What is John Innes compost and how to make your own!

Updated: Jul 6, 2022The term John Innes is very widely recognised by the general horticulturist as a word associated with growing media (compost). These John Innes growing media are loam-based (soil based) and as such are a good choice for cultivating containerised plants that are native to habitats with loamy soils.

So many people are familiar with John Innes media, and know how to use it, very few will actually know what it is or why it is named such. Some people believe there is a company named John Innes, others believe that it was invented by a horticulturist called John Innes, but neither is true.

John Innes himself was not a horticulturist and was not a private company as some gardeners think, but was a wealthy philanthropist who donated money to set up a research institute in Norwich (pictured below).

In the early days of the institute, the scientists decided to tackle one of the major issues of the horticultural industry; the poor quality and inconsistency of growing media.

Before John Innes formulas were developed in the 1930s, all growing media were mixed locally, often from secret recipes held by growers and based on mixtures of composted material and soil. As a consequence, mixes varied widely between growers. Plus, as the soil was not always sterilised, they often contained lots of weed seeds. To overcome these issues The John Innes Centee in Norwich standardised contents and quality for growing media and produced a number of recipes for growing media that producers could follow.

As ‘John Innes’ is a recipe and not a brand, standards vary between the companies selling it. In addition, as a John Innes growing medium is essentially a recipe/formula, it is indeed possible to make your own by following these instructions:

• Extract some loam from the top 25cm of topsoil below turf or a growing bed. This will will make up the vast majority of your growing medium. Excess topsoil can often be obtained from neighbours who are undertaking extensions to their houses. Alternatively, if you do not have a convenient source of loam available you could buy in topsoil from a commercial supplier.
• Heap the topsoil into a pile up to a maximum of 1.5m high or produce a wide stack and cover it in perforated plastic sheeting.
• If possible heat the topsoil to 60°C for 30 minutes to pasteurise the medium. This kills weed seeds and any pathogens present. This is usually only practical for commercial growing media producers though.
• Once cooled, the topsoil then needs to be mixed with moss peat to increase the permeability, aeration, and the water and nutrient holding capacity. Alternatively, more environmentally friendly forms of organic matter can be used such as coir (coco / coconut fibre) or fine bark. This should then be followed by an addition of sharp sand (or grit). The ratio of the mix should ideally be 7:3:2 topsoil:peat:sand. If a large volume of growing medium is being produced then a cement mixer will assist with the mixing process.
• A base fertiliser then needs to be mixed in at a rate of 3.25kg/m3. You can buy a commercial standard base fertiliser from a garden centre or you can mix your own using equal weights of superphosphate, hoof and horn (or any other slow release nitrogen-rich fertiliser) and potassium sulphate (sulphate of potash). Base fertiliser can also be substituted with a granular controlled release fertiliser. Again, an organic fertiliser could be substituted here.
• Ground dolomite limestone then needs to be added at a rate of 0.75km / m3 in order to neutralise excess acid in the peat. The addition of limestone should thus be omitted when making medium destined for ericaceous plants. Ensure that the limestone is finely ground and not in small chunks.
• Mix well.
• The completed mix then needs time to ‘mellow’ for around two weeks; this allows the nutrients and lime to be absorbed onto the loam and organic material. Ideally the mix should then be used within three months.

Clearly the balance of the components used will not exactly match those of the standards used in this recipe, as the properties of the loam used will vary between locations. Therefore you will probably have to experiment with a few batches to optimise the medium for your needs.

John Innes growing media are sold in a number of different grades, optimised for cultivating different plants. John Innes #1 is formulated for use when germinating seedlings and has a lower nutrient content and a smaller average particle size. Sieving should also be performed to ensure that no large clumps have formed in the medium. The base fertiliser used in John Innes #1 is predominantly triple superphosphate in order to aid the development of the initial roots on seedlings. John Innes #1 has a tendency to become waterlogged if planted in anything other than seed trays or small containers, so for species that are known to suffer from damping off disease extra sand should be added. John Innes #2 and #3 have an increased nutrient content and the aggregate particles are larger respectively. John Innes #2 is formulated for use on developing plants and John Innes #3 was developed for maintaining the health of established plants. When formulating John Innes mixes for use on ericaceous plants, ideally you should ensure that the base loam is acidic, add Sphagnum moss, and do not add any lime.

Most commercial peat-based growing media are made using recipes that incorporate both peat and sand and these were developed into standard recipes by the University of California, Los Angeles (UCLA) in the 1930s. These are known in commercial horticulture in the USA as ‘UC mixes’ and have subsequently been adapted for more temperate climates elsewhere. However, the term ‘UC mix’ for organic media is not as well-known in the UK as ‘John Innes’ is for loam-based media.

If you need extra information on growing media and the fertilisers that are compatible with them, then please get in touch.

Eutrema Ltd can help you source all the raw materials listed above such as coco fibre, dolomitic lime, and organic fertilisers.

Dr Russell Sharp

Eutrema Ltd