Here is a fact that astonished me when I heard it: Up to 40% of sugars produced by some trees through photosynthesis is sent to its roots to be exuded to feed soil organisms. Yes, it does all of that work making sugars from carbon dioxide and light just to push it out into the ground. Why do plants go to all this trouble? I shall elaborate.
Lately, I have been a bit bored. I used my intellect to make my living for over three decades and mostly now I do not. I mow lawns, I build raised beds and build wildlife habitats. Hence my intellect languishes largely idling apart from such direction as my idle curiosity bids it to take. And so, I decided to do some online courses to keep myself engaged with the world of thinking and learning, lest I otherwise become slack-jawed and monosyllabic. There are some excellent free online course and I have learned many fascinating facts such as that above. And I have begun to get some sense of how we might, in some small way, make our little patch of the Earth healthier and, anthropocentrically, more productive.
Everything we eat has, in one way or another, its origin in soil. Plants obviously grow in it (though hydroponics can also be a way to raise plants). Animals graze on vegetation which grows from soil and are also fed from other plant products. And of course, we ourselves eat plants grown as crops and some people eat animals fed from plants. Soil is the origin of all of our food, ultimately.
Strange then that soil is given so little thought. Moves are afoot to bring about a realisation of soil's importance and to protect and nurture soils, identifying soil as a resource which is largely though not completely, finite. Recognition is also growing of the damage caused by common and accepted agricultural practices such as ploughing and digging which persist despite an emerging understanding of their contribution to erosion and degradation of soil structure.
Looking with a little more focus at the dirt beneath our feet, I discover we have been underestimating it for some time. It turns out that soil is a lot more complex than the stuff you tread all over the carpet when you come in from a walk in the countryside. It is not a simple, homogeneous substance that fills in the gaps between rocks or sits passively at the side of the road with grass growing upon it. Certainly, it contains mineral particles (The "parent material") and this is what most people think of when they talk about "dirt". But in addition, soil contains a world of living complexity which is still not fully understood. What is understood however is that it is a community, a living ecosystem of its own and how to nurture, feed and care for it is becoming clearer as research progresses.
So, back to our small but explosive factoid from my opening line. Plants push out sugars into the soil to feed bacteria, fungi and a class of intermediate organisms called "actinomycetes" from which we get many of our antibiotics. These latter seem largely to be similar to bacteria but produce spores and filamentous networks like fungi.
Fungi and bacteria do not photosynthesise and so have limited ability to produce their own food from their surroundings. Certainly they break down wood and other materials for food but a plant offering a shortcut to actual sugars is very welcome to fungi. It is very difficult not to anthropomorphise about this, but essentially fungi undertake trade for these carbon resources. In return via their networks of mycelium they transfer to the plant roots elements such as phosphorus and other substances necessary for plant growth. Indeed so intimate are the relationships with some of these soil organisms that they have interfaces within the cells of plant roots where these exchanges can be easily transacted: A bit like a subcontractor having an office in the building of a major customer. It is estimated that the surface area of mycelial networks on a plant root have seven times the surface area and reach of the roots themselves, extending the plant's reach to gather nutrients massively beyond what the plant itself could manage.
Fungi like carbon. They can't get enough of the stuff and to be provided with it by a plant is a huge ecological boost. Similarly, plants need elements such as nitrogen which certain bacteria like rhizobium, will supply in return for food and a comfortable place to live such as in nodules on the roots of beans or birch trees.
We have then, networks below the ground where symbiotic relationships flourish to the benefit of all concerned. Millions of years of evolution have ensured this process exists everywhere plants grow. If we let it.
And here is the thing: These wonderful networks of existence and transaction are fragile. And what do we do? We come along with spades or ploughs and we smash them to bits by turning over the soil. This is how most farms work the soil and to be fair, they have reasons for doing it this way which are quite compelling. Vegetable gardeners everywhere know that beans are good for the soil because they add nitrogen. But then Autumn comes and the plot is dug over and all those networks are disrupted and the tiny particles of carbon held in the soil are exposed to oxygen from the atmosphere resulting in the production of carbon dioxide. The carbon could have stayed in the soil, nourished fungi and bacteria and hence our plants, but it is now floating around in the atmosphere warming the place up. How much better to leave them in place for the next crop to take advantage of?
So, five years ago, or thereabouts (although not actually being aware at the time of these actual mechanisms) I ceased digging my vegetable garden. Every year, I just add 3 -4 cm of compost on top and I plant into this. This adds beneficial organisms and, of course, carbon, locking it up safely for a while and feeding the soil organisms that feed my vegetables. Also, this results in the soil having better water retention and being beautifully crumbly so that what few weeds do germinate are able to be pulled up very easily. Not turning the soil over also has the benefit of not exposing weed seeds so they stay in the soil and rot. My yields are higher and best of all: I don't have to dig! Win-win as they say.
Now, taking this a step further, I looked into the further significance of mycelial networks and discovered something even more profound: Plants communicate!
No really. You read that correctly. An experiment took bean seedlings and stripped them to two roots, placing one root in a test tube of nutrient solution with the single root of a neighbour. The plants were, in effect, "daisy chained" in a line.
A plant nibbled by a grazing animal will produce salicylic acid (a component of aspirin) in response to the damage. It was discovered in the experiment that if the first plant in the chain was damaged deliberately, all the others would produce salicylic acid in response. Obviously some information about threat had been passed down the line.
Even stranger it was discovered that when a pine tree was given carbon dioxide containing a radioactive isotope of carbon, those atoms appeared in seedlings near the "mother" tree. But they only appeared in seedlings who were offspring of the mother. They did not appear in unrelated seedlings of the same species at the same distance. Now I don't know about you, but I find this utterly profound and somewhat disconcerting. Trees exchange information as well as resources along networks of mycelium. Giving one seedling food but not another, even if it is the same species, implies a decision of some kind. To call it "intelligence" might be premature. But is does seem to suggest trees don't just stand there unaware of their surroundings.
What are we to make of this then? Well, firstly, soil is far from inert but is bustling with life, information and trade. mycelial networks are vital to vegetative commerce and healthy crops. Also that plants are not passive but respond almost "intelligently" to their surroundings. I am not sure if this will make me regard my plate of spinach, carrots and chickpeas in any way differently but it is, if you'll excuse my shameless pun, food for thought.
