Many composts on sale in garden centres are labelled ‘with added John Innes’. Our own SylvaGrow with added John Innes is no exception. But why do so many products have this added extra ingredient and what exactly is it?
We explained in a previous blog the origins and nature of John Innes composts – see, http://127.0.0.1/melcourt_old/what-does-john-innes-mean/ . Essentially John Innes composts are those based on sterilized loam and nowadays (as opposed to the 1930s when they were first developed), many such as ours, use peat-free formulations.
So it is now possible to buy John Innes composts in the four standard blends, Seed, no. 1, no. 2 and no. 3, containing a slightly higher fertilizer levels as the numbers rise so that the seed compost has the lowest fertilizer level in order to facilitate seed germination and no. 3 has the highest level and is suitable for mature plantings. John Innes composts are noticeably soily and heavier than loamless growing media.
But this article is trying to explain why ordinary soil- free mixes are frequently sold ‘with added John Innes’. In other words, not the full-blown John Innes composts, but rather, multi-purpose compost with a little bit of John Innes – or sterilized loam and grit – added.
So what does adding a small amount of sterilized loam bring to a compost formulation? Loam is the term used to describe a soil with a roughly equal balance of sand, silt and clay in its makeup. Generally we can think of loam as a good, well-balanced soil that will be productive and fertile in the garden and not too difficult to manage – in other words, a good thing.
Most multi-purpose composts on sale in garden centres are made with no soil ingredients. Properly formulated such as our own SylvaGrow Multi-Purpose http://127.0.0.1/melcourt_old/products/sylvagrow- multi-purpose/ , they work extremely well across a range of garden applications from seed sowing to final potting. But in recent years there has been a trend to offer multi-purpose composts with a small percentage of sterilized loam – for example up to 10%, together with a small addition of sand or fine grit. The addition of loam and sand makes the compost just a little bit heavier, with a more substantial feel, holding slightly more moisture. And being more chemically active, soil typically has a greater resistance to nutrient leaching than most other compost ingredients such as peat, bark, wood fibre and coir.
On a practical level, a good sterilized loam is an expensive ingredient so ‘added John Innes’ mixes will generally cost a little more. Also, soil is around three times as dense as most soilless ingredients so ‘added John Innes’ products will feel heavier, even with the relatively low level that is added to most versions.
Trials comparing multi-purpose composts both with and without ‘added John Innes’ will typically show very little difference in plant performance. The differences are more about ease of management and personal preference. Some gardeners prefer the slightly more substantial feel of an ‘added John Innes’ formulation whereas others prefer the lighter, less consolidated feel of a straightforward loamless mix.
Whichever version is your preference remember that composts only have sufficient fertilizer for the first 4 to 6 weeks in use. Supplementary feeding after this will always result in the best plants.
