Wednesday 18 February 2015

Soil Carbon Capture

Nature’s glue and superglue: humus and glomalin 


Most of the gardening press portrays bulky organic matter as humus. Wrong. It is not. Humus is a small and very distinct component of soil organic matter. Most organic matter whether added by grower or nature is destined to quickly decay. Not necessarily a bad thing as valuable nutrients are released when the carbonaceous component returns to carbon dioxide and water.
Generally humus is much more long lasting. Sometimes for hundreds of years when intimately mixed with soil particles, particularly clay, it binds particles together in what we hope are ‘water stable crumbs’.
Even science is fairly ambivalent as to what humus actually is and in different circumstances the term has different meanings. You might remember from your geography books, forms of humus that occur in nature called mull, moder and mor. Yes, it bored me too!  
Mull is formed when plant residues mix evenly into the surface profile - rather similar to a gardener’s soil without the dubious benefit of cultivation but with the benefit of worms. Mor is accumulated partially decayed organic debris that builds up on the surface, is usually acid, relatively stable and any breakdown is dominated by fungal action.

Even the humus I refer to today, in terms of  definition, is something of a ‘mish mash’. Perhaps that is because it is a bit of a mash up in chemical terms! 
Scientists used to say they could not properly define humus because its extraction destroyed its true nature. They talked about humic and fulvic acids. They still talk about its colloidal nature.
Fresh plant and animal organic remains undergo a process of gradual decay. I think of it as a journey taken with poor navigation. Numerous organisms and processes are involved. They might include those in the gut of a vertebrate such as a horse, to those of the  smallest soil-living bacteria. Invertebrates, bacteria and fungi will all join the party. It is not just breakdown, it is synthesis too, when organic matter is food for an organism and becomes part of its substance, not to mention it’s faeces with all that enzyme action. The process might start by being rasped by a snail’s radula or the decay caused by grey mould. It ends with black humified material usually destined to quickly oxidise completely away to simple inorganic chemicals.

The mystery is why humus, as shown by radio carbon dating, can benefit the soil for hundreds of years. The reason is not completely clear. One factor might be that this mangled organic debris has no regular structure. Unlike fresh organic material where molecules join together in a series of regular patterns, these are no longer a feature of humus. Bacterial enzymes which are capable of organic matter breakdown usually work on ‘lock and key principles’. No regular structure means no easy way in.
Another factor is the very small size of some organic materials. Not only can they intimately mix in with and glue together clay particles, soluble humates might even penetrate within the lattice structure of clay where the spaces are too small for bacteria to enter.
Some ‘particulate’ humus is merely tough and hard organic material derived, for example, from woody material.
Sorry to be vague but soil organic matter does have that reputation of ‘muck and magic’.

Mycorrhiza are the source of glomalin

That a third of the world’s stored soil carbon weighing thousands of millions of tons was unknown until barely twenty years ago is truly amazing!  It is a constituent of every soil examined in what is now worldwide investigation. It is a tough glycoprotein and is a superglue that sticks sand, silt and clay together into wind and water resistant highly fertile  crumbs. It is a product of the tough linings of hyphae and spores of dead mycorrhizal fungi. Known as the arbuscular fungi of the genus glomales, all rely on symbiosis with plants to derive their carbohydrate content. Glomalin is a rich store of iron and nitrogen and is very stable with a life of as much as forty years. Its iron content is thought to be significant in a plant’s ability to fight pest and disease.
Although the amount of glomalin in a soil is highly variable it’s not unreasonable to estimate an average soil content at least five times that of humus. Humus content is partially protected by glomalin.
Needless to say no-till methods of farming that are now starting to proliferate worldwide  - although not in the UK - hugely enhance glomalin formation. 
Not only does minimum cultivation encourage glomalin production that hugely improves soil fertility and plant yield, it sequesters a lot of carbon.

For most of my life I did not realise that soils sometime contain significant amounts of pure carbon, the long lasting remains of historic fire. When vegetation is burnt most of the carbon is released in the form of carbon dioxide. But not all when char falls to the ground. Sugar cane soil for example is often very fertile as a consequence of annual burning.(A process unfortunately not free from atmospheric pollution). 

