Wednesday 2 November 2016

More on char as an ingredient in potting composts

The fact that it is bonfire night in three days time is merely coincidence
Peter Williams still chunters about when I extinguished his lovely fire. I wrote about it here
We had both sweated all day for all of seven hours feeding a hungry bonfire as fast as we could go. It was the residue of Cathi’s huge overgrown hedge.
Half an hour after Peter had gone home and left the huge pile of burning embers I got out the hosepipe….

That day I gained seventeen piled barrow loads of the lovely black stuff.
What a waste it would have been to let the bonfire burn through. In this case it would have taken several days of burning and smoking out neighbours. All that extra carbon dioxide and nitrogen oxides entering the atmosphere, probably doubling that released by the original very hot rapid firing. All to gain a little potash fertiliser which is a weak source of lime and very little potassium. Zero potash if later left in the rain!
Instead I have a long lasting nutrient absorbing, physical soil improver and ingredient for my compost.
You might have doubts about pouring water on a fire. In my own usual circumstances I burn three or four months of accumulated woody prunings and use three or four watering cans to douse my fire and generate between one and two barrow loads of ‘my own kind’ of char.
You might imagine it is a dangerous process pouring water on your fire. In fact it is very benign. It is the proverbial damp squib and the hot burning embers turn immediately black. No spitting or anything nasty!
I have become quite skilled when the last wood has been thrown on the fire when after a few minutes I extinguish perhaps a third! I comb the black with my garden fork to push over any ‘lumps’ onto the still burning fire. A couple of repeats at say ten minute intervals and all that is left is a still burning corner. That can be extinguished and used to start my next fire. Alternatively the very small pile can be carefully teased to one side and left to burn through!

My method adds perhaps half an hour supervising my bonfire. It does have the safety advantage of not leaving burning ashes unattended.

Regular readers know that I normally make my own seed and potting compost with my own sandy soil as the sole bulky ingredient. I do not recommend this generally because most garden soils are unsuitable from several standpoints. I do however feel that most gardeners can benefit from using garden soil when they grow in large tubs, say ten litres or more.
I originally took the view that the char I was making was pretty useless when I added it to my compost. That was before I realised  that raw char needs to be improved by a process of weathering and needs to be fortified with a large quantity of nutrients. After all charcoal is the great absorber. When charged with nutrients these become a reservoir for future plant needs and once ‘in the system’ can be used many times. Oh yes, I do reuse my composts many times over before eventually they are returned to the garden.
If I can repeatedly reuse the soil in the garden without ever replacing it why not the same for my compost in a pot?  Unlike regular compost, charcoal does not degrade.

Ready to go
You can access more information about my way of thinking by using the links in my theme column. There are three posts under the title ‘biochar’ which proceed  from Amazonian terra preta to modern interest in biochar and my own unusual method of making my own charcoal.
Under ‘Seed and Potting Composts’ I write about why I use soil as a compost and under ‘Seed and Potting Composts (2)’ is a series of four posts that deal with compost generally.
My attitudes to garden hygiene are revealed when I confess to not washing my pots!

I explained last year how my seventeen barrow loads of charcoal were stacked for a year mixed with organic waste and how I repeatedly top dressed the heap with a total of four kilogram of YaraMila
My writing today about my use of char is anecdotal. I stand by my recent post that trials done by gardeners are invariably useless. So too are many ‘proper trials’ when done by ‘interested parties’ or reported after the publicity department has got hold of them or worse when a reporter has added his own shallow veneer.
My use of charcoal that I am about to report was on only sixteen pots of tomatoes. There were as is normal for me several varieties. Eight spent early Summer outside before I had room in my greenhouse. Eight were inside all the time. Although I tend to water and feed at the same time I micromanage individual plants to get the best result I can.  There was no experimental control. My only comparison was with my own expectation of how my tomatoes should perform after fifty years experience of growing them. As I say anecdotal!
If I even state that I was pleased with my crop it might be demonstrating the benefits of the higher level of nutrition than anything to do with charcoal!
The most I can actually show is that charcoal can be used to grow a good crop of very nice tomatoes. (And my definition of ‘nice’ might be different to yours if you prefer the pap from the supermarket).

Using charcoal as the principal ingredient in compost
Having persuaded you that my experiments are of little comparative value I will tell you about my experience with my homemade bio char as an ingredient in potting compost this year! Rather than piddling about with small charcoal additions I decided to go the whole hog on this year's crop of tomatoes. Eight of my ten-litre pots were made up of 100% nutrient enriched charcoal and eight with equal volumes of charcoal and soil. 

I fully expected to find nutrient deficiency in the total char treatment. There was none, nor did any develop. My YaraMila Mila fertiliser which I used as a base and top dressing was the same as used for the earlier char fortification. YaraMila’s analysis shows that it contains the major plant nutrients and all essential trace elements. It worked for me.

