Before I meet Andy Goldsworthy, I have a wander round the retrospective of his work being constructed at the Yorkshire Sculpture Park, near Wakefield. Goldsworthy creates moments of wonder out of local rocks and earth and trees, and this wandering prompts several questions, which I jot down in my notebook: are all farm animals abstract expressionists? Is one dry-stone waller's work distinguishable from another's? Just how do you suspend these three oak trees in mid-air below ground in the middle of a field? And, is sheep shit more user-friendly (for smearing on gallery windows) than cow shit? Goldsworthy is 50 and, as these questions suggest, back in his element. Lately, the British countryside's most engaging propagandist has been pursuing his vision all across the world. He has made unlikely cairns in Des Moines, a monumental Holocaust memorial in New York (for which he planted oak trees in giant boulders). A return to the green, green grass of home feels overdue. He grew up not too far from here, on the Harrogate side of Leeds, in a house edging the green belt. He was a guest artist at this sculpture park way back in 1983, when he was still asking himself whether there might be a career at all in making piles of stones off the beach look like Brancusis or in taking vast Scottish snowballs down to London and observing them melt.

In the time since, he has collected a team of craftsmen and labourers who follow him around the globe, humping wood and carving stone. This morning I come across several of them, working in small groups on the various ingenious constructions that Goldsworthy has set in motion. Five men are out in a copse making a circular dry-stone structure that will obstruct a right of way and offer no entry or exit; a stubborn comment on the Enclosure Act of 1801, among other things. The foreman, Gordon Wilson, is on the phone to Goldsworthy, clarifying whether the copestones of the enclosure will be done in the Yorkshire style, rough and ready, or the Nottinghamshire, curved; another group on a different hill is making a complex sheep pen. 'I've been reading The Observer for 40 years,' Dave Griffiths tells me, 'and I've been waiting for you to do a story on dry-stone walling....'

Dave's patience has not been in vain. His crafted pens of quarried rock have at their centre an eight-and-a-half-ton block of sandstone on which visitors will be invited to make 'rain shadows'; this process will involve waiting for a likely looking rain cloud and then, as the first drops begin to fall, lying full length on the rock and allowing a body-shaped silhouette to form, which the prostrate pilgrim will photograph and contribute to an archive. I imagine a queue of cagoule-clad ramblers gazing at the horizon, invoking drizzle. The perfect English day out.

The tour across the park - which also takes in 'paintings' made in mud on canvas by sheep feeding around a circular trough - is a preamble to the subterranean weirdness that Goldsworthy is creating in the gallery itself. In five large adjacent rooms underground, he is reproducing some of his greatest hits. Almost filling the first room is one of his enormous egg-shaped 'black holes' made of mossy, random curved logs, held together only by the artist's uncanny defiance of gravity and a kind of ancient energy; in the next are an unsettling colony of 11 stepped clay mounds, each with a vacancy at the top, that seem like the extraordinary efforts of avant-garde termites; beside these, in what has the feel of a medieval workshop, art students are mixing clay with sackfuls of human hair diligently collected from nearby salons and slapping it on the walls; as this hirsute plaster dries out it will crack and crumble and be held together by myriad strands of local DNA.

The fourth room is waiting for a coppiced dome with a 20ft diameter that I'm told Goldsworthy will knock up in the next few days - I've been inside a previous dome he made at the Albion Gallery in London and can still recall the otherworldly claustrophobia of it, like finding yourself in the stomach of a tree. In the final room I come across the artist himself up a stepladder working on a beautiful filamented curtain stretching the full height and width of the gallery that up close turns out to be made from horse-chestnut twigs held together with thorns, each one - more than 10,000 in all - painstakingly jointed by hand. Goldsworthy comes down and, over his umpteenth big custard tart and mug of latte of the day ('this kind of thing burns up the calories'), tells me what he is up to.

