Following is our recipe to mix 100 litres of our general purpose potting compost.
Peat
Coarse Peat
For small pots under 1 litre – 80 litres of fine/medium grade peat
For larger pots over 1 litre – 80 litres of coarse peat. We want plenty of large fibrous chunks to help maintain the structure over time and stop the compost slumping.
Composted Bark
20 litres of composted bark, not bark chips.
We want the composted bark to help drainage, hold nutrients in the mix and inhibit some fungal diseases but if you can’t get hold of any increase the above peat content to 100 litres.
You can increase the bark content if you wish to reduce your peat content but be aware that the mix will be difficult to keep damp and you may encounter nutrient “lock up”.
Lime and Fertiliser
200g of preferably dolomitic lime (AKA dolodust, dolomite, mag-lime) but standard garden lime is fine if you can’t get it. This reduces the acidity of the peat to a more desirable level.
400g of 12 month controlled release fertiliser (Available here in our shop) which looks like little brown/yellow balls. This will provide all the nutrient requirements for 12 months. You can also use CRFs that last 6 or 8 months as is your preference.
80g of Vitax Q4 or bone meal (Available here). This is a starter fertiliser, the CRF above will not start to give out for about two weeks.
Mix well!
Mixing on a board with a shovel is the norm. I have seen cement mixers used and even a nursery using a 20 ton excavator, use whatever you have to mix it as thoroughly as possible.
Please be aware that when using CRFs you need to use up the compost that you’ve mixed within 4-6 weeks before the CRF gives out too much nutrient and makes the compost too “salty” for use.