Joop Corbin - swomp wrote:
the charcoal is supposed to retain more humid and micro-organisms. but, and than im lost, is so good in doing this that is increases without human intervention and using this soil means you never have too manure anymore....
What does that mean? without mulching?
...
And i can understand that charcoal in the soil keeps nutrients longer without leaching, and water i presume. I can get that terra preta will enable you to grow for much longer, and would make your (soil)system more efficient.
Hi, I can provide some explanation of the chemistry involved in charcoal as a soil amendment and the differences between temperate vs subtropical vs tropical soils which should shed some light on terra preta.
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Plant matter is generally mostly water by weight and, under a microscope, has a complex structure of various cell types arraigned into layers and those layers arraigned in different shapes and patters depending on the plant source, be it grass, herbaceous perennial, palm, bamboo, or woody plant. Density is highly ununiform and there are a lot of different microscopic and macroscopic structures. Once pyrolysis occurs, the water is mostly evacuated and the former cell walls and structural components make up the bulk of the remaining weight, along with small amounts of trapped ashes and tar like residues. In addition to the pre-existing venation, porosity, an cavities in the plant matter, pyrolysis leads to extensive crack propagation as steam produced inside the drying and hardening plant tissues causes ruptures, as well as the shrinkage from the drying itself causing buckling and checking.
The end result is a material that has a very complex structure with a variety of porosities ranging from microscopic to macroscopic.
Chemically, the material is roughly speaking an amorphous carbon species with substantial impurities. Incomplete dehydration, dehydrogenation, cracking, and similar reaction processes mean that rather than sheets of pure carbon like in graphene, there are a whole bunch of different residual organic (in the chemical sense) molecules. In areas of more complete reaction processes leave behind graphitic masses, vitreous carbon, and even fullerenes sometimes. To some degree there is also some carbon black and soot content. The degree of pyrolysis and the temperatures and durations involved will lead to a variety of polycyclic aromatic hydrocarbon (PAH) levels, and similarly a variety of carbonyl, carboxyl, and other functional group levels. All of these aromatic hydrocarbons and functional groups provide bonding sites and increase the ion exchange capacity (mostly CEC, but also AEC).
Typically, the charcoal is then broken up into large particles somewhere between coarse sand and pea gravel in size. At least when people are making "biochar" anyway.
What you're left with at the end of all that are these particles that with low weatherability and degradability that are highly porous with excellent internal and external drainage and a fairly high CEC.
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Now let's talk about soils.
The formation, composition and behavior of soil is
highly climate dependent. To the point where, basic "common sense" notions that are true in one region are false in others. As a great example of that, and as an intro for why charcoal can be useful, we can talk about soil organic matter.
As plant and animal detritus break down in the soil, the easily destroyed materials are quickly used up, leaving behind whatever materials are more difficult to break down. This is typically lignins, oils, and waxes, and some amount of highly complex carbohydrate structures. After some time, these structures tend to form what is loosely called "humic substances." This is an absurdly broad category of chemicals which can generally be said to be the soil organic matter component that is to some extent resistant to microbial activity. In soil science, they are broken down into three rough categories depending on molecular weight and solubility in alkalis and acids, fulvic acid, humic acid, and humin. It's very important to note that these three are not individual chemicals or made of specific molecules, they are just broad categories for describing residual organic matter in soil. You basically have these big, high molecular weight complexes of phenolic rings, quinones, polysaccharides and long-chain carbohydrates, etc. all stuck together. Typically, in heavy soils these macromolecules will form colloids with the clay particles, where the outsides of lumps of humic subtances will have clay particles stuck to them, whereas in lighter soils they will be adhered to the sandy particles. Imagine pouring tar into a container of flour or a container of tennis balls if you will to get an idea of the difference there.
