October 2003

Hydroponic Tomatoes

Tomatoes, the biggest hydroponically produced crop on a
worldwide scale, are complex in their physiology and response to crop
management techniques since vegetative growth, flowering and fruiting all need
to be continually maintained simultaneously on the plant. Obtaining economic
yields of high-quality fruit while minimizing the use of pesticides and other
agrichemicals has put commercial tomato growers under increasing pressure, and
many are now looking to modified hydroponic systems where higher profits are
possible. Many of these new tomato-growing techniques involve the production of
"spray-free" crops and using organically based systems. Today’s
selection includes a wide range of new, fresh tomato products, such as low-acid
fruit; on-the-truss, cluster or vine-ripened fruit; and many new varieties of
colored, plum and Italian fruit that serve to supplement the traditional round
beefsteak types.

While product diversity in commercial hydroponic tomato
production has been one way many growers have maximized profits from their
greenhouse operation, these newer systems require a greater degree of skill and
understanding of the plant and its production. With the development of rapid
and accurate nutrient analysis services now available to growers, nutrient
formulation and monitoring has become a vital tool for commercial producers who
aim to keep fertilizer wastage to a minimum.

Markets for fresh tomato fruit will continue to evolve,
making it essential for growers to keep up with new developments, research and
trialing different cultivars and techniques in their own production systems.

Recommended Cultivars

Often, one of the most confusing aspects of hydroponic
tomato production is selecting good varieties to grow. The tomato is a crop
that has been the subject of extensive plant breeding, and selection over a
long period of time and the genetic diversity of tomato types to select from
seems endless. Many cultivars have been selectively crossed over many
generations to create plants for different types of growing systems,
environments and fruit types. Also, plant breeders are continually bringing out
new, improved hybrid cultivars for greenhouse production. Often, running trials
on new cultivars each season is one way commercial growers can select improved
varieties.

Lists of recommended tomato varieties for hydroponic
production tend to rapidly go out of date. However, there are a few industry
standards that have been proven in many different systems and climates to
continually perform well in hydroponic systems.

Indeterminate Beefsteak Varieties. A continual favorite is ‘Trust’, a F1 hybrid grown
by many commercial hydroponic tomato growers. This fairly large-fruited tomato
is favored in the United States for its size and meaty texture. Average fruit
size is 7-10.5 oz. under good growing conditions, and it is used as a
single-harvest — as opposed to truss-harvested — fruit. Trust fruit have a
good shelf life and firm texture and handle well after harvest. This variety is
also resistant to some mold strains, including Fusarium crown and root rot. The
seed, and F1 hybrid, is more expensive than many of the open-pollinated types.
However, small quantities of this commercial variety can be purchased by
smaller growers. Since Trust is an indeterminate variety, it’s often grown over
a long season (10-18 months) and layered. It can also be grown as a
shorter-term crop by removing the growing point when it reaches the top of the
training wire.

Another large, round type that performs well under
hydroponic production with a smaller average fruit size than Trust is ‘Daniela’,
another F1 hybrid. The average fruit size of Daniela is 5-5.3 oz., which is
suited to either single or truss harvesting. The fruit ripens very uniformly
under most growing conditions. This variety has an excellent shelf life and
fruit firmness and is worth trailing — particularly for
"vine-ripened" fruit production.

Heirloom and Open-Pollinated Types. While older, heirloom, open-pollinated tomato
varieties frequently don’t yield as high as F1 hybrid types, some growers
prefer to grow them for fresh market sales where firmness and shelf life come
second to the flavor and aroma of the fruit. Fruit quality and disease problems
are common on some open-pollinated types, so selection of suitable cultivars is
important.

In recent greenhouse trials where a number of different
heirloom varieties were grown hydroponically, some were found to have market
reject rates as high as 90 percent due to fruit cracking, splitting and color,
size and shape deformities. However, the best selections did produce fruit that
was marketable, although shelf life and firmness were never as high as F1
hybrids. The better heirloom varieties in greenhouse trials proved to be
‘Moskvich’, a large, globe-shaped fruit with a high flavor score, and
‘Thessaloniki’, which produces rather large, beefsteak-type fruit. Both
Moskvich and Thessaloniki can be obtained as organically grown seed.

