of Greenhouse Vegetables
Hunter Johnson,Jr., George J. Hochrnuth, and Donald N. Maynard
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida
John T. Woeste. Dean
* Hunter Johnson, Jr. is Adjunct Professor, Vegetable Crops Department, Gainesville. His
permanent position is Extension Vegetable Specialist, University of California, Riverside.
George J. Hochmuth is Assistant Professor and Extension Vegetable Specialist,
Gainesville,. Donald Ni. Maynard is Professor and Extension Vegetable Specialist, Gulf
Coast Research and Education Center, Bradenton.
Soilless Culture of Greenhouse Vegetables
Hunter Johnson, Jr., George J. Hochmuth, and Donald N. Maynard
Production of field-grown fresh market vegetables is a billion dollar
industry in Florida. Because of its subtropical climate, Florida is not
generally thought of as a greenhouse vegetable producing state.
Nonetheless, there are many commercial greenhouse vegetable opera-
tions throughout the state. They range in size from a single house of a
couple of thousand square feet to operations that are over 25 acres in
size. The most commonly grown crops are tomatoes and European-type
Many of the state's commercial greenhouse vegetable enterprises
employ some form of soilless culture.in their production scheme. The
nutrient-film technique and bag culture are both commonly used in
Florida. This publication is intended as a guide for current growers and
those anticipating entering the business. For those in the latter
category, the following words of caution cannot be overemphasized.
1. Commercial production of greenhouse vegetables using soilless
culture methods is a capital-intensive, high management
agricultural enterprise. Seek advice from competent authorities
such as your local county extension agent before final plans are
2. Develop a marketing plan before committing to a soilless culture
system. This is especially necessary for one not currently in the
produce business and for one who will not have sufficient volume
to effectively market.
Soilless culture of greenhouse vegetables can be a profitable business
in Florida, but not without adequate pre-construction planning.
Justification for Soilless Culture
Almost all of the vegetables we find on grocery store shelves are pro-
duced either directly or indirectly in open-field soil. However, soil itself
isn't necessary for plant growth only some of its constituents.
Field soil serves two basic purposes: it acts as a reservoir for essential
elements and water and it provides physical support for the plant.
Artificial means can also provide these requirements for plant growth
with equal (and sometimes better) growth and yield compared to field
soil, although at substantially greater expense. Well-drained,
pathogen-free field soil of uniform texture is the least expensive
medium for plant growth, but soil doesn't always occur in this perfect
package. Some soils are poorly textured or shallow, and provide an
unsatisfactory root environment because of limited aeration and
restricted drainage. Pathogenic organisms are a common problem in
field soils. When adverse conditions are found in soil and reclamation is
impractical, some form of soilless culture may be justified.
Defining Soilless Culture
Soilless culture is an artificial means of providing plants with support
and a reservoir for nutrients and water. The simplest and oldest method
for soilless culture is a vessel of water in which inorganic chemicals are
dissolved to supply all the elements required by plants. Often called
solution culture or water culture, the method was originally termed
hydroponics (i.e., "water working") by W. F. Gericke in the 1930s. Over
the years, hydroponics has been used sporadically throughout the
world as a commercial means of growing both food and ornamental
plants. "Ibday, it is used widely in research facilities as a technique for
studying plant nutrition. Various modifications of pure solution
culture have been used. Gravel or sand is sometimes used in soilless
systems to provide plant support and retain some nutrients and water.
The retention of nutrients and water can be further improved through
the use of sphagnum peat, vermiculite, or bark chips. These are the
most commonly used materials, but others such as rice hulls, bagasse
(sugar cane refuse), sedge peat, and sawdust are used sometimes as
constituents in soilless mixes. Straw bales have been used as a growing
medium here and in England and Canada. Rockwool (porous stone
fiber) is used in Europe, and its popularity is increasing in the United
Since the major constituent of the media in artificial growing systems
may be solid or liquid, it is appropriate to use the term soilless culture in
reference to this general type of growing system and reserve the term
hydroponics for those in which water is the principal constituent.
Soilless-culture methods may thus be classified as either solid- or
Types of Soilless Culture
Liquid-medium systems are further differentiated from solid-
medium systems by method of operation. Liquid systems are generally
closed circuit with respect to nutrient-solution supply: the solution is
recirculated from a supply reservoir either continuously or intermit-
tently for a period of days or weeks. The two most common liquid
systems in use today are nutrient-flow technique (NFT) and gravel-bed
NFT growing systems consist of a series of narrow channels through
which nutrient solution is re circulated from a supply tank. A plumbing
system of plastic tubing and a submersible pump in the tank are the
basic components. The channels are generally constructed of opaque
plastic film (Figure 1) or plastic pipe (Figure 2); asphalt coated wood or
fiberglass also have been used. The basic characteristic of all NFT
systems is the shallow depth of solution that is maintained in the chan-
nels. Flow is usually continuous, but sometimes systems are operated
intermittently by supplying solution a few minutes every hour. The
purpose of intermittent flow is to assure adequate aeration of the root
systems. This also reduces the energy required; but under rapid growth
conditions, plants could experience water stress if the flow period is too
short or infrequent. Therefore, intermittent flow management seems
better adapted to mild temperature periods or to plantings during the
early stages of development. Capillary matting is sometimes used in
the bottom of NFT channels, principally to avoid the side-to-side
meandering of the solution stream around young root systems, but it
also acts as a reservoir by retaining nutrients and water during periods
when flow ceases.