I have written previously about ancient, fabulously fertile, very deep, Amazonian black earth soils which are known as terra preta. They were created by bygone farmers who over numerous generations added charcoal to their soil.
Science has never quite accepted the claim of modern South American farmers who sometime sell a thin layer of their soil to garden centres for compost(!) and claim that it grows back. This is not so silly as it sounds because charcoal with it’s vast porous internal spaces is an ideal substrate for arbuscular mycorrhizal fungi. If the rate of glomalin production is speedier than its decay the volume of the soil’s carbon grows!

My own interpretation of some recent research

There is now considerable interest in adding ‘biochar’ to agricultural soil. Made by pyrolysis from unwanted woody and vegetative remains it has the potential to create soil fertility and at the same time to sequester carbon.

Much research is taking place. The result of a recent trial at Southampton University worries me. Please indulge me for exploiting my platform of ‘writing a blog’ to make a suggestion!
The Southampton work is a very fine piece of research that has had much recent publicity. Done by geneticists using lettuce and thale cress, that darling of researchers, it showed that biochar/soil mixes gave healthy growth as a result of favourable genetic stimulation. Unfortunately they also found  a suite of genes that switch on a plants ability to fight pathogens to be completely inactive. The fear that char might make a plant vulnerable to pest and disease would be the death nell to this nascent technology being adopted. The research gave no indication that the plants actually suffered from any infection. 

I think they have drawn the wrong conclusion  

It has been my understanding that a plant’s defences are switched on when pest and disease threatens. We keep reading about how plants signal the presence of predators to mobilise their fighting resources. Every phyto-chemical synthesised to fight a pathogen comes at ‘a cost’. If a plant is healthy and unthreatened it does not need these genes to be working!  My own interpretation is that biochar encourages healthy growth but the trial control of less fertile agricultural soil causes stresses that alert the plant’s defences.

Even if as is likely, I have got it all wrong, we should not jump to conclusions. It is my understanding that field studies generally support the notion that plants grown in biochar are healthy. It is thought to be one of the huge benefits of terra preta soil!

I do think researchers have a problem with biochar in that the fresh material of this porous and strongly absorptive material has very different properties to older samples that have weathered, become charged with nutrients and teem biological life.
As an ingredient of a growing compost it seems in my own experience that fresh unprepared charcoal is pretty useless. But when I scoop up a spadeful from my charcoal enriched soil it gives superb results in a pot!

Making my own ‘charcoal’

I have reported before that I go out on a limb in how I prepare what I claim to be char. I do not let my bonfires burn through. I immediately extinguish the final burning embers by dousing with water. What a thrill to to see the immediate transformation to jet black shiny flaky remains.

Peter and I have recently had a project in Cathi’s garden. Her sixty meter long hedge had been uncut for all of ten years! Peter worked all day with his chain saw whilst with the ‘help’ of my son I dragged the wood away to create a thirty meter long pile. 
Last week it was dry enough to burn. 
We created a really hot firm blazing compact base and together spent ten man-hours continuously dragging branches to the hot blazing pyre. Peter later retreated and left me with at least a cubic meter of ember. I spent a very happy half hour with the hose pipe. Never before have I had a fire big enough to give me twelve piled barrow loads of lovely black gold! My normal fires produce a barrow of bounty and only take a couple of cans of water to extinguish. Not this one!

I make no apologies for having a fire. We live in the country and there are no legal restrictions. All our neighbours have domestic fires. Peter and I were always going to burn the wood. It would have cost a thousand pounds to have it recycled at the local composting plant. We see lorries burning diesel bringing all manor of wood many miles for disposal at the cost of hundreds of pounds a load! 
I do not of course have a pyrolyser which is the proper way that regular char is prepared. Pyrolyzing is of course non polluting and retains fuel residue. It is not an option for me. You might decry my methods but at least I have sequestered a lot of carbon rather than generated even more carbon dioxide. My char will sequester in the ground many years after all that wood would have rotted away to carbon dioxide and water.
I have amended my methods with the char that I generate. Not only do I now mix it with compost materials such as the copious herbaceous litter that my garden produces and the organic debris when I clean my ponds, I speed the breakdown of the soft vegetation and ‘charge up’ the charcoal with generous use of a high nitrogen general fertilizer.

