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Greenhouse Tomato Nutrition
By Dr Lynette Morgan
Suntec International Hydroponic Consultants
The Tomato Magazine
December 2006
One of the most important aspects of
greenhouse tomato production is nutrition.
Tomato plants, particularly modern
hybrid cultivars, have the potential to be
extremely high yielding compared to tomato
crops grown a few decades ago. Top producers
can now expect to obtain over 40 Kg per
square meter per year of high quality tomato
fruit compared to only around 20 Kg square
meter 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 environment growing
structures.
Tomato plant nutrition is one factor
which is continually changing with the use
of new cultivars, systems and methods of
production. There is no one `ideal’ or ‘optimal’ nutrient
program for greenhouse or hydroponic tomato crops, as each crop is different
and requires continual monitoring of
the nutritional status of the plants.
One Major Advantage
One major advantage of hydroponic
cropping is that it allows very precise control
and monitoring of tomato plant nutrition.
Growers can gauge nutrient uptake with
regular solution or leachate analysis and with
computerized records, patterns of nutrient
uptake, adjustments, and resulting yields can
be plotted and used for reference in further
crops.
Recirculating hydroponic systems are
a useful tool to examine nutrient uptake in
tomato crops, and much data has been published
on this subject. However, every grower’s
situation is different, and the uptake of
each element varies depending on a number
of factors. For this reason, commercial
growers need to know both the basics of
tomato nutrition and how this applies to their
particular cropping situation. Use of regular
nutritional analysis should be part of any
commercial tomato production operation so
that decisions are based on what the current crop requires at the present
time, not just following a `blueprint’ for crop production.
Tomato Crop Nutrition During Development
While the average tomato fruit is well
over 90 percent water at harvest, certain
nutrients are essential for fruit growth and
quality. Tomato fruit require good amounts
of nitrogen, phosphorus, high amounts of
calcium and often extremely high levels
of potassium if fruit quality is to be maximized.
Large amounts of phosphorus (P) are
required for seed formation within the fruit.
A fruiting tomato plant absorbs proportionately
more phosphorus than a non-fruiting or
vegetative plant.
In tomato crops, the requirement for potassium is about the same as for nitrogen in the early crop stages (from seedling through until fruit development). After this, the requirement for potassium keeps increasing with fruit load while nitrogen levels off. While nitrogen is important and is used in large quantities for vegetative growth, potassium is the predominant cation in tomato fruit and has major effects on fruit quality.
The majority of the potassium absorbed
by the plants during the active fruiting stage
ends up in the fruit. This is why ratios of
potassium need to be maintained at higher
levels during the fruiting stages than during
the vegetative and fl owering stages. Thus,
as crop load on the plant increases, so does
the requirement and absorption of potassium
which will become part of the fruit
tissue. If potassium becomes deficient during
the fruiting phase of a tomato crop, both
yield and fruit quality will suffer greatly.
Potassium is directly related to fruit quality,
via the acidity and fl avor of the fruit, firmness,
ripening disorders, color and shelf life.
As such, it is vital to maintain high levels
during development.
Maintaining Adequate Levels of N
Despite the importance of potassium during
fruit development, levels of nitrogen also
need to be maintained particularly during the
pre-flowering stage. It has been shown that
the concentration of nitrogen before initiation
of the fi rst fl ower truss was of crucial
importance in determining yield.
Studies have also found that hydroponic tomato plants growing under optimal conditions, carrying high fruit loads, can take up 140 - 230 mg of potassium per day. Similar figures for nitrogen are in the range of 80 - 110 mg per plant per day and 22 - 35 mg per plant per day of phosphorus. The potassium requirement of a fruiting tomato plant is highest about the time the ninth truss is in flower; this is when high fruit loading is occurring and when potassium depletion in many systems becomes most common when levels are insuffi cient. Fruit deficient in potassium have a lower overall flavor and shelf life quality and can also suffer from ripening disorders such as blotchy ripening, gray wall, cloud, lack of good coloration and can be described as `watery’.
Calcium is another mineral essential for fruit growth and development. The supply of calcium is more critical during the phase when there is rapid visible size increase as it is required for the formation of new cells and for strong cell structure. A lack of calcium transportation into the fruit can rapidly result in the development of blossom end rot.
