As a young undergraduate studying agriculture, soil science was not one of my favourite subjects, presented by lecturer who was so shy he couldn’t even bear to face the class when supposedly talking to them. Something about colloidal suspensions is just about all that I remember of the science bit, but I did pick up a sense that Australia was not so much farming as mining its soils. Rising salinity along the breadbasket Murray-Darling basin as ancient salts were brought to the surface by irrigation water; erosion from the impact of the hard-hoofed cattle and sheep brought with white settlement; desert winds sweeping away marginal lands being ploughed up for wheat.

It was only when, as a journalist, I visited a permaculture farmer, who went into the middle of a wheat field, yellow and crispy, about ready to harvest, and with no appreciable effort pushed a spade into the earth, turning over dark, moist, hummous-rich soil crawling with earthworms (considerably better than I would have done in my own garden) that I started to grasp how alternative approaches to the industrial farming about which I had been taught might have a point. And that the soil was the foundation of all, and that the chemicals, the minerals balanced out was only the start – that soil was indeed a whole complex ecosystem of its own – microbes, invertebrates and all, a complex mix that wasn’t going to be “fixed” by getting the nitrogen/phophorous balance right.

The introductory chapter of David R. Montgomery’s Dirt: The Erosion of Civilzation cover those basics nicely, beginning with the touching tale of how Charles Darwin, in his last book, working with his sons, demonstrated the vital role of earthworms in soil formation and the maintenance of soil fertility, concluding that “all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canal of worms”. (This despite some scorn about the “minor” issue on which he was working.) He calculated, more or less correctly, that English and Scots worms moved half a billion tonnes of soil a year. (Although, Montgomery points out, he didn’t know about isostasy, the process by which rocks lift to replace those eroded, so his figures on the age of mountains were wildly out.)

From that foundation, Dirt moves on to exploring the place of soil in early human civilisation. It includes the latest research in archaeology and genetics — in its account of the origins of agriculture, which Montgomery places at the headwaters of the Euphrates from about 10,000BC, and finds best explored around the settlement known as Abu Hureyra. For the 3,000 years before this it had been an Edenic environment of oak forests interspersed with stands of wild grain, he says, but when there was a sudden dry period, the communities of hunter-gatherers who had become settled there chose to begin to domesticate wild varieties of rye and wheat. After about 1,000 years rainfall improved, and barley and peas, and other crops were added to the mix, allowing the village population to reach some 5,000 within a couple of thousand years, the largest concentration of humans yet known.

This was the foundation of the great flourishing of Mesopotamia, but it faced two great problems: salinisation from irrigation (just as Australia has done in the past few decades), which did for the Sumerians, and silting up of irrigation channels as the growing population moved into easily eroded upland areas in search of farming land, a huge problem for China for the past couple of thousand years. Such erosion was also a problem in Bronze Age and classical Greece. Montgomery quotes Plato on the region around Athens: “The rich, soft soil has all run away leaving the land nothing but skin and bone.”

If the detail is fascinating, and some of the areas new, this is familiar territory, which Jared Diamond has already popularised. It is when Montgomery moves into the modern era that he reaches new, and frightening, territory.

The effects of inappropriate farming and drought of the American dustbowl era are well known, but Montgomery goes back, and forward, from that. He looks at how tobacco farming, based on slave labour, locked its proponents into a cycle of declining soils (with its clean-till cultivation promoting erosion), which required an expansion across the frontier, which had to be into slave states – otherwise the value of the “asset” (the slaves) would collapse. And slave labour, with unwilling, low-skilled workers, was no way to work to counteract these effects.

And then he looks forward: how after the mechanisation of agriculture after the second world war, the same kind of careless, mass production agriculture, that ploughed across the landscape with no regard to its local characteristics, had much the same effects. In the Palouse region of eastern Washington, a 1950 survey found that all of the original topsoil was missing from 10 per cent of farmland, and between 25 and 75% of the soil was missing from an additional 80% – just 10% was in something like its original condition. Slope is a major factor in this, and less than 50% of US croplands have a slope of 2%, and therefore are at relatively low risk of erosion. “The steepest 33% of US cropland is prjected to fall out of production over the next century.”

And that’s a worry for more than the US. Before the second world war, Montgomery says, western Europe was the only grain-importing region. Latin America produced nearly double the quantity of North America in the 1930s, the Soviet Union was also a major exporter, Africa was self-sufficient. Now, the only major grain exporters are North America, Australia and New Zealand. It is worth quoting the summary of the current state of play:

“Worldwide, over two billion acres of virgin land have been plowed and brought into agricultural use since 1860. Until the last decades of the twentieth century, clearning new land compensated for loss of agricultural land. In the 1980s the total amount of land under cultivation began declining for the first time since farming reached the land between the Tigris and the Euphrates. In the developed world, the rate at which new (and generally marginal) land was brought under cultivation fell below the rate at which land was being exhausted. Although we use a little more than a tenth of the Earth’s land surface to grow crops, and another quarter of the world’s surface for grazing, there is little unused land suitable for either. About the only places left that could be used for agriculture are the tropical forests where thin, highly erodible soils could only briefly support farming….
Acorss the planet, moderate to extreme soil erosion has degraded 1.2 billion hecatres of agricultural land since 1945 – an area the size of China and India combined…. The United Nations estimates that 38% of global cropland has seriously degraded since the Second World War. … average cropland erosion of 10 to a hundred tons per hectare per year removes soil about 10 to a hundred times faster than it forms.”

And then there’s the threat of global warming. The west of the American Midwest breadbasket is already marginal cropping land, while predicted vigour in the hydrological cycle is going to increase soil erosion from rainfall.

These are frightening figures, and figures on which there has been little or no serious focus. It is a pity really that Montgomery didn’t write a book that focused more on these – the historical stuff is fascinating and curious, but that previous civilisations fell because they degraded the land on which they depend is widely known and not generally controversial. That our potential to provide the food a growing world population needs is in question is not widely known, and no doubt will be challenged by those who believe science can do miracles.