A large proportion of plants are grown and sold in containers. The nutrition of these plants is probably more vital than of those growing in the open ground.

The reasons for this are as follows:

a) The roots cannot explore a large area of soil/compost.

b) It may be desirable to obtain maximum growth and yield.

c) Leaching (ie downward movement of nutrients) from the container can be significant where overhead watering is carried out.

d) The growing media may have a low nutrient status eg if using loamless composts consisting of one or more of the following which have little or no food value:-

Bark, peat, perlite, vermiculite, sand, grit.

Plants grown in a loam-based compost eg John Innes Potting Compost No. 2 generally require less regular feeding because of the nutrient reserve in the loam.


a) Drv Fertiliser Feed

i) Mixed with the compost.

ii) Given as a top dressing: this can be carried out satisfactorily on a small scale but is time consuming on a large scale.

b) Liquid Feed

Applied mainly to the containers in which plants are growing. Fertilisers used are mainly inorganic e.g. potassium nitrate (15%N 44%K20) though the organic fertiliser dried blood (11-14%N) can be used. The fertiliser is mixed with a small amount of water to form a concentrated stock solution: a typical ratio for dilution would be 1:200 I.e. 1 part stock solution to 200 parts water.

White polystyrene pots help to keep the roots cooler – of great benefit in the summer period as heat stressed roots do not exploit the compost as successfully as those in soils below 22°C. Polystyrene foam pots and units are popular and travel well.


These are plastic squares in units made to fit inside seed trays and are used mainly for bedding plants. There are several sizes ie 15, 24, 40, 60 in a unit.

Four or five compartment Vacapaks are also used for bedding plants, and are also fitted inside standard sized seed trays. They are mainly used for plants sold by the strip. These have been very popular with the public who receive plants with all their roots intact and also with the nurserymen who have found the system generally more profitable.

With the increasing use of capillary benches, and of capillary matting laid on benches, plants grown in plastic pots and peat/sand composts have become very popular because they may take up water more readily than those in clay pots.

Clav pots

These have been in use for many years and are still used but their chief disadvantages, compared with plastics, are that they are easily broken and are more difficult to stack and handle generally. Also cost, cleanliness and weight are major factors to be considered.

Paper (and similar type pots

These are mainly used for plants which are to be planted out later, as their life is short. With the thick papier mache type, the bacteria which break down the pots use up some of the nitrogen and extra feeding may be needed to compensate for this. Pots made from heavy gauge bituminised papers are sometimes used.

There also exist the Japanese paper pots which come in a strip folded flat and open out like a Christmas decoration to form maybe 180 compartments or pots. These are used for seedlings by specialist nurserymen.

Peat pots

Jiffy pots are a well known example of this type and small sizes in strips are much used in propagation, e.g. for sowing seeds direct, or inserting cuttings direct into the modules.

Peat modules, made from peat milled and mixed to cohere when wet and compressed into a suitable size of cube is a popular commercial alternative.

A peat blocking machine on a small nursery growing say 100,000 lettuce each year would quickly pay for itself compared with buying ready moulded peat modules like Jiffy pots.

Polythene pots

The larger lav-flat polythene pots are often used for shrubs and conifers for garden centre sale, and smaller sizes for Ericas and herbaceous plants. The smaller sizes are also used for growing on young shrubs. They are the least expensive type of pot. (Often called polythene bags inch). Care should be taken when potting to ensure that the compost is made adequately firm, and that the base of the pots is made level. Not all types on the market have either good enough drainage holes or adequate holes for capillary watering.

Examples of such plants are Chrysanthemum and Cyclamen, which may do better in J.I.P.2, I.e. where 225g of Base is added to each bushel instead of 1 lOg. If 340g is added the compost becomes J.I.P.3.

For ericaceous plants it is better to omit the chalk in the base, and use flowers of sulphur in its place. The possible use of urea formaldehyde in place of hoof and horn in the base fertiliser mixture should be noted. Cyclamen don’t like lime particularly.