Hydroponic growers need good knowledge
of average plant uptake rates of each of
the major nutrients as the crop comes into a
full fruit load. This is of even greater importance
where recirculating systems, such as
NFT, are used for a fruiting tomato crop.
Levels of potassium, in particular, can be
stripped from the solution within a few days
if not continually monitored and maintained.
Commercial growers will often have regular
solution analysis carried out to determine
mineral uptake levels for the crop during
different growth stages throughout the year.
This process provides valuable data for
formulating and adjusting nutrient solutions
which then ensures the plants do not become deficient in one
nutrient and a
good balance is maintained.
Tables 1 and 2 show how the ratios of nitrogen and potassium change during the life of a tomato crop.
While Table 1 shows an
overall trend in increasing
potassium uptake as fruit load
progresses, these same levels
will not hold true for all tomato
crops. Crops growing under
higher light have been shown to
take up much greater quantities
of potassium. It is highly likely that other
cultivars also have slightly different uptake
rates. Growers need to work through a process
of determining the best ratios of potassium
to nitrogen at these different growth
stages for their own production systems
under different seasonal conditions. This
information, however, can form the basis of
a `starter’ nutrient formulation for a tomato
crop with adjustments made in the potassium
to nitrogen (K:N) ratio as crop growth
progresses.
An example of how nutrient solution
levels need to change for season and stage of
growth is summarized in Table 3.
Nutrition in Different Types
of Soilless Production Systems
Recirulating systems such as NFT
(nutrient fi lm technique), DFT (deep
flow technique) or any system where the
nutrient is continually applied, drained
and recollected and reapplied to the crop
require a different nutrient formulation and
management system to `drain to waste’ or
non-recirculating media based systems.Recirculating systems tend to start
off with
a well balanced nutrient formula which
contains plenty of each of the elements
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,
thus resulting in imbalances in
the nutrient solution which the
grower may not recognize. As
the EC is adjusted each day
with more stock solution and
additional water, some of the
nutrients under heavy demand
are replaced—sometimes not to
sufficient levels. One example
is potassium. In recirculating
systems where tomato plants are carrying
a heavy crop load, this element can be
depleted to very low levels within a few
days despite frequent top ups of concentrated
nutrient stock solutions.
In non-recirculating systems, which are
often seen as wasteful of nutrient solution
as the excess drains to waste, fresh nutrient
solution is applied at each irrigation; thus,
there is less chance of long-term depletion
of elements such as potassium if 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 leachate (excess nutrient that
drains out the base of media systems) often
show depletion in potassium or some of the
other macro nutrients during certain crop
stages. Management of non-recirculating
hydroponic systems can also be complicated
by the growing media used, some of which
may retain nutrients, particularly in the early
weeks of use.
Growers using coir (coco peat) growing
media should be aware of the CEC, potential
for nitrogen draw down and calcium retention
in the fi rst few weeks of use and make
regular use of leachate analysis and nutrient
adjustments to counter such problems. In
the later stages, leachate analysis will assist
in fine tuning nutrient programs to avoid
potassium depletion and prevent deficiencies
long before they can impact on yields and
fruit quality.
Nutrient Sampling for Analysis
While there is data available from a
number of studies on nutrient uptake rates in
tomato crops, this can only provide a rough
model or estimate of what is actually happening
in a grower’s present crop. This is
because the variables which effect nutrient
uptake, such as cultivar, light, temperature,
humidity, stage of growth, aeration and crop
health, differ considerably between growing
systems. It is often just not applicable to
take crop nutrient uptake data from another
country or climate and apply it to a crop
grown in a different area due to differences
in light and temperature levels. In this situation
a grower’s best tool is regular nutrient
analysis, which can be used to make adjustments
as required and ensure deficiencies
do not occur. This data can also be used to
predict how the next crop will react under
similar conditions in the following season.
Interpretation of Nutrient Solution
Analysis Results
The solution analysis report provided by
the lab will usually give the amount of each
element in ppm (parts per million) or mg/l
(milli grams per litre) equivalent. EC and
pH should also be given, and often other
variables such as total alkalinity, sum of
cations/anions and comments or
comparisons to `ideal’ levels of
each element. Ideally, the data
on the solution analysis report
should be compared back to the
levels of each element that the
original formulation contained.
