Seed and potting compost: Part 4
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The compost for this bletilla orchid is half charcoal |
Because I prepare my biochar in an unusual
manner people tell me that it is not proper charcoal.No matter, perhaps the
black stuff used by the ancient Amazonians to make terra preta wasn't proper
charcoal either.
'Proper' charcoal is made by a slow process of
combustion and modern commercial biochar is made by pyrolysis in special
equipment which also captures volatile organic fuels.
As I have previously explained I make my own
black stuff on a very hot blazing fire when I immediately douse the burning
embers with several buckets of water. As for for the huge fire I made last year
when we cut down Cathi's overgrown hedge it required extended visits with the hosepipe.
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It looks like charcoal to me |
I now regard it as a crime when a gardener lets
his bonfire burn through for several hours to make ash. My beautiful black
contains the same level of potash and lime as next to useless wood ash and has
the physical bulk to absorb air, water and nutrients and be a substrate for mycorrhiza
which in the long term will potentially produce glomalin. It also will
hopefully last in my soil for hundreds of years as sequestered carbon.
My black stuff is quite lumpy and for nearly
ten years now I have added barrow loads to my vegetable garden and the nature
of my soil is starting to change.
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My char mix from my heap is quite coarse |
Hitherto other than an earlier half hearted earlier
failure, I have not used my charcoal to make up potting compost. I now wonder whether my
previous unpromising experience was a result of not adding enough nutrients.
Charcoal is a very powerful absorbent. Some of my older readers might remember
'hush puppy' shoes where charcoal in the lining was used to absorb odours!
Charcoal's ability to absorb and hold very large amounts of nutrients is one of
its merits but it would seem to need ‘priming’.
I must be careful however not to confuse
charcoal’s ability to hold large organic molecules with the capture of nutrient
ions. Nutrients are held on the large surface area of charcoal as a result of
negative electrical charge which gives it a cation exchange capacity. When char
is matured in the soil this capacity
increases.
The ancients probably matured their charcoal in
their middens of plant debris and animal faeces and in my own version the
eighteen barrow loads of char from Cathi's fire last year has been composted
with organic waste from my garden ponds and cut tops of herbaceous perennials. Over
the year my huge heap has been gradually fertilized with a total six kilograms
of 20:10:10 agricultural compound fertilizer.
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My mixing was quite crude when I deposited the char on a thick bed of decaying herbaceous tops and topped it with waste from my ponds. Whenever I spread fertiliser in my garden I gave it several handfuls. |
Over its nine months so far my pile has
decreased little in volume. It was a fifty/fifty mix of organic matter and
char. The charcoal of course does not decay and despite the extra fertilizer
nitrogen which might be expected to create a favourable carbon/nitrogen ratio
for composting, the mix is still very fibrous with tough ‘straw like’ organic
matter.
I intend to ‘play’ next year with this mix to
make up potting compost! My results will be entirely subjective and will have
no scientific validity. It’s just that I garden this way, always trying
something different!
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Readers probably regard my methods of growing vegetables as very unorthodox |
My existing vegetable garden surface is now
quite black and last season I scooped up char rich soil from the surface of my
veg garden soil to make up my ten litre pots of tomato compost. I have been
impressed with this crop of tomatoes but
there are far too many variable factors for me to draw any conclusions.
I estimate that my tomato soil is between five
and ten percent charcoal. Tomatoes need high levels of nutrient and I have
fertilized them freely.
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I have been pleased with my tomatoes Shirley and Albenga |
Perhaps adding small amounts of char is enough
to amend some otherwise unsuitable soils to use as compost?
Charcoal as a seed and potting compost ingredient.
This is no wild idea and charcoal has long been
used. I believe it is sometimes used in Japan and some orchid composts are
thought to benefit from its physical and absorptive properties.
I understand in South America, extracted terra
preta is sold as compost in their garden centres. I bristle with disapproval at
potential destruction of historic fertility but imagine it is wonderful
compost.
Amazonian farmers claim that it renews itself!
