Methods of Hydroponic Growing

HYDROPONICS HAS BEEN VARIOUSLY DEFINED, a strict definition being “the growing of plants in an aerated nutrient solution in which all the essential mineral elements — nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn) — are included in that nutrient solution.” Modified hydroponics could be defined as “the growing of plants in an inert rooting inorganic (sand, gravel, perlite, rockwool) or organic (peat moss, pine bark, coir, sawdust) medium in which a nutrient solution is periodically added.” But for most, hydroponics is known as “a technique of growing plants without soil.”

The word “hydroponics” was initially suggested by Dr. W.A. Setchell, a faculty member at the University of California, who combined two Greek words — “hydro,” meaning water, and “ponics,” meaning work — therefore describing a system of plant growing that used “working water.” The word “hydroponics” first appeared in a February 1937 issue of the scientific journal Science (178:1), an article authored by Dr. W. F. Gericke on the subject of soilless growing of plants. The earliest books on the soilless culture of plants were published between 1920 and 1940. In a series of articles in the 1920s the world was intrigued by the concept that systems of crop production could be done without soil. With the worldwide economic depression in the 1930s and WWII this technique of growing was kept from being developed economically. However, during WW II the U.S. Army built hydroponic farms on various islands in the Pacific to supply troops fresh lettuce and tomatoes.

THE HISTORY OF HYDROPONIC GROWING

The state of the art of hydroponics up to 1985 can be found in articles published in the proceedings edited by Savage (1985), and that to 1994 in a review article by Parker (1994) and in the proceedings edited by Rorabaught (1995). More recently, Jensen (1997) published a review on hydroponics, and descriptions of the various hydroponic growing systems of the past and those currently in use today can be found in the books authored by Resh (2001) and Jones (2005).

HYDROPONIC GROWING SYSTEMS

There are basically three commercially viable hydroponic growing systems: flood-and-drain (also known as ebb-and-flow), Nutrient Film Technique (known by its acronym NFT), and the rooting of plants in either rockwool slabs or perlite-containing bags or buckets with a nutrient solution applied by drip irrigation. Each of these hydroponic systems has specifically designed applications for crop type when growing cucumber, pepper, tomato, lettuce, herbs, strawberries, and ornamental plants.

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

The flood-and-drain system consists of two components: a growing bed containing an inorganic substance, such as gravel or course sand, and a nutrient solution sump (Fig. 1). A specifically formulated nutrient solution is pumped periodically from the sump into the growing bed, flooding it for a short period of time (five to 10 min), and then the nutrient solution is allowed to drain back into the sump. This system was used by the U.S. Army during WW II to produce vegetables (mainly lettuce and tomatoes) for troops operating in the Pacific. Its commercial application followed in Florida and in other semi-tropical regions of the world (Eastwood. 1946).

Although simple in its design and function, this method is inefficient in its use of water and plant nutrient elements. Being a “closed” system1 (the nutrient solution is recirculated), repeated use of the nutrient solution can lead to disease and nutrient element imbalances. In addition, there occurs an accumulation of unused nutrient elements (referred to as “salts”) and precipitates (mainly calcium phosphate and sulfate) in the gravel or sand bed that will begin to significantly affect the nutrition of the plants. Periodic leaching of the rooting bed is needed to remove accumulated “salts” and then, after a period of use, the growing medium itself must be replaced or thoroughly washed clean to remove the precipitants.

Fischer et al. (1990) have described an intensive tomato production system using the flood-and-drain technique. A tomato plant is grown in a large rockwool block that is placed on a table periodically flooded with nutrient solution. This single-truss system has been described by Giacomelli et al. (1993) and Roberts and Specca (1997).

Initially the flood-and-drain method (or modifications of the method) was the hydroponic procedure in use from the late 1930s into the 1950s (Eastwood, 1946; Resh, 2001; Jones, 2005). This method is still in use today, primarily in hobby-type growing systems (van Patten, 1994), but its commercial application has been abandoned.

