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’ – ie 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.
|NPK (many sources, types)
Bone meal, seaweed, etc
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
Compost (many types - green, food waste, manure, biosolids
Clay (many types)
SAP water gels
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.
• Peat is widely available and highly consistent, but we now recognise is use is not sustainable and environmentally damaging.
• Some of the above items (eg perlite, vermiculite) are quarried and undergo high-temperature heat treatment to achieve porosity.
• Others (eg Biochar) can be made from sustainable wood sources.
• Some of these materials (eg 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
This only tells part of the story – we need to know how the solid parts form into aggregates to make the growing media matrix. We can view this as aggregation using the following diagram
The aggregate structure controls aeration (oxygen to roots) and water retention for root uptake. But aeration and water are counterbalances. Items in the mix that hold water tend to be very fine (micron-sized) items such as clays and colloidal humus. But, tiny particles fill the air spaces. Items that support porosity (granular, sands, gravel) allow water to rapidly flow through and out.
When organic matter dries out it repels water (becomes hydrophobic). Some commercial composts and growing media will have wetting agents added to compost to help with water retention and rewetting. Some may have SAP water-holding polymer gels added.
The natural components that hold water are clay and colloidal humus. Soils and composts contain very little colloidal humus (1% and 5% reap). Here at SoilFixer, we have found a way to adjust the normal composting process to increase colloidal humus by adding a ‘compost humification agent'.
Table of soilless growing media ingredients and properties
SoilFixer had been working on using our knowledge of colloidal humus, biochar, compost and soil to create a fantastic container mix for hanging baskets and vegetable raised beds. Our aim is to help gardeners make their own potting mixes by adding SF60 super compost into potting mixes.
Should I add SAP water-holding gels?
This is going to be a personal preference. SAP gels are man-made plastics made in huge volume. The natural alternative is colloidal humus. Both hold and retain approx X10 own weight in water. Add too much and the mix can quickly become waterlogged which leads to poor root growth, disease and decay.
Should I add perlite or vermiculite?
Both are porous and offer good support to aeration. They are naturally occurring minerals, mined and then heat-treated at high temperature to improve pores. Here at SoilFixer we refer biochar – it has all the same properties, can be made sustainably, but vitally it also supports the soil biology and aide AMF/roots symbiosis
Can I re-use my potting mix? How long does it last?
This is interesting – biochar and colloidal humus last hundreds and in many cases thousands of years. Compost will be broken down and nutrients released within 12 months. The real question is how to I ‘top-up’ and keep using the following year.
Top up: preferred option: 1/3 high-quality compost plus a handful of chosen NKP fertiliser (eg organic blood/bone meal, or inorganic pellets, liquid feed). Mix in well and leave for at least a week before planting.