I used to be rather apologetic that my ‘shiny black’ was probably inferior to that made in the normal way. I wonder. I suppose those ancient South American natives prepared their charcoal from wood - not having the technology to prepare soft vegetation into the equivalent of modern biochar. Local Amazonian practices suggest even now, that soft organic vegetation and animal droppings were stored in urine rich middens. I imagine it all got together with the charcoal before being spread on the fields.

Boys form the black stuff!

It was dark by the time I extinguished the fire
Half of the prunings
We got a good blaze
It was a bit of a competition

 We kept a small fire going for several barrow loads of woody clearings the next day

It just might be better to crush the charcoal to a powder
It would have been better to mix it in

It won’t blow away but I won’t take any chances
My previously enriched soil. I won’t pretend this is a vertical profile! (yet)

Mulched two years ago from my previous compost/char heap

The creators of terra preta mixed pottery shards into the soil. It is merely incidental that my bricks are contributing!
It was a very hard day
Making char is just ‘my cup of tea’
(‘char’ is Yorkshire dialect for tea)

I have written a new post about the use of 'my char' in potting compost
USDA link about glomalin


  1. Roger you are way above my head there. I will have to re-read your post a couple of times to begin to understand it. Perhaps you can give me some advice. We have two wood stoves and twice a year I sweep the chimneys and put the result on the compost heap. This is black, charcoal-like stuff which is mostly creosote (we only burn wood in these stoves). Should I put it on the compost? Every sweep produces a full bucket.

  2. Well you are persistent if you read it again! I don't want top put people off!
    I think the popular gardening press often fails to take gardeners further than the elementary and there is a niche for more detailed articles. I am afraid as a former horticultural lecturer - with a special interest in soil management- I sometimes get carried away.
    I am afraid I cannot help with your problem in as much as creosote is usually toxic to plants. Old gardeners used to weather soot of the chimney sweep for at least a year before using it! I wonder if that is any kind of indication?
    I am inclined to be cautious and say its not for the garden! If anyone has any experience with this soot residue please help!

  3. Thank you Roger. Since creosote can be used as a preservative, it probably not a good idea to add it to compost.
    It is true that the gardening press does not take gardeners further than the elementary. I always enjoy reading your posts.

  4. I read it twice too - now I see why we had nothing to add to our soil after having a big fire - it burned for too long.
    Boys and their fires eh? I do think a little prior warning of the dressing gown shot would have been in order.

    1. Not much fire under the dressing down, Sue!

    2. In the frivolity I did not respond to your more serious point, Sue. Of course most gardeners do let their bonfires burn through to obtain a couple of quid’s worth of potash and lime. (which is also there in my char)
      To me that is so wasteful when char can do so much more for your soil - such as water absorption, aeration, store nutrients available to plants (seemingly nitrogen too which normally leaches), improves structure, increases biological activity and more. And it is permanent!
      I know of no other gardener who treats their bonfires as I do – but then fifty years ago everybody dug their borders! To my mind it is silly to add even more carbon dioxide to the atmosphere and destroy that beautiful black stuff.
      It is even safer than leaving an unattended fire overnight!

  5. For what it's worth, I do feel guilty when I let me bonfires burn to ash. But by that time I'm too knackered to get the hose out!

    I've had an area of garden that the worms have been working on for two years, which is where I have horsetail. As the worms love cardboard, and as the horsetail doesn't find it easy to get through the cardboard, I'm hoping it's a win/win. Every few months it gets a new layer of cardboard and some compost on top. I dream that one day I will wake up and the horsetail will have given up, and my clay soil will be crumbly. So far there is a definite colour change to the clay, its a much darker brown, but it's still wet sticky clay. Now I'm wondering how many more years before I can plant it, and if I should try burying organic matter, or just leave it alone. Digging holes might do more harm than good. Perhaps I should give the worms another year...hopefully they are breeding and increasing the work force!

    p.s. Before anyone tells me that horsetail thrives in dank acid airless soil, and I'm making that worse, yes I know that! I'm trying to deal with that too, but a few years without daylight will hopefully knock it back.

