Born from Rock
Ordovician life, the importance of dirt, and tips for being a lazy gardener
Dirt. Brown, crumbly, possibly wet, and bound to leave streaks on white surfaces. For most city people, dirt is more or less anathema. We definitely don’t want it around us.
Dirt is dirty.
But the thin layer on the ground that many call dirt is, in fact, very valuable for us.
Dirt is vital for plants, who need a soft material to anchor their roots in and stay stable. They also, of course, need a source of nutrients, of which soil is the main provider. And so, by extension, that dirty soil is a main source of nutrients for us humans. If it didn’t exist, then neither would we.
But why is there dirt in the first place?
How come there’s so much of it?
And where did it come from?
To find out, let’s roll back the clock by half a billion years. The sea is sparse, the land is bare. Welcome to the Ordovician.
The land of the Ordovician is empty. There’s nothing here. You might think of volcanic islands or deserts as empty, but this is at a different level. If you haven’t seen the Ordovician, then you don’t know what emptiness means.
Deserts may seem empty on top, but dig a little bit under the surface and you’ll eventually come upon soil. This is where the cacti and palm-trees draw their nutrients from; it’s where snakes, worms and smaller creatures burrow down to escape from the scorching heat.
But here, there are no creatures to burrow, and even if there were, there’d be no soil for them to go to.
In those days, “land” consisted of mostly rock, rock and more rock. You may find tiny pieces of powdered rock, perhaps fine enough to be called “soil” but nowhere near the always-available, ever-present state it is in today. As for life, there was nothing except crusts of bacteria and maybe a few fungi.
Everybody else was in the sea.
If you’re still wondering where soil comes from, the answer’s lying right under our feet: the rocks.
There exist three kinds of rock, and soil inherits its components from all three kinds. Volcanic or “igneous” rock is the cooled-down edition of lava or magma coming from the centre of the earth. Sedimentary rock is created by the accumulation and compression of sediments over thousands of years ; it might also contain dead organisms (though probably not so much before the Ordovician). And when any kind of rock gets buried deep, deep underground, the heat and high pressure can make it melt and reconstitute, turning into a whole new kind of rock — the metamorphic rock.
The minerals inside these rocks contain different chemical elements, and it is from these that the nutrients in soil come.
The minerals that compose soil are inherited from rocks that are ever-so-slowly worn down by wind, water, and sun. Over time, these elements weather down rock and create a thin layer of broken minerals.
The sun causes rocks to expand and contract because of heat and crack, breaking them down into smaller and smaller pieces. Wind blows particles against rocks, further turning them into sand and dust.
Water weathers rock in two ways: chemically and physically.
Chemical weathering changes the chemical composition of a mineral, either by adding or removing atoms from it. Some minerals are more or less resistant against this chemical alteration, depending on the specific chemicals that make them up. Moreover, chemical weathering is more important in wet and warm places, such as the tropics, because chemical reactions are quicker in higher temperature.
Physical weathering doesn’t change the chemical structure of a mineral but break it down into smaller pieces. In the cold of winter, water that has infiltrated into rock changes into ice and cracks it down.
But while weathering forms the baseline for creating soil, it’s only the first step. We still need some way for the chemicals and nutrients to stay together, rather than getting washed away, and be collected and maintained in an organised fashion.
That’s where the plants come in.
As minerals are worn down into soil, the nutrients leech out for the plants to use. However, this process of nutrient release takes time, and that’s one of the reasons we use fertilisers.
Natural areas like forests and grasslands don’t need added fertilisers; they have ways of their own. And you can copy them too: just pile up your food waste in a corner of your garden. (If there are hungry animals around, put those wastes in a box).
Your food waste will be decomposed by earthworms, insects, bacteria and other charming organisms. Decomposition takes time and your compost will be ready after a minimum of six months, which if you think about the scale of things is actually not much. And then, use in a pot or spread in your garden.
This works because, while rocks were the only source of nutrients in the early days, there’s now an even more abundant source they come from: dead animals and plants.
When dead bodies—both animal and plant — decompose, they break up into extremely fine elements like fulvic acids, humic acids and humins. Now, the technical terms aren’t especially important to know. What’s important is the idea that these elements are extremely efficient at storing plant nutrients — and especially cations, positively charged nutrients.
Together, this mix of nutrients is what is called “humus”.
Apart from storing nutrients, humus also acts like a cement: it sticks soil’s building blocks together, making it resistant to erosion. Soil can also retain more water and resists drought. Plants can grow roots more easily in a well-structured, humus-rich soil because it’s softer to dig in to.
Until recently, people thought soil formation was something that happened anyway, and plants were just the beneficiaries. But recent research suggests it may be otherwise.
In February 2012, Nature Geoscience published a special issue on the earth being shaped by plants. One of the examples was about rivers: it was long thought that running rivers eroded rocks to form the rich soil on their banks. That’s partly true — but it’s trees along the banks that holds the soil together, preventing it from washing out to sea. What’s more, when the trees died, their wood became an important part of the makeup of the soil.
Meanwhile, another team looked back at the late Ordovician, when plants first made the leap onto land. It was their roots which stabilised the landscape, allowing things like dirt and mineral to collect. And it didn’t stop there: the extra plants pulled so much carbon-dioxide out of the atmosphere, it caused the Earth to cool down rapidly, triggering an ice-age which helped grind down the planet’s rocks.
Nobody can imitate the Ordovician, but we humans can take help from plants in our own pots or backyards. If you happen to be cutting grass, you can use the straw to cover the bare soil between your plants, in the process known as “mulching”. You can also buy chipped wood in garden stores, or use the dead leaves of a tree.
Mulching makes you work less because you’ll not have to weed your garden: weeds won’t come anyway if they don’t get enough light to grow. But the main point of covering the soil is to protect it against rain and erosion, especially when there’s nothing growing on it during the winter.
During the winter, when you’re not growing anything, seeding grasses to cover your garden during the unproductive season is another good way to avoid erosion. The roots of the grass keep the soil aggregates altogether. Think of those ancient roots, holding soil from washing out into a mighty river, and you’ll know why it works. And in spring, you can just cut the grass and use it as a mulch.
To conclude: be a lazy gardener and let biodiversity fertilise your soil.
Humus is part of a larger component called soil organic matter which also includes living organisms like roots and bacteria, and not decomposed molecules like sugars, fats and proteins. The most abundant molecules are lignins and cellulose: they’re the main components of plants enabling them to stand straight, and only few bacteria and fungi can decompose these large molecules. Moreover they are slow to decompose and so are the main component of soil organic matter.
Thanks to soil living organisms, undecomposed molecules can be broken down, mineralized and made available for plants.
Soil organic matter is important because it contains still active molecules enhancing plant growth and plant pest resistance.
If soil and dirt is so important, then why are people so averse to it? Perhaps it’s because city dirt is often contaminated with drain spill-off, factory run-off, car exhaust and other pollutants. If you go to the countryside, you’ll find it’s much cleaner, and more filled with useful nutrients. (Some people even like to eat it!).
Dirt started out as bits of crumbled rock. Plants gave it a new form, by holding it together, freezing it apart, and adding new nutrients to the mix. Earthworms, fungi and bacteria breathed life into what was once a static dead mixture.
And now, we humans are adding our bit.