Specialty Varieties.
Plum or Italian varieties are becoming more popular as specialty lines for many
hydroponic producers. Often, with these types of elongated fruit, blossom end
rot is more common than in round, red varieties. Many growers find this to be a
significant problem in warmer growing conditions. Trialing new varieties of
greenhouse plum types, rather than those usually grown outdoors, is one way of
reducing this problem.

‘San Marzano’ (F1) is one commonly grown plum or Italian
type that features good fruit quality under hydroponic production. The fruit
are large, averaging 4.6-5 oz. and can be used for fresh consumption or as
"fresh paste" fruit. ‘Azafram’ is a yellow truss F1 hybrid tomato,
averaging around 3.2-3.5 oz. and is a good variety for on-the-vine cluster
sales of five to six fruit per cluster.

Cocktail, Cherry and Miniature Types. Cocktail tomatoes, averaging 0.7 oz. in weight, produce
lower yields than larger cultivars and have a higher labor requirement for
harvesting, grading and packaging. The industry standard types for red cocktail
fruit are ‘Cherita’ (F1), ‘Flavorita’ (F1), ‘Gardeners Delight’, ‘Sweet 100’
(F1) and ‘Chiquita’ (F1). All of these produce similar fruit and yields under
hydroponic production, although there have been advances in disease resistance
with some of the newer varieties, such as Flavorita.

Currant, grape, olive, golden and pear-shaped versions of
small-fruited tomatoes are also available, many of which have potential in
specialty markets. ‘Sun Gold’ (F1) is a yellow cocktail tomato that performs
well under greenhouse conditions. Red and yellow currant and red and yellow
pear are some of the non-hybrid, small-fruited types worth experimenting with
for fresh local sales.

NFT and DFT

Nutrient film technique (NFT) and deep flow technique (DFT)
are two media-free systems of production that feature a flow of nutrient
solution inside growing channels that contain the root system of the plants.
NFT and DFT are used in many commercial and hobbyist tomato production systems.
Tomato plants thrive in well-run water culture systems. However, like with any
recirculating system, monitoring nutrient levels is essential for commercial
production.

NFT systems rely on a thin flow of nutrient along the base
of the nutrient channel. Channels are often constructed of rigid PVC and
specially designed and manufactured for hydroponic crop production or formed
from thick plastic film that’s folded up to form a triangular-shaped channel.
Rigid PVC channels are more common for hydroponic tomato production. However,
film-based channels are less expensive and can be disposed of between crops to
prevent disease carryover. NFT channels for tomato production need to be larger
than those used for lettuce, herbs, strawberries and other small hydroponic
crops. Channel dimensions of 6 x 8 inches are common in larger tomato
operations as a large area is required to contain the root systems of long-term
crops. Channels with removable lids are convenient for plant removal and
cleaning the inside of the gully.

DFT is less common than NFT for hydroponic tomato production
and relies on a similar system of channels that are filled with a deep flow of
nutrient solution rather than a thin film. DFT systems rely on the introduction
of oxygen along the entire length of each growing channel so that oxygenation
rates in the root zone are continually kept high enough for good root growth.
As with NFT, the nutrient solution recirculates continuously and EC, pH and
often temperature levels are adjusted at the main nutrient reservoir. Both NFT
and DFT systems can produce yields and fruit quality similar to plants in
media-based systems.

Tomato seedlings for both NFT and DFT systems are usually
raised in an inert media, such as rockwool propagation cubes or small
containers of soilless media. Rockwool propagation blocks are most commonly
used because they don’t release small particles of media into the recirculating
system that could cause irrigation blockages. Many tomato seedlings are raised
to the point of flowering on the first truss before planting out into NFT
systems and the propagation cube provides support for the young plant for the
first week until training has begun.

Other Systems

There are a number of other systems that are often used for
hydroponic tomato production on small and large scales. Ebb-and-flow, continual
flow gravel bed, capillary, soilless organic and aquaponic systems, which
integrate fish and crop production, are all used for hydroponic tomato
production, often with good success. As with other systems, the way the plants
and nutrients are managed tends to be dependent on whether the system is
recirculating or runs to waste and the method of irrigation.