These humic substances also tend to have lots of aromatic rings, different functional groups, and in general lots of bonding sites, so they have a high CEC. They also can both physically hold or trap water and can undergo hydration and dehydration reactions and so chemically hold or release water. The "sticky" effect described before also tends to increase the porosity of the soil's mineral content to some degree. Finally, they are described as resistant to microbial decay, but just resistant. Granted, dead microbes can also contribute to soil organic matter to an extend, partially closing the cycle, but not entirely. Over time, soil organic matter is converted to CO2, which diffuses out of the soil and blows away in the wind. Some amount is also lost via leaching as water soluble low-molecular weight compounds dissolve into water and are washed away. Absent new depositions of animal and especially plant matter, the soil organic matter will eventually all, or nearly all, dissipate into the atmosphere. If you place a container of dark garden soil somewhere it will have some water percolating through it but remains oxygenated, and most importantly stays warm and moist, and you prevent any plants or algae from growing in it, that nice dark garden soil will over a few months or years degrade into just plain mineral soil with almost no organic matter.
If the container is warm and wet enough, the breakdown of soil organic matter will actually occur fast enough that new plant and animal matter will struggle to replace it, with new matter getting mostly broken down quickly. Since higher organic matter levels means more microbial activity, the equilibrium point isn't 0% but something a little above that, as microbial activity dramatically slows down once you get to very low soil organic matter levels.
If you do the same experiment but increase the temperature and water flow, ideally under somewhat acidic conditions, even a lot of the mineral content will leach out and you'll be left with just aluminum silicates, iron oxides, and a few other highly resistant, highly oxidized minerals. And that's basically how you get Oxisols (wet tropical soils) and, in less extreme cases, Ultisols (southern red clay).
Now let's put that all together. For soils that are fairly dry and generally cold, but that support, for example, highly productive grasslands that quickly produce new organic matter and quickly deposit it in the soil, the equilibrium point for the soil organic matter will be very high, maybe as high as 15%. That's how you get Chernozem. Eastern Ukraine and southern Russia are almost semi-arid, and are bitterly cold with fairly short summers where the soil takes a long time to warm up, especially given the weaker sunlight at those high latitudes. It's basically the perfect conditions for maximizing plant matter deposition and minimizing soil organic matter loss. The warmer and wetter the soil, and the shorter the winter and faster the soil heats up, the lower the soil organic matter equilibrium point will be. In the wet tropics, that equilibrium point is barely above zero, in the wet subtropics like the US South, it's oftentimes only 1-2%. In cold temperate regions, it can be higher, perhaps up to 5% or more. Soil texture plays are role as well, since clay colloids can protect the soil organic matter longer term compared to the humic substances adheared to sandy particles that are completely exposed to microbes, water, and oxygen.
This of course dramatically alters the effectiveness of artificially raising the soil organic matter content (e.g. adding compost or mulch). In traditional New England gardens, which are moist but cold, or in classic West Coast gardening areas like southern California and the inland PNW and Mountain West, which are summer-dry and have
relatively long, cool winters compared to similar USDA zoned areas in the rest of the country, artificially raising the soil organic matter content is highly effective at increasing fertility, since the natural equilibrium point is fairly high and the breakdown rate is quite low so even if you push the soil well above the normal equilibrium point, it will take a very long time for it to adjust back down to more natural levels. In contrast, someone gardening in Florida could theoretically have all artificially added soil organic matter decomposed and leached/blown away in the wind within a single growing season. Having grown up on an organic farm in eastern NC with sandy soils, as a teenager I never understood why we could apply compost and goat and chicken and cow manure to the garden over and over and over for more than a decade and mulched everything as thick as we could and
still the soil was just this deep sand that retained its pale, infertile color and never developed that dark color and earthy smell we were promised by all the gardening books (written by folks in Maine, Idaho, or the UK...).
Now I know. All that organic matter was just blowing away in the wind a few weeks to months after we added it.
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There are a few other equilibria that need mentioning.
