40: Soil and Carbon Stocks

It’s not just mud. Soil is pretty important, for all kinds of reasons including us being able to grow food and flowers, and not having to walk around on rocks all the time. But what is soil, and what’s in soil, and how can that brown stuff that we walk over every day be partly responsible for not only plant life, but the life of animals and even on the whole climate? This week we’ll be exploring all of the above including appreciating earthworms and looking at why planting lots of trees is normally a great thing but can actually be a bad idea in some places. Oh, and somehow I’ll try to make all these things tie in together. Enjoy!

Soil changes the deeper underground you go (Picture: UC Urban Forests Council)

What is soil?

Broadly speaking, soil is a mixture of air, water, tiny rock particles and dead plants and animals. The amount of rock particles or water, and the degree to which the dead stuff is broken down and decomposed, as well as the types of plants and animals that have decomposed, all determine what type of soil is present in an area. This can include things like nutrient rich peat which has no rocks and where dead material isn’t fully decayed, clay which has little air and is often quite waterlogged, chalky soil through which water passes quickly, and sandy soil which has large rock particles and so plenty of air between them. Through their content the soil can provide plants with water and nutrients from whatever minerals and elements are present.

However, soil is not simply a dead pile of nutrients. It is actually a dynamic habitat of its own that is teeming with life. As you’ll have noticed, despite the majority of soil being dead stuff you generally don’t see bones and internal organs lying around mixed with stone particles when you head out to the back garden… I hope. That’s because these things have been decomposed, meaning there needs to be a steady supply of decomposers sitting in the soil ready to turn anything dead into a broken down nutritious form that plants can access.

There are many different types of soil, often most simply divided into the proportion of silt, sand and clay that they have within them. However there are more soils than just mentioned here.


Who are these decomposers? There’s a few. The top layer of soil, the dark brown stuff, is called humus (pronounced ‘hew-mus’, not to be confused with the tasty Middle Eastern hummus). Here is where you might find some recognizable chunks of decomposing leaf and such like, and where you find the big creatures of the soil world. This includes things like ants, slugs, earthworms, and millipedes, just to name a few. They carry out their decomposing work in a very simple way, much like we do. They find the dead matter, or sometimes kill live matter (think of those murderous slugs attacking a leaf), digest it and then defecate out the broken down remains.

This is an important first step in the nutrient cycle. If you place a leaf next to your potato crop, the potato plant won’t be able to suck the nutrients from it because it can’t handle such big particles in certain chemical compositions, and so having it broken down by other creature first turns it into manageable chunks for the potato to reach and use. Note to gardeners, this is why having a soil full of earthworms is important, they are your and your potatoes friend. Little side note though, be careful what type of worms they are. New Zealand flatworms have started invading new places and killing local earthworms, and so are in the long run bad for your soil.

Soil is more than just worm poop of course. It is broken down by many other creatures into smaller, finer particles too. This includes things like microscopic algae, fungi and bacteria. Lots and lots of these things in fact. It is thought that in 1g of soil you can get 1 billion different species of bacteria, around a million different species of fungi and tens of thousands of algae species. These creatures vary with the type of soil, with fungi being happier in untilled or undisturbed soil (part of the reason some gardeners and farmers prefer not to turn over their soil) and bacteria being able to tolerate a bit more soil disturbance. Soil is very much alive then, and these microscopic creatures make up a big part of it.

On the left our friend the humble earthworm, on the right the evil New Zealand flatworm. Okay, it’s not evil, but it does attack the earthworms so better to keep out of local soils…

Nutrient cycling and breathing

So, microbes in the soil break down plants and animals, releasing the nutrients to live plants growing in the soil. This allows all the nutrients taken from the soil by e.g. plants to be recycling back into it. Fertiliser is used in agriculture to replenish the nutrient stocks as we take all the plants away for processing and eating rather than letting them fall, decompose and recycle their nutrients back into the ground. When we look at important soil nutrients we tend to focus mainly on nitrogen and phosphorus, though there are other important nutrients there too. However, carbon is something getting growing attention, especially when it comes to carbon stocks.

Earth is made up of carbon based life forms, so carbon is an essential component of living things. Humans, for example, are made up of 18% carbon. Plants and animals also have a lot of carbon in them, which means when they die and are decomposed this carbon is transferred into the soil. Having carbon sitting in the soil, or in a tree trunk or a goat is all fine and well. However, many microbes in the soil breathe just like us, taking in oxygen and releasing CO2, so having more active microbes mean they take up more carbon from the soil and convert it into more CO2. This is then released into the atmosphere which in high amounts can affect the temperature regulation of the planet.

Peat soils are not very active as they are often quite acidic and have little air spaces for soil life to breathe, so they have lots of carbon but it’s not really used up or released making them an important carbon storage area. For this reason in some areas people are being encouraged not to burn peat/turf as that then releases the stored carbon. In some more active soils, carbon is taken up by plants and used to build their stalks, trunks and leaves. This storage of soil carbon within the e.g. tree, and the fact that the tree breathes the opposite way than we do taking in CO2 and releasing oxygen mean that these plants are great at collecting and storing carbon too. That’s why a recent high profile environmental initiative has been to plant millions of trees everywhere to reduce the amount of CO2 in the atmosphere. That is partially an amazing idea. Trees do so many great environmental regulation services, this included, and so encouraging them is generally a great idea. However, it may not be the best idea everywhere.

File:Global carbon cycle.webp
The carbon cycle, showing how carbon moves throughout the environment. (Image: Jack Cook/Woods Hole Oceanographic Institution/ CC BY-SA 4.0)

Tundra soils

In the Arctic, especially in the treeless tundra the soils are very slow and not too active, so they store lot of carbon. However, if lots of trees are planted, it starts to speed up the turnover of nutrients and makes the soil more active. Some of the carbon that is released is picked up and stored in the structures of the new trees and plants, but there is so much more carbon in the soil than can be stored in above ground structures, especially in the Arctic where soil carbon is generally quite high. This means that by having more plants in the soil, making it more active but not absorbing all the newly freed carbon, planting trees might lead to more carbon being released into the atmosphere.

This is all at the very at the forefront of science and still being researched, but is a good example of the simple fact, rarely is something the same everywhere. The natural world is filled with variations and we have to be sensitive to that when studying, changing or working with it, rather than throwing blanket rules and regulations out there that might not always be appropriate.

A map showing soil carbon stocks around the world with darker colours meaning more carbon. As you can see the Arctic is one of the high carbon storage areas in its soils. (Diagram: Ruesch and Gibbs 2008)

So there we have it. Soil is basically essential for life, recycling nutrients allowing us to grow food and relying on worms and bacteria to make that happen. How active or inactive these soils are affect the whole carbon cycling of the planet, but the location of these soils on earth and their original carbon stocks are also important to keep in mind. What we do to this soil could have a big impact on our ability to get food and to allow stable regulation of the planets climate. So, as I said at the start, it’s more than just mud.

For more info:

Different types of soil, especially relevant for gardeners: https://www.rhs.org.uk/advice/profile?PID=179

Cover picture: Soil association

Music: kongano.com

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