Soil and Biodiversity

One of the most striking things about soil is that it harbours a remarkable diversity of life. A handful of soil from any well-kept garden, forest, or agricultural field, can contain literally billions of individual organisms and thousands of species. In some cases, as much as 10 per cent of the soil’s total weight could be alive, although in most cases it will be 1–5 per cent. The bulk of these organisms are microorganisms, which aren’t visible to the naked eye: the bacteria, fungi, and algae. But the soil is also home to many animals, including microscopic nematodes and protozoa, and large faunas such as springtails, earthworms, spiders, and even moles. The diversity of all these organisms is vast, with some scientists estimating that soils probably contain as much as one-quarter of the living diversity on Earth. The importance of soil organisms for soil fertility has long been known. The philosopher Aristotle (384–322 BC) referred to earthworms as ‘the intestines of the earth’, and Cleopatra (69–30 BC), the last pharaoh of Egypt, declared them to be sacred because of their contribution to Egyptian agriculture. Darwin detailed the importance of earthworms for soil fertility in his last book, published in 1881. He commented: . . . It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures. . . . Also, the benefits of leguminous plants for soil fertility and crop growth have been known since Roman times. But it wasn’t until the late nineteenth century that it was discovered that nitrogen fixation is down to microscopic bacteria (Rhizobium) that live in small modules in roots. Around the same time, it was also discovered that bacteria that live freely in soil, outside plant roots, also fix nitrogen from the atmosphere and boost nitrogen supply to soil.

Soil and the City

I have spent most of my living and working life in the countryside, surrounded by open fields, woodlands and hills, and in close contact with the soil. I recently changed my job and moved to the University of Manchester, which is in the centre of one of the largest cities in England. Because of this move my contact with soil is much less; in fact, as I walk each morning to my office, there is hardly a handful of soil to be seen. But is this really true of the whole city? Concrete, asphalt, and bricks certainly seal much of the ground in Manchester, as in most cities and towns. But soil is in abundance: it lies beneath the many small gardens, flower beds, road and railway verges, parks, sports grounds, school playing fields, and allotments of the city. In fact, it has been estimated that almost a quarter of the land in English cities is covered by gardens, and in the United States, lawns cover three times as much area as does corn. As I write, I am on a train leaving central London from Waterloo Station, and despite the overwhelming dominance of concrete and bricks, I can see scattered around many small gardens, trees, flowerpots and window boxes, overgrown verges on the railway line, small parks and playing fields for children, football pitches, grassy plots and flower beds alongside roadways and pavements, and small green spaces with growing shrubs outside office blocks and apartments. The city is surprisingly green and beneath this green is soil. Throughout the world, more and more people are moving to cities: in 1800 only 2 per cent of the world’s population was urbanized, whereas now more than half of the global human population live in towns and cities, and this number grows by about 180,000 people every day. This expansion has been especially rapid in recent years.

Soil and War

My first visit to a battleground was during a family holiday to Scotland. We were staying in Applecross, a small, isolated village on the west coast of the Scottish Highlands that looks over the sea towards the Island of Raasay. On the way back we passed through Inverness, the most northerly city in Scotland. To break the long journey we decided to stop off at Culloden Moor, the site of the Battle of Culloden in 1746, between the Government forces, which were mainly English, and the Jacobite army, made up of Scottish Highlanders led by Bonnie Prince Charlie. I had never visited the site before, but I recall thinking that it was an odd place for a battle; it is exposed moorland and the ground is rough and boggy, which would be difficult ground on which to go to war. I later learned that Bonnie Prince Charlie’s choice of this site for battle was catastrophic; not only did the exposed ground leave the Jacobite forces vulnerable to the superior artillery of the Government forces, but also the boggy soil hampered their attack, rendering them even more exposed. These factors led to the slaughter of the Jacobite forces and the collapse of the Jacobite campaign. I don’t know exactly how much the boggy soil contributed to the outcome of this war but it certainly played a part. For centuries, soil has played an enormous, and often unexpected, role in the outcome of war. War can also leave lasting and often irreversible scars on soil, leaving it churned, riddled with battle debris and bodies, polluted with heavy metals, toxic dioxins, oil and radioactivity. In many cases, it is left unusable. War can also indirectly affect the soil, for example through the need in Britain, during the Second World War, to cultivate gardens and city parkland for food. And the current growing demand for food, coupled with environmental pressures related to climate change, will place increasing pressure on soil, potentially leading to future wars. This chapter will look at how war is affected by and how it affects soil.

Soil and the Future

If the importance of soil for human lives hasn’t leapt out of the previous pages, this book has failed in its goal. Soil touches so many aspects of human life, often in ways of which we are not even aware. There are the obvious, such as when we dig up the soil to grow vegetables and flowers, or when a farmer takes a plough to a field. But there are also the less obvious, such as the role of soil in dampening climate change, filtering the water we drink, and breaking down and recycling the billions of tonnes of dead plant remains that annually fall to the ground. Soils have also played their role in warfare, thwarting military advances and providing underground shelter to those under attack. I could go on, but I think the message is clear: earth matters. Looking to the future, a major challenge for humans will be how to deal with rapid soil change. I emphasized at the start of this book that the natural rate of soil formation is spectacularly slow; it takes literally thousands of years for a mature soil to develop. But within just a few years, or decades, humans can completely transform the structure, chemistry, and biology of soils, often leading to their degradation. This degradation of soil can be catastrophic, for example when soils are over-cultivated or overgrazed, or when unstable hill slopes are deforested and left exposed to the erosive forces of wind and rain. Or it can be progressive, such as that caused by climate warming which, in some places, such as the Arctic, is gradually speeding up organic-matter decay and carbon dioxide release from soils. It can also be abrupt, such as when land is sealed by asphalt and concrete during the construction of expanding cities, or during war when major offensives obliterate the fabric of soil. As I stressed earlier in this book, the causes of soil degradation are complex: population growth, poverty, poor delivery of information to farmers, conflict, shortage of land, and climate change all play a role.