He talks with a precise animation, at odds with his Yorkshire vowels, and a constant sense of mischief in his face. 'They are calling this a retrospective,' he says, 'but actually I'm only interested in developing new stuff. Take this,' he gestures at the horse-chestnut curtain, 'I discovered this here in 1987 when I picked up a few horse-chestnut stalks and pinned them together with thorns, and I found that holding them up to the light was really beautiful. I wondered if I could span a couple of trees with them, and I was amazed that I could. Now here I am 30 years later making a mesh that spans a room 12m wide. I wanted to put this in to show the way things have grown, the technical things, you know....'

One of Goldsworthy's talents is to make such intricate stunts look easy. At one point he quotes Whistler's notion that a work of art is not finished until all signs of the effort of making it have been removed. He likes that idea. I suggest the 'black hole', the great cairn of oak branches he has created up the corridor, as a good example of that.

He laughs, in the way you might when thinking of the challenges of disciplining a high-spirited child. 'Stone to some extent has a system to it,' he says. 'But with wood every branch is totally different. I always look at the branches laid out on the grass before I begin and I think, "Oh fuck, here we go." I used to do them in a day. I can throw them up. But I took my time with this one, three days. To start with, you don't know what character it will take. If the base gets too wide it can be very sort of lumpen.'

What he is trying to bring out, he says, is something like the same quality that existed in the original trees. 'That effortlessness. A tree is so perfect in its profile but it is underwritten by this enormous daily struggle over years and decades. That is the energy I am aiming for.'

Goldsworthy is a land artist in the tradition of the great American earth-movers like Robert Smithson who created Spiral Jetty at Salt Lake, Utah. Richard Long, who imported that tradition to Britain, is another mentor; like them, he wants to get away from two-dimensional representation of landscape in a frame, and give you the thing itself. That's the theory. But he is also strongly in the tradition of everyone who has ever had memorable days making dens in parks or sandcastles on beaches. He preserves such ephemeral creations, icicle statues on rocks, brilliant forest dramas made with autumn leaves, in exquisite photographs. Goldsworthy's books are, reportedly, the biggest-selling art books in the country.

It's tempting to think of him as a naive kind of artist, returning us to childhood communion with nature. I tell him my eldest daughter, who is seven, keeps his book Passage by her bed, endlessly intrigued by how he makes things with twigs and stones, wondering why her dad can't do the same. He says he used to hate it when people referred to his work as childlike, or worse childish, believing himself to be a heroic conceptualist. 'I used to say, "Hey, I'm a grown-up and this is grown-up art." But since I have had my own children,' he has four, 'and seen how intensely a child looks at things, you really can't describe that looking as naive. My work is childlike in the sense that I am never satisfied to look at something and say that is just a pond or a tree or whatever. I want to touch it, get under the skin of it somehow, try and work out exactly what it is.'

This necessity got to Goldsworthy early. From the age of 13 he worked on farms as a labourer. Most of the lads wanted to drive tractors but he never fancied that much, rather he liked the repetitive quality of farm tasks, which he likens to the grind of making sculpture. 'A lot of my work is like picking potatoes,' he says. 'You have to get into the rhythm of it.'

He always assumed that he might have to work as a gardener or a farmer for the rest of his life, which he says would have suited him fine as long as he could do his work. He was learning all the time. 'Farming is a very sculptural profession. Building haystacks or ploughing fields, burning stubble. And it is a brutal thing, too. Go round the back of any farm and there will be a pile of dead lambs. Farmers see more death than anyone.'

Goldsworthy engages and worries about the consequences of our general disconnection from the land and the food chain. He wants to confront the fact that for urban people the country is just something nice to look at on a Sunday out.

'Some of my work addresses that very directly,' he says. 'I did a series of photos which were just of me skinning a rabbit, just that. The smell and the blood and the shit. You have no idea. It is not gratuitous, it is like the sheep paintings in there, done by sheep on canvas. I hope people will look at them first as landscape paintings, and then make the connection that farmers are creating sculptural landscapes all the time. The gallery window here overlooks the farmed landscapes. One of the main reasons it looks so green and beautiful is the amount of sheep shit on it. I'm going to smear the windows because I want people to look at the landscape through shit and see the connection between the two.'