The J.I. Base can be bought ready made up, but the formula is as follows:

Parts by weight: 2 hoof and horn meal

2 superphosphate 1 sulphate of potash

A note on volumes

The bushel measure has largely gone but it holds 36 Its (8 galls). The metric system with a cubic metre holding 1000 litres is quite easy to use because the weight of fertilisers needed for a quantity less man 1 cubic metre can be calculated. For 1 litre of mix only 1000th of the weight per metre is required so if one had a wheelbarrow mix of say 75 litres and if the recipe required 3.5Kg of fertiliser per m then the amount needed would be:

3500 grams x 75 = 262g



There is a wide range of types of pots used in horticulture and the past few years has seen a great increase in the use of plastic pots. These have replaced the traditional clay, peat or paper pots for many commercially grown pot plants. Brief details of the main types of pots available at the present time are as follows:

Plastic pots

These are light to handle, easier to stack than clay pots and are virtually unbreakable in normal use. Watering of plants grown in plastic needs close attention, as the compost does not dry out as quickly as that in clay pots. Pots of less than usual height are used for certain plants, e.g. A.Y.R. chrysanthemums, and extra deep pots for some others like Clematis.

Whilst the peat and sand in the J.I. composts tend to buffer unsuitable soil textures, it I desirable to obtain good fibrous loam.

The loam should be partially sterilised by raising the temperature to 80°C. Sand (some sands and many peats are routinely sterilised) need not be sterilised. Since the peat is naturally sterile, it may be used as it is. After the loam has been sterilised, it should be mixed with peat and sand, adding the fertilisers at the same time. Recent work on potting composts has been concentrated on eliminating loam because of its variability, and loamless composts are being used by many growers.

Commercial growers tend to sterilise all the compost bulky materials, and then add the fertilisers. This reduces still further the disease risks.

The two composts recommended for general use are:-

Seed sowing and pricking out

2 parts of bulk by medium loam ) adding to each bushel

1 part by bulk of good moss peat ) of this mixture:

1 part by bulk of coarse sand ) 42g superphosphate of lime and

20g ground limestone or chalk

or Superphosphate 1.2 grams per litre Ground limestone 0.6 grams per litre


7 parts by bulk loam ) plus J.I. Base 0.11 Kg )

3 parts by bulk peat ) ground chalk 20g ) per bushel

2. parts by bulk sand )

or J.I. Base 3.0 Kg chalk 0.6 Kg per cubic metre

This potting compost is the standard compost, and is suitable for most plants. Those requiring a richer soil, should be potted into the standard compost to which an extra dose of J.I. base and lime have been added (J.I.P.2) or in some cases three doses of J.I. Base has been added.

2. Virtually no need to sterilise.

3. Uniform physical condition/composition if mixed correctly.

4. Lightweight – handles more easily.

5. Better aeration and drainage.

6. Improved water-holding capacity.


These rarely have natural.porosity or sufficient drainage and aeration to cope with the greater amount of water that is required by container-grown plants.

Sand, if used, should contain no free lime. Always analyse a sand at source prior to purchase and check its lime content. For a loamless compost the particle size is not as critical as the grade. A uniform grade within the range of 0.5 – 3mm is acceptable. If a sand has a wide range of particle sizes, it may pack tightly, reducing air spaces and have an adverse effect on rooting and drainage. Keep sand free from contamination by weed seeds and other debris.

Peat should have a moderately coarse texture with particles grading evenly 3 – 10mm and be free from dust-like particles. The most suitable peat is sphagnum. Irish, English and Scottish are better overall than the much younger Russian, Polish and Finnish types. The pH should be in the range 3.0 – 5.0. Any peat outside these limits should not be used.

Granulated pine bark, used correctly, offers advantages for potting and rooting composts. It helps to open up the structure of otherwise fine mixes with the result that improved aeration promotes better root activity and can reduce waterlogging.


Many growers still use the John Innes Composts, which are free from weed seeds, pests and diseases, and contain a balanced supply of plant foods. They depend on supplies of loam at reasonable prices. Very often good loam, neither too heavy nor too light, and not too calcareous, is difficult to buy at economic prices, and the position is unlikely to improve. Sand or grit varies. The best consists of about 70% particles of 1.5 – 3mm in size.

Loam should be medium in texture : very light sandy loams should be avoided. A soil with a pH of 5.5 – 6.5 is best. Where analysis shows that the phosphate is less than 20 ppm, and potash less than 170 ppm, potash and phosphate should be added to correct the deficiency.


Equal parts by volume of moderately coarse textured peat and sand with a uniform grade 0.5 to 3mm. To each cubic metre add:-

0.4 Kg potassium nitrate

0.75 Kg powdered superphosphate (18% P205)

3 Kg ground chalk or limestone

For 100 litres divide the weights by 10.