NFT channels are frequently designed for a single row of plants with
a channel width of 6 to 8 inches. Wider channels of 12 to 15 inches have
been used to accommodate two rows of plants, but meandering of the
shallow solution stream becomes a greater problem with greater width.
'Ib minimize this problem, small dams can be created at intervals down
the channel by placing thin wooden sticks crossways in the stream, or
the channel may be lined with capillary matting. The channels should
be sloped 4 to 6 inches per 100 feet to maintain gravity flow of the solu-
tion. Flow rate into the channels should be in the range of 1 to 2 quarts
per minute. Channel length should be limited to a maximum of 100 feet
in order to minimize increased solution temperature on bright days.
The ideal solution temperature for tomatoes is 68o to 77" E.
Temperatures below 59' or above 86' F decrease plant growth and
tomato yield. Channels of black plastic film increase solution
Figure l. NFT culture using plastic film to hold plants and supply nutrients
through a recirculation systern.
temperature on sunny days. During cloudy weather, it may be
necessary to heat the solution to the recommended temperature. Solu-
tion temperatures in black plastic channels can be decreased by
shading or painting the surfaces white or silver. As an alternate to
channels lined with black polyethylene, 4 to 6 inch PVC pipe may be
IW "W KalIP
Figure 2. NFT culture using PVC pipe to hold plants and supply nutrients
through a recirculation system. Note heat delivery to a tube at root level
between PVC pipes in a double-row system.
Figure 3. Tomato roots in NFT showing rock wool block in which seedlings
used (Figure 3). Plant holes are spaced appropriately along the pipe.
The PVC system is permanent once it is constructed compared to the
polyethylene-lined channels which must be replaced for each crop. On
the other hand, initial costs are higher for the PVC and sanitation be-
tween crops may be more difficult. Cooper (1979) provides greater detail
on NFT management.
Gravel bed culture utilizes a waterproof trough filled with pea gravel
(or some other inert material of similar size), which is plumbed to a
nutrient solution reservoir (Figure 4). Gravel particles retain very little
water and nutrients, so the system must recirculate solution from the
supply tank to the beds several times per day with a time clock and
submersible pump. Some gravel systems are designed to be supplied
from the surface through perforated pipes, and drained at the base of
the trough through a slitted drain line; others are both subirrigated and
drained through a single pipe at the bottom of the bed. The advantage to
the two pipe system is that any root growth into the drain line will not
interfere with the uniform distribution of nutrient solution to the bed.
In either case, however, root growth will eventually clog the drain line
and rotary cleaning equipment must be used to remove it.
Gravel bed troughs are generally 24 to 36 inches wide and 8 inches
deep. Pea gravel must be thoroughly washed before use to remove par-
ticles of soil or other material that might clog the drain line. Care
should be used in selecting a gravel supply that is free from pathogenic
organisms. 'I1-eatment of the gravel by steam sterilization or an
/ SOIL SURFACE
FILL & DRAIN LINE
PUMP % 2 '
V SHAPED TROUGH
Figure 4. Hydroponic culture in a gravel-f illed trough.
appropriate fungicide is a wise practice when the condition of the
material is uncertain.
The nutrient solution supply tank should be large enough to hold a
volume of solution about twice that required to fill the beds; this pro-
vides a good margin of safety. The plumbing system's lines and sub-
mersible pumps should have the capacity to fill the beds in about 15
minutes, and allow complete drainage in 30 to 45 minutes.
When managed properly, NFT and gravel bed systems are capable of
growing good crops, but there are some disadvantages that should be
taken into consideration. The nutrient concentration of the recir-
culated nutrient solution is in a continuous state of change because
plants are removing elements at different rates. Therefore, some
means of monitoring and replenishing must be used to avoid deficien-
cies (and perhaps toxicities from excesses of some elements). This
increases the cost of equipment and laboratory analysis. Recirculated
systems are power dependent. If electrical energy is disrupted, there is
little reservoir of water and nutrients to protect the plants from stress.
Recirculation of the solution is an ideal means of spreading any
pathogenic organism (such as water mold fungi) which may be
inadvertently introduced to the system. For these reasons, more
management care, experience, and capital will be necessary for success
with recirculated liquid-medium systems.