  6. I guess if a refined lady like yourself can get 'knackered' so can I! I enjoy making my char so much I get a new lease of life when I put out my fire. It is quite an art to not waste any wood that is still burning. I migrate my fire with the help of my fork across the heap until it is all char. Usually takes about half an hour, but the huge one described took rather longer. Indeed Cathi came out in the evening to give it some more water just to make sure.

    As to your horsetail I have not blogged about this yet. It takes at least three years with careful and persistent glyphosate use- and even then most people fail!
    Your method is ok but I am not sure how long it will take. Make absolutely sure none raises its green head above the parapet of your cardboard!
    One suggestion is you put on another generous application of compost (And card if you wish) and a layer of polythene+/- a pretty mulch and stand some container grown plants there for a couple more years. I think by then your horsetail might be gone and your soil much improved - but of course clay does not miraculously disappear - it just develops a better structure with all those worms!
    Good to have you back Sarah!

    1. ps your method of suppressing the marestail will only work if ALL the clump is covered - none coming round the edge

  7. The sneaky ones that attempt to escape get an upturned flower pot over them, until I have time to spray them. However the woodlice often move in and finish the job for me, they seem to think horsetail is yummy. But I fear that every time a stem dies, the next root bud in the chain kicks into life, and there could be a hundred on a root. My theory (based on no evidence whatsoever) is that the reason people say that trying to dig it out makes it worse is because broken roots wake up more nodes. So as long as you kill those that grow, it probably helps.

    But I should wait until you blog on horsetail, I'm going off topic!

    1. You have got me thinking about doing a post for next month Sarah!

  8. Mull, moder and mor sounds like a firm of solicitors or three witches! I always learn from and enjoy your posts!

  9. @Roger

    Well now you have met another gardener who 'treats his bonfires as you do'. I even store up my woody waste until I have enough for a good char fire. My only difference in technique is that I start off by digging a shallow fire pit and pile the removed sods around the edges of the pit making a small wall about 12" high. As the burn progresses, I rake out the pile of embers to the sides of the pit against the makeshift wall, and douse them with the hose pipe. This cuts down the air supply and so I end up with more of the black gold.

    One problem is that the char is very alkaline from all the potassium and calcium oxides and nitrates. If you have a wormery, you might like to try this little trick. Bag up the char for a week, then pour the 'Tea' from the wormery over the char. The acidity and bacterial brew in the 'Tea' sweeten the char, while the char absorbs most of the richness of the tea.

    This mix really is 'Black Gold'. Incorporate it into the topsoil or your potting mixes, but don't let it dry out as you will tend to kill off most of the biomass it now holds.

    1. Great to hear the news I am not alone and your experience of charging the char is very interesting.
      Like yours my char is alkaline although I think some of the commercial inputs feed stocks give a neutral or even acid material.
      Other than the calcium and potash which I assume are carbonates I am not aware of any natural nitrogen content in raw char

  10. @Sarah with the clay.

    Unfortunately, worms and cardboard are more likely to turn your clay into a slimey stinking anaerobic hell, rather than into the wonderful loam that is the potential of nearly all heavy clay areas.

    The reason is that worms need more than just organic material to break up clay. Most importantly, they need Calcium (in the form of ground limestone). But in addition to the Calcium, clay needs frost. I don't know if you would agree with me Roger, but for me, clay is the one soil type that will benefit from digging, in order to allow frost and limestone to penetrate and do their work in a timespan that is useful to us short lived humans.

    Clay is an amazing material, and I would like to introduce you to a book I consider to be the bible of agronomy - 'Science in Agriculture - Advanced Methods for Sustainable Farming' by Dr Arden B. Andersen. ISBN 0-911311-35-1 (chapter 9 - Clay Chemistry) It is an amazing read, this guy really understands the importance of the 'life' of soil.

    Andersen explains how Calcium ions are needed to diffuse between the tiny clay crystal plates in order to hold them apart and in so doing, allow the clay to granulate and yield its magnificent potential fertility (Magnesium and Potassium bind clay up, but Calcium breaks it open).