I imagine the true explanation is that for terra preta soils which might be as
much as two metres deep, scraping some away seems to be insignificant.
The claim of renewing itself might not be
entirely bogus in that arbuscular fungi associated with the carbon and in
mycorrhizal relationship with the plants might accumulate glomalin.
As I have hinted I wonder if fresh char might
be an absorber of nutrients rather than a provider. Hence my heavy fertilization
of my composting pile. There are other ‘straws in the wind’ that ‘preparing’
char might be a good thing. Fresh char contains volatile organics that have
deposited on its inner surfaces which with time will be degraded by fungi and
bacteria. Also cation exchange capacity is thought to improve in the soil over
decades!
What I will be trying next year is to use some
of my large pile to make up some seed and potting composts. I will use some
‘neat’ and some mixed fifty/fifty with my soil. I will fertilize them as
described in my previous post.
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My charcoal pile has been marinating for eight months now. |
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When I fork into it there is a lovely clean fungal smell |
I have included charcoal in my series on
composts out of my own interest and curiosity. I don’t think any of my readers
should conclude they should rush out and buy charcoal! I suspect most
commercial sources of charcoal such as those for the barbeque and fish tank
cleaning are useless to the gardener!
There are no poisonous impurities in char made from woody prunings and no
trees have been chopped down to obtain it
Comments on some recent research
There was some interesting data about the
effects of biochar from Southhampton University last year. They grew laboratory
plants in charcoal up to the equivalent of 50 tons per acre. This is well over
twice the level that I have found to be suggested as a soil additive but
comparable with the levels I will try in my compost. The experimental plants
were lettuce and thale cress - that weed that is the traditional darling for
scientific research.
Their results showed exceptional
stimulation of plant growth by as much as 100%. Unfortunately they also showed
that genes responsible for pest and disease resistance were ‘switched off’.
They have no data whatsoever that the charcoal grown plants actually suffered
from pest and disease.
This latter finding has been deemed to be a
bad thing and might spell the doom of growing with biochar.
This supposition is complete nonsense but does need careful
investigation. I wonder whether the genes being switched off might merely be an
indication of healthy growth and that active genes switched on is a waste of
resources when there is no pathogen threatening.
After all we keep reading about plants
signaling the presence of pathogens and switching protective genes on.
Southhamptons’s suggestion of doubling of
growth is remarkable albeit not surprising considering the extremely high
yields of terra preta soils. It is not unusual for lettuce to grow very rapidly
indeed when conditions are ideal or apparently ideal as in such as in hydroponic
production. Wiki says that thale cress completes its life cycle in six weeks.
It seems to me that this weed sets seed within ten days from a young not yet
flowering plant. My point is that plants are extremely plastic in their growth
rates depending on a multitude of growing conditions. Perhaps they should trial
what happens to gene expression in peat and other organic composts.
I presume Southhampton’s trial must have
had a range of rates of biochar addition and I would expect the range to include zero char added. But added to what? I imagine it must be just
plain agricultural soil as stated in the very brief reports I have seen.
Although I seem to spend my life telling people
how good my own soil is as a growing
compost this is not true for most soil textures. Is the claimed doubling of
growth the comparison between plain soil and the very high maximum char? If so, I am not surprised at the vastly
improved growth. After all, all gardeners ‘know’ that soil in a pot is inferior
to compost! Perhaps good compost increases growth stimulants too?
I am reminded of my old foreman’s opinion
of the new fangled peat loamless composts when they were introduced sixty years
ago. He noticed that plants grown in the peat composts grew much quicker than
in the traditional soil based composts. His composts were potted very firmly
(ugh) and growth was stiff and slow.
He used to talk about ‘soft’ and ‘hard’
growth and always claimed that his soil based John Innes plants were healthier!
Shades of genes being switched on and off I
wonder.
I must do a post on whether hard and soft
growth is a real phenomenon in gardening! (I have now written this post - but no one seems to read it!)
You can read more about my unorthodox
methods with biochar and information about glomalin by clicking on these themes
in the right hand column