Solid-medium soilless culture may employ any one of several suitable
media in various types of containers. Basic requirements are a material
of uniform texture that drains well yet retains some nutrients and
water, a container in which the material is confined, and a means of
supplying nutrient solution. A well-drained sandy loam could be used
as a growing medium, but the supply of very uniform soil in the volume
required may be difficult to find, and the weight of the soil is much
greater than other types of material. Sand has been used in soilless
systems in which the entire floor of the greenhouse is filled a foot or
more in depth, but it is rarely used in container systems because of its
Full-floor sand culture has been successful for vegetable culture in
greenhouses and is considered a good means of providing plants with a
uniform, well-drained rooting medium (Figure 5). Installation requires
excavation of the greenhouse floor to the intended fill depth, and
grading (about 4 inches per 100 feet) for drainage. First, the graded area
is covered with 10-mil plastic sheeting to prevent root penetration into
the underlying soil. Then a system of drain tubes at the spacing of the
plant rows is laid out on the plastic and connected to a common drain at
the lower end of the house. Sand is then filled to the intended depth over
the plastic sheeting and drain lines. Be careful to select sand according
to its particle size distribution, and its freedom from pathogens and con-
stituents that might be toxic to the crop plants. Particle size distribu-
tion is an important consideration in order to maintain a good balance
between drainage (aeration) and nutrient and water retention. Particle
sizes should be in the range of 0.1 to 1.0 mm with an average of 0.25 to
0.50 mm. Because of the permanent preparations for full-floor sand
culture, it is recommended that sand intended for use in the system be
given a growth test in containers before actually filling the greenhouse
to determine if the sand meets the basic requirements.
Sand-culture systems for tomatoes or cucumbers are typically
irrigated and fertilized by trickle irrigation. The nutrient solution
should be supplied at each irrigation because of the relatively low
nutrient retention of sand. Irrigation frequency will vary with the crop,
its growth stage, and the temperature, but will range from two to
several times a day. Depending upon plant size and temperature,
tomato and cucumber plants will require from 1/2 to 4 quarts per plant
Soilless culture in bags, pots, or troughs with a lightweight medium is
the simplest, most economical, and easiest to manage of all soilless
systems. The most common media used in containerized systems of
soilless culture are peat-lite (Boodley and Sheldrake, 1977), or a mix-
ture of bark and wood chips. Container types range from long wooden
troughs in which one or two rows of plants are grown, to polyethylene
bags or rigid plastic pots containing one to three plants. Bag or pot
systems using bark chips or peat-lite are in common use throughout the
SAND ILL INES POLYETHYLENE LINER
Figure 5. Full-f loor sand culture.
United States and offer some major advantages over other types of
soilless culture: (1) these materials have excellent retention qualities
for nutrients and water; (2) containers of medium are readily moved in
or out of the greenhouse whenever necessary or desirable; (3) they are
lightweight and easily handled; (4) the medium is useful for several suc-
cessive crops; (5) the containers are significantly less expensive and less
time-consuming to install; and (6) in comparison with recirculated
hydroponic systems, the nutrient-solution system is less complicated
and less expensive to manage. From a plant nutrition standpoint, the
latter advantage is of significant importance. In a recirculated system
the solution is continuously changing in its concentration and its
nutrient balance because of differential plant uptake. In the bag or pot
system, the solution is not recirculated. Nutrient solution is supplied
from a fertilizer proportioner or large supply tank to the surface of the
medium in a sufficient quantity to wet the medium. Any excess is
drained away from the system through drain holes in the base of the
containers. Thus, the concentration and balance of nutrients in solu-
tion fed to the plants is the same at each application. This eliminates
the need to sample and analyze the solution periodically to determine
the kind of necessary adjustments, and avoids the possibility of solution
excess or deficiencies.
In the bag or pot system, the volume of medium per container varies
from about 1/2 cubic foot in vertical poly bags or pots to 2 cubic feet in
lay-flat bags. In the vertical bag system, 4-mil black poly bags with
prepunched drain holes at the bottom are common. One, but sometimes
two, tomato or cucumber plants are grown in each bag (Figure 6). Lay-
flat bags accommodate two or three plants (Figure 7). In either case, the
bags are aligned in rows with spacing appropriate to the type of crop
being grown. It is good practice to place vertical bags or pots on a narrow
sheet of plastic film to prevent root contact or penetration into the
underlying soil. Plants in lay-flat bags, which have drainage slits (or
overflow ports) cut along the sides an inch or so above the base, would
also benefit from a protective plastic sheet beneath them. Greater
detail on lay-flat bag culture is provided by Bauerle (1984).