    I am not lucky enough to have any clay in my garden (it is all Lincolnshire silt), but where I have needed to treat a heavy clay, I have processed it as follows:-

    Autumn time, dress with ground limestone.
    Trench dig, tipping organic, more limestone and char into the trench and dressing the rough sods with more limestone.
    Then leave it for the winter to work its magic.
    In Spring, when the land is dry enough to walk on without causing compaction, use a drag to break down the clods to a tilth, and enjoy the wonderful fertility of clay.

    Of course, worms will massively benefit from the digging aeration and incorporation of organic and limestone, especially if you use worm tea treated biochar. Then, with the return of the worms, if you are especially lucky, you might even get a mole to grace your plot and aerate the clay still further.

    Roger, I appreciate that this is near blasphemy on your blog, but I would very much like to hear your thoughts on bringing clay back into production.

    1. Sarah, close your ears to this blasphemous man. He has already led me astray on the subject of evolution. ☺. Carry on mulching!

      @fool hilly
      Yes I too would like to have a clay soil to demonstrate the wonders of clay and also to demonstrate that clay soils benefit MOST OF ALL by minimum cultivation. Indeed I will soon be writing a hard hitting post to say so. (rather than the mild one you will find if you put ‘clay’ in my search box at the bottom)
      I do mean clay SOIL however not a wodge pure clay subsoil in an ill drained low lying position. You cannot make a silk purse out of a sow’s ear and radical rebuilding might be in order – but even then I would tend towards building an organic improved layer on top of the clay and let the roots and worms penetrate down into it. The thorough treatment you recommend might be ok as a one off but that wonderful temporary tilth you describe – which I agree frost gives you – soon slakes down and return you to the usual problem and need to dig again!
      (Seem to have seen a post recently about something called glomalin sticking particles together! ☺ )

      I agree with you that lime will improve the structure of clay soil, but as I will explain in my post I am not sure about the conventional explanation of flocculating particles. More the benefit to the worms of the lime? But it does not need to be dug in.

      I will very shortly read the reference that you have given me.

      You have given us a clue to your identity by mentioning your local Lincolnshire soil. (Are there really hills to live on?)
      My spies had wrongly identified you as a radical Scottish blogger!

    2. My primary aim it to weaken the horsetail. However I did lime in the autumn, partly for the clay, but also because it was another weapon worth trying against the enemy. I did so cautiously as I wasn't going to dig it in. Compared with using plastic or newspaper, the corrugated cardboard is probably the least likely to exclude oxygen. The only downside is it needs replacing fairly often, and Amazon have cut down on the excessive use of large boxes for very small items, so I need a new supplier!

    3. Yes my polythene suggestion was only a way to keep on top of the marestail and at the same time stage a display of pots. Whether or not Mr Foothill is right about poor oxygenation in your clay I cannot see anyway that polythene can exclude oxygen short of an impossible complete seal. The air will just diffuse around and under it

    4. Ah Ha, you have me!! Indeed, I live in Long Sutton, famous for King John's Treasure and Dick Turpin, right on the edge of the Wash. The land around here is flat - flat - flat. But - it is said that 'In the land of the blind, the one eyed man is king', and so, I believe, it is with hills...

      In the reclaimed marshes that extend from the Bourne Ridge and Peterborough, right up to the sea walls penning in the Wash, you will find a number of very old towns, like Long Sutton, that are built on small hills rising out of the silt marshes. If you run the Google Earth pointer over Long Sutton, you will see the elevation above sea level soar to a staggering height 6 feet above sea level !! Enough to keep even a highwayman's feet dry... Our famous church and our nearly famous Couch House, the Bull Hotel, are both built at the very top of this hill.

      But I am not one of the gentry living in the (elevated) town centre, I have an old farmhouse built at or about sea level. So I thought I would build my own hill! Over the years, building work has led to the creation of quite a nice sized spoil heap which now towers to a magnificent 5 feet above sea level. So, early mornings, this fool can stand attop his private little hill, watch the sun tint the clouds and then come racing across the flat lands and hit me full on while I listen to the birds welcoming in a new day. Armed with a steaming cup of tea, it is a grand place to stand, watch, listen and think foolish thoughts...

    5. But back to reality... on the subject of clay vs clay soil, we are both very much of the same mind, with two provisos.

