Summary and discussion– Morgan 2003
“Hydroponic Tomatoes: The Complete Guide to Soilless Success
Part Two: Production Systems and Crop Management”
L. Morgan
Published 2003, The Growing Edge 15(1), pp 60-73.
http://www.growingedge.com/magazine/back_issues/view_article.php3?AID=150160
Summarized by Anita (Teena) Hayden, PhD (UTK, June 2007)
This is the second in a series published in the popular
magazine The Growing Edge (
Types of Tomatoes
Recommended for Hydroponic Production - Dr Morgan begins this lengthy
article with a review of the different types of tomatoes developed specifically
for hydroponics. The variety ‘Trust’ is the
commercial standard favored in the
Specialty varieties such as
Soilless Systems – Media-based Systems
Selection of media should be based
on what is locally available and cost effective. Rockwool slabs are used
world-wide, are pre-sterilized and have excellent water and air holding
capacities. Rockwool does not interfere with the nutrient formulation, however
on a commercial scale, disposal of old rockwool is an environmental concern.
Media from natural sources, such as sawdust, bark, gravel, coir, vermiculite,
sand, perlite, pumice, expanded clay, rice hulls, and others may be preferable.
These substrates are often used in re-usable containers (Dutch pots or PVC
channels), or disposable bags. Organic substrates, such as compost or
vermiculture (worm castings) are gaining popularity in smaller systems, however the nutrient management of these systems is
quite challenging compared to traditional hydroponic systems.
Substrate Selection Criteria
Economics – compared other greenhouse operation costs (heating,
cooling, labor), growing media costs are not high,
however media can be bulky and growers should consider using locally available
media when possible.
Pests, Diseases, and
Contaminates – Media contaminated with
disease spores, pests, or high levels of toxic salts (such as sodium in coir)
can be a serious problem. Compost, in particular, can contain very high levels
of heavy metals, such as copper, that can retard growth. Sawdust from timber
that has phytotoxic compounds or treated lumber should also be of concern.
Pests, such as fungus gnat larvae, can damage roots of young seedlings.
Physical Structure and
Consistency – The two most important
characteristics of substrates are its water-holding capacity and aeration. Both
of these can change over time as the root systems grow and the media breaks
down. Root-zone problems can show up in the crop as wilting during the middle
of the day, brown roots, flower or fruit drop, leaf yellowing, or overall lack
of plant vigor. The life-span of the media is an important selection criteria, but growers should take into consideration the
labor involved in maintaining the media. Rockwool has been used for as many as
six to seven tomato crops before replacement, and perlite is also successfully
re-used for many crops. Organic substrates have much shorter life-spans in
greenhouse systems.
Nutrient Interactions
Organic substrates, such as
compost, sawdust, or peat-based media can interact with the nutrient solution
and interfere with plant uptake of nutrients by binding with certain nutrients
or affecting the pH. “Inert” media, such as rockwool or perlite, are more
consistent.
Disposal & Labor
Although smaller growers do not
necessarily need to worry about disposal of media, commercial growers should
not discount the cost of disposal. Cost of labor to fill containers with media
is another factor that some start-up operations fail to consider.
Systems
Most media-based systems use drip
irrigation that run several times per day, and are non-recirculating, passing
the solution through the media once with excess solution running to waste.
However, more growers are realizing the need to recover and recirculated the
solution, achieving zero waste. The skills needed to successfully manage these
systems are very high. The environmental advantages of recirculating systems are
obvious, but another advantage is the ability to over-irrigate in order to
permit adequate leaching of the media and prevent salt build-up without the
worry of wasting fertilizers.
The frequency and amount of
irrigation to apply is a common problem for inexperienced growers. The amount
of irrigation needed is dependent on the media used, stage of growth of the
crop, and environmental conditions. Young seedlings may only need one or two
irrigations per day, whereas a mature crop may need a dozen or more in order to
supply sufficient water and nutrients to the crop. Commercial growers often
rely on computer-controlled systems based on sensors placed throughout the
crop, but simple collection trays under drippers and plants located throughout
the house that permit the grower to monitor the volume of drainage may be
sufficient. Generally, obtaining 10% drainage at each irrigation cycle should
be sufficient for most conditions, but certain media or environments may
require additional drainage to keep the root zone in good condition.
