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How to make the best plant growing media (compost)

Tony Callaghan 30/10/2016

How to make the best plant growing media (compost)

This blog looks at how to make the best growing media for germinating seeds, potting on into containers and for use in hanging baskets. (sometimes referred to as: seed compost, compost potting mixes. We look in detail at what ingredients (components) are used in the mix and why. Finally, we explore new ingredients such as colloidal humus and biochar and how “supper composts” such as SF60 Biochar Super Compost (Soil Improver) can enhance growing media.

Growing media are just mixtures designed to replace soil or improve performance of soil. 

Here are a few examples of growing media mix recipes

  • John Innes: 1:1:1 topsoil / peat /sand  
  • Classic 1:1:1 compost / sand / topsoil (this is a version of John Innes where compost replaces peat and perlite often replaces sand)
  • Peat free: 1:1:2: 0.5 Coir / vermiculite / compost /worm casts 
  • SoilFixer recipe: add 1 part SF60 to 4 parts commercial growing media. (SF60 is a ‘super compost’ made from biochar, colloidal humus, compost, trace & NPKs).

When making your own mix, we would recommend sieving the ingredients to remove bits above 6mm. [A cheap hand-riddle sieve can be made by stretching a piece of squirrel wire mesh over a wooden frame].  Measure the required parts by volume using jugs/containers onto a table. Mix by hand and then add water. The wet mix can compact on storage - make enough for each planting session and avoid storing for long periods.

So far it all looks easy enough. However, when you read gardening blogs, there is lots of frustration around “failed” germination and poor performance of container ‘compost’ mixes. To explain why they often fail, we need to look a little more closely at the ingredients and what impact they have on the mix. (Before we do this, also remember: the term ‘compost’ covers many things. If buying ‘All Purpose Compost’ do not assume this is OK for seeds/potting. Check the label! Pure compost is a soil improver and not a good substrate for growing. Potting composts are usually blended mixes (see above) of compost and other items.

 We have tried to keep this non-technical, but we do drift into some technical soil science terms!

What ingredients are used to make a great growing media – and why?

A recap on what we are looking for in a growing media:
•    Retains oxygen so roots can respire (which is akin to our breathing) 
•    Retains water ready for roots to use – but not so much that the soil becomes water logged – and hence anaerobic (no oxygen for roots or microbes)
•    Is easy to ‘re-wet’ – i.e. absorbs water without it draining straight through
•    Allows NPK nutrients to be stored and transported to the roots
•           Limits nutrient leach out on watering!
•           Limits pace of nutrient release
•    Holds and supports the root and plant

List of possible ingredients used in plant growing mixes (media)

The table below contains a list of some possible ingredients. We have grouped them based on major properties we are trying to achieve: aeration/percolation; water retention/glue; nutrient supply. Some ingredients do more than one thing.

Nutrients Aeration Water Retention
NPK (many sources, types)
Bone meal, seaweed, etc
Trace minerals
Lime (added to aid NPK uptake)
Rock dust (flours)
Sand and gravel 
Perlite & vermiculite
Volcanic & expanded rocks
Biochar* (many types) 
Wood chip and wood fibres
Bark chip
Coconut coir
Compost (many types - green, food waste, manure, biosolids

Clay (many types)
Colloidal humus*
SAP water gels
Worm casts

Notes: *You might not have seen these items before – they are natural and sustainable and easy to use. If you need to know more about them follow the hyperlinks.  

Why is each ingredient added to the growing media mix? This is quite a challenging question. It does not just come down to the function (what it does), many other factors such as environmental impact, availability and economic cost all play a part.

For example:
•    Peat is widely available and highly consistent, but we now recognise is use is not sustainable and environmentally damaging.
•    Some of the above items (e.g. perlite, vermiculite) are quarried and undergo high-temperature heat treatment to achieve porosity.
•    Others (e.g. Biochar) can be made from sustainable wood sources. 
•    Some of these materials (e.g. coir) have enormous ‘carbon air miles’ as they come from the tropical regions.
•    Some materials are only available in thousands not the required millions of tonnes.
•    Uniformity, consistency, disease and plants pathogens control become critical when looking at high volume horticulture.
At this time, the big challenges appear to be removing peat from growing media and finding a way to make compost from recycled green and food waste in a consistent way so it can be blended into mixes.  We will leave these challenges for another day and focus on the properties of each ingredient so you can decide which to use in your mixes. 

If we look at the make-up of soils using pie chart diagrams we can get a general picture