Nutrient solution is delivered to the containers by supply lines of
black polyethylene tubing, spaghetti tubing, spray sticks, or ring drip-
pers in the containers. Application devices have different wetting pat-
terns and are available in different flow rates. The choice of application
system is important in order to provide proper wetting of the medium at
each irrigation. 'Ixture and porosity of the growing medium and the
surface area to be wetted are important considerations in making the
choice. Spaghetti tubing provides a point-source wetting pattern,
which might be appropriate for fine-textured media which allow water
to be conducted laterally with ease. In lay-flat bags, single spaghetti
tubes at individual plant holes will provide good wetting of peat-lite. In
a vertical bag containing a porous medium, a spray stick with a
90-degree spray pattern will do a good job of irrigation if it is located to
wet the majority of the surface. Ring drippers are also a good choice for
vertical bags although somewhat more expensive. When choosing an
application system for bag or container culture, remember that the
objective of irrigation is to distribute nutrient solution uniformly so
that all of the medium is wet. Since a root system cannot function in dry
medium, dry medium is wasted medium.
The growing medium used in container culture must have good
nutrient- and water-holding characteristics yet provide good aeration
to the root system. Light weight is another important consideration so
that filled containers can be easily handled. Growing media should be
free of pathogenic organisms and substances that are toxic to plants.
The principal materials that meet these requirements are peat moss,
bark, shavings, sawdust, vermiculite, bagasse, and rice hulls. Table 1
provides a summary comparison of the characteristics of these
materials. Some should not be used alone, but have one or more
characteristics that make them valuable constituents when used in a
mixture. Bagasse is low in porosity and high in water-holding capacity,
which would lead to poor aeration and drainage if used alone. Because
rice hulls have low water-holding capacity and high pore space, plants
would be vulnerable to water stress when rice hulls are used alone.
Both vermiculite and sawdust are poor choices as sole constituents
Figure 6. Cucumber
plants in vertical poly
because their high water-holding capacities can lead to saturation and
poor aeration if excessively irrigated. Vermiculite particles also tend to
collapse with time, resulting in compaction and volume loss. Sawdust
(except for cypress and redwood) and bagasse have high
carbon:nitrogen ratios (C:N ratios), and require extra nitrogen fer-
tilizer to avoid the competitive demand for nitrogen between
microorganisms and the plants. Bagasse, rice hulls, sawdust, and ver-
miculite possess useful characteristics when used in mixes with other
materials in the range of 20 to 50 percent of the total volume. Because of
their high C:N ratios, bagasse and most sawdust material should be
limited to no more than 20 percent of the total volume of the mix.
Table 1. Physical and chemical characteristics of materials used in soilless culture.'
Bulk Water Cation Decomposition
Material density holding Porosity exchange rate
(weight) capacity capacity (Carbon:Nitrogen)
Bagasse L2 H2 L M2 H
Sawdust L H M H H
Rice hulls L L H M M-H
Shavings L M H M M-H
Vermiculite L H M H L
Peat moss L H H H M
Bark L M M M M
Sand H L M L L
'Adapted from R. T. Poole, C. A. Conover, and J. N. Joiner 1981; Foliage Plant Produc-
tion. Prentice Hall, Englewood Cliffs, NJ.
2Low 0.25 gm/cmJ 20% 5% 10 meq/100 cm3 1:200
Medium 0.25-0.75 20-60% 5-30% 10-100 1:200-1:500
High 0.75 60% 30% 100 1:500
Figure 7. Tomato plants in lay-flat bags.
Care should be used in the kind of wood material selected for soilless
culture. Cedar, walnut, and eucalyptus may have components that are
toxic to plants. Fresh redwood also affects the growth of some plants,
but this effect becomes negligible with aging and leaching. The causes
of toxicity from wood materials are not clearly understood, but probably
vary with the type of plant being grown and the type and age of wood
being used. Wood materials are generally acidic and any toxicity from
their use may be due to the effects of acidity on the availability of some
nutrients to the plants. In redwood, the toxic component is transient
because it decomposes or is leached away during composting. Materials
such as pine sawdust decompose rapidly because of the high C:N ratio
and, if supplemental nitrogen is not provided or is present in insuffi-
cient amount, the deficiency that develops may give the impression of
toxicity. The barks of pine, fir, and redwood (and possibly others) can be
safely used without growth-retarding effects, but cedar and walnut
bark should be avoided. Sawdust and shavings ofpine, fir, and redwood
can make good, safe amendments when composted with nitrogen at 13
pounds per cubic yard for two to three months.
Mixes should not be made merely to take advantage of availability or
low cost, but should be made considering the basic factors of weight,
nutrient retention, water-holding capacity, pore space, and C:N ratio.