2. NUTRIENT FILM TECHNIQUE (NFT)

In the early 1970s Allan Cooper (1972) introduced his nutrient film technique (NFT), which changed the basic concept of hydroponic growing. His system was relatively inexpensive to install and maintain and was quite precise in its control of the nutrient-root environment as is described in Cooper’s most recent book (Cooper, 1996). For tomato, a rockwool cube in which a young plant has been germinated is set in a sloping trough (or channel) of flowing nutrient solution. One type of trough consists of a plastic sheet that is pulled up over the plant-containing cube, enclosing it in a pyramid-shaped tent (Fig. 2).

But the technique was found to have a major flaw. As the root mass in the NFT trough increased in size, filling the trough, the flow of nutrient solution is impeded and an anaerobic condition develops, followed by the partial death of roots within the root mass. Altering the design of the trough to a “W” configuration significantly changed the potential for root clogging of the trough and the incidence of root death, but the technique still presented problems maintaining a suitable environment for best root function and plant growth.

The slope and length of the trough, as well as the rate of nutrient solution flow down the trough, can have a significant effect on a plant, depending on its position at the head or the foot of the trough. The article by Smith (2004) on the short history of the NFT gully (trough or channel) construction makes for interesting reading.

Being a “closed” hydroponic system1, the returned nutrient solution has to be treated to restore its original nutrient element composition and to remove any foreign substances, and be sterilized to kill disease organisms. Some NFT systems are designed so that air, or even pure oxygen (O2) gas, can be bubbled into the nutrient solution to increase its oxygen content.

The NFT technique works best for lettuce, a short-term crop (40 to 50 days) because plants are ready to harvest before their root mass fills the trough.

3. ROOTING MEDIA-DRIP IRRIGATION

With the introduction of drip irrigation combined with fertilizer injector systems, the placement of water and/or a nutrient solution at the base of the plant on a regulated basis became possible. With this system of water/nutrient solution control and delivery, the use of either rockwool slabs (Resh, 2001; Jones, 2005), or perlite-containing bags or buckets (Day, 1991; Jones, 2005), became possible. These two growing media systems are today the primary hydroponic growing systems used for the production of greenhouse peppers, cucumbers, and tomatoes, as well as some ornamental plants. Operated as an “open” nutrient solution system2, the amount of water/nutrient solution delivered to the plant can be regulated to provide just what is needed.


ROCKWOOL SLAB DRIP IRRIGATION GROWING SYSTEM

The rooting medium is a large (3 x 8 x 36-in.) plastic encased rockwool slab (Fig. 3). Rockwool3 has excellent water-holding and aeration characteristics, making it a very desirable rooting medium. Small holes or cuts are made along the base of the plastic envelope, which allows excess nutrient solution to flow from the envelope while keeping a small depth of nutrient solution in the bottom. The nutrient solution or water is delivered at the base of the plant, rooted in a rockwool block, with sufficient flow so that the solution will flow into the slab. The nutrient solution in the rockwool slab is monitored for its EC, and when the EC reaches a certain level the rockwool is leached with pure water applied through the drip irrigation system. Therefore, an environmentally acceptable means of disposal of the effluent from the slab is needed.

The management of the growing system, in terms of nutrient solution composition and the frequency and amount of nutrient solution delivered to the plant, is based on environmental conditions such as air temperature, light intensity, and stage of plant growth. Details on the use of this system of hydroponic growing have been provided by Papadopoulos (1991), Resh (2001), and Jones (2005). A rockwool slab can be reused several times and then it must be discarded. For the hydroponic growing of tomatoes, rockwool is the most widely used rooting medium worldwide (Fig.4) (see www.grodan.com).


PERLITE BAG DRIP IRRIGATION GROWING SYSTEM

The rooting medium is perlite4 placed in a plastic bag of about the same dimensions as the rockwool slab described above. The use of these perlite-filled bags for tomato plant growing is as follows. A tomato seed is germinated in either a rockwool or Oasis® cube. 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. Small holes or cuts are made along the base of the plastic bag, which allows excess nutrient solution to flow from the bag while keeping a small depth of nutrient solution in the bottom. The operating procedures described for the rockwool slab method are the same. The perlite in the bag can be used to produce two crops and then must be discarded.

