The skin of life denuded

Soil erosion is a threat akin to global warming, says Graham Elmes

The scourge of malaria ruined many a life in Ancient Rome, but it was largely a man-made problem. In 500BC, the Roman Republic was built around small labour-intensive farms, and as Rome grew people had less time for farming and turned to the plough. Ploughing greatly increases soil erosion. At first the farmers barely noticed the loss of soil, which slowly silted up the rivers around Rome, but by 200BC the problem was obvious. Many hillside farms were mostly bare rock, the notorious Pontine Marshes had formed, malaria was endemic and the falling food production spurred Roman expansionism.

This example of an ancient civilisation held hostage by its agriculture is one of several outlined by David Montgomery in Dirt: The Erosion of Civilizations . However, it is not a history book. Rather, it aims to show how fragile the world's soil system is, and in a low-key way it outlines yet another environmental crisis courted by modern society.

The first chapter, "Good old dirt", sets the scene. The second chapter, "Skin of the Earth", puts soil into biological perspective - "Soil truly is the skin of the Earth, the frontier between geology and biology" - and asks whether we should be worried that we are "stripping the skin" from our planet. Of course we should worry, because without soil there would be very few higher plants, and without plants there would be very few animals and certainly no humans.

We take the atmosphere and soil for granted because they are always there. The greenhouse gas debate changed the former, and with this book Montgomery hopes to change the latter. Soils are dynamic. Over time, exposed rock weathers into small particles that, without plant cover, would slowly wash down the hillside into streams, rivers and finally the sea, where eventually they would reform into sedimentary rocks. Vegetation slows the rate of transport, plants hold some of the sand in place, and when they die their decaying remains are incorporated, and gradually fertile topsoil is accumulated. Earthworms and other animals turn it over, slowly burying remains of human activity.

Since we stopped being part of the natural ecosystem as hunter-gatherers and turned to farming, we have slowly used up the accumulated soil resource, just as in recent times we have used up fossil fuels. In theory, small groups farming at low intensity can be neutral in the soil cycle. Unfortunately, the more productive groups thrive, increase and develop culturally, inevitably demanding more efficient cultivation, hence the plough. Therein lies the rub - this great agricultural invention increases erosion, as seen in the Roman example. The Romans became very sophisticated farmers who believed in technological solutions but they could only slow the process of erosion, not stop it.

The next seven chapters chart similar, even more dramatic self-induced calamities that various civilisations brought down upon themselves. "Rivers of life" explores how great civilisations were established and then flourished on the fertile flood plains of great rivers where fresh soil and minerals are provided by annual floods. This mode of agriculture persists only when the crop cycle is in harmony with the flood cycle. As soon as crops needing prolonged irrigation are grown, salty minerals accumulate, destroying the soil fertility. This is happening today in Egypt after millennia of successful agriculture.

Chapters four and five document the role of agriculture and soil in the building of empires and the exploitation of colonies ranging from the Greek and Roman, through the Mayan, British and other European expansions. Many of the ancient empires simply conquered and enslaved other people whose strong agricultures were literally ripe for picking.

The next couple of chapters recall the different story that unfolded when virgin territory was annexed by people equipped with modern technology. The number of colonists in the first states of America increased at a time when the fertility and productivity of the farms of New England, won by forest learance, declined and the tobacco-driven slave economies of the southern states demanded ever more fresh land. Farmers moved west into the lush prairie grasslands where the light soils, held in place by the grass, dried out, denatured and blew away after some years of ploughing.

By the 1930s, great dust storms denuded some farms and buried others under dune-like mounds. On May 9, 1934, a third of a billion tons of topsoil from Montana and Wyoming was carried eastwards at 100mph, shedding dust over the eastern states. The resulting starvation and mass migration to California are well known from American novels and plays of that period. Yet the causes and consequences were not unanticipated. Montgomery records how many agriculturalists and scientists in the US could see disaster looming, yet at the time it affected too few people directly for authority to bother to do much to prevent it.

If dirt is not washed or blown away, then gradually its nutrients are exhausted. The only recourse is to add fertilisers - either our own dung, as did the peasant farmers of China, or animal dung, especially the nitrogen-rich bird deposits of guano built up over millennia. When this was exhausted, we developed artificial fertilisers that take a lot of energy to produce.

Chapter eight, "Dirty business", highlights some consequences of artificial fertilisation, including some criminally irresponsible practices. In the 1990s a resident of Quincy, Washington State, discovered that heavy-metal toxic waste was mixed with fertiliser and sold very cheaply to farmers who poisoned their soil for who knows how long. This case was the tip of an iceberg in the US, and it was probably repeated elsewhere.

Closed island systems can graphically illustrate ecological cause and consequence, and "Islands in time" describes the over-exploitation of soil by several island cultures. Sadly, they are exemplars of what could happen on a global scale - after all, what is Earth but a closed system?

In the final chapter "Lifespan of civilisations", Montgomery pulls together some of the common threads from the previous chapters and suggests how we may slow erosion by using alternative methods of agriculture. Currently soil is lost from agricultural land 40 times faster than by natural erosion. One hardly needs to be a specialist to grasp that current practices are unsustainable and that using any land to grow biofuels is almost as irresponsible as continuing to burn the fossil fuels they replace.

Anyone interested in environmental issues should read this book. It is not too technical, and it succeeds in reinforcing the sense of urgency for major changes if we are to achieve a sustainable human presence on Earth. The way in which Montgomery draws together ideas from agriculture, geology and social history might be criticised by some specialists, but the great strength of the book is that it entertains and stimulates thought in non-specialists.

Graham W. Elmes is a CEH research fellow based at NERC's CEH Wallingford laboratories.