Mixtures of sphagnum peat and horticultural vermiculite (peat-lite)
have all of the required characteristics and make an excellent growing
medium. Proprietary peat-lite mixes are available, or growers can
prepare their own supply from the basic components (Boodley and
Sheldrake, 1977). Bark from pine, fir, cypress, and redwood have been
used successfully as growing media for greenhouse cucumbers and
tomatoes. Particle sizes of bark range generally from 1 to 10 mm in
diameter; and the distribution of particle sizes in most mill-run
material provides good aeration, and water- and nutrient-holding
characteristics. A bark medium can be used for several erops without a
significant reduction in volume due to decomposition. A supply of bark
with predominately large particle sizes should be amended with a
material such as sawdust, shavings, or bagasse in order to improve
The rockwool media, which is very common in Europe, is becoming
popular in the United States. In this system, the plants are grown in a
rockwool mat enclosed by polyethylene. The mats are usually 3 to 4
inches in height, 12 to 15 inches in width and 40 inches in length, accom-
modating two plants. Water quality is critical with the rockwool system
because of the low buffering capacity of the substrate.
Yields from rockwool, in general, have been similar to those from bag
culture and slightly higher than hydroponic systems. The mats can be
cropped a second time but yields tend to be reduced.
In field culture, the clay fraction of soils can be expected to supply
adequate amounts of at least some of the nutrients required by plants,
especially the minor elements. Fertilizer programs for soilless culture
systems must supply all elements required by the plants. Carbon,
hydrogen, and oxygen are provided from water and carbon dioxide in
the air. The grower will supply nitrogen, phosphorus, potassium,
calcium, magnesium, sulfur, iron, boron, copper, zinc, manganese,
molybdenum, and chlorine. Most media materials contain small
amounts of these elements; but they should not be considered in plan-
ning the fertilizer program because they are a small proportion of the
requirement, or they may be in forms not readily available to plants.
Liquid-medium systems such as NFT and gravel bed culture use
complete nutrient solutions prepared from soluble inorganic salts con-
taining various elements. Proprietary mixes of all required elements
are available which are simply dissolved in water to prepare the
nutrient solution. These mixes are available in various concentrations
and ratios of elements. Nutrient solutions can also be prepared by the
grower using readily available soluble compounds. Many complete
nutrient solution formulas have been developed and used successfully.
All contain the same elements and are generally prepared from the
same compounds, although in somewhat different proportions. No one
formula is necessarily the best for all plants, but all are capable of pro-
viding adequate nutrition. Special formulas are often recommended for
a particular crop plant based upon research under the prevailing
climatic and water quality conditions at a specific location. These for-
mulas are soundly based for those conditions, but none should be con-
strued as being the best under all conditions. They are good points of
departure in developing a fertilization program for the crop for which
they are recommended, and may be well-suited for use without altera-
tion. Successful managers seek as much information as possible from
reliable sources to develop a sound understanding of plant nutrition
and inorganic chemistry before attempting to alter published formulas
for their own imagined or perceived needs. Improper alteration of for-
mulas can lead to serious adverse effects due to excesses or deficiencies.
It is recommended that growers either utilize prepared nutrient mixes
obtained from reliable manufacturers or, if preparing their own mixes,
follow recommended formulas carefully. Competent assistance should
be sought before making changes.
Solid-medium systems such as bark or peat-lite can be provided
with nutrients by three methods: (1) entirely from a complete nutrient
solution; (2) from a combination of premixing some elements in the
medium and supplying others in a nutrient solution; or (3) premixing
all elements in the medium. The complete nutrient solution method is
commonly used in sand culture, and also for various mixed media or
bark. Nutrient solution is applied up to several times a day to maintain
Figure 8. One type of
for nutrient solution
the medium in a moist (but not saturated) condition. This system
requires either a supply tank for the nutrient solution or a ratio feeder
or fertilizer proportioner which prepares solution upon demand from
stock nutrient concentrates (Figure 8). For small greenhouses, the fre-
quent chore of replenishing the solution supply in a tank may be more
attractive than investment in a fertilizer proportioner. When using the
supply tank method for a recirculated growing system, the tank should
be large enough to hold a volume of solution about twice that required
to fill the system. This provides a safe margin of nutrient supply.
Formulas for several nutrient solutions are given in the Appendix,
along with methods of preparation. While they differ in concentrations
of the elements, all have been used successfully by commercial growing
operations, principally for the production of tomatoes. Formula 1 has
been widely used in research greenhouses as a general nutrient solu-
tion for a wide range of plants, and is a good formula choice where more
specific information is not known for a particular crop. It is possible that
adjustments in concentrations of some elements (particularly nitrogen,
phosphorus, or potassium) may be beneficial to the yield or quality of a
given crop. Until research is clear on this, however, it is best to adhere to
the basic formula.
The formulas in the Appendix list amounts of individual salts to be
dissolved in 100 gallons of water. This prepares the nutrient solution in
the form to be supplied to plants from a storage tank. When a fertilizer
proportioner is used, the amount of each salt must be adjusted to
account for the dilution rate of the proportioner. By this method,1liquid
concentrates of the salts are prepared that will be diluted for the final
nutrient solution as they are injected through the proportioner. For
example (in Formula 1), to calculate the amount of potassium nitrate to
prepare 50 gallons of concentrate to be used with a 200:1 proportioner,
divide 95 grams by 100 to obtain grams per gallon in the final nutrient
solution as shown, then multiply by 200 to obtain the amount in 1
gallon of concentrate, and finally multiply by 50 for the amount re-
quired for 50 gallons of concentrate. The amount of potassium nitrate
required for 50 gallons of 200:1 concentrate is 20.9 pounds.
