The bellwether, midwest state of Iowa is often described as the "American heartland".

It is the kind of place that even those who are born there find hard to hymn. Bill Bryson, who hails from its capital, Des Moines ("somebody had to"), described the natives as friendly ("everybody you meet acts like he would gladly give you his last beer"), "dopey", and "a tad slow" ("it's not because they're incapable of high-speed mental activity, it's only that there's not much call for it"). Iowans' wits, added Bryson, "are dulled by simple, wholesome faith in God and the soil and their fellow man".

This, then, is the inauspicious setting for the first volume of an ambitious trilogy by Jane Smiley, who is perhaps best known for her Iowa-based bestseller, A Thousand Acres, a retelling of King Lear which in 1992 won the Pulitzer Prize. In Some Luck we follow the progress of the Langdon family year by year, each of which is given its own chapter, from 1920 to 1952. Usually not a lot happens, other than the planting and harvesting of crops and the supplanting of horses with a tractor. Babies are born, grow up and go to school. World events - the Wall Street crash, the drought in the 1930s that turned swathes of America into a dust bowl, the Second World War - mostly happen offstage. Getting by rather than thriving is the imperative.

Through all of this Walter and Rosanna Langdon must shepherd their children, not all of whom will make it to adulthood. It is a quintessential American story in which love of the land is tempered with the siren call of the cities, where the possibilities are endless and the temptations are many. In the two volumes to come, Smiley will follow the Langdons into the 21st century, in the course of which change will not come dropping slow but like a drone, hurtling out of nowhere.

We meet in a hotel in central London just a few hours after Smiley had flown in from her home in California. She is 65, rangy and flaxen-haired, dressed in slacks and a sleeveless sweater, with a ready laugh and an engaging manner. Unlike many of the people she writes about, she is not a native of Iowa though, as she says, she did spend 24 years there, teaching at one of its colleges.

"Most Americans do have a mental picture of Iowa because of the Iowa caucuses and presidential races," she says. "And also, when you think about farming, that's where people think about. The other thing about Iowa is people leave."

Often, she adds, this has to do with inheritance, an overarching theme of A Thousand Acres. "There's always the wrong number of heirs to a farm. The Amish solve the problem by leaving the farm to the youngest rather than the oldest. That means that dad keeps farming on his farm as long as he can. So the oldest one has to go take up a trade and the youngest one, by the time dad is basically done for, is ready to take over."

As Smiley tells it, farming is not for the faint-hearted. Over the decades covered in Some Luck, the Langdons' farm grows in size and is slowly mechanised and professionalised and chemically fertilised. After one tragedy Rosanna finds religion and is "saved". Walter, meanwhile, two of whose brothers died in infancy while another was claimed in a flu epidemic, "comes across as a little depressed. Or maybe even a little depressive."

Not only is the work back-breaking and tying, the farm can be a dangerous environment where, if your mind's not on the job, it is easy to lose a limb or even your life. Frank, the eldest of the Langdons' children, wants to get away as soon as he can. He can see that if he does not he could be tied to it for ever, the consequences of which could be dismal and tragic.

"The thing you notice when you're in Iowa," says Smiley, "is people don't complain. They also don't talk much about their struggles or their inner life. They talk about practical things. In Minnesota, if you're worrying or arguing, you get in your car and drive around until you've gotten over it. There's no reason to complain in Iowa because there's too many things to complain about. I say that as the wife of a man from Philadelphia! If you let things get to you, you'll talk yourself into too many mood swings, too many ups and downs."

Smiley laughs, but as she is well aware farming is often no laughing matter. Suicide among farmers is relatively common, of which there is one instance in Some Luck. People who commit suicide, she says, may have gone years and years without saying much, bottling up their feelings. Once, she says, someone might recall them saying that they'd wished they'd gone to Chicago. "But only one time." It was the sole clue to their state of mind. "You keep it to yourself, that's small town life."

Smiley's upbringing was in a suburb of St Louis, Missouri. The writing bug may have been caught from her mother, a journalist. After graduating from Ivy League Vassar, Smiley spent a year hitchhiking around Europe with the first of her four husbands, to all of whom Some Luck is dedicated. She seems more than happy to give the lowdown on each.

"My first husband - we were together about three and a half years. We were very tall and neither of us had really dated anybody. He's six feet 10 and I'm six feet two. I'd at least gone out once or twice [with men]. I don't think he'd gone out with anybody. So I sort of plucked him off the basketball team. He could do this overarm plug shot from way out in the centre court and in it would go. He was the perfect tall girl's intellectual boyfriend and we're still on quite good terms.

"My second husband and I had kids so we were together about seven years. But we were ill-suited, which we both now recognise.

"My third husband had been a previous boyfriend. I thought that was the last one but he left me for a dental hygienist ... I think he would agree now that she had her eye on him, thinking that he had a lot of money."

And what about husband number four, the one she describes as "current"? Smiley laughs again, takes a sip of her tea and continues.

"He came along because I lived on this large property that required a lot of maintenance ... He was recommended by some friends. I went and met him and he had this table-saw and all of his fingers. He knew how to use it! He was a complete innocent in terms of intellectual attainments, because he'd been dyslexic as a child, but he was actually quite funny, and smart and good with his hands."

Such attributes, as Smiley, a sometime breeder of horses, acknowledges, are not to be sniffed at. A couple of her horses made it to the racetrack. Did they do okay? "No, but they survived." Apparently, one of the two let its jockey know that it didn't want to race any more because it didn't like to feel pain. "So we knew he was too smart to be a race horse. We decided that was a good insight into his personality."

As, indeed, are such anecdotes into Smiley's. She is a writer whose work exhibits an uncommon ability to switch literary horses, having written a detective story, a romance, a comedy and a tragedy. Her novels, like those of Balzac and Zola, and her beloved Dickens and Trollope, are symptomatic of an inquiring, restless mind.

Smiley, you feel, is interested in stuff that is best learnt firsthand. Reading Some Luck, you realise that when she writes about "detasseling the corn" or dealing with unwanted, worm-ridden puppies or that you must fix fences immediately to stop hogs escaping, she knows exactly what she's talking about. Certainly, hers is an Iowa that holds you fiercely in its grip and from which you've no desire to make an escape.

Some Luck is published by Mantle at £18.99