Potting mixture: 7 parts loam, 3 parts peat, 2 parts coarse sand. To each bushel add ¾ oz (20 g) ground chalk or powdered limestone and (for JI1) 4 oz (110 g) of a mixture of 2 parts (by weight) hoof and horn meal, 2 parts (by weight) superphosphate of lime, 1 part (by weight) sulphate of potash. For JI2 and JI3, double and treble the fertilizer amount respectively.
John Innes seed compost is used for sowing seeds, using seed trays or small pots.
The cuttings mixture contains no fertilizer and is for use when propagating plants from cuttings.
The potting mixtures are for potting on from the seedling and rooted cutting stages.
John Innes No. I (JI1) is for pots up to 4 in. (100 mm) diameter containing young or comparatively small plants.
John Innes No. 2 (JI2) is for potting into pots over 4 in. (100 mm) diameter and contains twice as much John Innes Base Fertilizer as JI1.
John Innes No. 3 (JI3) is for vigorous growing and large plants in containers over 8 in. (200 mm) diameter and contains three times as much fertilizer asjll.
John Innes-type composts have become modified over the years and often now contain peat substitutes and perlite or vermiculite as well as, or instead of, peat and sand.
JI composts are suitable for filling raised beds and other similar features. Because of the limestone or chalk in them, however, they are unsuitable for lime-hating plants, such as heathers, azaleas, rhododendrons, etc. If you wish to grow these in contained or raised beds, and many of them do lend themselves to this form of cultivation, you should obtain an ericaceous mix from which the limestone has been omitted, or replaced with a non-alkaline substance.
soil-less composts. These were originally developed because of the increasing difficulty of obtaining good quality loam, but are now used as a widespread replacement for soil-based composts. They are much lighter and therefore easier to handle than those containing soil, but watering is more critical — too much makes them waterlogged, too little and they dry out and become very difficult to re-wet satisfactorily, especially when they have been in pots for some time and contain a lot of roots. However, they are very useful for germinating seeds, potting on small plants, especially house-plants, and as a growing medium for temporary container planting — bedding plants, bulbs, etc.
Soil-less composts are based on peat, or a peat substitute, such as coir, bark or composted wood waste, to which may be added sharp or silver sand, perlite, or vermiculite, to improve drainage and aeration, and fertilizers capable of providing all the necessary plant foods for about 6-8 weeks. Many contain ground limestone or chalk to bring the pH up to neutral as without it the materials forming the compost tend to be acid, although many vegetables and ornamentals like slightly acid conditions, provided that a correct balance of plant foods is supplied to counteract any deficiencies because of the low pH. At one time different mixes were available depending on whether you intended to sow seeds, take cuttings, or pot on, but more manufacturers are now making a universal compost suitable for most purposes.