So, if a nutrient formation was
created that has 150 ppm N, and
the analysis of the solution after a few weeks in use came back with a
level
of 145 ppm N, then this indicates the initial
level was approximately what the crop has
required and taken.
This type of monitoring is important in
rapidly growing crops with a heavy fruit
load, such as tomatoes, where potassium
can become stripped from the root zone at
each feed if not supplied at the correct rate
in solution. Regular leachate analysis of the
solution draining from the growing container
at a mid day feed will soon show up any
nutritional problems with the original feed
formula.
Other factors which need to be checked
on the analysis report are accumulation of
any trace elements, particularly those which
are known to be present in the water supply,
and any sodium accumulation in recirculating
systems as this will determine when the
nutrient needs to be dumped and replaced.
Foliar Analysis
The role of foliar (leaf tissue) analysis
is different from that of solution analysis.
Growers will often have a foliar mineral
analysis carried out to determine what is
happening inside the plant with regard to
nutrition. Often, certain elements may be in
sufficient quantities in the nutrient solution,
but some external or internal plant factor
is limiting uptake and distribution to some
plant tissues. One example of this is calcium
where the distribution of this element in
plant tissues such as the tips of leaves and
fruit, can be highly dependent on environmental
conditions, rather than the level of
calcium in the nutrient solution.
Generally however, it is not a good idea
to use tissue analysis to determine what to
add to a nutrient solution. By the time a leaf
has formed, the sample taken and results
returned from the lab, it has usually been 2-4
weeks since that tissue growth occurred and
therefore that analysis refl ects the nutrient
status 2-4 weeks ago, and the solution composition
will have changed in the meantime.
However, looking at the tissue analysis figures
of a well-grown crop (and this includes
both foliage and fruit for fruiting crops),
still gives a good indication of the ratio of
elements required. Foliar analysis is also a
vital tool for diagnosing mineral defi ciencies
or toxicities in crops—many of which may
look similar and need confi rmation via foliar
sampling and testing.
Table 4 shows the foliar nutrient level
for tomatoes. This looks as if nitrogen and
potassium are required in the same amounts,
since 4.0 - 5.0 percent of both N and K were
found in the leaf tissue. Looking at this,
a grower might therefore think that equal
amounts of N and K in the nutrient solution
will give the best growth and yields. But
what vital information is missing from this
picture is the mineral composition of the
fruit, which would include high levels of
potassium, well in excess of nitrogen found
in the fruit tissue. In fact, with tomatoes,
80 percent of the potassium taken up by the
root system will end up in the fruit tissue,
whereas only 40 – 50 percent of nitrogen
uptake goes into fruit tissue. If we take both
fruit and foliage mineral levels into consideration,
this gives a much better indication of
the overall nutrient requirement of the crop.
Summary
Nutrient uptake ratios change significantly throughout the tomato cropping
cycle
and are also infl uenced by environmental
factors such as temperature, humidity and
light levels. Nutrition plays a mayor role in
determining both yields and fruit quality,
and a well monitored and regularly adjusted
nutrient program can also result in cost savings
and prevention of fertilizer wastage.
Growers who can monitor, adjust and fine
tune plant nutrition are at an advantage when
it comes to greenhouse tomato production,
and increasing numbers of producers are
focusing more on developing this skill.
Sources of information
*Nutron 2000+ Edition 3 Hydroponic Nutrient
formulation software website:
www.suntec.co.nz/nutron.htm
*Adams P and Winsor G. W., 1979. Nutrient uptake.Annual report of the
Greenhouse Crops Research Institute, 1978 page 85-85.
*Carpena , O., Masaguer A and Sarro M, J., 1988.
Nutrient uptake by two cultivars of tomato plants.
Plant and Soil, Volume 105, page 294-296.
*Chu C. B. and Toop E. W., 1975. Effects of substrate
potassium, substrate temperature and light intently on
growth and uptake of major cations by greenhouse
tomatoes. Canadian Journal of Plant Science
Volume 55, page 121-126.
© 2006 Columbia
Publishing
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