Today it is not always possible to obtain a good quality loam to make John Innes compost, so as a result many varieties of soil-less composts have evolved.
These are generally Peat based although there are moves by preservations organisations to change this, and use alternative products e.g. Coir and various other organic / recycled materials.
A frequently asked question is;
How do Loam-based composts compare against Soil-less composts?
Loam based composts normally contain a larger reserve of plant nutrients, and trace elements, meaning the plants need less additional fertiliser during the growing season, thus making feeding at a particular time less critical.
Some plants grow better in loam-based compost, however quality can be variable, and it can deteriorate in storage.
On the downside it is heavy to carry and less pleasant to handle when wet.
This added weight however can be useful in counterbalancing large potted plants, and in the event of the compost drying out it is easily re-moistened.
Soil-less compost is generally lighter to handle, more pleasant to use, and major brands are available all over the country, plus many plants prefer them.
On the downside, they can soon run out of plant food, and can be quite difficult to rewet.
Peat based composts have fertiliser already added, and a little lime to lift the pH value to around 6.5.
Peat by itself has a pH level of 2.5/4.5, which is considered to be far too low for most plants.
As a general rule, the fertiliser in the compost is only available to the plants for about 5/6 weeks.
After this period, supplementary feeding is generally required.
What type of compost do I need?
A rule of thumb is;
For plants that will remain in a container for up to 6 weeks, use multi-purpose compost.
From 6 to 12 weeks, use a potting compost e.g. Ji1* (or similar)
For each 6 weeks thereafter use stronger potting compost, e.g. Ji2*, Ji3* etc (or similar)
*The number denotes the fertiliser content e.g. Ji1 = 1 part fertiliser to the standard compost content, Ji2 = 2 parts fertiilser to the standard compost content, and Ji3 = 3 parts fertiliser to the standard compost content.
For hanging baskets add slow release pellets/tablets and water retention granules to Ji2 (or similar).
Most commercial peat-based seed/potting composts usually give good results.
However, there are some enthusiasts who choose to experiment and alter the original compost mix for various reasons.
This can be to give the compost a fertiliser content that is better suited to a particular group/s of plants, or to open up the texture of the original compost to assist drainage/water retention.
Here is an example mix for seed compost;
Sifted multi purpose Compost = 2 parts (by volume) plus one part Silver / Sharp Sand.*
*Sometimes the sand content of commercial brands of multi-purpose composts already contain a high sand content.
In cases such as this adjust the ratio to cater for this e.g. say 3 - 5 parts compost to one part added sand.
Compost / Sand ratio
Soil-less potting compost:
When making a soil-less potting compost there is no need to sieve the compost, and the ratio of compost to sand is circa 6-8 parts compost to 1 part sand.
To mix a soil-less potting compost equivalent to the fertilser content of Ji composts use the following formulae:
Ji2 equivalent - To every 10 litres of multi-purpose compost; mix in 18g (¾oz) of limestone and 90g (3oz) of John Innes Base Fertiliser*.
Ji2 equivalent - To every 10 litres of multi-purpose compost; mix in 35g (1¼oz) of limestone and 180g (6½oz) of John Innes Base Fertiliser*.
Ji3 equivalent - To every 10 litres of multi-purpose compost; mix in 55g (2oz) of limestone and 270g (10oz) of John Innes Base Fertiliser*.
It is very important that the chemicals are mixed thoroughly into constituent parts of the compost.
There are other similar products on the market that can be used in lieu of Ji base fertiliser.
Two that come to mind are Vitax and Chempak potting base.
There is also one to make ericaceous potting compost!
If you decide to use any of these alternative products; the recipe is quite different to that described above, for example one does not need to add lime / limestone if using Chempak but you will if you use Vitax.
Advice is; Follow the instructions on the product packaging to the letter.
Substitute Ji Potting compost:
On occasions there may be a need to use a soil based fertiliser but as mentioned above, good quality loam may be difficult to come by, and commercial products can be quite expensive.
To overcome this; one can use well rotted garden compost from the compost heap / bin in lieu of loam.
Method: add equal amounts by volume of garden compost to multi-purpose compost e.g. 50% -50%.
Subject to the quality of the garden compost it might be advantageous to seive it to remove any partially rotted material.
Mix the garden compost and multi-purpose compost thoroughly together, then substitute the resulting mix in lieu of the 100% mixed compost mentioned in the above recipes.
A word of warning!
The finished product will not be as sterile as commercially purchased Ji potting compost due to the addition of the garden compost. The difference is likely to be seen as the emergence of weeds in the compost.
Providing these weed seedlings are removed before they become established and compete for the nutrients in the container all should be well.
One could go try sterilising the garden compost prior to using it but this can be quite a difficult task, and you might still not be guaranteed sterile compost.
For this reason the writer has not detailed the sterilisation process required.
Finally; Store all resulting mixtures seperately in covered bins or thick plastic sacks.