Let’s talk about soils. Getting the scoop. Go down and dirty.
For most people, soils are pretty much an unknown; underfoot but not giving a lot of thought or attention. Or, I should say, not given a lot of attention by adults. Kids love playing in soil.
I am certainly not a soil expert. However, I have thought often enough about soils. They are very interesting.
One way to think about soils is as some giant science experiment. Ask yourself – what would happen if you crushed up an enormous amount of rock – pulverized it – and then covered it over with water, somewhat like the way you cover cereal with milk in a bowl? Would the rock dissolve, like a pile of sugar? Or, would it behave more like a pile of salt, ionizing and going into solution that way? Or would it not do anything – after all, it is just rock?
The answer to the above questions is, as I understand it, all of the above. Some of the rock will dissolve and some will become suspended, potentially staying in suspension for a long time. Rock, of course, is made up of a bunch of individual minerals, some of which come loose as the rock is broken up. Also, the way that the rock in the soil has been broken up is not just from being crushed or shattered – although glaciers and freeze-thaw cycles do a good job of physically breaking rock into pieces – there is also a fair amount of chemistry involved in the disassembling of these rich and large bits of raw material.
The main means by which the rocks that get converted into soils are changed chemically is through the influence of that complex chemical tool, water. There are other chemical tools involved as well, as we shall see.
The ideal composition of soil, as usually given, is that the best natural soils are about ¼ air, ¼ water, 45% sand-silt-clay (mineral component or the outcome of those crushed up rocks) and 5% organic matter. Saying soils is about one-quarter each air and water is effectively saying that a good soil is about 50% pore space – open spaces between all of the all of the other solid items that makes up the rest of the soil. This is important because soil is not just a thing – it is also a process. This is in the same way that a forest is not just a place, it is also a set of dynamics. Things happen within soil and this cavernous web of connected web of air and water. Especially, biological things.
It is what soils have to do with trees, of course, that most interests me. In the old view of soils, tree roots were seen in terms of how they would access necessary nutrients from the soil environment. Basically, tree roots were pictured as gently sipping from the widespread reservoir of dissolved minerals held in the soil, gathering not just the water it needed for photosynthesis, but also the essential nutrients the tree also required.
In this view, it was understood that tree roots live in the soil along with many other types of life – from large things such as moles and earthworms to all of those small to microscopic organisms, such as fungi, protozoa and bacteria. This led to an understanding of tree roots being producers as well as consumers, as the tree carries down into its roots certain products of its photosynthetic efforts above ground. As a root grows, it exudes a protective cap that allows the growing tip to work its way through the surrounding soil particles. This exuded material is something of a gift to those other living organisms in the soil, as it gives them food to feed on. This is also something of an investment on the part of the tree, as these other soil organisms contribute to providing a more favorable environment for the root. Tree roots also regularly sheds their fine roots – small, non-woody parts of the root that function in the absorption of water and nutrients from the soil. Whole sections of roots also often die, allowing the release of elements of these complex structures back into the living soil.
All of this biological activity helps to condition the soil, opening up wider pores and contributing to soil structure in another way. This organic material helps bind the various elements of the soil together into a characteristic structure that varies from one soil to another, but that is important to allowing the soil to function as it does. Respiring roots also influence soil chemistry as they give off carbon dioxide, which tends to have an acidifying effect on the soil solution.
This view of the soil, while energetic and dynamic, was also somewhat passive, as it tended to overlook the many sorts of interactions among soil organisms now being discovered and explored within this intricately interconnected world. As mentioned, trees, through their roots, are in the role of both consumers and producers within this sub-surface arena. Among the areas of great current interest is the role of fungi as intermediaries and as regulators, as they participate in determining how and where energy and nutrients flow within the soil. There is a whole range of fungi that engage in a complex symbiotic relationship with tree roots. These mycorrhizal associations have caught the attention of arborists, among others, as the fungal partner help the trees gain nutrients by being more adept at absorbing and then transporting certain nutrients such as phosphorous. The tree allows the fungus to perform this service by allowing the fungus to also tap into the food that the tree has produced for itself through photosynthesis. Since individual fungal organisms tend to be so extensive in their growth form, a single fungus may be associated with many roots of many trees. Since how nutrients flow within the fungus is under the control of the fungus, it can be said that the fungus is controlling which roots get which amounts of nutrients. Research tends to back this up, even indicating that the fungus may may be taking nutrients and food resources from one tree and directing it to another.
All of this changes how soils can be viewed. Rather than seeing them as simply a rich growth medium on which trees and other plants depend, it becomes possible to see soils as complex engines that help govern our world. Physical, chemical and biological processes work together in soils to shape the world that we see around us, by influencing both the plants and the animals that we are aware of. They also have a much more subtle role that should not be taken for granted.
As one example of what more soils do, a recent Scientific American article gives the estimate that there are some 2 trillion tons of carbon stored in soils globally. This is more than 3 times the amount of carbon currently in the atmosphere. In our current era, when we are so concerned about the effects of greenhouse gases in the atmosphere, we need to consider the soil processes that lead to the initial storage of that carbon and that keep it there. Understanding this is going to be critical to any successful effort to slow and eventually and stabilize global warming and climate change.
In another example of what soils do, an article published by the Philosophical Transactions of the Royal Society discusses how soils influence air quality by, among things, absorbing and chemically processing air pollutants. These include air pollutants that are absorbed as they come into direct contact with the soil, including those that wash off of the leaves of the trees, where these air-borne pollutants may have been originally captured.
Soils should be thought of as an important and separate resource, with each soil having their own intrinsic value and capabilities. I first began to become aware of this when I began think in detail about what it meant that so much of the forests of Connecticut and of most the eastern US had to grow back from scratch following the massive deforestation over the 17th, 18th and early 19th centuries. That these forests grew back as successfully, as measured by tree growth, is a testament to the power and capacity of these soils. If we have good soils and appropriate climate, we will have trees – we just have to allow it to happen. We must respect this intrinsic capacity of soils. There is very much a life force within those soils that we need to understand, appreciate and respect.
July 22, 2023
