I WAKE UP staring into the bluest blue I've ever seen. I must have fallen into a deep sleep because I need several seconds to realize that I'm looking at the Arizona sky, that the pillow beneath my head is a large clump of dirt, and that a near-stranger named Manuel is standing over me, smiling. I pull myself to a sitting position. To my left, in the distance, a Border Patrol helicopter is hovering. To my right is Mexico, separated by only a few fields of lettuce. "Buenos días," Manuel says.

I stand up gingerly. It's only my third day in the fields, but already my 30-year-old body is failing me. I feel like someone has dropped a log on my back. And then piled that log onto a truck with many other logs, and driven that truck over my thighs. "Let's go," I say, trying to sound energetic as I fall in line behind Manuel, stumbling across rows of lettuce and thinking about "the five-day rule." The five-day rule, according to Manuel, is simple: Survive the first five days and you'll be fine. He's been a farmworker for almost two decades, so he should know. I'm on day three of five — the goal is within sight. Of course, another way to look at my situation is that I'm on day three of what I promised myself would be a two-month immersion in the work life of the people who do a job that most Americans won't do. But thinking about the next seven weeks doesn't benefit anyone. Day three of five.

"Manuel! Gabriel! Let's go! ¡Vámonos!" yells Pedro, our foreman. Our short break is over. Two dozen crew members standing near the lettuce machine are already putting on gloves and sharpening knives. Manuel and I hustle toward the machine, grab our own knives from a box of chlorinated water, and set up in neighboring rows, just as the machine starts moving slowly down another endless field.

SINCE THE EARLY 1980s, Yuma, Ariz., has been the "winter lettuce capital" of America. Each winter, when the weather turns cold in Salinas, Calif. — the heart of the nation's lettuce industry — temperatures in sunny Yuma are still in the 70s and 80s. At the height of Yuma's growing season, the fields surrounding the city produce virtually all of the iceberg lettuce and 90 percent of the leafy green vegetables consumed in the United States and Canada.

America's lettuce industry actually needs people like me. Before applying for fieldwork at the local Dole headquarters, I came across several articles describing the causes of a farmworker shortage. The stories cited an aging workforce, immigration crackdowns, and long delays at the border that discourage workers with green cards who would otherwise commute to the fields from their Mexican homes. Wages have been rising somewhat in response to the demand for laborers (one prominent member of the local growers association tells me average pay is now between $10 and $12 an hour), but it's widely assumed that most U.S. citizens wouldn't do the work at any price. Arizona's own Sen. John McCain created a stir in 2006 when he issued a challenge to a group of union members in Washington, D.C. "I'll offer anybody here $50 an hour if you'll go pick lettuce in Yuma this season, and pick for the whole season," he said. Amid jeers, he didn't back down, telling the audience, "You can't do it, my friends."

On my first day I discover that even putting on a lettuce cutter's uniform is challenging (no fieldworkers, I learn, "pick" lettuce). First, I'm handed a pair of black galoshes to go over my shoes. Next comes the gancho, an S-shaped hook that slips over my belt to hold packets of plastic bags. A white glove goes on my right hand, a gray glove, supposedly designed to offer protection from cuts, goes on my left. Over the cloth gloves I pull on a pair of latex gloves. I put on a black hairnet, my baseball cap, and a pair of protective sunglasses. Adding to my belt a long leather sheath, I'm good to go. I feel ridiculous.

The crew is already working in the field when Pedro walks me out to them and introduces me to Manuel. Manuel is holding an 18-inch knife in his hand. "Manuel has been cutting for many years, so watch him to see how it's done," Pedro says. Then he walks away. Manuel resumes cutting, following a machine that rolls along just ahead of the crew. Every several seconds Manuel bends down, grabs a head of iceberg lettuce with his left hand, and makes a quick cut with the knife in his right hand, separating the lettuce from its roots. Next, he lifts the lettuce to his stomach and makes a second cut, trimming the trunk. He shakes the lettuce, letting the outer leaves fall to the ground. With the blade still in his hand, he then brings the lettuce toward the gancho at his waist, and with a flick of the wrist the head is bagged and dropped onto one of the machine's extensions. Manuel does this over and over again, explaining each movement. "It's not so hard," he says. Five minutes later, Pedro reappears and tells me to grab a knife. Manuel points to a head of lettuce. "Try this one," he says.

I bend over, noticing that most of the crew has turned to watch. I take my knife and make a tentative sawing motion where I assume the trunk to be, though I'm really just guessing. Grabbing the head with my left hand, I straighten up, doing my best to imitate Manuel. Only my lettuce head doesn't move; it's still securely connected to the soil. Pedro steps in. "When you make the first cut, it is like you are stabbing the lettuce." He makes a quick jabbing action. "You want to aim for the center of the lettuce, where the trunk is," he says.

