Where does the soil in your garden come from – the story of soil from rock to fertile ground
Where does the soil in your garden come from – the story of soil from rock to fertile ground
History lies beneath our feet
When we reach down and pick up a handful of soil from the garden, we are holding something that took thousands, and sometimes millions, of years to form. It is not simply “dirt”. It is a complex, multi-layered structure in which the history of climate, vegetation, animals and geological processes is recorded: everything that happened in this place long before any of us arrived. Soil is one of the most underappreciated natural resources on Earth, more complex than water, harder to restore than a forest, and absolutely essential to life as we know it.
Most of us treat soil as a backdrop. Something to walk on, to plant things in, and to fertilise from time to time. Yet one centimetre of fertile soil takes hundreds of years to form. Its loss, through erosion, paving over or agricultural degradation, is a process that is practically irreversible on a human timescale. Understanding where soil comes from and what creates it changes the way we look at it, and perhaps the way we treat it.
Everything begins with rock
At the foundation of every soil lies the parent rock. It might be granite, sandstone, limestone, shale or loess, depending on the geology of a given place. This rock is the starting point, but in itself it is not yet soil. For it to become something capable of sustaining life, it must pass through a long process of weathering: breaking down into ever finer particles under the influence of water, frost, temperature and chemistry.
Mechanical weathering is the effect of physical forces that break up rock without altering its chemical composition. Water seeps into cracks in the rock, freezes and expands, splitting the rock from within. Daily and seasonal temperature fluctuations cause minerals to alternately expand and contract, eventually leading to their disintegration. Wind carries grains of sand that abrade the rock surface like sandpaper. This is slow, unrelenting work by the elements, whose effects are measured in millennia.
Chemical weathering occurs in parallel. Rainwater, mildly acidified by carbon dioxide from the atmosphere, reacts with the mineral components of the rock, altering their structure and leaching out certain elements. Organic acids produced by plants and microorganisms accelerate this process. In this way, clay minerals and other compounds form from the parent rock, and these will later become the foundation of soil structure.
When life enters the picture
The mineral material alone, however finely broken down, is not yet soil in the ecological sense of the word. The pivotal moment comes when the first biological colonisers appear: organisms capable of living in an almost lifeless environment and beginning to transform it.
The pioneers are usually lichens and mosses. Lichens, being a symbiotic combination of fungi and algae, can establish themselves directly on bare rock, secreting acids that accelerate its chemical weathering. When a lichen dies, it leaves behind the first, microscopic layer of organic matter. On this, moss can take hold, retaining more water, creating a more humid environment and adding another portion of organic material when it dies. Layer by layer, over decades and centuries, primary soil builds up in this way.
In time, higher plants appear, and with them an entire soil ecosystem: bacteria, fungi, protozoa, nematodes, earthworms, millipedes, mites and dozens of other groups of organisms. Each participant processes organic matter, mineralises nutrients, creates soil structure and influences its properties. The earthworm, pulling leaves deep into the soil and excreting digested matter, is literally a builder of soil. A single earthworm processes several grams of soil per year, and the earthworm population in one hectare of meadow can weigh more than a herd of cattle grazing on the same area.
Humus – the heart of fertile soil
The key component of fertile soil is humus, also called organic matter. It is a dark, spongy substance formed by the decomposition of dead plant and animal material by microorganisms. Humus is not the same as compost: it is more thoroughly processed, more chemically stable and far more durable. It can persist in the soil for hundreds, even thousands, of years.
The importance of humus to soil is hard to overstate. First, it is a reservoir of nutrients: nitrogen compounds, phosphorus, potassium and trace elements that plants can draw on gradually, as needed. Second, it improves soil structure: it makes clay less compacted and more permeable, and helps sand retain water. Third, humus is one of the most important long-term stores of carbon. The soils of the world contain more carbon than the atmosphere and all terrestrial vegetation combined. The degradation of humus-rich soils releases this carbon back into the atmosphere, which is one of the underappreciated mechanisms driving climate change.
Building humus is a slow process requiring specific conditions: regular input of organic matter, adequate moisture, appropriate temperature and a rich biological community. Destroying it takes considerably less time. Intensive tillage, monoculture farming, overuse of synthetic fertilisers and removal of leaf litter: each of these practices accelerates the breakdown of humus and the degradation of soil.
Soil profiles – a vertical cross-section through history
If we were to cut through the soil vertically and examine the cross-section, we would see distinct layers, known as soil horizons. Each layer has a different colour, texture and composition, and together they form what is called a soil profile: a record unique to each place, encoding the history of the processes that occurred there.
