Pots and Composts For Greenhouse Plants

For convenience most greenhouse plants are grown in pots, though some large plants are grown directly in beds of soil on the floor of the house and this method is also commonly used for tomatoes.

Pots may be of earthenware (clay pots) or of plastic. Clay pots, being a little porous, tend to dry out more rapidly than plastic pots and this involves a different frequency in the watering of plants grown in them. If pots of both types are used for the same kinds of plant in the same house it can be confusing and it is really better to determine to use either all clay pots or all plastic pots. Plastic pots are more easily cleaned than clay pots and they are much less likely to break, but they do not ‘breathe’ like clay pots and some gardeners regard this as a disadvantage.

Pots are made in different sizes and are usually referred to according to their diameter at the top. Thus a 3-in (8-cm) pot is one that measures 3 in (8 cm) across at the top and a 6-in (15-cm) pot one that measures 6in (15cm) across.

Plants usually do best when their roots occupy all or most of the soil in the pots. Overpotting means putting a plant into a pot much larger than is needed to accommodate its roots and is usually to be avoided. As plants grow they are potted on. That is, moved to pots of larger size, and generally it is wise to adopt a steady progression, potting from 3-in (8-cm) into 4-in (10-cm) or 4l-in (12-cm) pots, then on into 6-in (15-cm) pots, then perhaps to 8-in (20-cm) or 9-in (23-cm) pots.

The Basic Ingredients of Compost

It is rarely that ordinary garden soil without additions can be used satisfactorily for pot plants. Usually the soil, whether from the garden or fresh soil brought in from a meadow – in which case it will probably be described as virgin soil or loam – is mixed with other ingredients such as peat, leaf-mould and sand. The resulting mixture is referred to as a compost, which can be confusing as compost is also the name given to rotted vegetable refuse, a very different material.

Potting and Seed Composts

Composts for use in the greenhouse fall broadly into two groups; potting composts and seed composts. The former are intended for mature or sem1 mature plants, the latter for the germination of seeds and the cultivation of young seedlings. Potting composts are usually richer and more complex than seed composts.

The John Innes Mixtures.

The name John Innes Compost is given to a range of potting and seed composts developed at the John Innes Horticultural Institution. This research station first produced these composts for its own use, but published the formulae, so making them available to anyone else who cared to make them. Many horticultural suppliers do this and sell their products as John Innes Compost, but this does not mean that they have been approved by the John Innes Horticultural Institution, nor is it a guarantee of quality. That will depend upon how carefully the manufac-turer has chosen the ingredients and how well they have been prepared and mixed.

Basically the John Innes Potting Composts are composed of 7 parts, by bulk, loam, 3 parts peat and 2 parts sand. It is recommended that the loam should be fertile, neither light nor heavy, but slightly-greasy when smeared without being sticky. The peat should be fibrous or granulated. Grading up to | in (075 cm) particle size with a preponderance of |-in (0-25-011) particles and /;H between 4-0 and 5-0. The sand must not be too fine and should be clean and reasonably lime free.

The loam is to be steam sterilized before use but not the other ingredients. When the mixture is prepared, a base fertilizer is added and this is prepared by mixing 2 parts, by weight, of hoof and horn meal. For ordinary purposes 4oz of this mixture is added to each bushel of the compost and this is referred to as John Innes Potting Compost No. 1, or J.I.P.I for short, to distinguish it from John Innes Potting Compost No. 2 and John Innes Potting Compost No. 3. which contain respectively 8oz (225 g) and I2 0z (340 g) of base fertilizer per bushel of compost. These richer composts are used for strong-growing plants or plants that have reached the larger sizes of pot -say 5in (13cm) and over.

One further addition has to be made before the compost is complete, of chalk or limestone or of flowers of sulphur. For ordinary plants 21 g of either finely ground chalk or limestone is added to each bushel of compost. For plants such as heathers and rhododendrons, which dislike lime, 1oz (21g) of flowers of sulphur per bushel is used instead of chalk or limestone. These quantities are doubled for John Innes Compost No. 2 and trebled for John Innes Compost No. 3.

The John Innes Seed Compost is simpler. Consisting of 2 parts by bulk of loam, 1 part peat and 1 part sand, the specifications for each ingredient being the same as before. The loam is steam sterilized. No base fertilizer is added but 11oz (40 g) of superphosphate of lime per bushel are added. And also 1oz (21 g) of chalk, limestone or flowers of sulphur, exactly as directed for J.I.P.I.

In these John Innes composts, practically every known kind of plant can be grown successfully. They represent an enormous advance on the haphazard and miscellaneous mixtures formerly used but, largely because of the variability of loam, even when it seems to conform to the standard laid down, gardeners have continued to search for composts with an even greater uniformity.

Soilless Composts As a result of this search for more uniform composts, a number of so-called soilless composts have been put on the market. These are mostly mixtures of peat and sand, or peat and vermicu-lite, or simply carefully chosen peat, with added fertilizers.

Soilless composts usually become ex-hausted more rapidly than loam composts and so established plants require more frequent feeding.

Traditionally it was the practice to place broken pieces of pot, known as crocks, or a perforated zinc disc in the bottom of each pot to prevent the drainage hole becoming blocked with soil. For some plants, such as cacti and succulents, which appreciate quick drainage, this may still be desirable but with the highly porous John Innes and soilless composts now used, crocking is becoming a thing of the past. Few commercial growers of pot plants use drainage materials and amateur gardeners appear to be gradully dispensing with them, too. In any case most plastic pots have several holes per pot instead of the single hole usual with clay pots, so that if one should become accidentally blocked there is still ample outlet for the excess water through the others.

Technique of Potting

Potting a plant is very similar to planting it. A little soil is placed in the bottom of the pot. The plant is held in position in the centre of the pot and prepared compost is run in all around it. Then, when the pot is nearly full, the compost is pressed in around the sides with the fingers and the pot is given a sharp rap on something firm (it is convenient to have a substantial wooden potting bench) which settles the plant firmly in the pot and distributes any remaining loose soil evenly over the surface. When finished, the level of the soil in the pot should be just a little below the rim of the pot so that, when it is watered, the water will not run off the surface but will be retained and soak down to the roots. These should all be covered by the compost but the uppermost roots need be only just beneath the surface.

Potting Large Plants Most potting can be done in this simple way. With the fingers only, but when it comes to big plants going into pots Sin (20cm) or more in diameter in soil compost, it may not be quite enough to get the soil evenly firm all round the plant. Then a rammer is used which may be any piece of wood about 1 ft (30cm) long and 1 in (2-5011) in diameter. A short length of old broom handle, rounded off at one end. Does well. This is used to ram the soil in round the edge of the pot before it is given its final rap to settle everything down.

Degrees of Compost Firmness

The terms ‘light potting’ and “firm potting’ are sometimes used to distinguish between soil that is left rather loose and that which is made fairly hard. Some plants seem to have a preference for one, some for the other, but these are refinements which can only be learned by experience and about which there is, in any case, considerable disagreement.

Peat composts are never made very firm but are simply pressed in lightly with the fingertips and then settled in by two or three sharp raps on the potting bench.

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