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13 Jan 2013
Seed Potatoes
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The supply of peat
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Tomatoes 2000

A professional grower will aim to produce around 50 kg of glasshouse tomatoes per square metre. That is equivalent to 500kg for a 12 x 8 foot glasshouse. Clearly the professional starts early in the year (December /January) and produces for a long season (to October/November) and it would be difficult to apply the same expert growing methods at home. Nonetheless, most amateurs probably fail to get anywhere near the potential of the tomato plants used either in terms of yield or in terms of quality. Yields can be low and Blossom End Rot (BER), usually blamed onto irregular watering, seems to be familiar to many home tomato growers (it causes blackening of the tomato centre underneath the fruit ie opposite side to the stalk).

Tomatoes are often grown in peat-based growing bags in a greenhouse. We see what happens if you use different volumes of compost, compost diluents and fertilisers and take a look at a hydroponic method for comparison.

What we tested

Typically three plants are grown in a growing bag containing a peat-based compost, plants are watered when required and a liquid feed applied through the summer. What could be wrong with such a simple method?
We decided to grow tomatoes in large containers and look at three main factors:

  1. Volume of compost
    - Three plants in the average growing bag means around 10 to 12 litres of compost per plant (unless the roots overlap?). Does it help to increase the volume of compost per plant?
  2. Type of compost
    - The three basic needs of the tomato root system are air, water and nutrients. The particular qualities of peat, bark, soil, sand and other growing media determine just how the air, water and nutrients are supplied to the plant. Can we improve the performance of a peat-based compost by mixing in different materials? We looked at soil+grit and perlite as diluents.
  3. Fertiliser/nutrient levels
    - Leaving aside, for the time being, the balance of NPK nutrients used (see the Bonanza Bolero Marigold game elsewhere on this site) more or less fertiliser/nutrients should make a big difference to the way the tomato grows. But its not quite that simple because a well-nourished tomato plant can produce lots of leaf and few tomatoes: a bit of a waste of time since its not a particularly beautiful plant, even when it has lots of leaves. So is the use of extra fertiliser worth the trouble and expense?
  4. Keeping it simple
    - What about trying a simple hydroponic method of growing tomatoes? Can you get reasonable yields and does it have any effect on problems such as BER?

No doubt the experts among you will feel that you know the answers to these questions. The inexpert will possibly have found it all too complicated to even care. However a lot of gardeners try to grow tomatoes - so, instead of trotting out all the traditional expert recommendations (which may or may not work) we tested a few ideas and you can see the results. You can then make up your own mind whether any of the ideas tested have anything to offer. The trial started in May 2000 and carried through into September/October.

Details of the trial

'Shirley' tomato plants grown from seed were planted into large identical containers when the first truss showed flowers. Plants were grown in 8 different ways. For convenience we gave each way of growing the plants a treatment number and a short-hand code to help remind you of the particular details of the treatment, ie the way the plant was being grown. The 8 treatments were:

T1 (Code P10L) - this is the standard for comparison with other treatments and is similar to growing three tomatoes in a normal growing bag containing 30 litres of peat compost. Each tomato in this treatment was planted in 10 litres of a good quality peat-based compost taken from a branded growing bag. The plants were fed with a branded liquid tomato fertiliser as directed on the product label.

T2 (Code P15L) - identical to P10L but with the plant growing in 15 litres of compost. Did the increase in volume (and of course the extra nutrient supplied) really increase the amount of ripe tomatoes collected?

T3 (Code P15PeL) - identical to P10L but with an added 5 litres of coarse Perlite mixed into the compost. So, the plant had an increase in root space to 15 litres but no added nutrient (Perlite is inert and contains no nutrients).

T4 (Code P15SGL) - identical to P10L but instead of adding Perlite we mixed 2.5 litres of clay loam soil and 2.5 litres of coarse grit with the 10 litres of peat compost. Possibly some nutrient added by the soil but the structure of the compost was different and so was behaviour of nutrients, water and air around the roots, But did this change make any difference?

T5 (Code P10L+CRF) - identical to P10L but with added slow release fertiliser (sometimes called controlled release fertiliser = CRF). The added fertiliser (designed to release nutrients for a period of 5/6 months) provided a steady supply of extra nutrients to the plant throughout the summer. Did this added fertiliser simply increase the size of the plant or did it actually provide more ripe tomatoes?

T6 (Code P10+CRF) - identical to P10L but the liquid feed is replaced by the CRF. Did such a fertiliser work as well as the weekly liquid feed?

T7 (Code PF10L) - plants grown in 10 litres of compost taken from a branded peat-free growing bag. Did the tomatoes do as well in one of the peat-free products? Plants liquid fed as for P10L

T8 (Code C15H) - tomatoes were planted into 15 litres of coir pith and fed with a proprietary hydroponic nutrient solution.

Key Dates

Event Date
'Shirley' tomato seed sown 15 March
Planted out into containers 14 May
Start hydroponic feed 19 May
 Start liquid feed 8 June
 Yield of ripe tomatoes end July
 Yield of ripe tomatoes end August
 Yield of ripe tomatoes end September

Materials used

  • Peat-based compost was taken from Levington Grobags
  • Peat-free compost was the J Arthur Bower's New Horizon Multi-Purpose Compost
  • Coir, ex Sri Lanka
  • Liquid tomato feed was Tomorite
  • Controlled Release Fertiliser was Osmocote


Perfectly normal plants can be grown simply in a balanced nutrient solution. Coir pith, a waste material from the coconut fibre industry is excellent for allowing free development of plant roots and will hold large amounts of water and nutrients. Nutrient solutions used in this trial were supplied by Growth Technology Ltd .

