Answer: True. The lower the tire pressure, the less it compacts the topsoil, says Ken Brodbeck, manager of Original Equipment and Export Sales for Firestone Ag Tires, Des Moines, Iowa. And the larger the tire, the lower the tire pressure needed to carry a given load. A larger tire at a lower psi has a longer footprint than a smaller tire at higher air pressure, which spreads out the load and reduces stress on the soil.

The proper tire inflation pressure depends on the tire size and axle load, Brodbeck says. Adding tires, or using taller or wider tires, reduces the carrying load per tire, allowing lower inflation pressure.

As the tire pressure decreases, so does the load-carrying capacity. Be sure to follow manufacturers’ recommendations so you don’t exceed the load-carrying capacity of the tire at that pressure. Ideally, Brodbeck says, you should run the vehicle over a scale in its heaviest condition to determine the exact weight before setting tire pressures. If that’s not practical, set the air pressure in the tires when the machine is resting on a firm concrete surface. When properly inflated, “one – or no more than two – tread lugs should be touching the concrete,” Brodbeck says.

Tire pressures are set for the heaviest load and road speed, DeJong-Hughes says. The optimum tire pressure in the field may be much lower, she says, but it’s impractical for farmers to change tire pressures for transport and field operations.