Air and Your Garden

Spring 2021

By Will Bonsall

Organic matter is basically composed of two types of elements: minerals and gasses. The minerals are obvious: they’re what’s left when you burn organic matter (like wood). The part that is gone is the gasses, four of them in all: hydrogen, oxygen, nitrogen and carbon. Now of course reduced carbon (as in charcoal) is a solid, but carbon dioxide, the form we usually deal with, is a gas. It has always struck me as a paradox that the vast bulk of living matter is so lacking in density, but not in importance. It is of course the stuff of humus, the vary basis of soil life. In animal life, the mass of organic matter is surpassed only by minerals of a structural nature, such as calcium carbonate (shells) and calcium phosphate (bones and teeth), otherwise we’re all largely composed of those four gasses. Not very flattering perhaps, but deal with it.

The main reason for even discussing these things here is that we need to keep supplying or replacing them in our gardens. But why? Aren’t minerals already in the soil without our needing to move them around? And isn’t the air we breathe full of those gasses just needing to be sequestered? Yes, but several things we do in agriculture cause those elements – minerals and gasses – to be lost from our soils, requiring replacement (via what are typically referred to as “amendments”).

I’ve always been critical of what I call “cake mix farming,” where we import some materials from here and from there (often with a huge carbon footprint), mix them up in the soil (again, lots of energy, usually carbon-derived), and send the results back to market. Consider that no one fertilizes the forest, no one spreads compost on the prairies. But those ecosystems don’t have anything removed from them except by “oxidation” (decay), and that which is lost is easily replaced by “reduction” (the re-sequestration of those elements by natural growth). On the other hand, we constantly stir cropland soils (through ploughing, harrowing and cultivating), which increases the rate of oxidation while suppressing the more efficient sequesterors – the weeds. That’s why we say soils “burn out” unless we keep adding to them. We also send much of the organic matter, in the form of crops, off to the market place whence it never returns. The latter is especially true of minerals, but that’s not what we’re considering now.


The difference between a pile of firewood and ashes? The gaseous organic matter has been removed, leaving only minerals. Photo by Mike Herrmann

The Four Gaseous Elements of Organic Matter

The carbon, hydrogen and oxygen are largely in the form of carbohydrates such as sugars, starch and cellulose (essentially a long polymer chain of sugar), and the passing around of those materials and the derivatives of their decay is what makes the living world go ‘round. The nitrogen is largely in the form of amine-based proteins which provide memory and information (as in DNA) as well as structure and other things. 

Given that those elements are all abundant in the air, why should we need to add them to soil? It all comes down to the sequestering of them in living form. Fortunately, plants, including those in our gardens, do much of that sequestration themselves with little if any help. But they’re not always up to the job especially given our tendency to destroy or remove the results of their work. Our most common agricultural practices are working against them and against ourselves. 

The usual strategy for restoring lost fertility is by importation from off-site, often from out-of-state – even from abroad. I’ve always thought it bizarre that farmers would pay dearly for nitrogen-rich guano from South America when 78% of the air we breathe is nitrogen. Can we really call that sustainable agriculture? 

What then to do about it? First let’s look at the loss of stuff to the marketplace. As long as certain people (farmers) in certain places (farms) are producing food for other people (consumers) in other places (cities), soils will need constant replenishment. That may be a great argument for subsistence farming ­– my own focus – but that’s not where most farmers are coming from. Now the loss of the gaseous components of organic matter is arguably less concerning than loss of minerals, given that all the gaseous components of organic matter are “in the air.” And the remedy is arguably simpler: some combination of green manure, mulch and living mulch can do very much to offset the loss. The use of ramial chips, or shredded brushwood (see “Too Many Wood Chips?” in the winter 2018-2019 issue of The MOF&G), is an exceptionally sustainable way of doing this. 