Ten minutes later, after a couple of other discouraging moments, I've cut maybe 20 heads of lettuce and am already feeling pretty accomplished. I'm not perfect: If I don't stoop far enough, my stab — instead of landing an inch above the ground — goes right through the head of lettuce, ruining it entirely. The greatest difficulty, though, is in the trimming. I had no idea that a head of lettuce was so humongous. In order to get it into a shape that can be bagged, I trim and trim and trim, but it's taking me upward of a minute to do what Manuel does in several seconds.

Pedro offers me a suggestion. "Act like the lettuce is a bomb," he says. "Imagine you've only got five seconds to get rid of it."

Surprisingly, that thought seems to work, and I'm able to greatly increase my speed. For a minute or two I feel euphoric. "Look at me!" I want to shout at Pedro; I'm in the zone. But the woman who is packing the lettuce into boxes soon swivels around to face me. "Look, this lettuce is no good." She's right: I've cut the trunk too high, breaking off dozens of good leaves, which will quickly turn brown because they're attached to nothing. With her left hand she holds the bag up, and with her right she smashes it violently, making a loud pop. She turns the bag over and the massacred lettuce falls to the ground. She does the same for the three other bags I've placed on the extension. "It's okay," Manuel tells me. "You shouldn't try to go too fast when you're beginning." Pedro seconds him. "That's right. Make sure the cuts are precise and that you don't rush."

So I am to be very careful and precise, while also treating the lettuce like a bomb that must be tossed aside after five seconds.

THAT FIRST WEEK on the job was one thing. By midway into week two, it isn't clear to me what more I can do to keep up with the rest of the crew. I know the techniques by this time and am moving as fast as my body will permit. Yet I need to somehow double my current output to hold my own. I'm able to cut only one row at a time while Manuel is cutting two. Our fastest cutter, Julio, meanwhile can handle three. But how someone could cut two rows for an hour — much less an entire day — is beyond me. "Oh, you will get it," Pedro tells me one day. "You will most definitely get it." Maybe he's trying to be hopeful or inspiring, but it comes across as a threat.

That feeling aside, what strikes me about our 31-member crew is how quickly they have welcomed me as one of their own. I encountered some suspicion at first, but it didn't last. Simply showing up on the second day seemed to be proof enough that I was there to work. When I faltered in the field and fell behind, hands would come across from adjacent rows to grab a head or two of my lettuce so I could catch up. People whose names I didn't yet know would ask me how I was holding up, reminding me that it would get easier as time went by. If I took a seat alone during a break, someone would call me into their group and offer a homemade taco or two.

TWO MONTHS IN, I make the mistake of calling in sick one Thursday. The day before, I put my left hand too low on a head of lettuce. When I punched my blade through the stem, the knife struck my middle finger. Thanks to the gloves, my skin wasn't even broken, but the finger instantly turned purple. I took two painkillers to get through the afternoon, but when I wake the next morning it is still throbbing. With one call to an answering machine that morning, and another the next day, I create my own four-day weekend.

The surprise is that when I return on Monday, feeling recuperated, I wind up having the hardest day of my brief career in lettuce. Within hours, my hands feel weaker than ever. By quitting time — some 10 hours after our day started — I feel like I'm going to vomit from exhaustion. A theory forms in my mind. Early in the season — say, after the first week — a farmworker's body get thoroughly broken down. Back, legs, and arms grow sore, hands and feet swell up. A tolerance for the pain is developed, though, and two-day weekends provide just enough time for the body to recover from the trauma. My four-day break had been too long; my body actually began to recuperate, and it wanted more time to continue. Instead, it was thrown right back into the mix and rebelled. Only on my second day back did my body recover that middle ground. "I don't think the soreness goes away," I say to Manuel and two other co-workers one day. "You just forget what it's like not to be sore." Manuel, who's 37, considers this. "That's true, that's true," he says. "It always takes a few weeks at the end of the year to get back to normal, to recover."

AN OLDER CO-WORKER, Mateo, is the one who eventually guesses that I have joined the crew because I want to write about it. "That is good," he says over coffee at his home one Sunday. "Americans should know the hard work that Mexicans do in this country."

Mateo is an unusual case. There aren't many other farmworkers who are still in the fields when they reach their 50s. It's simply not possible to do this work for decades and not suffer a permanently hunched back, or crooked fingers, or hands so swollen that they look as if someone has attached a valve to a finger and pumped vigorously. The punishing nature of the work helps explain why farmworkers don't live very long; the National Migrant Resources Program puts their life expectancy at 49 years.

"Are you cutting two rows yet?" Mateo asks me. "Yes, more or less," I say. "I thought I'd be better by now." Mateo shakes his head. "It takes a long time to learn how to really cut lettuce. It's not something that you learn after only one season. Three, maybe four seasons — then you start understanding how to really work with lettuce."

From the book Working in the Shadows: A Year of Doing the Jobs That (Most) Americans Won't Do ©2010 by Gabriel Thompson. Excerpted by arrangement with Nation Books, a member of the Perseus Books Group.