Grains of Dirt part 1
Ambivalent Nematodes
To refresh your memory very quickly— we had talked about the components of soil—the dirt, which is clay, silt, or sand, how that impacts drainage, aeration, and how they impacted the biology, or the fungi, bacteria, and micro-organisms in the soil. The relationships between these three living things in the soil largely impact plants’ ability to absorb water, oxygen, and nutrients. We also covered a bit about ratios of fungi to bacteria and how that impacts the plants growing in the soil, as well as why there are certain succession patterns in the soil— why weeds seem to be the only things that will grow in compact soil and not your new flowers, and so on. Hopefully this is all starting to come back to you.
Today, we’re gonna talk about soil disturbances. The no-till revolution has gained prominence over the past decade, despite the fact that my grandfather would have my head for suggesting I don’t till the garden. If you consider the soil like we have been, as this complex system with tons of intricate parts, then the idea of disturbance doesn’t seem like a great one, right? Nature has made a point that areas that are meant to experience massive disruption— think like, fire-prone areas, volcano areas, and so on— those areas actually do pretty well dealing with those disturbances. But tilling, no, that’s not a natural state, not something that happens often, and isn’t good for the complexity of the soil and tends to kill the life in the soil. Not only does no-till help the soil, but it helps support beneficial insects; compared to a conventional farm, for example, studies have shown that they will support up to 3 times more native species like local bees.1
Soil Organic Matter
There’s a term called Soil Organic Matter—S.O.M., which you might hear if you listen to ecologists in your free time, and what that means is what percentage of your soil is organic matter at any time. A typical forest, if you dug up a big bucket of soil, is about 4.3% SOM, versus a typical annual crop field, which is usually around 1.5%. There’s a fun test you can do called the cup test, actually i don’t know what it’s called, if you know, let me know, I learned about it and have used it a bunch but can’t seem to find it online to figure out what it’s actually called. What you do is drop some tilled soil in a cup and the unturned soil in another cup to see how they react to the water. What you’ll notice is that the cup full of no-till soil drops almost completely to the bottom and stays separated from the water, while the tilled soil will immediately make the water murky and the material fills the cup. In a perfect world, these two soils would be made up of pretty much the same exact minerals. What’s the difference? The life within the soils— the fungi produce glomalin, which is essentially a glue to keep their network together, which actually keeps the soil together as a mass, whereas the tilled soil has never developed those fungi structures or biological community, and is easily separated.
Well, that doesn’t make sense, right? We discussed how plants need those fungi and bacteria to make nutrients absorbable, so there had to be SOME in the soil, right? What happened? Well, let’s talk about the bacteria and fungi process again. They need certain things to be able to create a habitable environment, right? Water, insoluble nutrients, oxygen. What happens when you till? You aerate the soil to create easy access for the plants’ roots structure to get deep into the soil and to bring up new nutrients from deep within the soil. That’s the whole idea, right? However, one thing that happens— the reason why that only works the first few years, is because it over-aerates the soil. When you till, that exposure spurs lots of activity that breaks down the carbon and releases it back into the air. Not only does it spur massive, unsustainable biological activity, but the sun literally bakes the soil and the bacteria and fungi will die off from the exposure to the surface. Unprotected soil loses moisture quickly, and the dark color of the soil heats up. If you’ve ever dug a hole and realized the soil was way darker than you expected, you’ve probably also noticed that the same soil suddenly doesn’t look as nice a few hours later.
When the soil hits 113 degrees F species in the soil start dying, and by 130 degrees all of the moisture in the soil is lost. At 140 degrees, the soil bacteria is completely dead.2 Anyone that’s been outside and touched a dark road on an 80 degree day knows how quickly a dark surface can heat up, and that’s what’s happening to your soil. Even if you cover your soil so it doesn’t bake in the sun, you’ve begun the process of creating massive amounts of biological life in your soil by exposing new food that requires now massive amounts of inputs to sustain itself.
