I stumbled across a 19 page PDF at the Arnold Arboretum on plant roots. And permies seems to have a couple of threads on roots, but not getting into a lot of detail.
In the more conventional agriculture, if you are going to plant something from seed; you prepare the soil first. Which is a bunch of tillage and possibly other stuff. It could even mean running a subsoiler. I don't think even the biggest subsoilers get down very far, and at least up here some pipelines would be considerably lower than the deepest a subsoiler might get to.
I've run across specific plants which have deep roots; and they might be talking 2 feet. A particular sterile comfrey was said to put roots down almost 30 feet. I suspect that was local to a specific geography, geology and soil history.
Roots are complex, and no classification system seems to exist that is not without some problems.
That said, many plants develop "striker roots" which will go down to depths at which lack of oxygen is the controlling factor. (Question, is it oxygen, or air?) I suspect sandy soils allow oxygen to get to deeper depths and it seems likely that soils with a high organic content may also allow air to penetrate deeper. My soil is heavy clay, and I suspect the depth to which tree roots could go is limited because little air gets into clays if there is any water present. I think this is partly because clays tend to have disc like particles, which can pack efficiently. I think silty soils are probably more variable.
If one ignores these "striker roots", it seems most plants seem to have roots in the top 1 foot of soil. So, if someone puts in a swale which means digging substantially deeper than say 6 inches, they could easily damage a lot of roots. It may be that is okay. Certainly, I don't mind killing a few fescue or quack grass plants (is it possible to kill quack grass?).
It seems to be one of the things beginners learn, but if you lay wood chips on top of the soil (don't mix in), they will eventually loosen up the soil. And I would expect the process is similar to diffusion in solids, and so however one defines diffusion depth, it scales as square root time. It takes 4 times as long to go twice as deep. But this kind of process is what happens in forests, when trees die, fall down and re-enter the soil. This may loosen up the top inch or two of soil in a short time frame, it isn't going to get down to 1 foot very quickly. And it will be slow to get deeper than 1 foot.
On my farm, I am trying to to work with wood chips. I have about 40 loads of wood chips from last year and the year before. And this last week I seen the local guy for one of the big companies that chips wood, and told him I am still looking for more wood chips. And he will tell the other big company(s) in the area.
But, this is not fast.
We can do a little to speed things up. I am still learning, but tillage radish seems to be a tool one can use. It can potentially get down 2 feet (maybe a bit more?). It leaves a big hole in the ground when the radish rots the next spring (some people do eat daikon radish, this is a slight variation on that). But, if you let the tillage radish rot in the ground (it apparently can smell quite bad), all the nutrients that were in the radish are released to the soil in this deep hole. Eventually the hole will either refill from the side with whatever soil the radish pushed aside in growing, or debris (could be soil, could be organic, could be bird poop, ...) could enter the hole. I suspect that if a radish is growing down (and out) next to where a tree root is, it probably just pushes it aside. So now this tree root suddenly sees more access to air, possibly for an extended period of time depending on how much organic matter gets incorporated into the hole on closing. So, if we continue to seed tillage radish (I am not successful yet at getting a full grown plant, so there is a learning curve) or other similar annuals (I believe there is a turnip, I don't think it gets down anywhere near as deep as the radish) we keep poking these holes in the ground which allow for a little faster incorporation of organic matter into the soil. These locations where radish grows are random, it isn't the same location over and over. So the end result is that we are uniformly improving the organic matter content of our soil. And in the case of tillage radish, possibly helping striker roots (if the plant in question can have them) get deeper.
If one is using wood chips and growing tillage radish, do you want the wood chips falling into the holes left by the tillage radish? The problem with mixing wood chips with the soil, is that it takes too much nitrogen to break down the wood chips; which means there is less nitrogen available for anything to grow. At a low density of tillage radish, there is probably no reason to try and avoid wood chips getting into the holes. At a high density, it is no different that mixing with a rototiller. Where do things change from good to bad?
I think it was the Arnold Arboretum paper, but somewhere it was said that much of the production of nitrogen containing compounds was done in the roots. Chlorophyll has 2 5 members rings with nitrogen and 2 6 member rings with nitrogen. Does this mean chlorophyll is made in the roots? Alkaloids are a common protection mechanism for plants, they contain nitrogen. Does this mean the roots are also producing the protection alkaloids? An agent which stunts plants emitted from walnut roots is juglone; but it has no nitrogen. I haven't memorized other similar agents.
