Unseen and undetected, soil compaction steals yield like a thief in the night. At its worst, it can hack corn and soybean production by more than 40%.

Crop consultant Tom McGraw, Midwest Independent Soil Samplers, Buffalo Lake, MN, developed technology last year that detects how much and how bad compaction is in a field. He then converts the data into a computerized map. Using the same green-yellow-red color scheme that's on a hand-held penetrometer, the digitized maps paint compaction problems in living color.

To create the maps, McGraw developed software that takes readings from a load cell mounted on a hydraulically powered penetrometer. He mounted that equipment and a computer on a bean buggy and headed for the field.

McGraw drives the buggy the length of the field, taking a measurement every 100'. Then he moves over 200' and repeats the process. In total, he takes a little more than two samples per acre. The software records compaction in 1" increments down to 24". That data is then averaged to show the average of the top 6" of soil and the average of the plow layer from 7" to 10".

"We're still in the fact-finding part of our research," he says. "But we've learned already that there's so much variability, you can't make any generalizations about where compaction will likely occur. What happens with one soil in one county may not happen anywhere else in the U.S."

Last summer's research on more than 6,000 acres has confirmed some of McGraw's suspicions about the causes of compaction.

"Mudding a crop in in the spring doesn't cause severe compaction. It may hurt yields some that year, but it isn't cumulative," he says. "The real culprit is axle loads. Combines, grain carts and liquid manure tanks are the worst.

"In many cases, the best- yielding soils are the most compactible. The same clay content that holds water also compacts easily. Farmers tend to think they don't have compaction in those parts of the field, because that's where their yields are the highest."

McGraw's testing has shown that some areas of fields would actually be damaged by deep tillage. "We found some areas where the bulk density of the soil is already too loose," he says. "Ripping those soils just makes a bad situation worse."

In other areas, McGraw discovered that fine-textured soils have the same symptoms as compacted soil. "It's another situation where deep tillage won't do you any good," he says.

"We've also found that no-tilled fields can have just as much compaction as fields that are tilled. You still have compaction caused by heavy axle loads, but you don't have any tillage to break it up. Freeze and thaw will break up the compaction in the top few inches of soil, but it doesn't do anything for the compaction we're creating deep in the soil."

More than just a compaction detection tool, the computer maps show farmers where they need to V-rip and how deep they need to run the shanks to eliminate compaction.

"It helps you make decisions about where to spend your time and diesel fuel," McGraw says. "If you run deeper than you need to, it costs money and creates tillage sheer deeper in the soil."

McGraw believes farmers eventually will be able to use the computerized maps to vary tillage automatically just as some farmers now vary seed and chemicals.

"It all makes good agronomic sense," says Larry Eekhoff, sales manager for NEW Co-op, Ft. Dodge, IA. "Nobody really knows how much compaction they have. On fields we tested, you could see exactly where the farmer had V-ripped and where he hadn't. The one question we couldn't answer is, what do you do when the compaction is down 18" where you can't reach it with a tillage tool?"

McGraw sees two basic responses when he first shows farmers compaction maps of their fields.

"Some of them have a 'so what' attitude and wonder what you expect them to do about it," he says. "With others, you can see the wheels start to turn. They understand that compaction is costing them money and this technology will give them a competitive edge over guys who aren't interested."

Just like a yield monitor, compaction maps don't provide a final answer to yield problems.

"A yield monitor doesn't tell you what you don't know; it just quantifies what you do know," McGraw says. "A compaction map is just another layer of information. But it's an important piece in figuring out how to farm more profitably." ?