This is why first time gardeners will till and have a great garden the first year or two, and years 3 and 4 they can’t figure out what they’re doing wrong because they've ‘always had great harvests’. You’ve used up all of the content in the soil quickly, like opening up the flue on your wood stove, and now there’s nothing left to keep that blaze going.
Now there’s one thing here that’s not totally important— I mean, it is in a sense— but really, it’s kind of a tangent, and it’s the amount of organic matter you need in your soil. The average is around 4% or so, but it’s important to understand why some soils are more challenging to make ‘healthy’, and by some soils, I mean primarily clay soils.3 There’s two challenges with clay soil— the first is that it actually has some magnetic properties, and those properties can create challenges in nutrient absorption for your plants and fungi, and the second has to do with the amount of organic matter you need. A sandy soil with very little clay or silt may only need 2% organic matter to be healthy, and that’s largely because of the size of each particle of clay is nearly 100x smaller than one granule of sand. For the organic material to produce those aggregates to transfer nutrients across soil, more surface area needs to be covered to make those connections, meaning more organic matter needs to exist. Think of it like a house, it might take 5 gallons of paint to paint the outside of a house, but 20 gallons of paint to do the inside because each room has its own walls. Take that times ten. Since organic matter takes time to build up, clay soils can be significantly more work to develop into productive soil for annual crops.
Carbon:Nitrogen Ratios
The last kind of technical-ish area I want to touch base on really quickly is Carbon to Nitrogen ratios. So, where does new organic material come from in your garden-- if you wanted to do it without massive inputs? It would be from the dying plants, right? Whether it’s your tomato plants or cover crops, those plants die off and start to decompose back into the soil. I’m not talking just about the tomato stalks but the root systems as well, all of which add new material for the soil biology to break down and absorb. That said, A corn husk is gonna take longer time to break down than the root system to your arugula, right? Not all organic matter is equal— and I don’t mean that negatively. Much like everything else we have talked about, the value in organic matter tends to be in having a variety of it.
The ratio of the amount of a residue’s carbon to the amount of its nitrogen influences nutrient availability and the rate of decomposition.4 The ratio, usually referred to as the C:N ratio, may vary from around 15:1 for young plants, to between 50:1 and 80:1 for the old straw of crop plants, to over 100:1 for sawdust. Think of it this way; the longer it took to grow, the longer it's going to take to break down. For comparison, the C:N ratio of soil organic matter is usually in the range of about 10:1 to 12:1, and the C:N of soil microorganisms is around 7:1. The C:N ratio of residues is really just another way of looking at the percentage of nitrogen (figure 9.3). A high C:N residue has a low percentage of nitrogen. Low C:N residues have relatively high percentages of nitrogen. Crop residues usually average 40% carbon, and this figure doesn’t change much from plant to plant. This makes sense when you consider that most of our crops come from the same part of ecological succession, which is why they also want similar fungi to bacteria ratios as well.
On the other hand, nitrogen content varies greatly depending on the type of plant and its stage of growth. Young, smaller plants and root systems are able to break down more quickly than, again, corn husks, which means it is able to pump that nitrogen back into the soil quickly for new crops. When there’s too much of those high C:N organic materials—think like straw bales, the microorganisms in the soil will start to draw nitrogen from surrounding soils to stay alive limiting the amount of nitrogen available in the soil. Generally, as long as the residues you are using to break down stay under 40:1 C:N, you won’t be losing any nitrogen, and under 24:1 you’ll be bringing in new nitrogen.5 For context, most manures, cover crops, and compost are in the teens and slightly lower, so that’s why they are typically used as soil amendments.