      The first is that it is alarmingly easy to convert a good clay soil back into clay. Clay is tiny particles of silica, devoid of any nutrition for plants, but those flakes can store huge quantities of other mineral salts and ions between them (much like the activated carbon capabilities of char). In clay soils, calcium and other mineral ions have filled in between the silica plates and caused them to flocculate allowing air and water to penetrate, and along with air and water comes bacteria, fungi and plant roots. The plants are hungry for minerals, and salts such as sulphates, nitrates and phosphates. To get them, the plants pump sugars out of their roots to feed the bacteria which convert the sugars into acids. The acids release the minerals from the silica plates allowing them to diffuse to the root hairs where they are hungrily taken up by the plants, and in turn, hungrily consumed by grazing animals (or gardeners/farmers). The hard truth is that plants can strip calcium out of clay soils with alarming ease, and when this happens, the clay flocculates collapse back into dense clay, expelling water and air. The clay then rapidly goes anaerobic and becomes a no-go area for both plant roots and worms and all the other life that makes a soil a healthy place for plant roots.

      Put simply, calcium is the elixir which converts clay into one of the finest soils going. But without calcium, clay is useful only for lining ponds and making bricks and pots. Sadly, the prolific addition of organic material to clay soil can lead to the extensive production of acids which in turn leach calcium out of the clay soil and send it on its way back to clay...


    6. A few years ago, I worked with an Agronomist in Norfolk. One farm he tended had heavy clay soil. The yield from this land had been falling year on year despite the farmer following detailed advice on application rates of NPK, so the Agronomist had been called in to analyse the soil and correct the decline. He dug pits in the fields so he could section down through the 'soil' and see the progress of the wheat roots and soil structure. The farmer had been chopping the wheat straw and ploughing in the stubble in order to build up the organic structure of his soil. But there, 10" down was the black 'petrified' (or should that have been peutrified) remains of the previous crops, none of it had composted, not even the roots, it was below the aerobic zone ! The crop in fact was growing in just 3" of brick like 'topsoil'. Records showed the land had not had any limestone dressing in over 15 years, yet the crops had kept taking calcium out of the impoverished clay, spurred on by lavish applications of NPK - it was nearly hydroponic farming. There were no worms in the soil and the aerobic layer stopped barely 4" from the surface. The Agronomist's advice - apply a heavy dose of crushed limestone and disk it in with the chopped straw - NO PLOUGHING. Three years later, yields were racing up and the farmer had a healthy layer of soil instead of his brick rubble. Close your eyes now Roger - the farmer then started shallow ploughing in order to bring up some clay and increase the rooting depth available to his crops - this was literally a process of making a 'silk purse' out of a 'sows ear' by repeated application of limestone... But it highlights my second proviso - time.

      Given long enough, clay pans will be turned into clay soils as limestone dust brings the all critical calcium to the clay allowing it to flocculate and in turn attract plant roots and invertebrate and microbial life to create the soil - a process which can take hundreds of years. But we gardeners only have a few short decades and would like to help along the soil forming process a little faster - The challenge is to get air and calcium down into the clay. If we can get, and keep, air into the clay, then worms and plant roots can do the rest, provided there is a good supply of calcium nearby to work its magic on the clay and start its process back into soil.

      But yes, once you have a clay soil, all you have to do is mulch and add calcium to replenish that lost by acid leeching and plant needs, then let worms, moles and plant roots do the all important task of keeping the soil open and aerated..

    7. Where would the calcium go? I don't think it can leave the garden on it's own. It must end up on the compost heap, or in the bonfire ash, and then make it's way back into the soil.

    8. Roger, I like the idea of using pots for a while, but it's about 30m2. I do resort to plastic sometimes, but the worms attempt to eat that too, so I try not to.

    9. @Sarah

      Calcium is an element and cannot be broken down into anything else and it is not volatile, so there are really only two ways it can be lost from the clay.

      If organic acid is present, it can be solubilised and washed away the same way nitrates are lost from the soil. But the most significant way it is lost is by plant uptake and removal as crop.

      If plants are simply composted, then again their calcium can be solubilised as organic salts and again washed out of the compost before it is returned to the garden.

