When a society strips vegetation and tills slopes, erosion removes topsoil faster than weathering and biological activity rebuild it, typically depleting the agricultural base over several centuries. Montgomery argues this slow depletion, rather than a single dramatic event, repeatedly undermined the productive foundation of societies from Mesopotamia to Greece, Rome, and Mesoamerica.
Fertile topsoil behaves like inherited capital: it accumulates slowly over millennia but can be spent quickly through farming practices that export it faster than it regenerates. Treating soil as expendable income rather than a slowly replenishing asset lets a society live beyond its agricultural means until the account runs dry.
Viewing political and economic history through the condition of its soil reframes events usually attributed to kings, wars, or ideas as partly consequences of agricultural land use. Famines, migrations, and territorial expansion appear less as choices and more as responses to a steadily eroding resource base beneath farmers' feet.
Rising population pushes cultivation onto steeper, marginal, and forested land, which accelerates erosion and lowers yields, which in turn demands still more land to feed the same people. This self-reinforcing cycle can convert temporary abundance into a trap where each attempt to grow more food degrades the capacity to grow food at all.
Sharing the island of Hispaniola, Haiti deforested and eroded its hillsides into near-barren terrain while the Dominican Republic retained far more forest cover and soil. Montgomery uses this contrast to show that policy and land-management choices, not geography alone, determine whether a landscape stays productive.
Practices that keep soil covered, minimize plowing, and return organic matter can halt erosion and actually increase topsoil depth and fertility within a human lifetime. Montgomery presents this as evidence that soil decline is not an inevitable fate but a reversible outcome of specific farming methods.