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