Seed and Potting Composts

I can well remember the time when it was necessary to mix up different composts for almost every kind of greenhouse plant. But this has not been necessary for many years now, for in the 1930s The John Innes Horticultural Institute, after long research, gave the gardening world the standardised seed and potting compost formulae for which they are renowned. Instead of a multitude of composts a single seed compost and a few variations on a potting compost suffice for the vast majority of pot-grown plants, and these are readily available at gardening shops and centres. A word of caution is, however, necessary for not all dealers in these composts are scrupulous in keeping strictly to the formulae, so be sure to go to a reputable supplier.

More recently, there have been the introduction of the soilless composts, the development and introduction of which was triggered by the increasing difficulty of obtaining loam, one of the ingredients of the John Innes composts, of the desired quality. These certainly have their place in the scheme of things and I shall have more to say about them later. For the moment I want to explain in some detail the composition of the John Innes composts.


There are, as I have already noted, two basic formulae, one for seeds, the other for potting. Both make use of loam, a term rather loosely used for soil that contains some clay, sand and humus. The definition of the loam needed for John Innes compost is medium, neither too heavy (clay) nor too light (sand), with a pH of 6.5 or thereabouts. One way of providing loam is to obtain turves cut thickly with two or three inches of Soil from a good meadow or building site. This should be stacked, grass side downwards, and left for some months (as much as a year if possible) so that grass and root fibres decay.

The second bulk ingredient of the John Innes composts is horticultural grade peat which should be granular and reasonably free from dust. When purchased this is often very dry, and dry peat resists moisture. As it is essential to get the peat moist before use it should be spread out and liberally watered. It may be necessary to repeat this several times, turning the peat after each watering and then spreading it out again.

The third bulk ingredient is sand, defined for this purpose as coarse and damp, with particles grading up to 1/8 in. in size. Cornish river sand is ideal.

Mixing Composts

If one wishes to mix up one’s own composts at home this is not particularly difficult. The loam should first be sterilised by standing it over boiling water in a saucepan or copper or in one of the special sterilisers which can be purchased for the purpose. The idea is to raise all the soil to a temperature of 93°C. (200°F.) and maintain it at that for 20 minutes.

The John Innes Seed Compost consists of 2 parts loam. 1 part peat and 1 part sand, all parts by loose bulk. To each bushel of these combined ingredients is added 1-½ oz. superphosphate of lime and ¾ oz. of either finely ground chalk or limestone. First the loam is passed through a 3/8-in. mesh sieve to remove stones. Then the peat and sand are added and finally the chemicals, carefully measured out, are scattered over the top. The heap must then be turned several times so that all the ingredients are thoroughly mixed together.

The preparation of the John Innes Potting Compost is similar but the proportions are different – 7 parts loam. 3 parts peat and 2 parts sand. A base fertiliser is added to this mixture. This can be purchased ready mixed, or it can be made with 2 parts of hoof and horn meal, 2 parts superphosphate of lime and 1 part sulphate of potash, all parts by weight. This is added to the other ingredients at the rate of 4 oz. per bushel for No. 1 Compost. 8 oz. per bushel for No. 2 and 12 oz. per bushel for No. 3. to No. 1 add ¾ oz. of finely ground chalk or limestone per bushel of mixture: double and treble this amount is needed for No. 2 and No. 3 respectively.

Compost Uses

As a basic guide the No. 1 Compost is used for all initial pottings. No. 2 Compost for older plants in pots over 4 in. in diameter and No. 3 Compost for very strong growing plants in large pots and tubs.


The soilless composts, sold under proprietary names and consisting of peat, or peat and sand, with fertilisers added, are very useful, I find, for pots up to 3-½-in. In diameter, but I do not like them for larger ones than this as there is little weight in the compost and large plants tend to tip over. Seed and potting mixtures are available, and I use them for propagation purposes and for growing smaller plants. Saintpaulias, for instance, grow particularly well in such mixtures.

Soilless composts need much more careful watering than ordinary compost, for with their heavy peat content they are naturally absorbent and hold water for much longer than the traditional kinds. One soon gets used to this, however, and they are a valuable addition to the range of growing mediums.

01. March 2012 by admin
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