Details on the use of this system of hydroponic tomato growing have been described by Brentlinger (1992), Gerhart and Gerhart (1992), Resh (2001), and Jones (2005). Also, a detailed description of this hydroponic method of tomato production can be obtained from CropKing (www.cropking.com).


PERLITE BATO BUCKET DRIP IRRIGATION GROWING SYSTEM

The perlite-containing bag is being replaced by a perlite-containing BATO Bucket. The BATO bucket is a sturdy reuseable container that has a reservoir in its base (Fig. 6). The base of the bucket is so designed that the overflow opening in the base of the bucket can be attached to a nutrient solution drain line for the easy collection of effluent. The same general operational procedures are used as with perlite-containing bags. BATO buckets planted to tomatoes in the greenhouse are shown in Figure 7.

OTHER HYDROPONIC GROWING SYSTEMS

There are two other hydroponic growing systems that can be used to grow plants, though neither is well suited for commercial application.


STANDING-AERATED HYDROPONIC GROWING METHOD

In this hydroponic growing system, the plant roots are suspended in a nutrient solution that is being continuously aerated (Fig. 8). This method of plant growing is primarily for use in plant nutritional studies because the composition of the nutrient solution can be easily manipulated. Although the standing-aerated system is not suitable for large-scale commercial production of plants, it has been used for the production of lettuce and herbs, with the plants set in openings in Styrofoam sheets floating on agitated nutrient solution (Morgan, 2002).


AEROPONIC HYDROPONIC GROWING METHOD

In an aeroponic hydroponic growing system, the plant roots are suspended in a fine mist of nutrient solution that is applied on a continuous or intermittent basis. Aeroponic growing systems have been described by Soffer (1985, 1988), and commercial details on the method has been given by Adi limited (1982); however, the aeroponic technique has yet to be found economically suitable for the large-scale production of plants (Chow, 2004; Morgan, 2005). Recently, a new home-use growing system, the AeroGarden (see: www.aerogrow.com, and Alexander, 2007) has been introduced that employs the aeroponic method.


ROOTING MEDIA

Researchers and growers are experimenting with other rooting media because rockwool has a significant disposal problem, even though methods are being explored for refurbishing used slabs. Expended perlite can be added to soilless mixes or disposed of by mixing with soil. Coconut fibre (coir), a relatively new rooting medium, is now available in blocks and slabs for use like rockwool blocks and slabs (Morgan, 2003). Various other substances, such as composted bark, sawdust, and rice hulls, have been used in place of perlite and rockwool with varying degrees of success. Some of the physical and chemical characteristics of hydroponic substrates (rooting media) are given in Table 1.

A new potential rooting medium, Fytocell, has a number of desirable characteristics and is available in slabs as replacements for rockwool, and as loose particles in 50- and 100-L bags for use in BATO buckets. Fytocell is of interest because it is biodegradable, whereas rockwool and perlite are not.


SUMMARY

Although both rockwool and perlite hydroponic growing systems are in wide use, these systems of growing are being studied and modified to make them more efficient in their use of water and nutrient elements, as well as being made adaptable to varying growing environmental conditions (i.e., adaptable to conditions in space, extreme environments, outdoor applications).

REFERENCES:

Adi Limted (1982) Aeroponics in Israel. HortSci. 17(2):137.

Brentlinger, D. (1992) Tomatoes in Perlite: A Simplified Hydroponic System. Amer. Veg. Grower 40:51–52.

Chow, K.K. (2004) A New Frontier for Hydroponics. The Growing Edge 16(1):72–75.

Cooper, A. (1976). Nutrient Film Technique for Growing Crops. Grower Books, London, England.

Cooper, A. (1996) The ABC of NFT Nutrient Film Technique. Casper Publications, Narrabeen, Australia.

Day, D. (1991). Growing in Perlite. Grower Digest 12, Grower Books, London, England.

Eastwood, T. (1946) Soilless Growth of Plants. Reinhold Publishing, New York, NY.

