Menfro Silt Loam

The Menfro silt loam is Missouri's official state soil. It sits on the upland ridgetops and hillslopes above the Missouri and Mississippi Rivers — the loess-covered bluffs that define the eastern and central parts of the state.

Menfro is an Alfisol: a soil with a clay-enriched subsoil that forms under forest cover and holds nutrients and water exceptionally well. The loess parent material — fine silt blown from the glacial Mississippi River floodplain — runs 6 to 20 feet deep. The clay-enriched argillic horizon built within that loess extends nearly six feet below the surface, one of the thickest of any state soil in the country.

The Menfro series is named after the small town of Menfro in Perry County, along the Mississippi River in east-central Missouri. Soil scientists established the series there when they first systematically mapped and described this distinctive loess soil.

Menfro covers more land than any comparable loess soil series in Missouri — more than 780,000 acres across 40 counties — and it underlies some of the most historically important ground in the state. The current state capitol in Jefferson City, the first state capitol in St. Charles, and the governor's mansion all sit on Menfro soil.

The Missouri Legislature designated Menfro the official state soil in 2004 through House Concurrent Resolution 10. The same year, when USDA, the Soil Science Society of America, and the Smithsonian Institution created a traveling exhibit of state soil monoliths, Menfro was chosen to go on display first.

Menfro has four main zones. Dig into a Missouri River bluff and you find a thin dark surface layer, then a pale transitional zone where clay is being washed deeper, then — for nearly six feet — a dense dark clay-enriched subsoil. Below that, the original loess continues.

That thick argillic horizon is what sets Menfro apart. Most soils develop a clay layer of 20 to 40 inches. In Menfro, the clay zone runs close to 70 inches deep, built up over thousands of years as rain slowly carried clay particles down through the deep loess deposit. This makes the subsoil exceptionally good at retaining water and nutrients for crop roots.

Menfro silt loam covers more than 780,000 acres across 40 counties in Missouri. It follows the corridors of the Missouri and Mississippi Rivers and their major tributaries — the ridgetops and slopes above the floodplains, where loess blown from the river sandbars settled thickest.

The soil runs from the northeast corner of the state, where Missouri meets the Mississippi River, down along the eastern border and back west along the Missouri River to Jefferson City and beyond. Counties along both river corridors have Menfro soil on their uplands.

Beyond Missouri, the Menfro series also appears in west-central and southwestern Illinois, wherever the same loess deposits from the glacial Mississippi River reach the bluffs across the river.

Menfro silt loam is prime Midwest farmland. The thick argillic subsoil holds moisture through dry stretches and releases nutrients steadily to crop roots. On slopes under 6 percent, it is classified as prime farmland — the USDA's highest productivity rating. The main crops are soybeans, corn, small grains, and hay.

Steeper slopes stay in timber. The natural vegetation of Menfro soil is deciduous hardwood forest — bur oak, white oak, chinkapin oak, black walnut, white ash, shagbark hickory, and in the bottomlands along the rivers, pecan. Where these forests were cleared and the slope was gentle enough to plow, they became some of Missouri's most productive fields.

Managing Menfro on slopes requires care. The same fine, silty texture that makes the soil fertile also makes it erodible when groundcover is removed. Farmers and conservationists have worked for generations to keep contour farming, cover crops, and buffer strips in place on the steeper Menfro ground.