Potting Compost

A good potting compost should provide container plants with a growing medium that is both water-retentive and well aerated. The first loam-based standardised potting composts were formulated by the John Innes Horticultural Institute of Great Britain in the 1930s, with loam-less (or soil-less) potting composts arriving on the scene in the 1960s.

1. Loam Based

Loam based potting composts are made up from high-quality sterilised loam, grit and organic matter such as peat and bark fines. The loam is produced from turf that has been stacked for at least six months, then sterilised to kill weed seeds and pests by heating to a high temperature or by adding chemicals.

Loam-based composts offer plants a steady supply of nutrients essential for long-term growth. They are also free-draining, with good structure and aeration, making them less prone to waterlogging or drying out than peat-based versions. As they are soil based, they are also an excellent choice for plants that are to be transplanted to the garden.

2. Peat Based

Loamless peat-based composts are lightweight and clean to use, combining moisture retentiveness with good aeration. They are ideal for pricking out, potting up plug plants and for seasonal tubs and baskets. However, as peat is low in nutrients, plants must be fed more often than with loam-based compost. Peat composts also dry out rapidly and are hard to re-wet, becoming waterlogged if plants are over-watered. Some peat-based composts have up to 20% bark fines and composted green material added to improve drainage and to reduce peat consumption. Plants grown in loamless compost (either peat-based or peat-free) may also find it difficult to adapt when transplanted to garden soil.

3. Peat Free

Peat free composts have become increasingly popular over the last few years as they reduce the environmental damage to endangered peatlands. Alternatives may be based on a blend of composted wood, coconut fibre, bark and green material with added organic nutrients. Plants grown in peat-free compost may take a little longer to become established, but once they do, they usually last longer and tend to be bushier with a better shape. Find out more about recycled compost here ...

A 70 litre bag of potting compost will fill the following:

* 8 x 30cm (12 inch) hanging baskets OR
* 20 x large seed trays OR
* 34 x 15 cm (6 inch) pots OR
* 56 x 12 cm (5 inch) pots OR
* 130 x 10cm (4 inch) pots OR
* 260 x 8cm (3 inch) pots

It's always better to use fresh compost, so buy a smaller bag if you don't use much each season.

Standard Potting Compost

Standard potting mix may be either loam-or peat-based with varying amounts of fertilizer to suit the varying needs of plants. For example:

John Innes No.1 is a loam-based potting compost with relatively little fertilizer, which makes it the ideal mix for pricking out;
John Innes No.2 potting compost has twice as much fertilizer making it a good choice for potting on fast-growing plants and vegetables;
John Innes No.3 potting compost has three times the amount of fertilizer, useful for the permanent planting of trees, shrubs and hardy perennials.

You can make your own soil-based potting mix using the following ingredients:

7 parts loam
3 parts peat (or peat-substitute)
2 parts sand

Add these materials together, sieving or breaking up any lumps as necessary. For every 10 litres of compost you will also need to add 18 g of ground limestone plus 90 g of John Innes Base Fertiliser. Whilst it is vital for the chemicals to be mixed in throughout the whole compost, you must not over-mix or the peat particles will become too fine. Store in a covered bin or in a thick plastic sack.

Special Composts

There are a number of potting composts designed to use for specific growing requirements. These include:

African Violet Compost

Contains a special blend of nutrients to promote flowering.

Aquatic Compost

A loam-based mix with added grit to prevent float-off in ponds.
It has a low nutrient content to restrict algal growth.

Bulb Fibre Compost

Often made from un-decomposed sphagnum moss, producing a medium ideal for growing bulbs in containers - even if they have no drainage holes.
Charcoal and crushed oyster shell is also often added.