Mud. Muck. Dirt. Although we have plenty of words for it, we rarely give soil a second thought. But without soil, we would certainly be dead.
Soil is crucial to almost every aspect of life on land, from water storage and filtration to climate regulation, flood prevention, nutrient cycling and decomposition. The dirt beneath our feet is also an exceptionally high source of biodiversity: some estimates suggest that at least one quarter of all species live in or on the soil. And we are still discovering its treasures: in January 2015, scientists announced that the first new antibiotic in 30 years had been found in soil bacteria.
“Soil biodiversity is largely unseen but is crucial for healthy ecosystems, and ultimately healthy humans,” say Tandra Fraser and Diana Wall from the Global Soil Biodiversity Initiative.
The UN has named 2015 the Year of Soils and 5 December also happens to be World Soil Day. If there was ever a time to celebrate this underappreciated substance, it is now. But where did soil come from originally, and why is it so fundamental to life on land?
At the birth of the solar system, before our planet formed, the building blocks of soil were lurking in the inky blackness of space. Evidence for this comes from meteorites known as carbonaceous chondrites that date from the dawn of the solar system and that are rich in the clay minerals that made up the earliest terrestrial soils.
Following the formation of Earth, about 4.6 billion years ago, these clay-rich primeval soils developed across our young planet. But conditions were harsh: frequent and massive meteor impacts would have melted and pulverised large volumes of these early as quickly as they formed.
Almost from the moment of its origin, life began to influence – and be influenced by – soil
“There is debate about whether the whole surface of the Earth was melted,” explains Gregory Retallack, an expert in ancient soils from the University of Oregon in Eugene, US. He supports the theory that no more than half of Earth was molten at any one time.
Around 3.8 billion years ago, conditions on Earth began to stabilise. The constant meteorite bombardment that had made the planet an inferno until that point began to subside, and liquid water could condense, forming lakes and seas. This marked an important point in the soil story. The liquid water weathered and eroded Earth’s rocky crust, generating mineral matter and forming more permanent soils.
The first life on Earth probably appeared a little later, about 3.5 billion years ago; some of the earliest evidence comes from fossilised structures that formed on rocky shores and resemble microbial mats called stromatolites, which are still found on Earth today.
Almost from the moment of its origin, life began to influence – and be influenced by – soil. For instance, those first microbial mats were built up from photosynthetic organisms, which could produce huge volumes of organic material using energy from the sun. This organic matter gradually built up on the shoreline, where it mixed with the minerals freed up by eroding rock to create what was arguably the first true soil.
But this was not soil as we know it. These soils were poor at storing the water and nutrients that can sustain life. The storage capacity of soil depends on pores that form between grains; the simple structure of early soils meant that they drained quickly, washing out nutrients in the process. Because of this, the land remained an inhospitable habitat, and life was restricted to the shoreline where water was more readily available.
No single organism had the adaptations necessary to move away from the shore and fully colonise the poor-quality soils. The key to colonising the land was cooperation – more specifically, the appearance of lichens between 700 and 550 million years ago.
Lichens were critical for the colonisation of land by plants
Lichens are quite remarkable organisms. Their tissues are formed from a mutualistic relationship between algae and fungus, and sometimes bacteria too – organisms representing three different kingdoms of life. Lichens are extremely resilient and adaptable because of this unique symbiotic relationship.
Algae can photosynthesise, providing the lichen with energy, while the fungus collects water, preventing the lichen from dehydrating. Fungi have long, thin filaments, which are extremely good at collecting water from the environment, and they can also recycle water during respiration. Even more importantly, lichens containing photosynthetic bacteria called cyanobacteria are able to take up nitrogen from the environment, which is released when they die, fertilising the soil.
By working together, these diverse organisms combined their skills to colonise the bleak lifeless soils that covered the continents half a billion years ago. Even today, lichens are among the most adaptable organisms on Earth.
“Lichens can colonise bare rocks”, says Paul Falkowski from Rutgers University in New Jersey, US. “They also produce organic acids that increase rock weathering”, he says.
This means the lichens didn’t just move into Earth’s early soils – they also changed them. By accelerating the weathering of rocks, lichens released even more nutrients into the soil, making it more fertile and paving the way for other forms of life to move onto the land. “Lichens were critical for the colonisation of land by plants,” says Falkowski.
Scientists believe this mutualistic relationship was essential to the evolution of land plants
This second wave of colonisation began about 440 million years ago – and early land plants soon set about substantially altering the soil themselves. “They created a more marked soil structure,” explains Retallack – and they aided the release of nutrients such as phosphorus and potassium into the soil. “This had a trickle down effect of fertilising both the land and the sea,” he adds.
Key to the fertilising power of plants were the fungi in their root systems. These “mycorrhizae” evolved around 500 million years ago, before plants had even evolved roots.
Much like the fungi in lichen, mycorrhizae gained energy by cooperating with photosynthetic plants – and, again as with lichen, the benefits ran both ways: the mycorrhizae grow filaments, extending a plant’s reach and making it more stable, and enabling it to absorb nitrogen and other nutrients from the soil.
Mycorrhizae filaments also burrow into rock, releasing nutrients such as phosphorus, calcium and iron, and helping the volume of soil to grow.
