You can't see the damage a tandem disk creates in a field. But the data make it obvious.

Ten years of on-farm comparisons revealed that farmers drop corn yields an average of 13 bu/acre when they use disks for fall tillage.

"Our farmers asked us to provide data to help them make decisions for increasing yields," says Lance Murrell, Erny's Fertilizer, Logansport, IN. "So we set up 80 field strips as wide as one round with a combine, or about 1 1/2 acres. Forty strips were in cornfields and 40 in soybeans so we could see the effects of rotation."

The study included annual soil tests at harvest. To gauge compaction, resistance to the soil probe was rated as slight, moderate or severe when Murrell collected soil samples. "We didn't take a quantitative measurement," he says. But the numbers tell the story.

In plots where farmers had disked bean stubble the previous fall, compaction and disease increased and yields decreased (see printed article). Using the disk again in spring just made matters worse.

The data are similar to those from many other compaction studies, according to Wayne Reeves, research agronomist at the USDA-ARS National Soil Dynamics Lab at Auburn, AL.

"There's research across the board to back up the fact that a disk causes compaction," says Reeves. "When we want to create a hardpan in our research plots, we wait until the soils are good and wet and then disk them."

Murrell adds: "That's why you see road crews using big disks when they're building roadbeds. It's tough to beat a disk and a water tank if you want to compact soil."

Disking isn't always bad, says Purdue University agronomist Gary Steinhardt. "Done at the wrong time, it can cost you as much as 20 bu/acre," he says. "But a lot of it has to do with soil conditions. If you get enough moisture, you can get away with murder."

Murrell's customers witnessed the opposite side of that coin this year as drought and high temperatures made compaction problems apparent.

"The last few years we've had nearly ideal conditions and really haven't seen any effect of compaction," he says. "Farmers are asking what they did this spring to cause compaction and the answer may be they didn't do anything. They're just seeing the results of previous years' tillage that didn't show up until the crop really got stressed."

Steinhardt points out that, if you disk lightly and fast under dry conditions, there's not much of a problem. Unfortunately, those are rarely the conditions farmers face.

"The disk can be quite a compacting tool," says Steinhardt. "You've got narrow blades carrying a very heavy frame."

The compaction noted in Murrell's study was worse in lighter soils with 1 1/2-2% organic matter (OM) than in darker soil where OM ranged from 2 1/2 to 3 1/2%.

"The OM seems to add a buffering effect," he says. "The worst compaction was in transition soils that tend to stay wet longer. Water tends to move down to the compacted layer in the lighter soils and then move horizontally until it reaches transitional soils lower on the hillsides."

Murrell's farmers have replaced disks with V-rippers for working soybean stubble in fall.

"We don't have a single client using a disk. It took some longer than others to believe the data. Some were determined they were going to win with the disk. But they all knew it was causing compaction. Ultimately, it's hard to argue with the data when it's coming from your own field.

"There's been a shift toward just V-ripping the soils where compaction is the worst, rather than tilling the entire field," Murrell adds. "When you run the V-ripper 13-14" deep, it costs as much as moldboard plowing."

In corn stubble, most of Murrell's clients now either moldboard plow or chisel stalks.

"A chisel works okay if they're rotating to soybeans. But if the field is going back to corn, they're better off using a moldboard plow and burying the residue. We have more problems with disease in cornfields that are chiseled and then planted back to corn."

The effects of disking on soybean yields are less clear-cut. Murrell's data show a disadvantage to the disk, although the disk shows a slight advantage over a chisel or no-till system (see printed article.) Purdue's Steinhardt, however, hasn't seen similar soybean yield loss in his plots. The compaction from tillage is there; it just doesn't seem to lower yields as much as in corn.

"Soybeans seem to be more tolerant. It's due partly to their growth habit," Steinhardt says. "Soybeans are far more likely than corn to wait for better growing conditions. In compacted fields, you'll see some very irregular growth patterns. But at harvest the differences disappear when you look at yield."

No tillage system eliminates compaction, points out Murrell. Some are just less likely to compact soil than others.

"You can't eliminate compaction, you just have to work hard to minimize it," he says.