When National Geographic caught up with Bogard by phone at his home in Minnesota, the author explained why Iowa is the most transformed state in the U.S., how soil is alive but we’re killing it, and how places where terrible things happened can become sacred ground.

You write, “We are only just now beginning to understand the vast life in the soil, what it does, and how our activities on the surface may affect it.” Talk us through some highlights of the new science—and how you became so passionate about dirt.

It began with this statistic: that those of us in the Western world now spend about 90-95 percent of our time inside, in our houses, work places, in our cars. We’re living our lives separated from the natural world. When we walk outside, many of us walk on pavement. There’s this literal separation from the natural ground, from the soil, the dirt. It made me think, what are the costs of this separation? And it struck me as symbolic of our separation of these many different kinds of grounds that sustain us. Our food, water, energy, even our spirits come from these different grounds.

One of the first scientific discoveries I found was the hypothesis that human beings need to be exposed to the biota in the dirt, in the ground, especially when they’re kids, as a way of inoculating us to diseases that appear later in life. Kids these days are not being exposed to dirt because they’re not allowed to play outside. Their parents think dirt is dirty. But both the newest science and the oldest traditions tell us the same thing, which is that the ground is alive. The ground gives us life. And in the book I tried to touch on both of those things.

How Dirt Can Help Slow Climate Change Soil can store so much carbon that it can help to curb greenhouse gases.

You begin your journey in Manhattan. Tell us about Eric Sanderson and what new insight his maps of New York can give us.

These are maps of the island as it was when Henry Hudson sailed into New York harbor: the river and natural ground of Manhattan, as it was before it was colonized, urbanized, paved over. Eric was a great person to talk to because he can see the ground beneath us as we’re walking in present-day Manhattan. He can look over and say, “Over there, under that section was a hill,” or “Here, in Times Square, there was a pond. People used to come duck hunting.”

One expert you quote says, “asphalt is the land’s last crop.” Talk about “soil sealing” and how roads and suburbs are literally eating away at the ground beneath our feet.

Soil sealing is one of the most shocking things I learned about. When we pave over the natural ground, we cut it off from the air and water that the life in the ground needs to stay alive. We essentially kill that ground. There is an argument that, if we pulled up the pavement and worked hard to rejuvenate that ground, we could bring it back. But the scientists I talked to said, when you pave it over, it’s the last crop, the last thing that’s going to grow there. We’re not moving in the direction of pulling pavement up. We’re moving in the opposite direction where we’re paving some of our most fertile ground, the ground that we’re going to need to feed a growing population.

One of the casualties is Civil War battlefields—tell us about your search for Ox Hill, in Virginia. And why Gettysburg represents, for you, “hallowed ground.”

The search for Ox Hill was fascinating. In the States, we have a certain number of Civil War battlefields left, which we can still access. But it’s being eaten away every year by development and urban sprawl. You have areas, in this case in Northern Virginia, where important battles in the Civil War took place but they’re being paved over for parking lots, condominiums, and streets.

Ox Hill is a great example of this. It’s not one of the most famous battles, but it was significant because a couple of Union generals were killed and the Confederates were advancing on the capital. The battle took place on what was then rural, sprawling hills. Now, all that’s left is one small park surrounded by streets, condominiums, and shopping malls. It’s not even one city block. It’s like this little postage stamp.

View Images Reenactors demonstrate artillery at Gettysburg National Military Park on the 150th anniversary of the U.S. Civil War battle. The place is hallowed ground, says Bogard, due to the tremendous sacrifice of life for an ideal. Photograph by Matt McClain, The Washington Post via Getty Images

If you ask an American, “What is hallowed ground?” Gettysburg is going to come up fairly quickly. It had a personal connection for me, in that the regiment of soldiers from my home state of Minnesota played an important role in the battle. I was able to go to the exact spot where they made their charge on the second day of the battle, which arguably saved the Union, and resulted in the regiment being almost entirely killed or wounded. To be able to walk down the slope of the field where they charged at the same time of day as they made their charge, was very powerful. As one of the folks I interviewed said, “Why is it hallowed? It’s hallowed because they sacrificed themselves for an ideal, the ideals that we, as Americans, stand for.”

You also had childhood affection for Iowa. But when you went back to research your book, you changed your mind. Why?