People often talk about microbes being supported by soil organic matter and how as they consume that organic matter they release nutrients for plants to take up. This is technically true, but it misses the main part of it. Far and away the most important thing organic matter does, once it reaches the more decomposed stage, is it provides a medium for growth and nutrient holding and a buffer for moisture. The lumps and sticky masses and films of organic matter are in a lot of cases just providing surface area for microbes to live on as well. It is also being broken down and consumed by some soil microbes, yes, but the vast majority of soil microbes are simply living on and in it, not consuming it. Indeed, the vast, vast majority of soil microbes can't even eat organic matter anyway. Instead, those microbes are doing whatever it is their kind do, be it reducing sulfur, oxidizing sulfur, solubilizing phosphate, absorbing soluble phosphate and making it insoluble, reducing nitrites to nitrates, reducing nitrates to ammonia, converting ammonia into nitrates and nitrites, denitrifying nitrates into nitrogen gas, fixing nitrogen gas into nitrates, turning living potatoes into mushy dead potatoes, fermenting alcohols and lactic acid, etc. Notice how most of these chemical activities are cyclical? (Also, notice that the soil itself fixes nitrogen? Yeah, it's not only legumes, and the dozens of non-legumes nitrogen fixing genres of plants that for some reason never get any love, just regular dirt will fix nitrogen all by itself, it's just that the equilibrium point might not be as high as if there are plants pumping sugars down to feed microbes doing the fixing, but regardless, soil, unless completely sterile, will never actually run out of nitrogen--though it can get pretty darn low). Depending on the species and the conditions, the soil microbes will be running these reactions one way or the other, or in reality both ways, with some equilibrium point depending on conditions. And plants are part of those cycles, because they're doing things like uptaking soluble ions, secreting sugars, dying, releasing highly digestible forms of organic matter, etc. Most plant roots, for example, are primarily made up of cellulose, and cellulose degrades in the soil due to the activity of certain microbes that enzymatically break it down into disaccharide and further down into just simple sugars. Which means that little or even no humic substances might result from a plant root dying and being consumed, for example. But that process will still cycle some carbon, and more importantly cycles sugars into the soil as well as organic forms of mineral nutrients, which will eventually get consumed and turned into inorganic forms, which plants then pick back up. And the humic substances, because of how well they moderate water and how well they chemically bind ions, as well as provide pH buffering and such, really help keep conditions favorable for microbial activity and provides a reserve so to speak of chemicals in the soil that's constantly getting added to and removed from.
So a phosphorous ion might get chelated by some phenols and just hand out in the soil for a while, then gets picked up by some bacteria that uses it in a cellular process but then gets eaten by something else that gets eaten by a fungus which passes it over eventually to a plant that uses it in a leaf that gets eaten by a bug that dies and gets eaten by ants and ends up in an ant that gets killed by a fungus which eventually decomposes and passes to a bacterium that secretes it as part of a biofilm in the soil where it adheres to a grain of sand for a few more months before another bacteria comes along and breaks down the organic compound that it was part of and then releases the phosphorous ion as a salt again which begins leaching during a rain storm but then binds to some organic matter in the soil for a while until the soil temperatures change which alters the pH and releases the ion which gets taken up by a plant root directly and... etc. Sure, there are also some phosphorous molecules bound up deep in a tar-like lump of humin stuck to some clay, but that phosphorous doesn't enter the cycle until that humin gets oxidized and broken down, which might be months or even a few years later, or never in a bog with no oxygen. Most of the nutrient, energy, and water cycling that's happening in the soil is taking place in conditions provided by degraded soil organic matter, but it's not from the final decomposition of degraded soil organic matter. Perhaps think of a very tall glass of water. You can scoop a spoon of water off the top and into another cup, put the water back, and repeat over and over. Eventually, the water at the bottom of the glass will also get mixed up and ends up at the top of the glass where it gets scooped up, but it'll take a very long time. It's the water at the top of the glass that's doing the most cycling.