The uppermost layer, just at or below the surface, is the humus horizon. It is the darkest, biologically richest and most fertile. It is precisely this layer that determines the productivity of the soil, and it is precisely this layer that is the thinnest and most vulnerable to degradation. Beneath it lies the eluviation horizon, where water carrying mineral components leaves characteristic traces. Deeper still lie the mineral accumulation horizons and, finally, the parent rock from which everything began.
Reading a soil profile is like reading an ice core or the growth rings of a tree. Each layer says something about the conditions that prevailed in the past. Geologists, soil scientists and archaeologists can extract from such a profile information about ancient climates, vegetation and even human activity going back thousands of years.
Polish soils – a record of glaciations and winds
Polish soils have their own history, deeply marked by the last glaciations. The glacier that covered much of the country several tens of thousands of years ago left behind specific materials: boulder clays, glaciofluvial sands and gravels. When the glacier retreated, vast, vegetation-free plains were exposed, across which the wind scattered fine loess dust. It settled in layers across the south of the country, forming the basis for some of the most fertile soils in Poland: the chernozems and loess alluvial soils of the Lublin Upland and Lesser Poland.
In the north of the country, sandy soils and podzols dominate: less fertile, more acidic, characteristic of dune areas and outwash plains. In river valleys, alluvial soils formed: young, regularly replenished by river floods, and the foundation of Polish riverside agriculture for millennia. In hollows where water accumulated and stagnated, peat and bog soils developed: stores of carbon and valuable habitats, today largely drained and degraded in Poland.
This diversity of soils is both a richness and a challenge. Different soil types require different agricultural and forestry practices, different tree and plant species, and different conservation strategies. A one-size-fits-all approach to such a varied resource is one of the mistakes whose consequences we feel most acutely in the context of Polish soil degradation.
Forest soil versus urban soil
Not all soil is equal, and the difference between the soil in an old forest and soil in a city is striking. Forest soil is a structure shaped by millions of years of evolution: rich in humus, full of biological life, aerated by roots and the tunnels of organisms, moist and permeable. The forest litter, a layer of leaves, twigs and dead wood on the surface, is the soil’s natural protection against erosion, desiccation and extreme temperatures.
Urban soil is often its opposite. Compacted by foot and vehicle traffic, stripped of litter, cut off from natural organic matter, and frequently contaminated with heavy metals and petroleum derivatives. Urban trees grow in such a substrate like pot plants in too small a pot: they can survive, but they do not have the conditions for full development. This is the source of the short lives of urban trees, their susceptibility to disease and the difficulty they have in taking root.
Restoring health to urban soil is one of the most difficult but most important tasks in the context of cities’ green infrastructure. It requires not only the addition of organic matter and a reduction in compaction, but a fundamental change in the design of urban space: one that gives soil and roots room and conditions to function. One More Tree Foundation takes this context into account when planning every planting event in urban spaces, selecting species and locations so that trees have a genuine chance of long-term growth, not just an impressive start.
Soil is not a renewable resource – at least not on our timescale
One centimetre of fertile soil forms, depending on conditions, in anywhere from one hundred to one thousand years. Meanwhile, intensive wind and water erosion, driven by deforestation and poor agricultural practices, can destroy that same layer within a single decade. According to FAO estimates, more than one third of the world’s soils are considered degraded, and the pace of degradation far exceeds the pace of natural regeneration.
This means that soil is a resource we treat as renewable, even though it is not, at least not on a human timescale. The protection of soil should be taken as seriously as the protection of water or air. Practices that degrade it, such as deforestation, excessive tillage, monoculture and paving over land, have consequences whose repair will take generations.
Trees are, in this context, the soil’s key allies. Roots maintain its structure and protect it against erosion. Leaves create litter that nourishes the microbiome. Dead wood and roots build channels for water and air. A forest is not merely a collection of trees: it is a machine for building and protecting soil, operating on principles that humanity is only beginning to fully understand.
A handful of soil, thousands of years
The next time we pick up a handful of soil from the garden, a forest or a nearby park, it is worth pausing for a moment to imagine what is hidden in that seemingly ordinary clump. Minerals from rock that weathered over centuries. Organic remains of plants and animals from dozens of generations. Billions of living organisms, most of them invisible to the naked eye. Traces of a climate that prevailed here thousands of years ago. And a particular arrangement of all these components that makes exactly what grows here grow here, and nothing else.
Soil may be the most underappreciated wonder of nature. It does not dazzle like the ocean, does not impress like mountains, does not move us like an ancient forest. But without it, none of those things would exist. It is the foundation on which all terrestrial life stands: patiently built by nature over millions of years, and asking of us only one thing, that we stop taking it for granted.
