What is John Innes potting compost?

John Innes potting media, or composts, are widely sold and offer significant advantages over soil-free potting media due to the weight, trace element content and the effect of soil content in avoiding undue chemical and water fluctuations (buffering activity). 

Peat is used in the original recipe, but peat substitute may also be used and is environmentally preferable. As most peat substitutes are much less acid than peat, less ground chalk is needed to raise the pH of the mix.

The currently available formulations are:

John Innes Seed Compost

This mix is used for sowing and cuttings until the seedlings are ready for pricking out or the cuttings have rooted. To mix your own (all ‘parts’ are by volume):

2 parts by loose bulk medium sterilized loam
1 part by loose bulk good peat or peat substitute
1 part by loose bulk coarse sand

To each 9-litre (2-gallon) bucket of the mixture add:
10g (1/3oz) superphosphate
5g (1 teaspoon) ground chalk

John Innes Potting Composts

a) Soil mix

7 parts by loose volume medium sterilized loam
3 parts by loose volume good peat or peat substitute
2 parts by loose volume coarse sand
 
b) Fertilizer mix (John Innes Base)

 2 parts by weight hoof and horn meal (or 2/3 of a part by weight Nitroform*)
 2 parts by weight superphosphate
 1 part by weight sulphate of potash

John Innes Compost No 1

This mix is suitable for young plants. It is used for pricking out or potting-up young seedlings or rooted cuttings and short-term potting such as for bedding plants or vegetable transplants which will subsequently be planted out.

To each 9-litre (2-gallon) bucket of soil mix add:
28g (1oz) John Innes Base
5g (1 teaspoon) ground chalk

John Innes Compost No 2

This mix contains double the amount of nutrient in John Innes No 1 to suit established plants. It is suitable for most houseplants and vegetable plants in containers.

To each 9-litre (2-gallon) bucket of soil mix add:
56g (2oz) John Innes Base
10g (1/3oz) ground chalk

John Innes Compost No 3

This mix is the richest (triple the nutrients in John Innes No 1) and works well for established plants, trees, shrubs and climbers, including fruit trees, bushes and vines, and mature indoor plants which are to remain in the pots for some considerable time. No 3 Compost is also suitable for vigorously-growing plants such as tomatoes and chrysanthemums.

To each 9-litre (2-gallon) bucket of soil mix add:
84g (3oz) John Innes Base
15g ground chalk

John Innes Ericaceous Compost

A recently developed product that is suitable for use with ericaceous or ‘lime-hating’ plants.

Peat-based or peat-free media with ‘added John Innes’

Adding John Innes potting media to soil-less media is sometimes done to improve buffering, trace element content and weight of these potting media. As John Innes potting media contain many fine particles gardeners should be wary of adding John Innes materials to other media as the ‘fines’ may fill or block the air spaces within the media that are vital for drainage and aeration of the root zone. This is no doubt taken into account by manufacturers of media that is labelled as having 'added John Innes' and formulations adjusted for adequate drainage and aeration.

*Nitroform (urea formaldehyde) is available through the post from:

Garden Direct
The Garden Centre
Birchall Lane
Cole Green
Hertford
SG14 2NR
Tel. 0845 217 0788

Is John Innes potting compost always the same?

Variability

Purchased composts may legitimately vary to a considerable extent in texture as the medium loam recommended to make the composts may contain varying amounts of clay ranging from 10-20 percent and amounts of silt ranging from 30-50 percent. In addition, texture will differ depending on whether stacked, rotted turf is used or whether the source is an arable field. The sand content may also vary in the range of particle sizes and sharpness according to source.

Loam is currently in short supply and the soil-based material used appears to be highly variable and not necessarily consistent with what is commonly understood to be a medium loam. There is no legally binding standards for John Innes potting media ingredients.

However, many compost manufacturers are members of the John Innes Manufacturers’ Association whose aim it is to provide reliable John Innes potting media that will support good plant growth even though not all ingredients match the original specification. GIMA (Garden Industry Manufacturer's Association) members’ packs all carry the insignia or mark of the association.

Making and sterilising loam

Gardeners who wish to formulate their own John Innes materials can use garden soil or better the loam derived from stacking turf upside down until it rots. Sterilising loam at home is however quite difficult.

Unsterilised loam or soil might contain weeds, pests and diseases. Soil can be sterilised in the oven. Using moist soil, it should be sieved through a 5mm sieve and then placed in a baking tray up to 8cm (3 inches) deep. This should then be heated for 10 minutes at between 71°C (160°F) and 79°C (175°F) which should eradicate most plant-damaging or competitive soil-borne organisms.

Supplementary feeding and storage

Supplementary feeding

With all the composts, supplementary feeding using liquid feeds can be carried out once plants are well established in their pots - usually 6-8 weeks after potting when the roots are through to the sides of the pot.

Storage

When prepared composts are stored, chemical changes take place in the organic matter and hoof and horn meal. Seedlings grown in a stored compost may show a check in growth when compared with those grown in fresh compost. It is sensible therefore to avoid buying or preparing more than required for immediate needs.