Making a fantastic raised bed / container compost mix

This blog was updated in March 2023 to reflect latest information on the new SF40 biochar multi-purpose compost.

SoilFixer has been supplying Biochar and SF60 (Biochar Super Compost) for many years. Our biochar-humus-rich compost is added to your topsoil to improve soil health and plant growth - it's the closest means yet for you to make your own Terra Preta!

Many customers grow plants and vegetables in raised beds, containers, planters, and hanging baskets using "all-purpose compost" rather than soil.  (We are not fans of using the term compost to describe a soil-less growing media (mix) - especially when very few of the growing media mixes use any green waste compost. Most are formulations of peat/coir/wood fibre!  see our compost definitions). 

Our goal is to find a better-performing raised bed mix that is also environmentally friendly - not just peat-free, but actually carbon negative (i.e. it locks carbon into the mix for hundreds of years). With a bit of annual care, our mixes should not need replacing each year.

Below we cover our journey to date.

Raised bed and container mix challenges

Container mixes present special challenges - they lose water and dry out faster than soil and the roots have a limited space to grow.  If you add lots of material that holds water (e.g. clay or humus), the mix can become 'airless' which restricts the root even more leading to stress, and disease problems like damping off (stem rot). It is not easy - ask any of the big growing media companies who have spent millions working on peat-compost alternatives. 

We have been using our knowledge of compost, biochar, soil, and colloidal humus to create a fantastic raised bed container mix. We have looked at two routes: adding SF60 to the soil, and creating a wholly new mix from biochar and compost (now called SF40).

We tested the two options last year:

• Raised bed one: two parts finely sieved PAS100 green waste compost, 1 part SF60, to give a 10cm layer on clayey soil base
• Raised bed two, two parts PAS100 and 1 part biochar, 10cm layer on clayey soil base.

The differance is SF40 is a mixture, SF60 is made by co-composting biochar with green waste, then sieving the final compost-biochar mix.

SoilFixer raised bed mix benefits

Both work extremely well - giving 100% increase in plant growth. There are differences in water retention and growth in different plants towards the end of the season. 

• Enhanced growth results
• Strong water retention and resistance to drying out
• Good re-wetting ability
• Viable cost for the "biochar raised bed mixes"
• Re-usable year on year (with some minor care)
• 100% peat-free
• Climate positive - unlike other raised bed mixes, biochar-based will lock carbon for +100 years

What next?

• SF40 is our preferred option for supplying a growing media - in part because it is much easier to make and control
• We need to test SF40 at a much larger scale... ongoing
• We believe SF60 will be the default for soil improvement