When preparing concentrates for proportioners, two separate concen-
trates are required to avoid precipitates. One contains only calcium
nitrate and the iron compound; all other ingredients are in the other
concentrate. The concentrates are kept in separate tanks and must be
used in conjunction with a twinhead proportioner. When activated, the
proportioner draws equal volumes from each tank and mixes them with
an appropriate volume of water to provide the dilute nutrient solution.
When using a proportioner, it is good practice to monitor its operation
on a regular basis to be certain that solution of correct concentration of
elements is being provided to the plants. This should be done in two
ways. A water meter attached to the outlet side will record the volume
of solution mixed for a particular period of time. The depletion of con-
centrate volume in each tank over the same period of time and the
volume of solution supplied should be in the same ratio as the dilution
ratio of the proportioner. Another check on the system is to compare
periodically the total salt concentration of a physical dilution of both
Figure 9. Portable
meter for measuring
of nutrient solution.
concentrates with water. As an example, mix 200 ml of water with 1 ml
(using a 1-ml pipette for measurement) of each concentrate. Salt con-
centration can be determined by an analytical laboratory or by a
portable battery-operated instrument that measures electrical conduc-
tivity (Figure 9). During preparation of the concentrates,
measurements of the individual salts should be made very carefully so
that the final solution will contain amounts of individual elements as
intended in the formula. Mistakes can be made, however, and for this
reason it may be wise to sample the final solution from the proportioner
and have a complete analysis made in a laboratory on a periodic basis.
Fertilizers Mixed with Media
When premixing fertilizer materials with bark, or bark and sawdust,
compounds that supply phosphorus, magnesium, calcium, sulfur and
all minor elements may be added to the growing medium prior to plant-
ing. A small amount of nitrogen may also be mixed with the medium,
but most and sometimes all of this element is supplied in the irrigation
water. Nitrogen is generally supplied in the nitrate form in the range of
100 to 200 ppm.' The ammonium form of nitrogen, if included, should
not exceed 10 percent of the total nitrogen supplied. Potassium also is
supplied routinely with nitrogen in the irrigation water at about 200
ppm. A nitrogen/potassium liquid-feed solution providing 150 ppm
nitrogen and 210 ppm potassium can be prepared by mixing 0.45 pound
potassium nitrate and 0.40 pound calcium nitrate in 100 gallons of
water. 'Ib premix the other elements in the medium per cubic yard,
phosphorus can be supplied as superphosphate (0-20-0) at 2 pounds,
calcium and magnesium from dolomitic lime at 10 pounds, and minor
elements from trace element mixes such as F-503o or Esmigran" at 5
ounces. Additional iron may be added in diluted form at 1 ounce of 138
The method of premixing all fertilizer materials in the medium
before planting may include the use of slow-release nitrogen materials
such as Mag-amp" Osmocotes isobutylidene diurea (IBDU), or sulfur-
coated urea (SCU). This method is not commonly used but, when com-
pared to the liquid-feed method, it is reported to produce equal or better
yields of tomatoes (Sheldrake, Dallyn, and Sangster, 1971). The premix
method offers important advantages by eliminating the need to
prepare nutrient solutions, and the need to purchase and maintain a
fertilizer proportioner. The potential disadvantage of slow-release fer-
tilizers that supply all of the nitrogen and potassium is that they may
not be able to release the elements at the proper rate to satisfy the
plants' needs. While slow-release fertilizers are widely used in the
'ppm (parts per million) = mg per liter. Calculation example: to obtain 100 ppm N from
potassium nitrate (14 percent N), 10010.14 = 715 mg potassium nitrate/Iiter = 0.715
gmiliter = 2.79 gm potassium nitratelgallon (1 gallon = 3.785 liters).
ornamental plant industry, their success rate and cost effectiveness
on vegetables in soilless culture has not been adequately established.
Until more information becomes available, it is suggested that their
use be limited to medium amendments in proven fertilizer programs.
Analysis of Solution, Tissue, and Media
Knowledge of the nutritional status of all components of a soilless
culture system is important for two reasons: (1) it is the only means of
judging how successful management practices have been in attaining
the objectives of the fertilizer program in terms of the availability of
nutrients and the nutrient levels in the tissue; and (2) it helps diagnose
the causes of any abnormal plant symptoms that may occur. The costs of
this knowledge are a form of insurance toward success. Periodic
laboratory analysis of the nutrient solution is imperative in recir-
culated hydroponic systems, especially if use of the solution is extended
over a period of weeks. It is equally important in solid-medium systems
in order to evaluate the operation of the fertilizer proportioner and the
preparation of concentrates.
