Today's tractors weigh up to 10 times more than their ancestors of 1960, and the two- to four-row combine of the 1960s has given way to the 12- to 16-row harvester of today.

That means bigger tires - and more of them - as the farm economy continues to expand.

At the 65-year-old Firestone Agricultural Tire plant in Des Moines, workers are raising parts of the venerable plant's roof by 10 feet to accommodate the new "Big Ag" fabricator that can make tires almost 8 feet tall.

The raised roof will be part of a $77 million expansion that also will add about 50 jobs to the work force of 1,400 at the plant by the end of this year, according to Ken Allen, president of Bridgestone Americas Agricultural Tire Operations.

Iowa's 92,000 farms are just the pilot light for the state's agricultural manufacturing economy, exemplified by Firestone and its competitor, Titan Tire of Des Moines, as well as Deere & Co., Vermeer Manufacturing of Pella, Kinze of Williamsburg, Art's Way of Armstrong and Sukup Manufacturing of Sheffield.

Firestone and other farm-related manufacturers provide up to 20% of Iowa's nonfarm employment and put about $50 billion annually into the state's economy, according to studies by Iowa State University Extension.

The ag industry has offered some stability as the recession has tossed manufacturers. Iowa has lost 13,200 factory jobs in the last year, Iowa Workforce Development reported.

The 62-acre Firestone factory on Second Avenue is the city's largest manufacturer - its employment is about the same as the work force at Deere's Ankeny works. The factory opened in 1945, just a dozen years after Harvey Firestone's newly invented rubber tire quickly converted America's farm tractors off their old steel wheels.

Through the years, the Firestone plant has kept its original mission, to sell tires primarily to original equipment manufacturers such as Deere, Case, New Holland and Caterpillar. Firestone buttresses its wholesale market with a 1,300-dealer retail sales network.

Factory helps compensate for auto industry's troubles

Allen, a Des Moines native and Lincoln High and Iowa State University graduate, has been president of the plant and the tire division since 2008. He acknowledged that he took over the plant at a good time for agricultural manufacturing.

Firestone and Allen guard the secrets of the Des Moines plant - such as the number of presses and new equipment pieces, as well as annual sales and profits and market share - with the doggedness of a defense contractor.

But Allen will say that after taking its lumps during the agriculture slump of the late 1980s and 1990s, the Ag Tire division is now a shining star in the Firestone/Bridgestone firmament.

"The car people have had tough years recently because of the downturn in the automobile industry," Allen said. "But we've been increasing output to meet demand."

Deere & Co. recently reversed earlier Wall Street expectations by announcing that low commodity prices wouldn't prevent sales of big agricultural equipment to be at least as strong as last year.

Most of those sales will be in the heaviest, top end of the market. Tractors today now run up to 60,000 pounds. Combines can be 40,000 pounds and capable of carrying 360 bushels (or two acres' worth of average corn production) in their bins.

That's why Bridgestone's proprietary, roof-raising new "Big Ag" tiremaker will be needed to roll the rubber and steel belts into tires.

The plant also will need more and bigger tire presses, which bake the rounded rubber forms at about 300 degrees for up to three hours to produce the biggest tires.

Through the 1990s, one tire press could make 16 tires - enough for two tractors - per day. But the increase in bigger tires has cut down the daily productivity of a tire press to seven tires daily.

The monster tires that will be needed for future generations of tractors will likely be able to be produced at the rate of only about five per day.

"It's a productivity problem and we'll definitely need more presses," said Allen, who came to Firestone in 2002 from Goodyear's farm tire business, which is now part of Titan.

Another dilemma for the tire designers is that while agriculture is getting bigger, the fieldwork has become more precise.

Farmers can use grids and satellite-based positioning to guide them through the fields. But the fields are more crowded.

For instance, the average plant population in an acre of corn has doubled in the last 15 years to about 32,000, and Pioneer Hi-Bred and Monsanto seed companies confidently predict the coming of the 40,000-plant corn acre.

It's left to Allen and his people to produce tires for the bigger tractors and combines that can delicately maneuver through those crowded fields without running over the plants.

The answer has been taller, dual-mounted tires spaced about 4 inches apart to provide enough room to get through corn rows as narrow as 20 inches apart. For that, the tire width drops from 18.4 inches to 12.4 inches but goes up 10 inches or more in height.

Plant manager Norvel Smith and operations manager Dovie Majors have to produce tires that are strong enough to carry a 60,000-pound piece of machinery over an Iowa field, yet soft enough to minimize the soil compaction that is the prime cause of environmentally damaging fertilizer runoff, not to mention spring floods.

To accomplish that minor miracle, tractor and combine tires are engineered to be inflated at between 6 and 12 pounds per square inch, vs. the 30 to 32 pounds per square inch in an automobile tire and 80 to 90 pounds per square inch in a truck tire.

"The idea is to make a softer imprint on the soil," Allen said.

Simple cleanup makes plant 'more pleasant' for workers

The Firestone plant has symbolized the blue-collar image of the northeast and east sides of the city for decades.

The factory has avoided strikes despite extended disputes over contracts in the last few years.

"Things have gotten a lot better here in the last decade," said Al Skinner, president of the Steelworkers local that represents the bulk of the workers. "A big part was simply cleaning up the plant. It's a lot more pleasant place to work now."

The average wage at the plant is $22 per hour, putting it near the top of the production trade wage scale. New contracts, however, have brought concessions on wages and benefits. Younger employees come in at lower wages now and workers can be used in more disciplines than before.

The Firestone plant has long run on a 24-hour schedule because it is more cost-efficient to run the tire presses without shutdowns rather than the time- and expense-consuming process of cool-downs and reheats.

The plant has been under the ownership of Bridgestone of Japan since its acquisition of Firestone in 1988, but it was only last year that the Bridgestone name went on the side of the building.

In North America, the products from Des Moines are still sold under the Firestone name.

"About 20% of our product is exported now, but if you take in the tires that we put onto major manufacturers, then the export total rises to about 30%," Allen said.