Pot plants need a growing medium that is both well aerated and water retentive, with a resilient structure that can withstand heavy watering.
For best results always use the most suitable type for purpose. Standardised potting composts were first formulated in the 1930’s by the John Innes Horticultural Institute of Great Britain. These are loam based and also contain peat and course sand.
Loamless (or soil-less) potting composts (mainly peat based) were not promoted until the 1960’s.
Loam based potting composts
These should contain high quality loam that is rich in organic matter. The loam is usually obtained by stacking turf for six months or more. This should be sterilised, either chemically or by heating the soil to a high temperature to kill pests, disease and weed seeds.
Loam based composts provide better conditions than loamless ones for long term growth. They contain a steady supply of nutrients are free draining, with good aeration and structure. They do not dry out as rapidly as peat based composts and are less prone to water logging. Plants also grow on readily when transplanted to garden soil.
Loamless Potting Composts
These are lightweight and clean to use. Many are based on peat, which combines moisture retentiveness with good aeration, and is relatively stable and long lasting. Peat is low in nutrients and does not hold them well compared to those with loams.
This means plants must be fed more often. Plants grown in loamless potting compost may also find it difficult to adapt when transplanted to garden soil. Most loamless potting composts break down rapidly, so that the volume of compost shrinks and the remaining material loses much of its structure. For long term container sized plants, the more fibrous the compost the better. Loamless composts dry out rapidly and are hard to re-wet. They also become waterlogged if plants are overwatered.
These have physical properties and nutrient supplies somewhere between those of seed and potting composts. They have a wide range of purposes, from growing on seedlings to potting many types of plant.
Special composts are used for plants that have specialised growing requirements.
Orchid composts are free draining and often contain charcoal and fine bark. Alpine and cactus composts are very free draining and sometimes poor in nutrients to suit the needs of the plants. Ericaceous composts have a low pH and are suitable for lime-haters such as rhododendrons.
These are peat formulations that are compressed when wet and then moulded to make planting containers and seed modules. These may be planted direct into the soil so there is little disturbance to plant roots.
Bulb fibre compost
This is made from un-decomposed sphagnum moss, which has an open structure, and is used to grow bulbs in containers that do not have drainage holes, when poor aeration may be a problem. It gives physical support and retains moisture reasonably well. Its lack of nutrients is unimportant as bulbs have food reserves.