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.