Soil Aeration

As for aeration of the soil, as by tilling, it has one great advantage which is also a disadvantage. The advantage is that it accelerates the breakdown of organic matter; the disadvantage, it accelerates the breakdown of organic matter. It is by that breaking down that nutrients are released from the dead stuff to nourish the living stuff – that’s good, right? But in breaking it down, it reduces the organic content by burning out the humus faster than can be replenished – that’s not so good, right? That’s all an argument for no-till or low-till agriculture. It’s also a great recommendation for the broadfork for unlike ploughing which inverts the soil and annihilates much of the soil community, broadforking merely opens up packed soil to allow some air and water to penetrate better without desiccating and trashing the soil’s natural crumb structure. Once loosened a bit, the normal percolation of rainwater, penetration of taproots and burrowing of earthworms will further the work without causing excessive aeration. 

Incidentally, one of the key components of soils in general and organic matter in particular is dihydrogen oxide, also known as water. Water contains not only dissolved oxygen but dissolved nutrients of every kind. In fact, plants cannot absorb and use any nutrients unless they are first dissolved in an aqueous solution so the presence and penetrability of water must always be a paramount consideration. However, like aeration, water may be in excess as it dissolves nutrients and carries them away from plants either through surface runoff or downward reaching. Given that much of my cropland is steeply sloped, I consider terracing essential to avoiding either extreme of erosion or waterlogging; excess rainfall can drain off, but not before sinking into the ground and leaving its dissolved treasure there. 

A particularly important component of the soil system is nitrogen, but atmospheric nitrogen (N2) is useless to most plants unless converted to nitrite (NO2) and nitrate (NO3). Much of this conversion is the result of nitrogen-fixing bacteria such as those colonizing the rootlets of legumes; though other plants, such as rice, maize and sorghum may also have the ability to fix nitrogen. (See “Rice, maize and sorghum may be able to fix nitrogen from the air,” in the February 23, 2019, edition of The Economist.) However, since they require aerobic conditions to operate, the soil must contain oxygen as well as nitrogen to form the nitrate. Without it, microbes convert that nitrogen to methane and other compounds that aren’t helpful to the soil community. But again, excessive aeration (as by too-frequent tillage) upsets the rhizobacteria and desiccates the soil. 

Too Much Humus?

I’ve been asked: Is there such a thing as too much humus? I’m no authority, but that never stopped me from having an opinion and here it is. Humus has a marvelous ability to regulate things ­– pH, water retention, ion-exchange capacity, crumb structure, etc. – and I’m inclined to believe it even regulates itself. The higher the humus level, the more oxygen and, in turn, the more rapid the decay. At some level of build-up, the rate of decay of humus will balance with the rate of accumulation. Unless of course something were to cause an anaerobic condition which impeded the decay, such as waterlogging. That’s how vast deposits of peat (fossil humus) are formed, and that’s why when sphagnum peat is added to aerobic soils (e.g., your garden), it resumes the decay that was interrupted by interment thousands of years earlier. At some point humus regains a balance, unless we continue to over-aerate the soil by tillage to where it is consumed by oxidation (read burned-out).

So far I’m only discussing sequestration of carbon, hydrogen, oxygen and nitrogen for purposes for building and maintaining soil tilth, which is really big; however, some will likely point out the importance for pulling a lot of carbon out of the atmosphere by incorporating it, with the other three elements, in humus as a means for mitigating global warming. Others will advocate creating biochar for sequestering reduced carbon for centuries. I have no argument with either of those practices, but I wouldn’t like us to get feeling so good about removing that relatively tiny amount of carbon from the atmosphere while blithely living our lives so that vast amounts of fossil carbon continue to be released. Even if we turn the top 12 inches of all our soils into pure biochar, it would be dwarfed by the burning of fossil fuels. I used to be in the mining business and I learned to appreciate the slogan: “Leave it there.” 

That’s all the more reason we need to examine our farming practices – even organic practices – with a very critical eye and constantly ask ourselves “whence and wither” questions like where did this stuff come from, where does it end up going, and what did it take to do all that. Only then can we approach a truly sustainable food system. 

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