The nutrient solution in a recirculated hydroponic system such as
NFT or the gravel bed method may be used on either a short-use or
extended-use basis. For short use, a fresh solution is prepared every
week or two with periodic replenishment from concentrates. This
system assumes that, during the short-use period, nutrient removal
from the solution will not reach deficiency levels as long as some fresh
concentrate is added every few days. This is a workable procedure and
has been used with some success, but it is wasteful of some nutrients
and, if the used solution is discarded on porous soils, it can be a source of
groundwater pollution. The alternative is extended use of the solution
for a period of several weeks or perhaps months. This method can also
be successful but requires close periodic monitoring to avoid deficien-
cies or excesses of nutrient elements, either of which can have adverse
effects on plant growth. Because the nutrient status of an extended-use
solution is continually changing due to plant uptake, frequent analysis
is necessary to maintain the solution close to the original concentration
of elements. At a minimum, a complete analysis is warranted every
three weeks with weekly analyses for nitrogen, potassium, and
phosphorus. Daily determination of the total salt content will provide a
useful estimate of the nutrient status of the solution although this can-
not substitute for complete analysis (Johnson, 1980).
The constantly changing nutrient status in recirculated solutions
and the necessity (and cost) for close control through solution analysis
to avoid deficiencies and excesses of elements provide a logical justifica-
tion for the preferred use of solid-medium systems in soilless culture. In
solid-medium systems, plants are supplied with a uniformly balanced
solution at each irrigation. Solution management problems are
reduced to measuring precisely when preparing concentrates and
monitoring the fertilizer proportioner to assure that it is in proper
Tissue analysis is the best method to evaluate the nutritional status
of the plants and the success of the fertilization program. Periodic
sampling and analysis of leaf tissue during growth of the crop will pro-
vide information that can be used to make adjustments in fertilizer
practices, as well as to help interpret any abnormal plant symptoms.
Whole leaves, leaf petioles, or leaf blades are the plant parts normally
used, although desirable levels for nutrients vary depending upon the
part sampled, its location on the plant, and the analytical methods
used. Desirable leaf concentrations have been reported for tomatoes,
cucumbers, and a wide range of vegetables (Ward, 1973; Wittwer and
Honma, 1979; Lorenz and Ty~ler, 1983; and Johnson, 1980). Growers who
wish to include tissue analysis in their management program should
seek assistance from their local county extension agent in choosing an
experienced agricultural analytical laboratory. The lab can help to
outline procedures for sampling and the handling of samples, so that
material provided to them is from the proper location on the plants, is
representative of the growing area, and in good condition upon arrival.
Usually the youngest, fully expanded leaf should be sampled from ran-
domly located plants throughout the area; this is generally the fourth
or fifth leaf from the growing point. An adequate sample size is 30
leaves. A single sample will be adequate where plants are uniform in
vigor and appearance. Abnormal plant growth should be sampled
separately. Samples should be delivered as soon as possible, in a fresh
condition, to the laboratory.
Allison, E E., and M. S. Anderson. 1951. The use of sawdust for mulches
and soil improvement. USDA Cire. 891.
Barragry, A. R., and J. V. Morgan. 1978. Effect of mineral and slow-
release nitrogen combinations on the growth of tomato in a
coniferous bark medium. Acta Hort. 82:43-53.
Bauerle, W. L. 1984. Bag culture production of greenhouse tomatoes.
Ohio Agr. Expt. Sta. Spec. Circ. 108.
Boodley, J. W., and R. Sheldrake Jr. 1977. Cornell peat-lite mixes for
commercial plant growing. New York State College of Agriculture
and Life Sciences. Informat. Bull. 43.
Cooper, A. J. 1979. The ABC ofNFTI Grower Books. London.
Cotter, D. J. 1974. Yields of successive cropping of tomato in sawdust
and bark media. HortScience 9(4):387-388.
Furuta, T. 1974. Environmental plant production and marketing. Cox
Publishing, Arcadia, CA.
Gartner, J. B., S. M. Still, and J. E. Klett. 1973. The use of hardwood
bark as a growing medium. Proc. Int. Plant Prop. Soc. 23:222-230.
Ingratta. E. J. 1979. Soilless culture of greenhouse vegetables. Ontario
Ministry of Agriculture. Agdex 290/518.
Jensen, M. H., and N. G. Hicks. 1973. Exciting future for sand culture.
Amer. Veg. Grower, 33-34, 72-74, Nov.
Johnson, H. Jr. 1975. Greenhouse tomato production. Div. of Agr. and
Nat. Res., Univ. of Calif. Leaflet 2806.
Johnson, H. Jr. 1980. Hydroponics: A guide to soilless culture. Div. of
Agr. and Nat. Res., Univ. of Calif. Leaflet 2947.
