A successful home trial of biochar

In an experiment at home, I compared the germination and growth rates of lettuce seedlings planted in either a biochar mix or in a conventional peat-based ‘John Innes’ seed compost.  Although the germination rates and the speed of growth of the leaves of the seedlings were slightly better in the John Innes seed compost, root formation was extraordinarily enhanced by the use of biochar. Good roots improve the future growth of plants because they enable faster take up of water and nutrients.

Biochar is the carbon-rich residue after any organic matter has been strongly heated in the absence of oxygen. Most charcoal  is made to be burnt as a cooking fuel, but biochar is manufactured to added to local soils. In depleted tropical earths biochar seems to add to fertility, reduce the need for fertiliser and improve water retention. Because biochar is highly chemically stable, these hugely beneficial effects may persist for long periods. Research results in fields throughout the world suggest that biochar may be an extremely useful addition to the top metre of soil.

In addition, biochar permanently, or semi-permanently, stores carbon which would otherwise have been transferred to the atmosphere in the form of CO2 or methane. Biochar may offer us the opportunity to make a significant reduction in the rate of growth of atmospheric greenhouse gases. If it helps agricultural productivity in countries with good soils and high rainfall as well as in depleted tropical soils, we can expect biochar to be extensively used around the world.

Research method.

I placed single lettuce seeds (Quatro Stagioni variety – a red leaved Italian lettuce) in forty small pots of about 20ml size. In twenty pots I used commercial John Innes compost bought from a large garden centre. The compost is peat-based with added artificial fertiliser. The other twenty I filled with seed compost from Carbon Gold, Craig Sams’s new biochar venture. The Carbon Gold mixture appears to use coir (from coconut husks) mixed with biochar and inoculated with beneficial micro-fungi.

Results from the experiment

The seeds were planted in mid March and the little pots were kept well watered in a sheltered outdoor location. Fifteen of the seeds grown in biochar germinated and eighteen in the conventional mixture. Growth rates were slightly faster in the John Innes, which probably contains added artificial fertiliser (NPK, nitrogen, phosphorus and potassium). The higher germination rates in John Innes probably arise because the compost constituents are very largely geared towards ensuring that the seed is closely surrounded by soil particles and is able to access water.

The John Innes compost required significantly more water in periods of drought. Coir and biochar kept its moisture very much more effectively. This is also an unsurprising result because seed compost is generally extremely friable because the soil particle size is so small and sand-like. Coir is much better at retaining water within its fibrous structure.

In late April, I took the seedlings out of the pots. The roots of the lettuces grown in biochar were very much larger and better established. The photographs are of two small plants of approximately the same total leaf area. (This may not be apparent from the photograph). The roots of the seedling grown in biochar are thicker, white rather than grey and very much longer in length. The average root thickness is at least twice as great as the seedlings grown in John Innes. The plants grown in biochar will be able to make very much faster progress in the soil. Now transferred to a plot on my allotment which was treated with large amounts of home-produced biochar last year, the seedlings from both sources are making slow progress because of the very low temperatures in early May. Under a fleece, the plants germinated in biochar seed compost are nevertheless growing more rapidly.


Disclosure. I was sent the biochar compost by Carbon Gold in response to my request. I was not asked to pay for the compost.