Even the most fertile garden soil is unsuitable for use in plant pots because in these artificial conditions it soon becomes tightly compacted. Consequently the vital air supply to the plant roots is reduced and their growth impeded. Furthermore, soil pests and disease flourish, posing serious risks to the plants. Special growing composts are therefore required which are specially designed for use in pots. Originally these were based on sterilised soil as in the John Innes growing composts. Nowadays, however, houseplants are more commonly grown in soil-less media which essentially means that the main ingredient is moss peat. Both types of growing compost are equally good, but nevertheless when re-potting becomes necessary it should be done with the type of compost used in the original planting.
These are available as the John Innes range of growing composts. They are made up of a mixture of sterilised loamy soil, peat and coarse sand with the addition of ground limestone or chalk together with a supply of fertilizer. This produces an open, fairly free-draining growing medium which is ideal for use in pots. This compost is less likely to dry out so rapidly and completely as a peat compost, and there is a better reservoir of plant food. There are various types of John lnnes composts to meet the full range of growing requirements. These are as follows:
JI. Seed Compost in which the added fertilizer is superphosphate. Used for seed sowing and for the rooting of cuttings.
JI. Potting Composts Nos. 1, 2 and 3 which contain different levels of a complete NPK fertilizer (NPK stands for the major plant nutrients: nitrogen, phosphorus and potash), No.1 having the lowest content and No. 3 the highest. No. 1 is generally recommended for use with seedlings whilst the higher numbers are suitable for use with established plants.
Ericaceous Composts which have no added chalk or limestone are designed for use as growing media for plants such as azalea and erica which dislike lime.
Various brands of peat, peat/sand and peat/vermiculite growing composts are available from garden shops and garden centres. Some are described as general purpose composts which can be used for seed sowing, rooting cuttings and potting on. Others are specialised seed or potting composts. Peat-based ericaceous growing composts are also available.
J.I. type composts can be made at home provided that your soil is a good quality loam. The soil, however, must first be sterilised by heating to just below 100°C (212°F) and this presents difficulties unless you own an electric soil sterilizer. One method is to put a thin layer of moist soil on a metal tray and heat in the oven for 15 minutes. Alternatively you can sterilize the soil by steaming. To do this you suspend a sack of dry soil over boiling water in a saucepan for 30 minutes. Allow the soil to cool and then mix it with peat and coarse sand in the following proportions by bulk:
Seed compost Potting compost
Loamy soil 2 7
Mosspeat 1 3
Coarse sand 2
To complete the preparation of J.I. Seed Compost you then mix in log (¼ oz) of ground limestone or chalk and 40g (1½ oz) of superphosphate to 2 litres (8 gallons) of the mixture. When making J.I. p. 1 the superphosphate is replaced by bOg (4 oz) of J.l. base fertilizer which can be bought ready mixed. J.I. P.2 and 3 contain double and treble the quantity of both chalk and fertilizer.
These are much easier to prepare since none of the ingredients needs to be sterilized. Furthermore it is possible to buy prepared packs of combined chalk and fertilizer. Consequently all you have to do is to mix these with either peat or a peat/coarse sand mixture. You can also make up composts to your own specification. Granular perlite, for instance, can be used in place of sand to give a more open mixture such as is needed by cacti. Fertilizer packs suitable for the preparation of ericaceous composts are also available.