Recirculating vs. Nonrecirculating Systems

One of the most important aspects of hydroponic tomato
production is nutrition. Tomato plants, particularly modern hybrid cultivars,
have the potential to be extremely high yielding compared to crops grown a few
decades ago. Top producers can now expect to obtain over 1.4 oz. per square
inch per year of high quality tomato fruit compared to only around 0.6 oz. per
square inch per year from older varieties grown a few decades ago. This huge
increase in yield potential has also seen massive increases in the amount of
nutrients taken up by the plant to support these yields and vigorous plant
growth in controlled environments. There isn’t one ideal or optimal nutrient
formulation for hydroponic tomato crops. Each crop is different and requires
continual monitoring of the nutritional status of the plants.

Recirculating systems require a very different nutrient
formulation and management system than nonrecirculating systems. Recirculating
systems tend to start off with a well balanced nutrient formula that features
plenty of each element for plant uptake. However, by the time the solution has
passed through the root systems of the plants, certain nutrients may have been
taken out more than others, causing imbalances in the nutrient solution that
are unknown to the grower. As the EC is adjusted each day with more stock
solution and additional water, some of the nutrients under heavy demand are
replaced — but sometimes not to sufficient levels. One example is potassium.
In recirculating systems where tomato plants are carrying a heavy crop load,
potassium can be depleted within a few days despite frequent additions of
concentrated nutrient stock solutions.

In nonrecirculating systems, which are often seen as wasteful
of nutrient solution since the excess runoff drains to waste, fresh nutrient
solution is applied at each irrigation. Therefore, there is less of a chance
that elements will be depleted over the long term (as long as a well-balanced
nutrient formula is continually being applied). However, even in these systems,
the nutrient needs to be monitored on a regular basis to determine plant
nutrient uptake rates and to modify the nutrient formula. Tomatoes are
extremely heavy feeders, particularly when carrying a heavy fruit load, and
samples of runoff often show potassium and micronutrient depletion during
certain crop stages.

Single-Truss Cropping

Single-truss tomato production was originally developed in
the 1960s by Dr. Allen Cooper of the Glasshouse Crops Research Institute in the
United Kingdom and has been examined by a number of researchers over the last
four decades. In fact, the NFT method of tomato cropping was initially devised
for single-truss production.

In the single-truss system, the plants have the growing
point removed after the production of 2-3 leaves above the first fruit cluster.
All lateral shoots are removed as they form and the plants are grown at a high
density of 12-16 plants per square meter in order to maintain maximum yields. The
concept behind the development of this system was to produce at least four
crops per year per greenhouse and attempt to increase the efficiency of tomato
production.

Single-truss cropping is usually carried out on growing
benches at a convenient working height. Since the average single-truss plant
doesn’t grow any higher than around 30 inches tall, the plants can be grown in
small (4 x 6 inch) NFT channels on growing tables. This makes planting,
harvesting and crop care more comfortable for workers and also means that
artificial lighting can be used to maximum efficiency on the shallow canopy
that single-truss plants create.

Perhaps the most interesting advantage of single-truss crop
production is the improved quality of the fruit. Since the fruit truss develops
on the plant after the growing point has been removed, there isn’t any
competition for assimilates from new leaves, other flowering trusses, or fruit
on the same plant. Single-truss fruit tend to develop rapidly
larger-than-average fruit size and have extremely high compositional quality in
terms of sugars, acids, dry matter content and overall flavor. In fact, the
compositional Á quality of single-truss tomatoes has been proven to be
significantly greater than the same tomato cultivar grown under similar
conditions — even in poor light during winter production. The most significant
finding, however, was that single-truss fruit not only had a higher
compositional quality but that applying high EC levels to get increases in
quality didn’t result in the type of yield losses that occur in multi-truss
crops growing under high EC conditions. Therefore, single-truss cropping has
the potential to produce the highly flavored tomato fruit that consumers are
continually asking for without the yield losses to the grower than normally
accompany such quality.