But, if these important nitrogen containing compounds are made in the roots; then talking about roots only getting as deep as oxygen gets to may be misleading.
I believe alfalfa (lucerne to some people) is said to get considerably deeper than 1 foot. The growth of an alfalfa plant is not going to have the effect on the soil of growing a tillage radish. The alfalfa plant is perennial, so it doesn't rot in place the next year. It's root system may expand and contract with the seasons, so it may be forcing small roots into new soil and then dying back later. So it does tend to break up the soil and increase organic matter. But the process is different than what tillage radish allows. If root depth is limited by oxygen or air, it is entirely possible that alfalfa on my farm doesn't get anywhere near as deep as reports suggest. So the idea that planting a sterile comfrey will get roots down to 20 feet or more, is a pipe dream for most people. The roots will get as deep as the pore structure of the soil above it allows.
If I plant a tree, I want to have a reasonable amount of wood chips surrounding the tree (but not contacting the trunk). And then extending out a reasonable way. What is reasonable? I haven't a clue. Part of the reason for the wood chip surrounding, is to "insulate" the tree. If the tree is small (such as less than 1 foot tall), calling it insulation may be useful. Heat is streaming out from the core of the Earth all the time. When it starts to get close to the surface, it may start flowing differently depending on the heat resistance it sees "above" it.
If I don't put any wood chips around the tree, there is no change in insulation (except for the minuscule amount provided by the tree itself), and so the temperature profile in the soil will be about the same as some random place in the field. If I put wood chips around the tree, but not too close; at some point the heat coming up from the centre of the Earth sees that there is an insulated surface, and starts to divert around it. The heat rises a bit more, and it sees that there is actually a "hole" in the insulation. A path to escape the Earth. So, choose the correct size hole, and your tree could have enhanced heat flow. It will be a function of how much land is insulated in the vicinity of the tree, and how big the "hole" in the insulation is.
In a sense, this is passive heating of tree roots. I haven't been thinking much about it, but I have recently been trying to find a way to passively heat a root cellar. Which is not a root topic.
Hopefully this has helped some people, and isn't thought to be "wrong".
It's also worth considering that roots in a healthy soil system could deliver oxygen down into the root zone. The contribution of roots to the rhizosphere is supposed to be primarily carbohydrates. These are chains of varying lengths and complexity made up primarily of Carbon, Hydrogen, and Oxygen. If the soil flora and fauna are using these carbohydrates as food it seems likely that some of them are cracking them apart to get the carbon and letting the oxygen free. In theory roots could be doing this at their deepest depth and thus perpetuating an aerated zone as deep as they can penetrate.
wayne fajkus wrote:I may be simplistic in my views, but adding the organic matter to the top brings earthworms up, which brings air into the soil.
Different types of earthworms may go as deep as 20 feet, which they wouldn't if the ground was anaerobic. There was also a study where they tested common lawn grass against prairie grass and the prairie grass got roots down ten feet or more while lawn grass was less than 6 inches. Not sure what process they were using as I only glanced at it in passing. I've also had zucchini and tomato roots go at least three feet down. Even weeds often get down two feet or more (I usually don't chase them down that far unless I'm dealing with something like bindweed). Maybe clay soils have a shallower root zone, I don't know. I do know that roots go much deeper than 1 foot in my yard.
Zone 5b/6a, alkaline soil, 12 inches of water per year. For now the goal is a water independent urban homestead with edible landscaping and food forest.
I think a lot depends on your soil, my previous place was silt over solid clay. NO roots went down into the clay not even trees, the silt was between 3inches and 6ft deep, but became waterlogged at around 1ft. Some roots did go down into the waterlogged and anoxic (by the smell) soil, those belonged to reeds and bulrushes things that are adapted for it, I would guess that they took their own oxygen down with them as the smell and 70% organic matter would suggest no or very little at least oxygen.
When trying to research the 'rotting radish as fertilizer' technique, I keep finding reference to this research where, although radish locked up nitrates from agricultural ferts effectively, they weren't returned to the soil through the process of decay.
Radish Mop Crop
I suspect Fukuoka's success may be specific to variety, climate and microbiome.
Examine your lifestyle, multiply it by 7.7 billion other ego-monkeys with similar desires and query whether that global impact is conscionable.