But of course there's also an extent to which stuff degrading in the soil doesn't just get consumed and does form more resistant chemicals. Stuff with more lignins and waxes and whatnot ends up in the soil and a lot of that ends up contributing to the more long lasting forms of soil organic matter. But just as that stuff is being deposited, mostly by plants and animals, some microbes, and just free oxygen, is steadily breaking it down and turning it, ultimately, into CO2 that just floats away same as the sugars from the faster cycles that get metabolized into CO2 and diffuses off into the atmosphere. The first process depends mostly on the primary productivity of the plant life on top of the soil, and how quickly it's getting cycled into the soil (hence why grass is such a big deal), but the second process is mostly just a function of how much soil organic matter there is, how warm the soil is, and how moist but still oxygenated the soil is. So, to bring back the comparison mentioned above, cold temperate grasslands have great primary productivity, at least in summer, and put out a lot of organic matter that ends up on or in the soil that same year or even just a few weeks later, but the soil is either frozen or cold most of the year, and once it does finally warm up, is generally also at its driest since most temperate grassland regions of the world have relatively dry late summers. Hence the black dirt of the American and European prairies. Reverse that, and make the soil warm and moist almost all year, and it won't matter how fast the plants are growing, the soil microbes will be destroying most of the soil organic matter as fast as it arrives.
But then, with very low levels of soil organic matter, an issue arises. Soil particles, the mineral component anyway, are chemically pretty inert, and they tend to have low porosity and so don't hold air or water inside, just in the spaces between the particles, and they don't provide much pH buffering, hydrate buffering, salt buffering, etc. Clay particles and more weatherable minerals tend to be better, but they're still not amazing at these things. Sand particles tend to be pretty bad at it, and also just don't have much surface area for anything to happen on. And if the soil is extremely weathered and it's just some extremely chemically stable minerals left, then the soil is going to be almost inert. When that happens, leaching becomes a major drain on the system, and so many of the nutrient and energy cycles described above end up with low equilibrium points. Additionally, the soil itself will cycle much faster between too wet without enough air, to too dry, the pH can swing more rapidly or slide to more harsh extremes, and in general conditions just aren't as good and are much more chaotic. Hence the problem with tropical soils, and to a lesser extent with subtropical soils.
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We can finally come back to charcoal.
So what's the selling point of charcoal? Well, it has all the buffering, nutrient holding, water and oxygen storing, and related benefits of regular soil organic matter, and has high surface area via its wild porosity and so can provide a lot of media to grow on. But unlike soil organic matter, charcoal is extremely stable in the soil and has a far, far, far lower rate of degradation and oxidation. Sure, there are mineral nutrients that might be left over still trapped down in a glassy mass of vitreous carbon, just as there can be in humin, but as with the point above, it's not the few nutrients that are trapped in the long-lived forms of soil carbon that matter, it's all the nutrients constantly cycling on the chemical medium provided by those long-lived forms of soil carbon that matter, and all the good conditions it creates by moderating water, oxygen, etc.
There's a few other things charcoal does that are quite nice. Regular soil organic matter has excellent water holding capacity, and to the extent that it causes soil particles to aggregate by basically cementing them together it also improves internal drainage. By making some particles stick together, it effectively makes the soil more coarse than it actually is, and those coarser aggregated particles don't pack together as tightly, which channelizes the soil, allowing excess water to drain through it like it would drain through sand or gravel. But that soil organic matter is still trapping and absorbing lots of water which will then slowly release. This process only works to a point, though, because what happens if you keep adding cement to aggregate? You get concrete, which drains really, really slowly. Basically, there are few channels for the water to flow through because there are sticky masses of soil organic matter everywhere gumming up the works. Hence why boggy soil is so reluctant to drain even when ditches are cut through it. If highly, highly aerated, highly organic soils or media can drain, for example fresh compost, but once it packs down or collapses in on itself, the drainage becomes awful. Organic matter, depending on the form, can also become highly hydrophobic when very dry, and so can be difficult to wet, which also causes drainage problems of a different sort.
This isn't the case with charcoal. Charcoal has excellent drainage, and it retains that under almost any conditions. It works in two ways. Internally, charcoal particles are highly fractured and have lots of cavities, voids, and such that water can seep through, but charcoal particles also tend to be fairly coarse and tend to be very rough and don't pack well, so water also has lots of space to drain around the particles. All that internal and external drainage space also means that it has excellent aeration, and those voids and such mean that even submerged, charcoal will often still hold a whole bunch of air. What voids and such that do end up filling with water will generally be very slow to release that water due to water adhesion, especially since charcoal is chemically active enough that it'll try to hold on to that water through hydrogen bonding, van der waals forces, and a slew of other means. So it has great moisture holding capacity as well.
