rockwool and perlite, an accumulation that can eventually reduce plant growth. The procedure is to draw an aliquot of the solution from the slab, bag or bucket and measure its EC. When the EC exceeds a predetermined level, the rockwool or perlite is then leached with pure water through the drip irrigation system.

The challenge for the hydroponic grower is to maintain the nutrient element status of the tomato plant to keep it productive over an extended period of time. The initial composition of the nutrient solution, its rate of delivery, and adjustment in composition with both the changing status of the plant and environmental conditions are required management practices.

The influence of stage of plant growth is also a factor in determining what the elemental concentration ranges should be. As the stage of growth advances, there is an increase in the N, K, and Mg concentrations, while the other elements remain at constant concentration.

If a nutrient solution is recirculated, in addition to its composition being maintained, any accumulated organic material from the plant roots must be removed by filtering and the solution sterilized.   Sand-containing filters will remove many suspended materials while cartridge-type filters will remove molecular-sized substances, providing some degree of disease control. For sterilization, placing two 16-watt UV lamps in the path of the flowing nutrient solution at 13 L (3 gal/min) is effective as well as the use of ozone treatment.