Formulation of John Innes Composts

Mix Ratio

John Innes (JI) Composts are manufactured using sterilised soil (usually called loam), peat and grit-sand. Environmental concerns surrounding the use of peat are resulting in an increasing number of John Innes brands becoming available with ‘peat-free’ formulations. These are checked by the John Innes Manufacturers Association and only approved if they achieve the same standards as those containing peat. Traditionally, in the days of stacking grass turf to produce loams with a high organic matter, seven parts of loam, three parts of peat and two parts of grit-sand were hand-mixed and shredded.
As nowadays such labour-intensive methods are not practical, manufacturers vary the mix depending upon the type and quality of raw materials available, to end up with a product every bit as good as the traditional mixes of old, and in which loam is still the main ingredient.

The present-day John Innes Composts, whilst based on the original loam:peat (or peat-alternative):grit formulation, are lighter in weight, flow better, are easier to use and have more stable nutrients than the original mixes.


Loams vary considerably and it would be unrealistic to expect JI manufacturers across the UK to use identical formulations, indeed it would be undesirable. When a suitable source of loam is found, its physical properties are analysed so that the required peat or peat-alternative and grit levels can be calculated.

For instance, a very sandy loam or a heavy clay loam will need more peat or peat-alternative than a silty clay loam, whilst a coarse loam will need less grit. The final JI composts will be far less variable than the loams used to produce them.

Loam must be sterilised, or more precisely pasteurised, prior to use, in order to kill any weed seeds or propagules that might be present, and to eliminate plant pathogens, such as Pythium and Rhizoctonia, which are often found in field soils.

pH Level

During manufacture of the composts the pH has to be balanced and nutrients added. Peat or peat alternatives) are an essential part of JI composts, giving them structure and improved available water-holding capacities. Peat, however, is acidic, so ground limestone is also added to raise the pH to around 6.5. Manufacturers using peat-alternatives will add ground limestone levels that are appropriate for the material being used.


Nutrients are added at different rates depending on the proposed use of the compost. Seeds require few nutrients, whereas some full-grown pot plants and crops such as tomatoes require extra nourishment. JI Seed Compost has a low level of nutrients while JI Potting Composts Nos 1, 2 and 3 have an increasing amount of nutrients, as the numbers would suggest. Naturally, the plant nutrients have been updated to gain the benefits of improved fertiliser technology.

As an alternative to the traditional No 1, No 2, No 3 JI Composts, some manufacturers have produced a single "JI Potting Compost". As with all compromises, this single formulation JI Potting Compost has some advantages and some disadvantages compared with the separate No. 1, 2 & 3 formulations, which were developed to suit plants at different stages of growth.

Physical Properties

As manufacturers have to formulate their JI composts with locally available raw materials, it is not possible for JIMA to give details of the precise ingredients and all the various physical parameters. However, all John Innes composts should exhibit high available water-holding capacities and be sufficiently open to encourage air movement, so that a good growing performance can be assured. The table below gives the expected range for the most important properties of JI composts, as manufactured by members of the John Innes Manufacturers Association:

Typical Properties of John Innes Composts (when packed):

Bulk density grams per litre 800 - 950
Moisture content % by weight 15 - 30 %
pH when packed - 6.0 - 7.0


Nutrient Levels (when packed)

The conductivity of compost is an indication of the total level of available nutrients - the higher the figure, the greater the nutrient levels. Typical values for the different grades of John Innes Composts are as follows:

John innes grade conductivity (micro-seiemens / cm)
John Innes Seed 180 - 360
John Innes Potting 360 - 720
John Innes No.1 360 - 540
John Innes No.2 480 - 720
John Innes No.3 600 - 840


NOTE: Manufacturers cannot be held responsible for the condition of their products after prolonged storage.
(See below on Storage of Composts.)


Material Safety Data

Control of Substances Hazardous to Health (COSHH) regulations

Within the context of the current COSHH regulations all John Innes Composts supplied by members of this association are regarded as: "non-hazardous"


First aid measures:
Contact with eyes wash out with clean water
Ingestion no special action is necessary
Skin contact users with sensitive skin should wear gloves






Storage of John Innes Composts

All composts, whether loam-based or peat-based can be subject to major detrimental changes if stored incorrectly.

Substantial biological and chemical changes can occur in composts after they are manufactured. The most important from the users' (and the plants') point of view are the changes in pH (acidity) and salinity. Under normal conditions there is an initial increase in pH, followed by a gradual decrease as the organic nitrogen is mineralised. These are perfectly normal reactions and occur in all composts, including those made from peat.

However, it is desirable that these changes occur as slowly as possible, and to achieve this the following points should be observed during storage:


Storage guidelines

  • The compost must be kept dry and cool.
  • It is best stored inside in cool buildings. Keep away from radiators, heating ducts, light bulbs, etc.
  • If it is stored outside, avoid direct sunlight and rain. Try to site it against a north-facing wall, under a fixed canopy or porch roof.
  • Keep dry by covering with white or light-coloured plastic sheeting; black is not suitable as it causes the compost temperature to rise.
    If the compost is not covered it will absorb water through the breathing holes.
  • If the compost is not kept on pallets, ensure that the bottom layer of bags is raised above ground level.
  • If the compost is re-stacked, ensure that air can circulate within and between the stacks.
  • Always rotate your stock - never put new deliveries on top of old stock.

If the above rules are observed the composts will produce the growing results for which they were formulated, and this, in turn, should ensure customer satisfaction.