Plows are one of humanity's oldest and most important technologies, dating back thousands of years. Innovation has come slowly. A major advance was the 18th-century moldboard, which added a curved piece of wood to lift and flip over a slice of topsoil cut loose by the steel plowshare.

After the great drought of the 1930s, many plains farmers abandoned the moldboard because it exposed too much topsoil to erosion. They adopted the new sweep plow, a five-foot-wide blade, shaped something like the wings of a small airplane, that was pulled along inches below the surface. The sweep plow broke up the soil and destroyed weed roots while leaving the surface largely undisturbed and still covered by the natural moisture-saving mulch from the residue of previous crops.

Unfortunately, as agricultural researchers eventually found, the sweep plow also smoothed the surface of the soil below the blade. Like a mason's trowel, it closed up the cracks and crannies that would catch rainwater or allow air to reach the valuable soil bacteria that turn soil minerals into plant nutrients.

Soil scientists at the Department of Agriculture have developed a new kind of sweep plow that overcomes this drawback.

The new plow, invented by the Agricultural Research Service's Lloyd N. Mielke, should help producers of rain-fed row crops such as corn and soybeans by saving soil and water and exploiting the activity of microbes.

Mielke's innovation was to add prongs that project six inches from the underside of the sweep plow. Breaking up the subsurface soil too much, however, let in too much water, keeping out much of the oxygen needed by the most desirable bacteria. Mielke tried various shapes and sizes of prongs, until he found the best design.

It takes extra power to pull a Mielke-modified sweep plow but the result is an erosion-resistant, moisture-conserving soil teeming with the microbial life that gives fertility.