Three parts by volume of peat and one part sand. To each cubic metre for the ‘B’ general use mix, add:

For immediate use and short term storage only:

0.4 Kg ammonium nitrate

0.75 Kg potassium nitrate

1.5 Kg powdered superphosphate (18% P205)

2.25 Kg chalk or ground limestone

2.25 Kg Dolomitic limestone (ground magnesian limestone)

0.4 Kg Frit No. WM

For longer term use – Mix E (High Phosphate)

1.5 Kg magnesium ammonium phosphate

0.2 Kg ammonium nitrate

0.4 Kg potassium nitrate

2.25 Kg chalk or ground limestone

2.25 Kg Dolomitic limestone (ground magnesian limestone)

0.4 Kg Frit No. M

Mix D for summer use has urea formaldehyde (Nitroform) in place of NH4NO the ammonium nitrate.

The amount of urea formaldehyde in the compost for immediate use (Mix B) should be increased to 1 Kg for summer use and even to 1.5 for gross feeders, but fertiliser levels in the one for long term use (Mix E) should not be increased. Medium grade sphagnum peat is recommended for the composts but fine grade may be used for the seed sowing mix. Sand should be free from lime and salt.

Today loamless composts offer many advantages to the grower:

1. Freedom from weeds, pests and diseases.

A. Elevator for unwrapped bales of peat/bark/grits as required.

B. Hopper weigher for bulk ingredients.

C. Very accurate scales for quantities of fertilisers and lime FTE + fungicides if required.

W. Water as required.

D. Large drum mixer.

E. Elevator to take the mixed compost to the point of use or transportation.


Several proprietary peat and sand composts are available, with nutrients added usually including trace elements. Some growers use their own mix e.g. 3 or 4 parts peat to 1 of a coarse sand plus a base fertiliser, either proprietary or similar; some of these may need watering before use, and individual product instructions should be carefully noted. These composts are used in propagation, in growing a wide range of pot plants, and they are also increasingly used for shrubs in containers for garden centre trade. The proportions of sand and peat may be varied to suit the subject being grown.

When peat and sand composts are made up on the nursery, variable results are sometimes obtained and poor mixing can often be the cause. Mixtures advised by the I.H.R. – the Institute of Horticultural Research at Littlehampton, Sussex, formerly the G.C.R.I. [Glasshouse Crops Research Institute] are as follows:

necessary nutrients need to be supplied. However, the initial soluble fertiliser levels should never be so high as to reduce germination or inhibit uptake of water by roots.

Present day composts are mainly made up from: (Bulk constituents only)


(a) 100% peat

(b) 75% peat 25% bark (normal potting mix)

(c) 60% Peat 40% bark (liner mixes).

Some producers will add sand (grit) for stability, others will add a small percentage of sterilised loam e.g. 5 or 10% to give the compost a buffering factor. The use of loam is waning as it is difficult to obtain good fibrous loam which is weed, seed and disease free. If a good source is available then it may be steam sterilised prior to use in a compost.


Hand mixing is rare in commercial practice, indeed many growers buy in their composts ready mixed. However it is useful to know how to make a good mix by hand.

Start with a flat, level heap Turn and turn back.

with even strata of peat/grit/ Keep a space between the

bark or loam etc. mixed and the unmixed


Mixing of the peat, bark, sand and the controlled release fertilisers and other ingredients must be thorough to give a uniform compost and more important, even growth. This can be obtained by pre-mixing the fertilisers with a little sand. This can then be added as required while the machine is in motion. Avoid increasing the time of mixing in a mechanical mixer to obtain a more thorough mix since peat can break down to a fine dust which will clog air spaces in the compost and reduce aeration/drainage.

The fertiliser content of potting compost should always be measured according to the needs of the specific crop for which it is going to be used. Trace elements are needed for peat-based composts because peat contains almost no manganese, boron, zinc, copper, iron or molybdenum.

The range of soil and soil-less mixes has evolved partly in the quest for a better compost, partly because of shortages of fundamental materials like sphagnum peat or fibrous loam and partly because of cost factors, as well as the desire to provide optimum growth.

It is probable that no one compost has all the answers for all the crops, but did nature ever intend that its plants should have such uniformity? It is reasonable to suppose that plants which root on trees may prefer to have their roots infinitely more exposed to the air than bullrushes growing at a stream side.