As a child I was enamored with the beauty of the green corn stalks, the black dirt, and what I thought was the natural topography. Coming back older and with a new understanding of the ground, it made me uncomfortable because Iowa is the most transformed state in the union. Some 97 percent of the natural ground has been altered, changed, or transformed. As one biologist said, “it’s an open air monoculture owned by monopolies.” So, instead of my romantic, childhood view of miles of corn stalks, the beauty of life growing, and the color green, I saw it as this monoculture where other life isn’t allowed to grow.

View Images A field of corn is ready for harvest in Iowa, where 97 percent of the natural ground has been altered. Photograph by Design Pics Inc, Alamy

The Prairie grasslands have been called “the most decimated ecosystem in continental North America.” But there are still some pockets of hope. Tell us about the Sandhills of Nebraska and its famous “potholes.”

The pothole prairie region is vitally important for many reasons, one of which is the duck population. Ducks that migrate through rely on that unique ground. Many of the potholes you see most of the year look just like little dips or swales in the ground. But at certain times of the year, they fill with water and invertebrate life, and the ducks come through right when that happens. To the uneducated eye it looks like you’re in the middle of nowhere. But you’re actually in the middle of an incredibly alive and life-sustaining ground for wildlife.

Cranes also use the area in the Sandhills of Nebraska, where they migrate through the Platte River. There’s a bottleneck of their migration, coming down from the North and up from the South, and you get some half million cranes coming through in March and April every year. I got up at dawn to see them. It was something I wanted to see because the sight and sound of these ancient birds, which have been making this migration for eons, is a magical, wonderful sight.

Americans love their lawns and spend billions of dollars keeping them green and weed free. But we are also paying a high price for this perfect turf, aren’t we?

Oh my! We really are, certainly ecologically, paying a high price. America’s greatest crop, the thing we grow the most of, is our turf grass lawns. And the amounts of pesticides and chemical fertilizers we dump onto these lawns, and the amount of water that we use to grow them, is enormous. As a result, we have problems with runoff draining into our rivers and the lawns themselves tend to become monocultures, where nothing else grows but the turf grass. What a massive opportunity is being lost! We could have lawns that are more biologically diverse and pollinator-friendly. There’s also evidence that a number of illnesses are associated with coming into contact with these chemical fertilizers and pesticides.

Golf courses are often regarded as the worst of the worst in their over-application of chemical pesticides and water, but interestingly I found that many of them are working overtime to conserve water and only apply chemicals to their turf when needed. The golf superintendents I talked to say we need to look at ourselves and our houses and the way we’re over-watering and over-using chemical fertilizers. Now, a lot of them are focused on how they can make golf courses into wildlife habitats and models in their use of water and careful pesticide application.

A Nazi extermination camp does not seem, at first glance, a promising place to discuss the term “sacred ground.” Talk about your journey to Treblinka.

That was something I hadn’t expected. My original motivation for going to Treblinka was that I wanted to have a first-hand experience of what it felt like to be on that ground where 900,000 people had been murdered. What’s remarkable about Treblinka is that, even now, there isn’t much in the way of memorial or instruction for your imagination on what to think and feel. You come into this clearing in the forest and ask your imagination to please picture what you know happened here. And that’s quite an experience.

The way I think about sacred ground came in reflection after this powerful experience. I also travelled to places that are designated as sacred, like cathedrals, on that trip. But the place that felt most sacred to me was Treblinka. It was the idea that sacred ground is ground that gives us a chance to understand the truth and power of what sustains us. In this case, it was the connections between human beings. Later, when I went to Alaska, it was the truth of our connection with animals and nature. For me, sacred ground is a place that instructs us about these connections that sustain us.

You end your journey back home in northern Minnesota. Why is that place so special to you? And what can our readers do to discover a bit of “sacred ground” in their own lives?

It’s special to me because this is the place where I buried my friend, my dog, who had been with me for 15 years. Our family also has a small house on a lake that I’ve been to all my life. It’s the most important place in the world for me. As I buried my friend there I realized this is my sacred ground, the most important ground I know, and I wanted her to be there forever. It’s the place that sustains me the most, spiritually.

Sacred ground is available to us in many places. I want to say everywhere. It depends upon the person. We can find the sacred in our backyards, in nature, or in the parks near our houses. It’s about understanding how this ground you’re on sustains you and helps you understand the connections that keep you alive. When we are spending 90-95 percent of our time inside now, the first step is to get outside onto natural ground—and go from there.

This interview was edited for length and clarity.