JEFFERSON CITY, Mo. -- After more than a century of digging into the matter, researchers have identified and mapped all 44.6 million acres of Missouri's soil.

The key finding: Missouri has about 5,000 different types of soil.

The project was conducted continuously since 1899, although it was accelerated over the past 25 years. Farmland in northern Missouri was the first to be surveyed and Ozarks timberland the last.

Government officials marked the project's completion Friday by sinking a soil probe into the earth near the Governor's Mansion -- a ceremonial gesture as the state celebrated Earth Day on the nearby Capitol grounds.

State officials estimate the soil project cost $120 million in today's dollars.

"This is my baby," said Vernon Renner, 86, a retired soil professor at Southwest Missouri State University who has been working on the project since 1952.

"I trained soil scientists so they would have the necessary education to go to work on this project," he said. "A lot of them spent their entire careers doing this."

Soil maps are used by contractors, because some soils are more likely to crack concrete foundations. Farmers also use the maps to plan crops. Under state law, soil types also are used in the assessment of agricultural property.

Dennis Potter, a soil scientist for the state Natural Resources Conservation Service, said the 100 years of research provided a unique look into history.

"I like to compare it to tree rings," Potter said. "You can tell a lot about the background of the soil from looking at the soil profile."

"It's a dirty job, but somebody has to do it," Potter joked.