      NB Calcium Nitrate is very soluble, so if the calcium ions in the clay link up with free nitrate, then there is the risk of loosing both valuable calcium and nitrate by leaching.


      I don't know if you have noticed this one Roger, but if you treat a lawn on clay soil with ferrous sulphate, the soil tends to get much heavier and more waterlogged. The ferrous sulphate quickly breaks down to ferric hydroxide and sulphuric acid which drags the calcium out of the clay, then precipitates it an insoluble calcium sulphate which is neither use nor ornament to the clay (at least until sulphate reducing bacteria set about breaking it down in order to steal the sulphur). I try to compensate for this by treating moss with a 50:50 mix of Ferrous Sulphate and ground limestone.

    10. Group 2 metal, I know my way around the periodic table :-)

      So are you saying it will end up in the nearest river, and so it needs replacing? Plausible that it will end up in a river, but in nature how would it be replaced at the same rate?

    11. I turn my back for five minutes and you two cause chaos!
      I shall break down my replies thus
      @fool on a mound (1)
      ah we have located you - great to have background and I love your eloquent prose describing your situation. I believe Dick Turpin (who also had associations with York!) caused chaos too

    12. @ foolmound (2)
      This is more difficult to answer, so please everyone forgive me for the detail.
      Clay is not pure silica but most sands are. Clay is not little pieces of sand as silt might be. Clay differs from sand in size, physical make up, chemical nature and in most cases clays can absorb water. I know you realise this Mr Hill from your later comments but I just wanted to clarify for readers. Clay has a negative electrical charge and holds on to positive charged ions such as K+ Mg+ Ca+ on its surface. These ions are held quite tightly but are nevertheless freely available to plants. In acid conditions these positive ions may be replaced by positive hydrogen ions but only in extremely severe circumstances is this of concern to the gardener.

      Deflocculated clay is the podgy mess you have already described. Soil flooded by sea water which was so severe in the South East UK floods in 1953 completely ruined good land by deflocculation. The land was recovered by adding calcium sulphate which provided calcium to displace the sodium ions but without creating alkalinity.
      We have been besotted by deflocculation ever since and every chemistry teacher shows a shaken up suspension of clay in a test tube to which with aplomb he/she adds lime water and miraculously shows clay particles sticking together. I believe this gives a complete false view as in normal soils I believe flocculation (a very good thing) is the natural condition and only in extreme circumstances does deflocculation occur.

      You mention the sugars that plants secrete into the soil with all manner of undoubted benefits. I still believe my book that says that simple ions are absorbed by diffusion – although I do realise that nutrient absorption is more sophisticated than just that.

    13. @hillmound 3
      Very interesting experience with your farmer. I have no idea of the precise circumstances but from your description I see the problem having been created by wet low lying soil, natural extreme acidity, heavy tractors causing compaction and poor fertilizer practice (not uncommon in the past) – and of course the very high clay content. Where artificial layers have been created in a soil because of a site’s very varied possible histories I have no complaint at all about mechanical mixing as I have discussed in one of my posts on why gardeners dig.
      I can very well see that in extreme acid conditions heroic amounts of lime might be needed.

      In contrast I think many gardeners over use lime. My own sandy soil is slightly acid and I do use a little lime in special circumstances such as making up compost or on my apple tree where bitterpit threatens. I am just reaching the end of my 25kg bag of dolomitic limestone but that is over five years

    14. @ on hill 4
      In your answers to Sarah I see leaching of nutrients differently to you. I was taught about ions existing as separate entities in the soil water and leaching as such. The only way that I can see calcium nitrate leaving the profile is in the unlikely circumstance of free calcium ions being released from the negative surfaces of clay and organic matter. This would only be where there were very severe acidifying influences on the soil
      If such influences on my own lawn of adding iron sulphate have lowered my pH to less than six (unlikely) I shall try your method

    15. Roger, I apologise for going off topic again, but I'm interested in this idea of calcium leaching. The science is all very well, but I have been wondering how nature copes with it. There must be some kind of equilibrium. I found this interesting article, which got me thinking. Are we trying to rid our gardens of the best calcium capturers nature can provide....snails?!!!! :-O
      This just leads to more questions! If I improve my soil with calcium, am I just asking for a snail population explosion? Most of the plants I inherited with the garden do not get snail damage, perhaps there is a good reason for that, they are not calcium lovers and therefore of little interest to snails? Should I leave my soil alone, and stick to 'no snail' gardening? That is quite a confusing idea :-/

    16. You and your off topics! I am already in the middle of writing a post on horsetail at your behest. In fact I think I will make your mulching a case study! In fact please tell me do you think your two years mulching has seriously weekend the horsetail?

