IT'S the line nearly everyone picks up on when they talk to David Wall and I bet he's fed up with hearing it by now.

But when you combine two such diverse careers as he does, such remarks are to be expected.

"How fast can you spray a field?" folk will ask. Or "How does the sprayer fit on a Ferrari?" Or "How long are the skid marks at the end of a row?"

David smiles, grimaces slightly, and tries to be kind.

I had to admit he looked more like an agricultural crop sprayer than a racing driver, but then I was talking to him on the family farm just outside Bromyard and not while he was levering himself into a £300,000 Formula One mean machine at Mallory Park.

Pit crews there can probably never picture him driving down a field of maize. They'd think the only Lamborghini he's familiar with is a sports car not a tractor. In which case they'd be wrong.

It's just that not many people spend half their week driving racing cars and the other half driving crop sprayers.

The two seem to be at opposite ends of the internal combustion spectrum and there seems no way they would reasonably interlock.

Except in the boyhood world of David Wall, who is now 59, where Farmers' Weekly and Autosport magazines lay side by side in his bedroom.

"I was born at Mathon, where we had a farm in those days," he said. "So I was brought up in a farming environment and probably always knew I'd work in agriculture.

"But I loved motor racing. People like Stirling Moss and Mike Hawthorne were my heroes."

After school, David went to agricultural college, but as soon as he could, he got himself a motor racing licence.

"We're talking about 1963," he added, "and it was a lot different back then. The RAC ran the sport and all you had to do was apply to them, assure them you hadn't got a limb missing and were of sound mind and they sent your licence back. It cost 3s 6d if I remember correctly."

Doing odd jobs, he gathered together £90 to buy a small, home-built, single-seater racing car.

"I bought it off Terry Scriven, who ran a Volvo dealership in Malvern. It was based on the remains of a Porsche Formula One car the American driver Dan Gurney had crashed at the Nurburgring in Germany in 1961.

"Someone bought the bits off Porsche and turned it into this single-seater."

David's first appearance behind the wheel was at the Llandow circuit near Cardiff in '63, where he finished 10th out of 28 starters in his first race and failed to complete the second when the fuel pump broke.

"What I remember most about the day was the grid start. It's still one of the most exciting parts of a race. All those cars so close, all straining to go. The terrific noise and the smell and then suddenly the whole lot moves together.

"There were no lights in those days, of course. You went on the drop of the starter's flag, a Union Jack."

The spell was well and truly cast.

Over the following 20 years he raced all over the country, when time would allow, at all the main circuits - Silverstone, Mallory Park, Brands Hatch, Castle Coombe and all the rest.

But this wasn't the day job. That was in the agro-chemical industry. With brother Dennis running the family farm, David opted for a career as an agronomist, a specialist in crop diseases and treatment.

He worked for a number of leading companies and it was while on one of those "corporate days", so popular in the 1980s, the two strands of his life finally came together.

"We were at Mallory Park with a French chemical firm. It was about the time racing schools were just starting up and we had a go in the cars there."

Not surprisingly, seeing as he was racing regularly, David won the best driver of the day award. Not only that, but he was also offered a job as an instructor at the newly-formed racing school.

Which is what he has been doing for the last 18 years.

Now he is senior instructor as Mallory Park for Everyman Motor Racing and also instructs for the Johnny Herbert School at the new Rockingham Indy-circuit in Northamptonshire.

As well as instructing on corporate and "experience" days, David is responsible for the licensing of new entrants to the sport and advanced high speed tuition.

"Within three to four laps of a circuit, 90 per cent of the population can be taught to drive a car very quickly indeed," he added.

"There are about one or two per cent who are undiscovered geniuses and there are about five per cent who should never really be let loose in anything on four wheels!"

Along with racing driver Tim Matthews from Stourport-on-Severn, he was recently involved in Channel 5's television programme You want to be a F1 driver? The finals of which took place on the famous Monza track in Italy.

In his other life, David has set up a crop spraying business with a special handy-sized machine capable of doing jobs the huge wing-span sprayers can't cope with.

So instead of roaring around Mallory Park keeping a close eye on an embryo Schumacher, he's trundling up and down rows of maize or cereals or spraying off thistles and docks on grassland.

"Mind you, the sprayer will do 35mph on the open road," he laughed.

And it's a lot handier than a Porsche on a ploughed field.