Greenhouse Growing Composts

Greenhouse Growing Composts

The need for good compost

Greenhouse Growing Compost The soil in the garden is usually satisfactory for growing plants there but it is not so suitable for use as a growing medium in pots or boxes, particularly under intensive culture under glass. The reason is that the soil will settle into a compacted state in the pot, especially when watered continually.

In the garden, the soil is constantly being turned and agitated by the action of worms and insect life within the soil (not to mention larger beings using a spade or hoe!) but these creatures are not generally present in the plant pot, the result being that the soil settles, becomes inadequately aerated and therefore increasingly difficult for water to penetrate; growth will be poor and eventually the plant may die. It is therefore necessary to improve the soil as a growing medium if it is to be used successfully in a pot. This is normally done by the addition of suitable materials to the basic soil and by correct treatment, but more recently composts made without soil have become important.

The John Innes Composts In 1939 W.J.C. Lawrence and J. Newell of the John Innes Horticultural Institution published the results of their intensive research into the characteristics of compost, and the John Innes Composts, which were to revolutionize the growing of plants in pots and boxes, were born. The gardener for the first time had standard formulae that could be reasonably well reproduced and when properly used would give excellent results. Prior to this, he had had no means of deciding what particular factor a failure might be due to, whether it was a bad loam or diseased leaf-mould and so on. The ingredients of these composts were neither expensive nor difficult to obtain at that time, and the mixing of them was a comparatively simple job which did not require the use of such variable material as stable manure or leaf-mould.

There are two basic John Innes composts, one for seed sowing and one for potting, differing in their physical and nutrient make up. Two were considered necessary because of the differing conditions required. In the seed sowing composts, the structure had to be free-draining so that the compost was warm and free of excess moisture which can cause rotting. The potting compost was more moisture retentive but still well aerated to ensure good root action.

The basic formulae are given here for the mixes but the specifications of the individual raw materials given later must be studied. These are well and strictly defined so that the composts will be as uniform as possible.


Seed compost

This consists of two parts sterilized loam, one part peat and one part sand, by volume (ie. using containers of known size). To this is added superphosphate of lime at 1g/litre (1-1/2oz) per bushel and chalk or ground limestone at 0.5g/litre (3/4oz) per bushel (36 litres/8 gallons). Metrication has embraced composts more than many other spheres of horticulture and soilless types are now invariably sold in litres. There are 36 litres in one bushel.

During their early life, most of the nutrients required by seedlings are present within the seed itself but phosphates are required for the growth of new cells and good root action. The chalk is added to neutralize the acidity of the peat.

Potting composts

The formula for John Innes potting compost No 1 strength is shown overleaf. The No 2 strength was made by adding double the amount of John Innes base and chalk, ie. 6g/litre (8oz) and 1g / litre (1-1/2oz) per bushel respectively, and the potting compost No 3 by adding three times the amounts, ie. 12oz of John Innes base and 1.5g/litre (2-1/4oz) of chalk per bushel of compost.

It should be noted that a mix of say 7 bushels of loam, 3 bushels of peat and 2 bushels of sand will not in all probability yield 12 bushels of compost and correspondingly if calculating in litres. The reason is that particles of loam and sand will enter the pore spaces in the peat, thus reducing the volume of compost produced. Therefore, the compost volume, after mixing, will have to be measured or estimated, and the John Innes base and chalk added at the rates given above per actual bushel or cu yd of compost produced.

For certain acid-loving plants such as the heathers (Erica varieties), etc, a special acid compost may be used. It is known as the John Innes seedling compost (A), the A standing for acid. As the name indicates, it is prepared by making the normal J.I. Seed compost — 2 parts sterilized loam, one part peat and one part sand, but the lime is omitted and replaced by 1g/litre (3/4oz) of flowers of sulphur per bushel of compost. The pH should then be between 5.0 and 6.0.


Measurement of volume

The most common unit of measurement of composts and their raw materials is the bushel. This is a volume, and one bushel is defined as 2,200cu in. For convenience, a standard bushel box may be made or purchased having a standard size 55 x 25 x 25cm (22 x 10 x 10in). One bushel is roughly equivalent to 36 litres (8 gallons) and, therefore, for convenience a bucket containing a 9 litres (2 gallon) mark may be used for measuring smaller quantities of compost ingredients as this represents one quarter of a bushel.

The other standard measurement of compost volume is the m3 or cubic yard. This is a volume of 1m x m x m (36 x 36 x 36in) and there are 21.7 bushels in a cu yd and 27 bushels (1000 litres) in a cubic metre. This unit is rather large for the amateur and working with litre or bushels is probably adequate for most purposes. Most compost in Europe is now sold in litres.

26. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Soil Cultivation | Tags: , , , , | Comments Off on Greenhouse Growing Composts


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