Composts for Greenhouse Plants

Composts for Seed Sowing and Potting

Several generations of gardeners have been brought up now on the John Innes composts, which, by standardising compost mixtures into just a few different categories for seed sowing and potting, have made it easy to get good results without remembering or looking up countless compost recipes. There are two basic formulae for the John Innes composts — one for seeds, the other for potting and the three ingredients used in them in bulk are loam, peat and sand.

Medium grade loam is needed for John Innes composts, neither too heavy (clay) nor too light (sand) with a pH of about 6.5. Such loam can be obtained from turves cut with 2 to 3in. of soil attached from a good meadow or building site. The loam is stacked, grass side down, and left for up to a year if possible so that the grass and soil fibres have a chance to decay. It should be riddled through a 3/8in. sieve before being used.

Composts-for-Greenhouse-Plants The horticultural grade peat used should be granular or fibrous and reasonably free from dust. It must be well watered and made thoroughly moist before use, the watering being repeated several times, if necessary, to obtain the desired result. The sand needed is defined as coarse and damp with particles grading up to 1/8in. in diameter. Cornish river sand is ideal for the purpose; never use builder’s sand.

This compost can be bought from garden stores and nurseries but it is as well to be sure of the source because some so-called John Innes composts offered by dealers bear little resemblance to the real thing. If you make your own the soil should be sterilised by standing it over boiling water in a saucepan or copper, or placing it in one of the special sterilisers which can be obtained through garden stores, etc. The temperature of the soil should be raised to 93°C. (200° F.) and kept at this level for 20 minutes.

The John Innes Seed Compost is prepared by mixing 2 parts sterilised loam, 1 part peat and 1 part sand, all parts by loose bulk. To each bushel of these combined ingredients is added 1-1/2oz. of superphosphate of lime and 3/4oz. of either finely ground chalk or limestone. If the compost is to be used for lime-hating plants substitute flowers of sulphur for the ground chalk or limestone, at the same rate. First, the loam is sieved to remove stones. Then the peat and sand are added and finally the other ingredients, carefully measured out, are scattered over the top. The whole heap must then be turned several times.

The John Innes Potting Compost is prepared in a similar manner, but the proportions of the main ingredients are different: 7 parts loam, 3 parts peat and 2 parts sand. A base fertiliser is added to this. It can be purchased ready mixed or can be made with 2 parts of hoof and horn meal (13% nitrogen), 2 parts superphosphate of lime (18% phosphoric acid) and 1 part sulphate of potash (48% pure potash), all parts by weight. This is added to the other ingredients at the rate of 4oz. per bushel for No. 1 Potting Compost, 8oz. per bushel for No. 2 Potting Compost, and 12oz. per bushel for No. 3 Potting Compost. To No. 1 add 3/4oz. of ground chalk, or limestone (except for lime-hating plants) per bushel of mixture; double and treble this amount for Nos. 2 and 3. the No. 1 compost is used for all ordinary purposes, No. 2 for older plants in pots over 4in. diameter, and the No. 3 for some very strong-growing plants in 10in. pots.


Soilless Composts

The increasing shortage of loam resulted in a search being made for an ingredient which could take its place, and it was found that a mixture of peat and coarse sand with fertilisers added provided good growing conditions for plants. These soilless composts are now sold under proprietary names. It is interesting that some plants with a reputation for being tricky to grow are proving easier to manage in these composts. However, at present they are most suitable for propagation purposes and growing smaller plants. Large plants like chrysanthemums are too heavy for these composts to support firmly. On the other hand, some plants, such as African Violets, do particularly well in them.


Hanging Baskets

Perhaps we associate hanging flower baskets most often with outdoor displays, but they are, of course, splendid for greenhouse decoration as well. Fuchsias, pendulous begonias, Ivy-leaved pelargoniums and Impatiens sultanii (the Busy Lizzie) are some of the best and most popular plants used for this purpose.

Early spring is the time to prepare such baskets and the moss lining, which should be thick, is the framework on which one starts to build. John Innes No. 1 Potting Compost is suitable for this kind of planting, and planting should proceed as the compost is built up. It is then easy to plant through the sides of the basket and end up with an extremely effective display of plants arching over the sides and building up to a well-furnished crown of more upright-growing plants. Some of the plants planted near the edge or top of the basket should be angled so that the new growth hangs over the side.

If polythene film, with drainage holes made in the base, or puddled clay, is placed round the inside rim of the basket this will aid watering, which in hot weather will need to be frequent. With so many plants packed into such a relatively small area (baskets of 12 or 14in. diameter are usually used) the demands on available soil moisture are heavy. For the same reason regular feeding with a soluble fertiliser is, I consider, necessary every seven to ten days during the summer. This is well worthwhile for, given such treatment, planted baskets will keep up an impressive display well into the autumn and give a great deal of pleasure.

Such baskets look best, of course, if they can be hung from a suitably rigid beam, but this is not always possible and the alternative is to stand them on the staging on large pots which provide a firm base.

01. December 2010 by admin
Categories: Greenhouse Gardening, Propagating Plants, Soil Cultivation | Tags: | Comments Off on Composts for Greenhouse Plants


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