Soil and Climate Change

The world’s climate is changing. Not only is it getting warmer, but also there are more extreme weather events, such as droughts, storms, and catastrophic floods. Humans are undoubtedly the cause of this change in climate, through the burning of fossil fuels, intensive farming, deforestation, and many other aspects of our industrious lives that increase the emission of greenhouse gases—carbon dioxide, methane, and nitrous oxide—to the atmosphere. In fact, over the past fifty years or so there has been an unprecedented increase in the release of greenhouse gases to the atmosphere, and, unless measures are put in place to cap emissions, this trend is likely to continue. So what have soils got to do with climate change? Put simply, soils play a pivotal role because they act as both a source and sink for greenhouse gases, and any disruption of this balance will affect the concentration of these gases in the atmosphere and hence the global climate, potentially making the situation either better or worse. Perhaps the most powerful illustration of this concerns the carbon cycle. Soil is the Earth’s third largest carbon store, next to the oceans and deep deposits of fossil fuels, and together with vegetation it contains at least three times more carbon than the atmosphere. Many worry that climate change will destabilize these carbon stores by stimulating the soil organisms that break down soil organic matter, releasing vast quantities of carbon dioxide to the atmosphere. This could shift soils from being sinks to sources of this greenhouse gas, thereby accelerating climate change. Scientists call this carbon-cycle feedback, and we will revisit it later. Let’s begin with the main actors of climate change, the greenhouse gases. The most abundant and well-known greenhouse gas is carbon dioxide. This gas is taken up from the atmosphere by plants through the process of photosynthesis, which occurs in the presence of light. Plants retain most of the carbon they take up and use it to grow and sustain their metabolism, but they also release a portion back to the atmosphere as carbon dioxide through respiration from both their shoots and roots.

Soil and the Grower

Throughout history few things have mattered more to humans than their relationship with soil. This is a bold statement, but since the dawn of civilization, the use of soil to grow crops has been of central importance to mankind. Not only have past civilizations relied on fertile soils to fuel their prosperity and growth, but also neglect of soil, leading to its degradation, has in many cases led to their collapse. Unfortunately, soil neglect isn’t just a thing of the past. Today, at a time when the need to produce food for a growing world population couldn’t be more acute, vast tracts of once productive land lie degraded and struggling to grow crops or support livestock. The causes of soil degradation are complex, with population growth, poverty, poor delivery of information to farmers, conflict, shortage of land, and climate change all playing a role. Whatever the cause, the consequences are the same: soil degradation causes food shortages, poverty, and hunger. But I don’t just want to discuss the neglect of soil and its dire consequences for mankind. I also want to consider what makes a soil fertile and able to support healthy crops year after year, and how soil can leave its fingerprint on the quality and taste of what we grow. I also want to consider some of the ingenious ways that humans have devised to maintain soil fertility and boost crop yields, and how this knowledge can be harnessed to restore degraded soils. Before I do, however, an important point to make is that the concept of soil fertility is largely agronomic: it relates to the ability of soil to sustain the growth of agricultural crops through the continued provision of nutrients, water, and anchorage. Soils that support some of the most valued natural habitats on Earth, such as pristine tropical forests or expansive arctic tundra, are very infertile from an agricultural point of view; tropical forest soils are highly weathered and nutrient-poor, whereas tundra soils are wet, acidic, and low in plant-available nutrients.

Soil and the Distant Past

Rainbow Beach is a small town on the coastal dunes of eastern Australia, near Brisbane. I had travelled there to meet with some colleagues to sample soils from the vast coastal sand dunes that surround the area. It might seem an unusual place to visit to collect soil, but a unique sequence of soils has formed in the sand dunes, which differ greatly in age. As you move inland from the sea, the soils get progressively older and deeper, and more weathered and nutrient-poor. The youngest soils are shallow, having only just started to form in recent sand dunes, whereas the oldest soils are around half a million years old and can reach 25 metres deep. These are among the oldest, deepest, and most weathered soils that I have sampled, and what I recall most vividly is how stunted and sparse the vegetation was that grew there, reflecting their struggle to grow in such ancient, weathered soil. The soils of Rainbow Beach are by no means the oldest on Earth. Hidden beneath ice sheets in Greenland, scientists recently discovered a soil that was 2.7 million years old, a remnant of the fertile tundra that covered the area before the ice sheets came. And scientists working in South Africa recently discovered a soil, now compacted in rock, that is 3 billion years old. One of the most fascinating things about soil is that it is incredibly diverse; soils vary enormously across continents, countries, and from valley to valley and field to field. Even within a small patch of land, such as a field, forest, or vegetable garden, the underlying soil can vary considerably. Over distances of metres, it might differ in its texture and depth, and in its pH, being acid in one patch of a field and neutral in another. Soils also vary greatly in the diversity of living organisms that live within them. I will go into more detail about the diversity of soil life later in this book; but for now suffice to say that it is vast. Soils also change with time.