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In the past it was usual to either buy or make compost that used the John Innes formula as a base.
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.
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, so 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 light to handle, 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.
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.
n.b. Peat by itself has a pH level of 2.5/4.5, so this is considered to be far too low for most plants.
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)
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 for seed compost;
Sifted mp Compost 2 parts Compost one part Sand Final mixture note; Sometimes the sand content of commercial brands of composts already contain a high sand content. In cases such as these an adjustment should be made to the above example to cater for this.
e.g. say 3parts compost to one part added sand.
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