Johnson, H. Jr., and G. W. Hickman. 1984. Greenhouse cucumber
production. Div. of Agr. and Nat. Res. Univ. of Calif. Leaflet 2775.
Jones, J. B. Jr. 1983. A guide for the hydroponic and soilless culture
grower. Timber Press. Portland, OR.
Larsen, J. E. 1973. Nutrient solutions for greenhouse tomatoes.'Ibxas
A&M Univ., College Station. Mimeo.
Lorenz, O. A., and K. B. Tyrler. 1983. Plant tissue analysis of vegetable
crops. In Soil and plant testing in California. Div. of Agr. and Nat.
Res., Univ. of Calif. Bull. 1879.
Lunt, H. A. 1955. The use of wood chips and other wood fragments as
soil amendments. Conn. Agr. Expt. Sta. Bull. 593.
Maynard, D. N. and A. V. Barker. 1970. Nutriculture. A guide to the
soilless culture of plants. Mass. Coop. Ext. Ser. Pub. 41.
Mass, E. E., and R. M. Adamson. 1980. Soilless culture of commercial
greenhouse tomatoes. Agriculture, Canada, Pub. 1460.
Pokorny, R. A. 1966. Pine bark as an organic amendment in the
production of container plants. Geo. Agr. Res. 7(4):8-9.
Poole, R. T., C. A. Conover, and J. N. Joiner. 1981. Soils and potting
mixtures. In Foliage Plant Production. Prentice Hall, Englewood
Riekels, J. W. 1973. Nutrient solutions for hydroponics. Ontario
Ministry of Agriculture. Agdex 200/532.
Sangster, D. M. 1973. Soilless culture of tomatoes with slow-release
fertilizers. Ontario Ministry of Agriculture. Agdex 291/818.
Sheldrake, R., S. L. Dallwyn, and D. M. Sangster. 1971. Slow-release
fertilizer for greenhouse tomatoes. New York's Food and Life
Ward, G. M. 1973. Leaf analysis for vegetable crops. Ontario Ministry
of Agriculture. Agdex 290-532.
Wittwer, S. H., and S. Honma. 1979. Greenhouse tomatoes, lettuce,
and cucumbers. Michigan State Univ. Press, East Lansing.
Appendix 1. Some useful formulas for complete nutrient solutions
Compound gl100 gallons of water
potassium nitrate 95
monopotassium phosphate 54
magnesium sulfate 95
calcium nitrate 173
chelated iron (FeDTPA) 9
boric acid 0.5
manganese sulfate 0.3
zinc sulfate 0.04
copper sulfate 0.01
molybdic acid 0.005
N P KCa Mg S Fe B Mn Zn Cu Mo
ppm 105 33 138 85 25 33 2.3 0.230.26 0.024 0.01 0.007
(Jensen, in Wittwer and Honma, 1979)
Compound gl100 gallons of water
magnesium sulfate 187
monopotassium phosphate 103
potassium nitrate 77
calcium nitrate 189
chelated iron (FeDTPA) 9.6
boric acid 1.0
manganese chloride 0.9
cupric chloride 0.05
molybdic acid 0.02
zinc sulfate 0.15
N P KCa Mg S Fe B Mn Zn Cu Mo
ppm 106 62 156 93 48 64 3.8 0.46 0.81 0.09 0.05 0.03
N P K Ca Mg S Fe B Mn ZD Cu Mo
ppm 172 41 300 180 48 158 3 1.0' 1.3 0;3 0.3 0.07
Compound gl100 gallons of water
potassium nitrate 221
magnesium sulfate 194
calcium nitrate 380
monopotassium phosphate 99
iron chelate (FeEDTA) 30
manganese sulfate 2.3
boric acid 0.6
copper sulfate 0.15
zinc sulfate 0.17
ammonium molybdate 0.14
N P K Ca Mg S Fe B Mn Zn Cu Mo
ppm 236 60 300 185 50 68 12 0.3 2.0 0.1 0.1 0.2
'When formula is to be used with a fertilizer proportioner, see procedure under
section on Nutrient Solutions.
potassium magnesium sulfate
chelated iron (FeDTPA)
phosphoric acid (75%)
91100 gallons of water
This publication was promulgated at a cost of $2,206.50, or 27.6 cents
per copy, to provide information about methods of growing greenhouse
vegetables through soilless culture. 11-8M-85
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORI-
DA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, K. R.
Tefertiller, director, in cooperation with the United States Department Il F"AS
of Agriculture, publishes this information to further the purpose of the
May 8 and June 30, 1914 Acts of Congress; and Is authorized to pro-
vide research, educational information and other services only to Indi-
viduals and Institutions that function without regard to race, color, sex or national orl-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulk
rates or copies for out-of-state purchasers Is available from C. M. Htnton, Publications
Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida
32611. Before publicizing this publication, editors should contact this address to deter-
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