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Wide, fat and soft is best, for tires that is

Wide, fat and soft is best, for tires that is
A soil compaction pit comparison found as much as 50% deeper compaction by bias-ply tires versus radial floatation tires on a spreader.

If you really want to ease up on soil compaction by manure and fertilizer spreaders, make sure you're rolling with radial floatation tires. That was the clear bottom line of a soil pit demonstration of radial versus bias-ply tires by Alliance Tire Americas at July's North American Manure Expo near Chambersburg, Pa.

The "pit stop" showed a roughly 50% difference in compaction depth solely due to tire type. So it should be no surprise that new spreaders and grain carts/wagons are increasingly equipped with floatation radials.

DEEPER BIAS: Steve Vandegrift, Alliance Tire product manager, examines the deeper bias-ply tire soil compaction layers, evidenced by sawdust lines in the compaction pit. The radial's footprint and soil compaction behind him was much shallower. Photo courtesy of Alliance Tire

How it went down
After excavating a four-foot-deep pit, it was refilled with four layers of soil and sawdust to highlight tire downforce of the soon-to-roll-over-it manure tanker carrying a 50,000-pound load. Tires on the 7,259-gallon tanker's left side were 28L26 bias-ply R-3 diamond tread tires, standard rubber for this type of equipment. On the right side were 28L26 Alliance 390 flotation radial tires.

Total axle load per tire was 12,500 pounds. Both were inflated to recommended pressures – 40 psi for the bias-ply and 26 psi for the radials.

Next, the tanker was rolled backwards onto the pit, then pulled forward. Then the trench was re-dug to reveal how the soil layers were compacted.

The sawdust lines tell the story. The bias-ply tires compacted roughly 16 inches or four layers down. The flotation radials distorted the first layer of sawdust, but soil disturbance disappear below the 8-inch depth. This tire-to-tire test was done in a heavy clay loam. Compaction differences would have been greater in a more porous, higher organic matter soils.

Surface bias-ply ruts were 4.5 to 4.75 inches deep, with a deep center flanked by sloping sides. Water from an overnight rain pooled in its footprint and infiltrated slowly over several hours.

Surface ruts from the flotation radials were about 3.5 to 3.75 inches deep, and flat in profile. In comparison, the rainwater infiltrated the soil rut more quickly.

Why footprint and inflation pressure matter
The tire footprint impact is multiplied many-fold as it rolls over your fields. "Reduced water infiltration can increase runoff during heavy rains, delay fieldwork or planting, and decrease the amount of water available to the crop later in the season," points out Steve Vandegrift, Alliance Tire Americas product manager.

"Compaction also squeezes water and air out of the root zone, making it difficult for roots to penetrate into the soil," he adds. Together, it can pose a very costly problem in a difficult growing season like this year – and a devastating problem when the damage is deep.

Note that the bias-ply tires were inflated to 40 psi, while the radials were at 26 psi. Tire inflation pressure corresponds within one or two psi to soil compaction pressure, says the tire expert. That means the bias-ply tires applied about 35% more force to the soil.

Then there's the footprint factor. Bias-ply tires tend to crown at the tread center, so they concentrate compaction force in a smaller area of the footprint, he explains. "Radials have a longer, wider, flatter footprint, so the load is spread much more evenly over a larger surface."

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