This starts with the origin of agriculture (at least in the Middle East), the civilizations based on fertile river valleys in Egypt and Mesopotamia, and the rise and fall of Greece, Rome, and the Maya and Pueblo.
"A common lesson of the ancient empires of the Old and New Worlds is that even innovative adaptations cannot make up for a lack of fertile soil to sustain increased productivity."
Montgomery proceeds to agriculture in medieval and early modern Europe (mostly Britain and France), also covering the history of European deforestation, the importance of colonies in providing food imports and exports of labour, and the political economy of agriculture. Turning to the Americas, the focus is on the 18th and 19th century United States, with glancing references to modern Amazonia: the westward movement of settlers seeking new soils is linked to crop types and political institutions.
"the rapid soil erosion and soil exhaustion produced by slave-based agriculture condemned the institution of slavery to continuous expansion or collapse".
He goes on to describe the dust bowls in the United States, desertification in the Sahel, and the effects of ploughs and mechanisation more generally.
The increasing use of fertilisers, from guano to the Haber-Bosch process and the Green Revolution, is reaching its limits; Montgomery argues for increased use of no-till methods and other organic farming approaches that reduce dependence on fertilisers and improve soil quality.
As in so many domains, islands offer informative case studies: soil erosion played a role in that archetype of environmental disaster, Easter Island, different soils and social systems on Mangaia and Tikopia contributed to different histories, Iceland experienced extreme deforestation and erosion, and in Cuba the state compelled a shift to permaculture after the loss of Soviet support.
In his conclusion Montgomery suggests there are no easy answers to the agricultural problems facing humanity, but reiterates the importance of soils.
"There are three great regions that could sustain intensive mechanized agriculture — the wide expanses of the world's loess belts in the American plains, Europe, and northern China, where thick blankets of easily farmed silt can sustain intensive farming even once the original soil disappears. In the thin soils over rock that characterize most of the rest of the planet, the bottom line is that we have to adapt to the capacity of the soil rather than vice versa. We have to work with the soil as an ecological rather than an industrial system... The future of humanity depends as much on this philosophical realignment as on technical advances in agrotechnology and genetic engineering."
Scattered through Dirt is information about soil scientists and others who studied erosion and lobbied for agricultural reforms: Marcus Cato, George Perkins Marsh, James Hutton, Walter Lowdermilk, and others. Quite a bit of space is also given to political economy, but there is relatively little on technology and economics.
Montgomery doesn't claim soil erosion is responsible for everything, but his enthusiasm leads him to make some rather broad claims which in a few places are simply wrong, as when he asserts that "to this day, the amount of surplus food available to nonfarmers sets the level to which other segments of society can develop". More convincingly, he suggests that many of the historical events attributed to climate change were actually driven by soil erosion.
Some striking halftones provide illustrations, but there are no tables or graphs in Dirt and few maps. In places these might have helped synthesize and integrate scattered details. Going the other way, some micro-geographic studies of individual farms or specific locations might have given a better feel for what actually happens "on the ground".
These are minor quibbles, however. Dirt is an accessible overview of an important topic and deserves a wide readership.