Which all sounds great, and to an extent it is. But remember that all of this is just providing good conditions for soil microbial life? Build and they will come is only true up to a point. Excellent soil characteristics will hit a wall of diminishing returns and the microbial life and nutrient cycling will end up being rate limited by something other than water and oxygen levels or surface area and CEC or pH buffering. Where is that point of diminishing returns? Depends on the soil, the plants, the climate, etc. It's likely somewhere around the normal soil organic matter content of most healthy, fertile, rich soils, which is between just 3-6% soil organic matter believe it or not. After that point, you're not getting much benefit in most cases.
And that means that in temperate zones where the soil organic matter will already be close to or at the ideal level, adding charcoal is unlikely to do much in terms of soil microbial activity. Unfortunately.
There can be other reasons for using it though. The rough edges and texture of charcoal can be useful for controlling or deterring certain pests, for example. And recall that the aeration and drainage of charcoal are far superior to that of soil organic matter at higher percentages? In some cases, waterlogged or anaerobic soil needs to be avoided at all costs, such as when growing succulents or dealing with disease pressures that occur in those conditions. In that case, adding a large amount of charcoal might tilt the soil more towards those conditions. Add enough charcoal and the soil might eventually get to the point where it's exceptionally well drained while still moisture retentive, to the point where standing water is almost impossible but plants still get plenty of moisture from the soil despite that drainage and oxygen levels remain high pretty much all the time. There's actually a real life soil type out there that's like that. Volcanic soils. It's also highly erodible, and DIYing a volcanic soil garden would mean applying something like six feet of charcoal to the whole thing and somehow tilling it in, which sounds a little unrealistic. So there's that. Chemically they're also not actually that similar but close enough for this example.
That point about drainage though also applies to a related product that is used in bulk to improve soil hydrological characteristics, PermaTill and other expanded slate products, and other more well-known expanded minerals like perlite and vermiculite which tend not to be used in bulk due to cost and other issues.
When it comes to those difficult soils in the wet subtropics and especially in the wet tropics, however, charcoal can have a significant impact on the fertility of the soil because high soil organic matter, the holy grail of conventional organic gardening doctrine, doesn't exist in the wet tropics, and is a fleeting, capricious thing in the wet subtropics. In the Amazon, and in the South to an extent, you have to manure and mulch over and over and over. And within a growing season or so, it's all gone already like dust in the wind or tears in the rain, or in this case CO2 in the wind and solutes in the rainwater. The advantage of terra preta is that it's made of things that are not biodegradable, but that have much or all the benefits of soil organic matter. The pottery, by the way, is similar to the charcoal, though probably a bit inferior since it'll have a lower CEC.
As for terra preta "regenerating" itself, sounds like hype to me. More than likely, it was just that after they scraped the topsoil away, charcoal particles that had been dispersed in the subsoil started coalescing in the new topsoil layer, turning it black again. Charcoal being very light, it will tend to collect near the surface unless buried deeply through some kind of animal or human driven mechanical action. They probably saw a pale subsoil that after a while miraculously "regenerated" the dark color and superior texture and thought the dark materials were somehow regenerating. Almost certainly not, they were just finally not buried in the highly stratified subsoil and could move around again, which mostly means collecting in the top few inches. What may have even happed was there could have been a mineral deposition formed hardpan layer in the subsoil at the aerobic to anaerobic transition point or something, but after they removed the topsoil that transition point dropped deeper into the soil, which enabled all those minerals that were cementing the hardpan to dissolve, loosening the soil and allowing those charcoal particles to move around. Who knows, but among the least likely of options was a chemical process that requires carbon feedstock, combustion, 800 F temperatures, and sudden but sustained oxygen depletion was somehow spontaneously happening in the soil after they scraped off the topsoil...
Charcoal as a soil amendment is already kinda miraculous, we don't need to be conjuring up even more miraculous properties that make a mockery of reason and well-informed good sense.