      I keep hinting that slugs and snails are not as bad as gardeners think (put slimy into my search box) You have given me more ammunition. I was originally concerned that my pond snails might go short of calcium but of course my hard water when we top up supplies it.
      On my sandy slightly acid soil I sometimes find myself thinking of calcium in my dolomitic limestone more as a fertiliser than as lime.
      I have no reason to think you have a calcium deficiency in your garden. I think Mr Hillfool has got you worried- the clay that he and I like does hang on to calcium very well. If you have any worries about extreme acidity do a pH test with a simple kit. I know you already grow rhodos.
      Snails are very common in my garden and although I might take action with the hostas I can be barely bothered to squash them elsewhere

    17. Unfortunately I haven't been at all scientific with the horsetail battle. As well as the cardboard I've used glyphosate, ammonium sulfamate, lime, wood ash, plastic sheet, upturned plant pots and some root removal. The cardboard has become my preferred option, as it doesn't require a daily check, it's not as ugly as plastic, and it is improving the soil. In terms of success, again it's hard to say. Apart from not having counted the shoots, I've got rid of lawn and paving where it could hide, and we've had some very wet seasons that seem to have encouraged it to pop up in new places. It was never a dense infestation, but it was spread over a big area, so counting would be the only way to check progress. So although I'd love to say I'm making a difference, I think it's going to take a bit longer before I know!

  11. Roger, your posts are always so interesting and makes me want to skip over to Google and look for more info about your topics. I just had to look up glomalin and on Wikipedia I found: “It requires an unusual effort to dislodge glomalin for study: a bath in citrate combined with heating at 250 F (121 C) for at least an hour.... No other soil glue found to date required anything as drastic as this.” Fantastic! I know a bit about glycoproteins in our bodies – and the antibodies to them, but have never thought about the fact that they of course are also found in the soil.

    I also must admit I haven’t given much thought to what else is in my soil in the garden and the compost I buy. I choose according to what type of plants it’s meant for and as for my garden soil I am just happy it is as lovely as it is. I would like to educate myself better on this, and funnily enough, I actually bought a soil tester a couple of weeks ago – still un-opened. After over 13 years in this garden the result will probably not come as a surprise though, my plants have told me a long time ago what kind of pH my soil is!

    As for making char, obviously not a past time for me living in inner city London, bonfires are banned in Victorian back gardens without rear access so barbeques and chimeneas are the closest we get to open fires!

  12. I don't really expect you need your soil testing kit - although it is one of the few soil tests for gardeners that is any good at all! Never use the things myself although I am wondering at the moment whether to test my lawn to see what all my applications of iron sulphate have been doing when I moss kill!
    You mention barbecues. My own char would be superb for a barbecue- but I won't be using it!
    That hits one of the problems with worldwide take up - it has the potential to be burnt as a fuel

  13. This is a very fascinating discussion but like Alain I may have to go back and read it a second time. I'm fascinated by glomalin as a soil glue that has been only recently discovered. I took a basic soil science class at CBG but don't remember glomalin being mentioned. But maybe I just wasn't paying attention.

    1. You are just too old Jason! It was less than twenty years it was discovered.
      You cannot doubt its authenticity when you read the link to the United States Dept of Agriculture who are the last word in soil science!

  14. Roger, I am sorry, this is way off topic, but could you have a look at this video please. I would appreciate your thoughts or email me on

    1. Watched the whole hour and a half. Very interesting, seems that Roundup Ready crops raise a lot of questions. If you want to start up a discussion on the blog I suggest you do it on a previous glyphosate post or the next one! Will e mail you today.

    2. I think I have a lot more reading to do, so will wait for your next one...


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