Fischer, D.F., G.E. Giacomelli, and H.W. Janes (1990) A System of Intensive Tomato Production Using Ebb-and-Flow Benches. Prof. Hort. 4:99–106.

Gerhart, H.A. and R.C. Gerhart (1992) Commercial Vegetable Production in a Perlite System. In: D. Schact (Ed.). Proceedings of the 13th Annual Conference on Hydroponics. Hydroponic Society of America, San Ramon, CA., pp. 35–38.

Giacomelli, G.E., K.C. King, and D.R. Mears (1993) Design of a Single Truss Tomato Production System (STTPS). Symposium on New Cultivation Systems, Cagliari, Italy.

Jensen, M.N. (1997) Hydroponics. HortScience 32(6):1018–1021.

Jones Jr., J.B. (2005) Hydroponics: A Practical Guide for the Soilless Grower. CRC Press, Boca Raton, FL.

Morgan, L. (2002) Raft System Specifics. The Growing Edge 14(2):26–38.

Morgan. L. (2003) Hydroponic Substrates. The Growing Edge 15(2):54–66.

Morgan, L. (2005). Build-It-Yourself Hobby Systems: Drip and Aeroponic Systems. The Growing Edge 16(4):46–53.

Papadopoulos, A.P. (1991) Growing Greenhouse Tomatoes in Soils and in Soilless Media. Agricultural Canada Publications 1865/E. Communications Branch, Agriculture Canada, Ottawa, ON, Canada.

Parker, D. (Ed.) (1994) The Best of the Growing Edge. New Moon Publishing Company, Corvallis, OR.

Resh, H.M. (2001) Hydroponic Food Production, 6th Edition. Newconcept Press, Mahwah, NJ.

Roberts, W.J. and D. Specca (1997) The Barlington County Research and Development Greenhouse. In: R. Wijnarajah (Ed.). Proceedings of the 18th Annual Conference on Hydroponics. Hydroponic Society of America, San Ramon, CA. pp. 19–27.

Rorabaught, P.A. (1995) A Brief and Practical Trek Through the World of Hydroponics. In: M. Bates (Ed.). Proceedings of the 16th Annual Conference on Hydroponics. Hydroponic Society of America, San Ramon, CA. pp. 7–14.

Savage, A.J. (Ed.) (1985) Hydroponics Worldwide: State of the Art in Soilless Crop Production. International Center for Special Studies, Honolulu, HI.

Smith, B. (1994) The Short History of NFT Gully Design. The Growing Edge 15(3):79–82.

Soffer, H. (1985) Israel: Current Research and Developments. In: A.J. Savage (Ed.). Hydroponics Worldwide: State of the Art in Soilless Crop Production. International Center for Special Studies, Honolulu, HI. pp. 123–130.

Soffer, H. (1988) Research on Aero-Hydroponics. Proceedings of the 9th Annual Conference on Hydroponics. Hydroponic Society of America, Concord, CA. pp. 69–74.

Van Patten, (1992) Hydroponics For the Rest of Us. The Growing Edge 3(3):24–33, 48–51.


DEFINITIONS:

Electrical Conductivity (EC): a measure of the electrical resistance of water, nutrient solution, or effluent from a rooting medium, used to determine the level of ions in solution and the potential effect of ion concentration on plant growth; the units commonly used are either millimho per centimetre (mmho/cm) or decisiemen per metre (dS/m).


ENDNOTES:

1 designates a hydroponic growing system in which the nutrient solution, after passing through the rooting medium, is recovered and reused.

2 designates a hydroponic growing system in which the nutrient solution, after passing through the rooting medium, is discarded.

3 inert fibrous material produced from a mixture of volcanic rock, limestone, and coke; melted at 1500ºC to 2000ºC; extruded as fine fibres; and then pressed into loosely woven sheets. The sheets are made into slabs 16 to 18 in. (15 to 46 cm) wide, normally 36 in. (91 cm) long, and 3 to 4 in. (5 to 10 cm) deep.

4 natural aluminosilicate mineral of volcanic origin that, when crushed and heated rapidly to 1000ºC, forms a white, lightweight aggregate with a closed cellular structure.