These are balanced mixtures designed to support healthy and active plant growth throughout all stages of propagation, pot and container culture and other forms of simulated growth conditions. They are also, on occasions, used for mixing into outside planting positions in order to give trees, shrubs and plants a good start.
There were at one time innumerable, highly individual soil composts, especially where potting was concerned, but some years ago the John Innes Horticultural Institution, decided to seek some standard formulations.
After considerable work and research, a series of standard potting and propagating composts was developed, making it clear that apart from certain modifications in special cases, such standard mixtures would work efficiently for an extremely wide range of plants. And they came as a very welcome simplification of what had been a highly complex, not to say hit and miss process; satisfactory for the skilled gardener, perhaps, but certainly not for those with little experience.
Five composts were devised, three for the various stages of potting, and one each for seed sowing and propagation by cuttings. The primary ingredients are partially sterilized loam steamed at 200°F (93°C) for twenty minutes, essential in order to destroy pests, diseases and weed seeds granular peat with a minimum of dust, and coarse river sand with nutrient materials in the form of hoof and horn, grist (13 percent nitrogen), superphosphate of lime (16 percent phosphoric acid) and sulphate of potash (48 percent pure potash) and, finally, ground limestone or chalk. For potting composts Nos. 1, 2 and 3, the basic formula is:
7 parts by loose bulk of medium loam -partially sterilized
3 parts of granulated or moss peat 2 parts of coarse sand
It is important to note that in each instance the measurements are by loose bulk and not by weight. The fertilizers and limestone or chalk are measured by weight, not by bulk.
These materials are mixed most efficiently by placing them in three complete layers, one upon another, and then turning this flat, sandwiched heap over, working from one end to the other and shovelling from ground level throughout. The heap should be turned at least twice out and back for maximum success.
Fertilizer mixed in the ratio of:
2 parts by weight of hoof and horn
2 parts by weight of superphosphate of lime
1 part by weight of sulphate of potash plus 21g (f oz) of ground limestone or chalk, making sure to mix all thoroughly throughout.
This complete mixture is suitable for most basic potting in small to medium pots, for delicate and hothouse plants and those which are not to be kept for any length of time.
Plants other than delicate and hothouse, when requiring a shift beyond the 10cm (4in) pot stage, should be potted into John Innes No. 2; made by adding twice the amount of combined fertilizer and chalk to each bushel of loam, peat and sand. John Innes No. 3. comes from adding three times the amount of these materials and is suitable for the more robust and long-standing plants requiring, perhaps, shifts into 20cm (8in) pots and beyond.
Compost made up for plants objecting to lime should obviously have the chalk or limestone omitted; it should be reduced where lime is present to any extent in the loam used in the basic formula. Ideally, this should not be so, with loam with a pH of 6.5, the kind to use if possible.
The John Innes standard seed sowing mixture consists of:
2 parts by bulk of loam partially sterilized
1 part by bulk of peat
1 part by bulk of sand
to each bushel of which is added only 45g (1 1/2 oz) superphosphate of lime and 12g (1/2 oz) ground limestone or chalk. The loam and peat may have to be rubbed through a 1cm (1/2in) sieve to make them sufficiently fine for seedling growth.
Certain plants require special composts; orchids, for instance, are traditionally potted in a basic mixture of osmunda fibre, sphagnum moss and charcoal.
Where many tiny alpines are concerned, extra grit in the compost is often required in order to allow them very adequate drainage, essential to many of them, especially in their early stages.
Sometimes commercial growers handling plants, trees and shrubs in containers for immediate planting, use a 90 percent peat, 10 percent soil compost for general expedience, nutrients being supplied in balanced liquid form.
Using the compost
Well-made corn-posts from vegetable wastes may contain more nutrients and more organic matter than good farmyard manure.
If you know that your soil is very low in humus, about 5kg (10lb) per square meter (yard) should be mixed with the topsoil for a year or two and in subsequent years give 2.5kg (5lb) per square meter (yard).