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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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