When any organic material is added to soil, it decomposes relatively rapidly at first. Later, when only resistant parts (for example, straw stems high in lignin) are left, the rate of decomposition decreases greatly. This means that although nutrient availability diminishes each year after adding a residue to the soil, there are still long-term benefits from adding organic materials. This can be understood as a “decay series.” In other words, crops in a regularly manured field get some nitrogen from manure that was applied in past years. So, if you are starting to manure a field, somewhat more manure will be needed in the first year than will be needed in years 2, 3, and 4 to supply the same total amount of nitrogen to a crop. After some years, you may need only half of the amount used to supply all the nitrogen needs in the first year. However, it is not uncommon to find farmers who are trying to build up high levels of organic matter actually overloading their soils with nutrients, with potential negative effects on crop quality and the environment
So, with all this information, what can we do to help our soil be all that it can be. We can compost. We mulch our soil. We provide cover plants. We plant understory plants that have similar needs to the trees we are covering— think back to those fungal ratios—which low-growing plants are common in the same areas as those plants you’re covering; they’re meant to grow together. And this is NOT the same as permaculture attempting to stack different fruit or nut producing plants, either. We’ll get into why that doesn’t work at a later time. This is the co-evolutionary trait that is nature creating efficiencies that we had talked about last episode.
Providing this long-term co-planting allows deep roots to supply exudates, helping to improve soil aeration, structure and ability to hold water. Further, these cover plants help prevent compaction in the soil, and provide balanced nutrients, reducing risks of disease.
Cover Plants
Natural nitrogen fixers like vetch and clove are used often as cover crops to protect soils from desertification and erosion, carbon capturing, and to provide nitrogen. Further, they are great non-native pollinator feeders, which, if you’re a beekeeper like me, is great. Bare soil in temperate climates is very often bad and increases water runoff while also reducing the health of the soil— very quickly. Mulching feeds back into the soil, and can be done with tons of different materials. I’ve used straw, wood chips— even pine tree wood chips— cardboard, heavy leaf collections, chop & drop plants, including tree crops like willow, and I’m sure there are tons I’ve never tried. If you’ve never used wood chips as a mulch to protect your soil, a foot of mulch will break down to 4 inches of actual soil over only a few years.
The thing is, soil doesn’t build quickly. You’re not gonna get black gold soil in a year or two. It takes years, decades to build a few inches of heavy, healthy soil with intense complex systems in place within the soil. If you’re working hard at it, you can build a few inches in 5 years, and that requires a lot of inputs. It’s one thing to throw down a bunch of mulch, it’s another to not only get the mulch in place, but to get plants and to build up the soil from below that mulch and to make the new wood chips part of the ecosystem.
Now, I want to circle back really quick to the idea of cover plants— what I said, and not what you might have heard, which is cover crops. Many folks are familiar with the use of clover, vetch, snow peas, and other various plants that are used to cover your garden during the fall to protect the soil and add nitrogen. Other plants like comfrey and sorghum sudangrass, as well as things like sunflowers help not only break up compacted soil but create organic matter quickly to turn into topsoil. The one thing that is important to understand when it comes to using these cover crops to build biomass is that for the organic matter to break down quickly and in a healthy way, do not till the new matter into the soil, or the bacteria will flourish to meet the new organic material decomposing, and ultimately the amount of bacteria will remain unsustainable and can create numerous problems in your soil. Also, this is a bit of an oversimplification, as techniques like discing have been successful for decades, but that’s a longer discussion than what we’re having right now.
And of course, if you need help breaking down that material, it’s a great place for animals to start getting involved in your nutrient cycling.
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https://www.sare.org/wp-content/uploads/Cover-Cropping-for-Pollinators-and-Beneficial-Insects.pdf
E;, P. J. P. M. B. (n.d.). Comparison of temperature effects on soil respiration and bacterial and fungal growth rates. FEMS microbiology ecology. Retrieved February 3, 2023, from https://pubmed.ncbi.nlm.nih.gov/16329892/
https://franklin.cce.cornell.edu/resources/soil-organic-matter-fact-sheet#:~:text=Soil%20organic%20matter%20is%20the,3%20and%206%25%20organic%20matter.
https://www.nrcs.usda.gov/conservation-basics/conservation-by-state/illinois/soil-tech-note-23a-carbonnitrogen-ratio-cn
https://advancecovercrops.com/resources-advanced-cover-crops/carbon-nitrogen-ratio/