By ANN HINCH
Assistant Editor

INDIANAPOLIS, Ind. — Use the smallest possible vehicle for any given job in the field, and make sure the tires aren’t over-inflated: This was among the advice Randy Raper gave farmers at the National No-Till Conference in Indianapolis two weeks ago.

The ag engineer also told them to be mindful of the water in their soil before driving across a field and further compacting the dirt. “If you don’t hear anything else I say today, hear this,” he stressed: “Only traffic when soil moisture is low.”

The National Soil Dynamics Laboratory in Alabama, where Raper conducts soil compaction studies, tests new tractors and other equipment and tires specific to the farm industry. In fact, the initial radial tractor tire on the market was tested there first.

Successful no-till depends on keeping soil at the right compaction so it doesn’t require turning to “breathe.” Raper defined compaction as a reduction in available pore space where water can reside, and it happens when too much pressure meets gravity.
“Water control is everything,” he explained, calling it “money in the bank … That determines our yields, more than anything.”

In 1948, he said the biggest piece of ag equipment tested weighed five tons; by 2000, that had shot up to 55,000 pounds, without an increase in the number of axles to distribute the weight. Axle load is important, since that helps control how far down soil may be compacted by a tire; Raper suggested limiting per-axle load to between 6-10 tons.

When testing various pieces of equipment, he said his lab determined even an empty modern combine still weighs more than what researchers would like to see driving across a field. He advised not taking equipment into a too-wet field (“Only traffic when soil moisture is low”), and using correctly inflated radial tires to help spread out the weight load.

“There’s just a lot of benefits of radial tires, if you use them properly,” he explained, adding while they cost more, it’s worth the expense in the long run.

He also noted that dual tire systems may preserve subsoil looseness, but won’t necessarily help near the top surface. In Australia, he said farmers are using tire inserts that widen the wheelbase of tractors, to spread out the total load.

Raper said using auto-steer with strip-tilling, using the same established rows planting after planting may also help against compaction, since the GPS will keep the tires out of the loosened rows and make certain the seed is getting into that soil.

In planting cotton, he said data have shown as much as a 16 percent reduction in yield if seeding is off by as little as two inches from the prepared row; for nine inches, yield can sink by almost one-third.

Soil can also be compacted by livestock; Raper said a single cattle hoof can apply twice as much pressure for its size than some tires. Hard rain, too, may compact soil. Determining compaction is the first step to fixing it.

He suggested using a soil cone penetrometer, which is a rod one pushes firmly and steadily into wet soil while reading a gauge on top that gives pressure per square inch (psi). Raper said a reading of 300 psi in saturated soil is too dense and “shuts down a root system,” since if a root system can’t get through wet soil, “you’ve really got a problem.”

“We do recognize there can be effective methods of removing compaction with strip-till subsoiling,” he said. His opinion is the bent-leg shank, one inch below the densest compaction reading, is the best blade to use since it only turns dirt where it’s needed most and uses less energy.