Scientists believe this mutualistic relationship was essential to the evolution of land plants – a hypothesis strengthened 15 years ago with the discovery of 460-million-year-old fossil mycorrhizae that date from before the evolution of land plants.
Water storage and filtration is one of the most important roles soil plays
“This mutually beneficial relationship helped plants to colonise the land before they had roots and before there was soil as we know it today,” explains Katie Field from the University of Leeds, UK. “As time progressed, plants evolved to become more structurally complex, developing extensive vasculature, leaves and rooting systems”, she says. This brought more organic matter into the soil and helped stabilise it against erosion.
Today, mutualistic relationships like these form the basis of global nutrient cycling, without which we would starve. More than 80% of modern plants form mycorrhizal relationships with filamentous fungi, and they are crucial for releasing nitrogen into the soil.
Mycorrhizae also form huge networks, which stabilise the structure of the soil and enable plants to communicate, gaining them the nickname “Earth’s internet”.
As plants gradually colonised the land and began to input large quantities of organic matter into the soil, its water storage capacity increased. Water storage and filtration is one of the most important roles soil plays, even today: we depend on it for our drinking water and agriculture. The water storage capacity of soil is also important in reducing flood risk, as well as providing an important buffer against drought.
By about 420 million years ago, terrestrial invertebrates were thriving
Water in the soil is given two names. Below the water table, where the soil is saturated, it is called groundwater; and above the water table, where there is less water, it is referred to as soil moisture.
Groundwater makes up about 20% of the world’s fresh water supply, although it represents less than 1% of all water on Earth. It is an important reservoir for our drinking water and irrigation systems, with 33,000 trillion gallons (125,000 trillion litres) stored in US soils alone.
There is one final chapter in the evolution of modern soils. Some time between about 490 and 430 million years ago, animals first emerged from the oceans and began to colonise the increasingly verdant land. By about 420 million years ago, terrestrial invertebrates were thriving – and, again, the soils changed as a consequence.
These early land dwellers were herbivores, devouring the algal mats and lichen that populated the land, and returning nutrients to the soil. They also began burrowing into and colonising that soil, churning up the dead organic matter and mixing it thoroughly with clays and other minerals weathered from the rocks. Their actions gave soil an even more distinctive structure, and helped plants continue to develop and thrive away from the water.
The variety of organisms living in the soil increased rapidly. New invertebrates appeared, including millipedes, springtails, mites, and early ancestors of spiders. By about 360 million years ago, soils were much like they are today, with the same range of varieties we can find beneath our feet – including swamp soils and forest soils.
Soil can be a direct source of greenhouse gases
“All major soil orders had appeared on Earth with the exception of grassland soils,” explains Retallack. Grasslands didn’t begin to appear until about 65 million years ago, after the extinction of the dinosaurs.
The history of soil has been shaped by physical factors and living organisms, through a dynamic web of interacting processes that began at the dawn of geological time, billions of years ago. And the story of soil is continuing to unfold as a consequence of our actions over the last few centuries.
Prior to 1960, the nitrogen cycle was roughly balanced across the world. Since then, the use of nitrogen fertilisers has increased some 800%. Too many nutrients can be just as harmful as too few – excess nitrogen is washed into rivers and streams where it can cause devastating algal blooms, leading to the release of nitrous oxide, a dangerous greenhouse gas and a hazard to human health.
Large swathes of Indonesian peatlands have been burning continuously for several months now
The change is the largest the nitrogen cycle has experienced in 2.5 billion years and it could have serious consequences for our food supply and climate.
Disruptions to key nutrient cycles in the soil are particularly worrying because the soil system tends to respond slowly to change – any harm done by humans now may take decades, even centuries to repair.
Soil can be a direct source of greenhouse gases too. By trapping organic matter, soils are one of the major stores of carbon, keeping it from becoming CO2 in the atmosphere. But when, for example, peatlands are burned, that carbon finds its way back into the atmosphere.
Large swathes of Indonesian peatlands have been burning continuously for several months now, releasing more greenhouse gases each day than the entire US, in what has been described as “the greatest environmental disaster of the 21st Century”.
Modern agricultural practices are also harmful to plant mycorrhizae, reducing the ability of our crops to gain vital nutrients, and degrading the soil structure in the process.
The security of our future food supply hangs in the balance
In effect, our agriculture is reversing billions of years of soil evolution and making our soils more vulnerable to erosion. In fact, half of the world’s topsoil, the most active and important part of the soil, has been lost over the last 150 years.
Eroded soil holds less water and nutrients, making it difficult to grow crops, and leaving our land more vulnerable to flooding and drought. The sediments from soil have to go somewhere, so soil erosion also clogs our streams and rivers, killing the organisms that live there.
The problem may only get worse. Intensification of agricultural processes is degrading soils globally, and with the population set to reach 9 billion by 2050, the security of our future food supply hangs in the balance.
The good news is that if we do begin taking better care of the world’s soils we can take advantage of their carbon storing capacity, among other things, to help combat the effects of climate change.
We might not give it much thought, but soil is silently keeping us alive. By acting now to protect soil as a key ecosystem worldwide we can ensure it continues to provide us with clean water, food and a hospitable climate far into the future.