I suggest to him that his preoccupation with meat and death links him to Damien Hirst, a near contemporary from Leeds. Goldsworthy uses the implications of those facts of life in a different way though: not as a negative, but as a suggestion of natural process and renewal. 'Well,' he says, 'I enjoy the raw shock of Damien Hirst. But for me art has to be more than shock. I would rather subvert things, try to make people look at them differently.'

After college in Lancaster, when he spent most of his time on the beach at Morecambe making rock sculptures, Goldsworthy settled in Dumfries: 'I had no money and it was cheap.' His agent offered him a small fortune in contracts to relocate to London, but he declined. It is as impossible to imagine Goldsworthy living in Hoxton as it is Gilbert and George in the Dales. He is a man of nature, and part of him wants to remind us, somewhere deep down, that we all are.

This urge has seen him described as a druidic figure, a mystic. He laughs at the idea, though he allows that his work has a kind of spiritual purpose. 'Everything has the energy of its making inside it,' he says. 'There is no doubt that the internal space of a rock or a tree is important to me. But when I get beneath the surface of things, these are not moments of mystery, they are moments of extraordinary clarity.'

Though he can make this connection with almost any landscape, he likes working in Britain because there is always the sense of people having had claims on the landscape before him. He is sometimes characterised as a wilderness artist but he rejects the thought, talking of the importance of negotiating with farmers and landowners. 'I hate this idea of people who want to be the first person somewhere, claim it, or the only person on the beach. When I am on my holidays I am out there with the crowds making sand sculptures alongside everyone else.' Proper holidays for Goldsworthy are few and far between. He works every day, or tries to, generally in the woods and fields near his home. 'I make an awful lot of crap,' he admits, 'but I have to be out there, trying things.'

This work ethic is maybe something he got from his parents, who were strict Methodists. From his father, who was a maths professor at Leeds, it is also tempting to suggest that he inherited his intuitive sense of the possibilities of form. 'My father had a practical side to him. He would dabble a bit. But he wasn't an artist, like I'm not a mathematician.'

There is an ephemeral quality locked into Goldsworthy's work just as surely as it is locked into nature. Many of his pieces only last a few hours, though they achieve an afterlife on film or in photographs (50,000 of which have been catalogued by his partner, Tina Fiske, an art historian whom he met when she came to work with him a couple of years after he separated from his wife). I wonder if as he gets older he feels the sense of loss, of mortality, implicit in this transience more keenly?

'I do,' he says. 'The great thing about art for me, and one of the dangers of contemporary art becoming too youth-orientated is that it can be a reflection of a person's entire life. I look forward to that. When you think of what Matisse did with those long sticks he used in his late years to draw with, it is the resistance, the difficulty that creates the wonderful energy of his line. I really hope I can do something like that. I am physically still very able, but I know that will change.'

For the time being, however, Goldsworthy seems cheerfully capable of almost anything he wants to do. To prove the point, he goes back to his improbable curtain of chestnut, which he wants to try to complete before the afternoon light goes. He also suggests the answers to the questions that I noted down at the outset: yes, yes, with great difficulty, and yes.

· Watch a slideshow of the installation here

Life on earth

Born 26 July 1956 in Cheshire. Grew up on the outskirts of Leeds, where he laboured on a farm from the age of 13.

Education Bradford Art College (1974-75) and Preston Polytechnic (1975-78).

Family His father, F. Allin Goldsworthy, was professor of applied mathematics at Leeds University. In 1982 he married Judith Gregson. They had four children and moved to Penpont, Dumfriesshire, where he lives with his current partner, Tina Fiske, an art historian.

Awards Scottish Arts Council Award, 1987. Honorary degree from University of Bradford, 1993. OBE, 2000.

· Andy Goldsworthy's new book, Enclosure, is published by Thames & Hudson in April. The Andy Goldsworthy retrospective is at Yorkshire Sculpture Park from 31 Mar to 6 Jan 2008 (01924 832631; ysp.co.uk)