I actually sowed all my tomato seeds in the fifty/fifty mixture. As my charcoal is un-sieved and a little lumpy the seedlings were rather unsteady! Another time I might sow a little deeper. In contrast I sowed very fine seeded annual mesembryanthemums on the same day when I scattered seed on the char compost surface. They germinated freely and were patched out to make very fine plants.
I must confess that charcoal is not very nice for the fingers and I needed my dibber.

By my normal standards this year's tomatoes have made very fine plants. Some are still cropping at the end of October. One in particular had lots of ripe and green fruit on truss number seven. That's good for me. 

Not very special but it was the first of November. This very strong plant was in 100% charcoal
I received Peter Williams' accolade when he watered this Summer when we were away!
Word from the kitchen was that the tomatoes were as tasty as ever and that Shirley was ‘putting on weight’ and bigger than usual and the Marmandes were huge. I am afraid quite a few of our tomatoes were wasted this year even though given to friends and relations. We had too many even with Brenda repeatedly freezing.

Marmande type Albenga was particularly well received in the kitchen
There have been suggestions that growing in charcoal might leave plants open to pest and disease. In my view this is complete nonsense and seems to be based on white coated laboratory experiments and not trials in the field. Indeed many field trials around the world seem to confirm healthy growth when charcoal is added to the soil. In respect of plant health my own plants grown with charcoal were perfectly normal. My experience adds less weight as I never  suffer whitefly and red spider mite which are completely absent from my garden as a result of careful isolation.

I have dabbled with charcoal in my composts quite a bit this year and most of the rest of my plants have had between ten and thirty percent additions. In practice this was loose handfuls of charcoal mixed into my soil when I did any sowing or potting! 
In one case I was intrigued to find six tiny pots of some strange red seedlings. I had forgotten I had sown a few beetroot to see what would happen in the charcoal. I popped in each potfull of five or six undisturbed seedlings into my vegetable plot. Never have I had better beetroot. They were much better than the same beetroot sown direct in the ground.

My peppers are always on a knife edge during propagation as I have no artificial heat in my greenhouse and are somewhat at the mercy of the vagaries of the season. Sown at the same time as my tomatoes - for their first fortnight in our heated conservatory - I lost all but two of my ten delicate seedlings. I had just one ten litre pot in a half charcoal mixture. 
The pot produced six lovely red peppers, better than ever before. Even Mark Willis would have been proud of them. Unfortunately Brenda was called away to her phone and they were cooked to a cinder (well actually char).

My single pepper was tucked away behind the tomatoes
I divided the nice orchid I pictured last year doing so well in a charcoal mixture. Unfortunately it did not like being cut into three and this year has sulked and barely survived. I live in hopes for next year

Is it worth using charcoal in a compost?
It is for me but then I don't like spending money on what these days is often dubious compost from the garden centre. I don't really like peat as it tends to dry out and these days I can't easily find any! As mentioned I usually get good results with my own garden soil. Ameliorated with charcoal for some plants it seems to be marginally better.
Possible advantages for me include that the 'cut off' between sufficient water and wilting on hot days in Summer is less sharp than in certain organic composts and being heavier my pots of tomatoes are more stable in wind. I qualify this last point as I have never actually weighed them and completely dry charcoal is quite light. Mixed with sandy soil my compost weighs quite heavy.
I did find that an inserted stake in my tomato pot is NOT very stable as my own lumpy charcoal tends to be loosely bound together. Lesson for next year is to put the 8ft. cane into the ground.
I tend to neglect my tomatoes towards the end of their life when we have more fruit than we can deal with. My main omission is feeding and I will say nothing about lack of trimming! I did find at the end of their season my tomatoes declined very gracefully.

The main surprise with the tomatoes was that there was little discernible difference between the two levels of charcoal.If anything, the best plants were in the all-charcoal. As I said earlier my efforts might show more about the feeding than about the char.

All the plants made very strong root systems
Charcoal when I dismantled my tomato

And yet another disclaimer
Although I would like to persuade you that my method of making charcoal is well worth doing if like me you are able to have bonfires, I have no experience with biochars sold on the market!
Biochar has huge international potential for soil improvement and is widely researched. I hope it has an exciting future. 

You can now buy biochar. It will be quite different to my own homemade char. New ideas bring in exploitive products. I am unable to advise you about them!
For what its worth it would be uneconomic and probably very foolish to buy charcoal marketed for barbecues to put on your garden!

My achimenes liked a half char mix

So did this rather large kohleria


I explained here why home trials are of limited value
I confessed to not washing my pots and denied any problems with damping off.


  1. This reminds me of The Martian, which I've just read. Have you read it, if not I'll get you it for xmas - it's right up your botany street

    1. Yes please Ben
      (Shooks, I rush to the post alerted there is a comment - they are rather sparse for this subject. - and its my son! Not sure what he is getting at, must read the book)

    2. rush? its been sitting there 2+ days! have you been actual gardening rather than blogging?

    3. I must have just thought the alert was one of your regular e mails.


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