Progress to end of May

Tomato plants are all well established and growing fast with first trusses flowering. The hydroponic plants are as tall as the other plants but less green and less leafy at this early stage, possibly due to the short period at establishment before feeding began. Some plants have set fruit.

A few whitefly have appeared and will be controlled by chemical means for convenience.

Progress to end of June

On July 3 the plants were measured for height (cm) and assessed for colour (depth of green) and vigour (on a 0 - 5 scale).

All plants have grown normally to the end of June.

All plants were similar in height at around 120 cm except plants growing in the peat-free compost(Code PF10L) which were on average about 10 cm taller.

In terms of colour, P10L plants were a lighter green than most of the other plants.

In terms of vigour, all plants scored 4.00 or more on a 0-5 scale.

Progress to end of July

Tomatoes were forming quickly by 11 July despite the generally cool dull weather

By the end of July all plants in the Tomato trial are growing vigorously with plenty of fruit developing. Perhaps because of the generally poor summer so far few of the fruit have ripened with from none to four ripe fruit per plant being harvested.

The standard T1 treatment (Code P10L) produced no ripe fruit on four plants. Nine ripe fruit were collected from the four plants grown in 10 litres of peat compost fed with the controlled release fertiliser (Code P10+CRF). All the other treatments produced from 1 to 3 ripe fruit on four plants.

Whiteflies have suddenly appeared and are infesting all plants. Chemical treatments are being tested to see if we can control or reduce the pest. We will report on any results we get.

Progress to end of August

During August all tomato plants have grown well and produced ripe fruit. By the end of the month the weight of good ripe fruit collected per plant off each of the treatments was as follows:

tomatoes graph

Note: BER = Blossom End Rot

The highest yield of ripe tomatoes to the end of August was produced where soil and grit had been added (Code P15SGL) to the basic peat-based compost. Although the yield was however only slightly above the standard 10 litres of peat compost (Code P10L) the addition of soil and grit appeared to reduce the amount of Blossom End Rot.

Increasing the volume of compost to 15 litres (P15L) unexpectedly reduced yield and so too did the change from liquid feed (P10L) to controlled release feed(P10+CRF). The use of both liquid feed and the controlled release fertiliser (P10L+CRF) reduced yield even further. The peat-free compost (PF10L) yielded less than the peat-based compost and the lowest yield was recorded on the tomatoes grown in a simple hydroponic system (C15H). As yield of good ripe tomatoes decreased so the amount of BER affected tomatoes increased.

From these results it looks as if too much fertiliser can reduce the yield of ripe tomatoes and increase Blossom End Rot.

Final Results at End of September

All ripe, green and BER (blossom-end-rot affected) tomatoes were collected from the plants on 28 September. The total weight of tomatoes collected from each of the different compost/fertiliser treatments was as follows:

tomatoes graph

All the plants in this trial grew vigorously and in good health throughout with only very small differences visible between them. The hydroponic plants were equal in appearance to the best of the other treatments.

Best result produced by adding soil and grit to compost
By the end of the season the 10 litres of growing bag compost to which soil and grit had been added (T4) produced almost 4.5 kg of good quality ripe tomatoes per plant and at the end of September carried the most good quality green tomatoes. During the full growing season only one tomato affected by blossom end rot (BER) had been found on the four plants grown using the soil+grit compost amendment.

Perlite a good compost dilutor
The second highest yield was produced where 5 litres of Perlite has been added to the 10 litres of growing bag compost (T3) and this mix only developed 8 BER tomatoes from four plants during the trial.

Good Results with a Standard Growing Bag Compost
10 litres of standard compost (T1) comfortably yielded over 4 kg tomatoes, equivalent to around 12.5 kg from a growing bag containing 30 or more litres (they usually contain 35 to 45 litres)

Increasing volume not worth it
Increasing the compost volume from 10 (T1) to 15 litres (T2) per plant did not increase yield.

CRF as almost as good as liquid feed but only one of them needed
On a statistical basis there was no difference between liquid feeding and using a CRF mixed with the compost before planting. Using both appears to depress yield significantly.

Peatfree could do better?
A peat-free compost produced a very respectable yield of tomatoes: nutrient content could need adjustment.

Hydroponics promising but needs more work
The hydroponic plants were as tall, as healthy and as vigorous and the tomatoes as big as any of the other treatments but the yield of ripe tomatoes was lower and there were more BER tomatoes. A small adjustment in the feeding regime to favour fruit production rather than leaf production could make a big difference. The system was simple and easy to use for tomatoes.

Whitefly - prevention better than cure?
We failed to get control of the pest particularly during the early stages of the trial and as a result suffered in late August and September when insect numbers increased despite repeated applications of different chemical products. By the end of September the leaves of the tomato plants were covered in sooty mould as a result of fungus developing on the honeydew produced by the whitefly and very large numbers of whitefly on the undersides of the leaves.

There is widespread whitefly resistance to the chemicals available to gardeners in the UK and it is therefore difficult to get control once the numbers of whitefly increase. The repeated introduction of the parasite Encarsia formosa early in the season when the whitefly are first seen would be a preferred method of attempting control.


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