Lower pressures are possible because radial tire construction provides greater strength and flexibility than bias ply tires. With more research, tire manufacturers were able to reduce the tire pressure. Reducing tire pressure distributes the weight of the tractor over more soil area due to tire flexing. Also, lower tire pressure reduces soil compaction as the pressure in the tire is closely related to the force on the soil surface. If there is a soil compaction problem, lower tire pressure may reduce the effect.
Fluid reduces the flexing of tires.
If fluid amounts are reduced, weight lost be need to be replaced by adding cast-iron weights. This must be measured in the field while pulling a load with the tractor. To adjust tire pressure properly, know how much of the load each tire is carrying. This information is available from tractor manufacturers, or by weighing the tractor. They figure that all the flexing will ruin their tires.