One of the main reasons why single-truss cropping has been
studied is the fact that the time from planting to harvest can be precisely
estimated, and computer models have been developed to calculate such figures.
This means that if a grower wants to target a particular market date with an
entire crop, it can be determined exactly when plants need to sown or planted.
Single-truss crops develop rapidly once the growing point has been removed and
have been used as filler short-term crops in greenhouses that are used for
other plants and are empty for a few months a year. This is also useful where
growers want to produce one last crop before winter sets in and growing becomes
economically prohibitive. It’s also possible to produce fruit continually by
planting out blocks of seedlings every three weeks so that there’s always a
block of mature fruit being harvested while others are in various stages of
development.

The single-truss system has proven to produce yields as high
as 1.4 oz. per square inch per year, which is compatible to top tomato
producers using the multi-truss system — although there’s a higher requirement
for growing seedlings, planting and plant removal.

Yields and Flavor Quality

There are numerous reports from all over the world stating
that consumers are often disappointed with fresh tomato flavor, and
out-of-season greenhouse tomatoes are often described as watery or tasteless.
This may be due to the fact that many modern varieties haven’t been bred for
flavor or because they’re grown intensively with emphasis on yield, good visual
quality and long shelf life. This is unfortunate since tomato crops can be
manipulated by the nutritional program used to improve fruit quality. Selection
of different cultivars also plays a major role when looking to improve fruit
flavor, and many growers are now producing "high flavor" lines.

A number of factors influence tomato fruit flavor: plant
genetics, light levels, temperature, water stress, raised salinity, fertilizer
additions and leaf area (as influenced by the training system used). Many of
these variables can be manipulated by growers to increase the flavor of tomato
fruit.

Light is the chief factor that determines the level of
photosynthesis of the plant and thus the amount of sugars and dry matter
available to the fruit. Poor light results in low sugar and dry matter content
by limiting photosynthate production. Researchers have found that the soluble
solids of tomato cultivars increase significantly under a 16-hour day as
compared to a 12-hour day but that fruit acidity isn’t affected. The effect of
season is linked to changes in both light and temperature, which have a major
influence on fruit quality. Both temperature and light affect color in tomato
fruit, while soluble solids are known to increase with a reduction in available
media moisture and periods of high temperature. High air temperatures (above
84° F) are detrimental not only to tomato fruit yields but also quality.
Fruit needs to be protected from direct radiation in areas of high light
intensity. High greenhouse temperatures have been linked to lower-quality fruit
and lower yields with a high incidence of soft fruit with a limited shelf life as
well as ripening disorders, such as blotch and crazing. Excessive leaf removal
around the lower fruit trusses is also another common cause of poor fruit
quality. Leaves are required to produce sugars for importation into the fruit
and leaf removal has been shown to significantly lower fruit quality on
surrounding trusses.

One of the easiest ways to improve tomato flavor lies within
the root zone. How the grower manages water and fertilizer inputs has a major
effect on both the yield and quality of the fruit produced. The simplest way of
increasing the flavor constituents of tomato fruit is to increase the EC of the
nutrient solution. This will help produce fruits Á with a higher
percentage dry

matter, sugar and acid and, consequently, better taste and
firmness. This has been found to be the case with both large-fruited and cherry
tomatoes. It has been found that both sugars and acidity levels increased in
the cherry cultivar Gardeners Delight with fruit grown at an EC of 10 mS/cm-1
as compared to an EC of 2.5 mS/cm-1. Other studies have reported that the dry
matter content, sodium content and acidity of fruit grown at an EC of 8 mS/cm-1
was greater than fruit grown at 3 mS/cm-1.

Increasing the EC to boost fruit dry matter content reduces
the rate of water accumulation and subsequent cell enlargement, so a loss in
yield is almost inevitable. Therefore, there has been little incentive for a
commercial grower who is paid by the pound to increase fruit quality — until
recently, with the increased consumer interest in fruit taste.

High Yields, Exceptional Taste

Hydroponic tomato production involves a number of complex
interactions between plant genetics, the environment, and management of the
crop, each of which plays a role in fruit yield and quality determination.
However, the tomato fruit provides many opportunities for growers — both large
and small — to manipulate both plant growth and the flavor quality of the
fruit with the use of nutrition, training and cultivar selection. With the
right information and experimentation, tomato fruit of exceptional taste and
high yields can be achieved from a wide range of different soilless production
systems.

This article was reprinted with permission of The Growing
Edge magazine.