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Enjoying the sort of adulation usually reserved for rock stars, Michael Pollan., author of In Defense of Food spoke to a standing-room-only crowd of about 900 last night at the Enoch Pratt Free Library in Baltimore.

Pollan elaborated on the themes he explored in In Defense of Food, which opens with this sage advice: “Eat food. Not too much. Mostly plants.”

Pollan claimed that the last sentence incurred him the wrath not just of the meat industry but also of vegetarians. He said this is typical of the kind of Manichean thinking that has come to predominate thinking about food in this country. “Why must everything be in black and white? What’s wrong with a little meat?”

Of course, it all depends on how that meat was raised. Pollan described visiting a feedlot, standing knee-deep in cow manure, viewing the sick and miserable animals all around him, and said the experience would be enough to make most people swear off meat forever. Still, he reminded the audience that the best way to extract protein from some kinds of land, such as rocky, hilly terrain, may be to pasture animals on it.

But he also counseled the audience to be skeptical of such labels as “free-range.” He related an amusing anecdote in which he visited one such “free-range” chicken producer in California. Some 40,000 chickens were confined in a giant concrete shed the length of two football fields. How is this free-range? he wanted to know. His host pointed out to him two tiny doors, one at each end of the enormous building.

Why aren’t they open? he asked. His host assured him that the doors would be opened once the chickens reached the age of five weeks.

How long do they live? he queried. Seven weeks, he was told.

Oooooh-kay. What happens when the doors are opened? Nothing, came the reply. The chickens have spent their entire life indoors. That’s where the food is. Why would they want to go outside? Pollan advised the audience to look for pastured chicken, which are kept outdoors all heir lives.

Pollan averred that modern petroleum-intensive meat production is a major source of greenhouse gas emissions, and that if we would all enjoy one meatless day a week, that would be equivalent to taking twenty million cars off the road. But he had high praise for grass-finished beef, which he said was one of the most sustainable ways to produce meat. He also reminded the audience that grass-fed beef is not the same thing as grass-finished beef. All cows are fed grass until they are six months old. Grass-finished cows dine on grass their entire lives.

Pollan noted that when cows are put out to pasture, an amazing thing happens. The grass, which initially is about 18 inches in height, gets clipped to a length of about two inches. The plant compensates for this by killing off a comparable amount of root mass, which then becomes food for earthworms, beetle grubs, fungi, and soil bacteria. This is how soil is built up. He reminded us that the activities of the vast herds of bison that once roamed the Great Plains built up a layer of topsoil over ten feet thick. And that is how carbon gets sequestered, slowing down the process of global warming.

Today, of course, most cattle feed on corn and soybeans harvested from giant industrial-scale farms that now cover most of the Great Plains. Growing this corn and soybeans is an activity that consumes huge amounts of petroleum, contributing to global warming. It also requires huge amounts of chemical fertilizers, much of which run off into the Mississippi River to the Gulf of Mexico, creating a dead zone which by now is larger than the entire state of Delaware. Pollan said one of the best things we could do for the environment would be to return that land to pasture and graze cows on it.

Pollan spoke highly of President Obama, who he described as very aware of how modern industrial-scale agriculture contributes to the environmental crisis, not to mention the public-health crisis. He also had kind words for the First Lady, who planted a garden on the White House Lawn that she publicly identified as an “Organic garden” and in doing so incurred the wrath of the pesticide industry.

But he described the President as being wary of expending his political capital on this issue, without a strong base of popular support. He said Obama once told someone who questioned him about such matters, “Show me a movement.”

Last night looked like a pretty good start.