‘Common as dirt’ is, so I am told, an expression that has many international equivalents. What could be more easily obtainable than earth? And for most of us, it is indeed common. After all, soil is one of the reasons why humanity is where it is. You want to build towns and cities in places where there’s plenty of arable land to feed everyone.
From a global perspective though, soil isn’t quite as readily available as the idiom implies. Most of our planet is water. Of the 29% of it that’s land, much of it is too dry and hot to grow much, or too cold and frozen, or too steep and rocky. We can use some of this for pastureland, but just 11% of the earth’s land is suitable for crops.
So that’s not so common, as it happens. Not when you consider that we rely on the soil to feed the world. Not when you remember that the world’s population is growing, so we need more from the soil than ever before. And since we can’t make more of it, at least not in a timeframe relevant to human civilisation, you’d think we’d be keeping a close eye on the soil that we have. But that is not the case.
As we’ve seen, soil is a self-maintaining system. In nature, it looks after itself. When humans intervene with agriculture, the balance can be lost and the processes interrupted. Soil works in tandem with the vegetation that grows from it, as a mutually reinforcing dynamic. Plants need soil, and soil needs plants. Unfortunately, we tend to clear the land completely in order to choose what grows from it, breaking that cycle. Then we haul away what’s been grown, keeping the grain as food and baling up the stalks, rather than letting the soil re-absorb the nutrients. The result is a gradual loss of fertility, and we have to make up the difference with chemical fertilisers.
Along with the return of nutrients and organic material, soil also relies on cover vegetation to keep it together. The roots of plants run through the soil, holding it in place and controlling the flow of water into the ground. If you clear the ground and turn the soil over, it is exposed to the sun, which dries it out. It is exposed to the wind, which whisks it away. And it is vulnerable to the rain, which will wash it downhill and into the rivers and the sea.
Farming has nonetheless involved clearing the ground in this way for thousands of years, ever since we first experimented with settled agriculture back around 7000 BC. Because we’ve been doing this for so long, history has plenty of examples of what happens when rising population meets neglected the soil. The Sumerian city states of Mesopotamia farmed areas of Iraq that are now desert. The Mayans may have succumbed to soil exhaustion. “Time and again, social and political conflicts undermined societies once there were more people to feed than the land could support” writes geologist David Montgomery in his book Dirt: The erosion of civilization. “The history of dirt suggests that how people treat their soil can impose a life span on civilization.”
In the early 1900s, Northern China was blighted by a succession of famines, with poor soil management a major factor. The US learned many lessons from the dust bowl conditions of the 1930s. Some argue that the story of Syria’s collapse into chaos begins with creeping desertification of the Syrian steppes, driven by a combination of overgrazing and climate change. Rural populations lost their livelihoods, and the metropolitan elites in government didn’t care. The Arab Spring set the spark, but the tinder was ecological.
There are a number of different processes that degrade soil, and eventually lead to exhaustion and eventually desertification. Here are some of the main ones:
- Erosion – soil erodes in the wind when it is loose, dry and exposed. It is at risk of water erosion on bare slopes. Because it takes from the top, it’s the most fertile topsoil that is washed or blown away, leading to gradual degradation. Some erosion happens naturally, but this is kept in balance by new soil formation. Conventional agriculture accelerates it, with erosion happening tens of times faster than new soil can form.
- Deforestation – woodland soil is rich and fertile, but if trees are cut down for firewood or timber, it is vulnerable to erosion. Deforesting hillsides is particularly problematic, as it leads to flooding as well.
- Overgrazing – when too many animals are kept in one place, the land doesn’t get a chance to recover and re-grow vegetation. In some places, cattle have been raised on land until it doesn’t grow enough to sustain them. Then sheep come in, and finally goats. Goats will eat anything, leaving the land barren and eventually desert.
- Exhaustion – when the land does not get a chance to rest, or when the same crops are grown repeatedly, the soil is drained of its nutrients – nitrogen, phosphorus and potassium in particular. These then have to be added artificially, and if the farmers cannot afford these or if they aren’t available, the land may have to be abandoned.
- Salinization – in dry climate, irrigated land can suffer from a build-up of salt, which gradually reduces fertility.
- Compaction – heavy vehicles or too many animals passing over the soil can compact it, pressing the air out of it and making it impossible for roots to penetrate. Water then runs off this harder soil instead of soaking in, leaving it dry and open to erosion.
- Urbanization – the most obvious way that soil is lost is just by paving over it and locking it away. That happens when towns and cities expand, when new roads are built, or when people choose the low-maintenance approach to their garden and put down patios and driveways.
As you can imagine, there are a range of the knock-on effects that we won’t have time to investigate in detail, but it’s worth mentioning that they exist and compound the problem. Erosion leads to silt build-up in rivers, loss of aquatic species and increased flooding – here’s Madagascar’s Betsiboka river, which runs red with eroded soil. Wind erosion creates air pollution from wind-borne dust, with all of its associated health risks. Soil loss impacts global biodiversity, climate change, and several of the planetary boundaries.
How bad is it right now? Let’s do a quick round-up of our global situation.
Globally, the equivalent of 10 million hectares of arable land is lost every year. In the last 150 years, we have lost half the world’s topsoil. When land is exhausted, farmers move on and start somewhere else. The FAO estimates that 20 million hectares of farmland is abandoned every year.
If it’s hard to imagine how run-off from fields adds up to this kind of global disaster, remember our key fact from earlier: it can take 500 years to make an inch of topsoil. If one millimetre of soil blows away every year from across a field, then a thousand years of soil creation can be lost in fifty years. It’s all about the speed of replenishment. Stung by previous crises, the US has soil management schemes and still loses soil ten times faster than it can be recreated. In China and India soil loss is running at 30 to 40 times the replenishment rate.
Directly or indirectly, humans get 99% of their calories from the soil. The world has enough malnourished people as it is, and a growing population. Food production must increase, piling yet more pressure on the soil. We can’t afford to lose it.
Fortunately, there are solutions. We shall come to them tomorrow.