I've read a similar claim in The Omnivore's Dilemma by Michael Pollan. According to Pollan, most fertilizer is manufactured with petroleum-based energy, so our agriculture is driven by fossil fuel consumption; ie natural gas -> fertilizers -> corn. This is not necessarily a vegan agenda; Pollan says we should move to solar-based agriculture by feeding animals grass instead of corn, which should incidentally make the animals tastier and more nutritious.