The pot and its compost have to provide:

Compost must be structured to permit air spaces; to hold water, to provide anchorage. (Desirable – to hold nutrients)

Drainage holes

(Desirable) – small feet to

assist water passage

Pots don’t have to be rigid or black, white or permeable. Each has some advantages

(desirable) – A watering space


In this system the plants’ roots are supported in rockwool.

This is very like the rockwool used in house insulation but it has usually been treated to balance the pH. Following propagation in small modules made of rockwool the plants are set out on to pillowcase-like wrapped rockwool modules. The rockwool is able to hold about 40% of its volume as water and the remainder is largely air.

There are no pests or diseases and hence the crops’ root requirements are largely met apart from basic nutritional fertility. These needs can be supplied through the liquid drip feeding technique. The composition of the liquid feed will be calculated taking into account sunshine, crop growth stage and temperature. Monitoring of the nutrient status of the fluid in the rockwool module is achieved by thrice weekly samples being analysed. It is readily obvious that this is a very specialist field of activity, but the yields of tomatoes can be colossal: even 300,000 Kg Ha (about 120-140 tons per acre) of marketed fruit.



A suitable compost must have physical structure, and be able to supply an adequate and balanced supply of nutrients. Composts should be free from weed seeds, pests, diseases and materials’ that may be toxic to plant growth. The physical structure will provide adequate porosity to allow air movement and be able to retain water for plants’ requirements.

Nutrients. The major nutrients required by the plant are supplied by fertilisers incorporated during preparation of the compost or subsequently by top dressings or liquid feeding. It is usual for a combination of these methods to be used for container grown plants. Peat, bark and sand are deficient in most nutrients, and when these are used without loam (soil) all the

Important factors for the ideal compost would be as follows:

A. The density of the compost is important to give the right amount of stability and

support to the plant.

B. It must be a source of nutrient for developing roots.

C. Free drainage. is essential to avoid waterlogging and any anaerobic conditions

developing. Excess water from irrigation should freely drain away.

D. Sufficient water retention for the needs of the plant.

E. Source of oxygen supply available for the roots by the density of the compost having

sufficient air spaces.

F. It must be low in any excess fertiliser salts and free from all toxic substances.

G. Free from pests and diseases, I.e. fungal spores, insect eggs.

H. Be the right consistency for expansion of root growth in the pot.

I. The correct pH (acidity/alkalinity).

As an alternative to trickle or similar irrigation laid on the benches the sand can be kept moist by water fed from a header tank fitted with a flat expanded polystyrene float and the level of water in the sand bed is kept at about 50mm below the surface.

The main aim with this method of watering is to cut down labour. If trickle irrigation is used, liquid feeding can be combined in the operation. The system is best suited for quick growing plants which have a fairly high water requirement.

With liquid feeding a salt concentration can build up in the surface of the sand due to evaporation, and leaching may be needed to prevent possibility of damage.

Capillary systems are helpful to amateurs because they (in theory) provide large or small plants with the water that they need to sustain full growth without overwatering (with the consequent loss of air – death of roots and a check to growth).


e.g. time controlled

CAPILLARY MATTING (often covered with microperforated black polythene)


This system works very well indeed, it relies on a thin film of very dilute liquid feed flowing down a darkened black polythene trough in which the roots of the crop plants (mainly tomatoes and cucumbers) are growing. The fact that the water film is so thin enables the roots to have sufficient oxygen aeration. The system is dependent on electrical supplies and time switches. The cost of running pumps and their reliability or otherwise, has tended to encourage growers to look for other growing systems and one of the successors is the use of rockwool.

To meet their growing needs plants have to have water available or there is an immediate check to growth and development. The water also carries dissolved salts essential for growth. The mechanisms for the provision of water may vary and include can and rose, controlled overhead spray lines, controlled ‘point irrigation’ (drip systems), capillary systems and flood benches, and the more specialised nutrient film techniques and rockwool systems mentioned later. The essential provision is water and the non-exclusion of air (oxygen). Air must be retained at the roots for the root hairs to stay alive.

Time-switched systems are helpful for the amateur because they are fairly reliable and are easy to set. The usual system is a mains or battery driven time clock and solenoid valve which turns the water on or off. This may then flood a capillary mat covering a level bench which in turn supplies the water required by the plants, or it could operate a spray line, or a point irrigation system (where each pot has a drip feeder).

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