Non-media based systems
Nutrient Film Technique (NFT) and
Deep Flow Technique (DFT) are two media-free systems that utilize a
recirculating flow or stationary pool of nutrient solution. NFT systems involve
channels of PVC or plastic sheets where a small amount of fertilizer solution
(the “film”) flows across the roots of the plants. NFT systems for tomatoes
need to be made larger than those for lettuce or strawberries. DFT is less
common for tomato production, and requires careful attention to the oxygen
levels in the root zone. Aeroponics is a system where the roots are sprayed
with the nutrient solution inside a dark chamber. Aeroponics is not widely used
for tomato production due to the high capital expense and maintenance required.
Ebb-and-flow, continual flow gravel beds, and aquaponic systems are other
non-media based systems that can be used for tomato production, but usually on
a small scale.
Nutrient Management
Huge yields are possible from
modern varieties grown under ideal nutrient management, however, there is no
one optimal nutrient formulation for all hydroponic tomato
crops, so the grower must continually monitor the status of the plants and
understand the chemistry of the fertilizers. Recirculating systems
change over time as crops pull needed nutrients from the solution. Although the
solution may be adjusted daily with fresh stock fertilizer or water, certain
compounds, often potassium, can become depleted without the grower’s knowledge.
Non-recirculating systems permit less chance of a fertilizer becoming depleted,
however even these systems may fail to deliver the needed nutrition to the crop
during certain crop stages.
Cropping Systems
Bush-type tomato plants
(“determinate” varieties) are usually grown in the field, whereas vine-type
varieties (“indeterminate”) are most common in the greenhouse. Commercial
greenhouse operations generally grow vine-type crops for at least 10 months by
layering, or leaning and lowering, the vines as they grow to keep the level of
the harvested fruit at a convenient height. As the plants are lowered, fruit is
harvested and older leaves are removed. The advantage of the long vine system
is that the majority of the time the plants are in the house, they are producing
marketable fruit. Disadvantages include the difficulty in maintaining
high-quality fruit as plant vigor declines with age, as well as the risk of
pests and diseases such whitefly or Botrytis becoming entrenched in a mature
crop. Also, growers in extreme climatic conditions may choose to grow
shorter-term crops to target the harvest period for the most profitable times
of the year.
Single-truss cropping systems are
one method to utilize the new vine-type varieties in growing systems that favor
bush-type plant architecture (such as NFT or ebb-and-flow systems). In
single-truss systems, plants are grown in very high-density plantings (12-16
plants per square meter, or approximately 1.5 plants per square foot), and the
plant is cut at the 2nd or 3rd leaf above the first fruit
cluster. The concept behind this system is to produce at least four crops per
year of the highest quality fruit. Fruit size can be exceptionally large in
this system, with better flavor characteristics than the same variety grown
under long-term conditions. Continual harvest can be achieved by seeding every
three weeks, with yields as high as 60 kg per square meter (12 pounds per
square foot) per year.
Yields and Flavor
Hydroponic tomatoes have the
potential to produce exceptionally high quality fruit with excellent fruit
quality and yields through the manipulation of the fertilizer mix and the
environmental control. Light is the main factor that determines the amount of
sugars and the size of the fruit. The soluble solids (sugar) content of fruit
produced during a 16 hour day is significantly higher than that produced during
a 12 hour day, while acidity is not affected by day length. Sugars are also
influenced by moisture and high temperature. High air temperatures above 84F
are detrimental to both yield and quality. Soft fruit and blotchy color are
both linked to high temperatures. Removing too many leaves from a plant can
also result in lower fruit quality, since the leaves closest to a fruit are
responsible for producing the sugars that ultimately end up in the fruit. Key
to fruit quality is managing the root zone. Increasing the electrical
conductivity (EC) of the nutrient solution helps produce firmer fruits with
higher flavor. Although this has been seen in both beefsteak and cherry tomatoes,
the cherry tomato varieties respond well to very high ECs (10mS/cm compared to
the usual 2.5 mS/cm). The downside of increasing fruit flavor through increased
EC is a reduction in yield. Therefore, a commercial grower who is marketing
fruit as a commodity and selling strictly by the pound has little incentive to
improve flavor if it means less yield. However, a smaller operation with a
roadside stand may be able to compete with grocery chain stores if they can
emphasize the flavor attributes of their product.
(Hyperlink to definition of
indeterminate -plants that have a vine-like character. Contrast to a bush-like
character, called a “determinate” plant. These terms come from the botanical
terminology referring to plants that cease growing larger as soon as flowers
are produced, so their size and shape is “determined” by flowering.
Indeterminate plants continue growing even after flowers are set)