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Why do the loess hills of Iowa need to be farmed in terraces?
Loess is formed by deposits of silt originated by the deposition of particles transported by dust storms.
19/06/2018

Terracing fields of loess soil in Iowa helps reduce erosion. / Photo: SV Fisk

Soil formed in loess, or wind-blown silt, is great for farming. Its soft texture contributes to its productivity, but also increases its susceptibility to erosion. To combat this issue, farmers implement many practices to ensure vulnerable soils are protected from wind, water, and gravity. Let’s dig into this substance called loess, and explore the Loess Hills of the Midwestern United States.

Contrary to popular belief, the state of Iowa is not flat! Iowans and those who have visited can attest to this fact. Minus flood plains, Iowa’s landscape is comprised of numerous rolling hills and bluffs of various geologic origins. The Loess Hills (pronounced “luss”) are a particularly stunning set of steep, rippled bluffs that erupt unexpectedly from adjacent lowlands. Few similar landforms exist in the world, making the Loess Hills a hidden gem and a source of pride to those who live amongst them.

This rare formation is identified as a narrow 200-mile strip of land that snakes along the western edge of the Missouri River flood plain. Its northern extent is in the middle of Iowa’s western border and stretches south into northwestern Missouri. The most dramatic landscape features, found closest to the river, include 60-200+ foot tall hills with near-vertical faces of exposed sediment. Further east of the river, the landscape quickly transitions to more dampened, soft rolling hills.

The material that makes up these hills – loess – is defined as silt that has been transported and deposited by wind. It is often pale brown or blonde in color, and has the texture of flour. The German word ‘löss’, from which its name was derived, translates to “loose”. Indeed, loess deposits are light and easy to dig into. 

An essential ingredient in the recipe for loess deposits is a silt source. Unvegetated landscapes such as glacial outwash plains, bare shales and siltstones, and deserts are major examples. The Loess Hill’s silt deposits of western Iowa and Missouri came from the wide barren floodplain of the Missouri River valley. Geologists determined the silt was deposited during the last two North American Glacial advances.

As winds move across barren lands, fine-textured sediments erode from the earth and lift into the air. Airborne sediments move with currents for as long as the wind energy is great enough to hold them. When wind energy does decline, the largest/heaviest loess particles are the first to drop back to the ground. The finest textured materials instead can be carried hundreds of kilometers from their original source before falling out of the air. 

Loess itself is not uncommon. It is thought to cover nearly 10 percent of the earth’s surface. The Midwestern and Northwestern United States, Alaska, Germany, Argentina, and Russia are just a few places that have deposits of wind-blown silts. Less commonly found are deposits deep enough to influence a landscape’s shape. The Loess Hills of Iowa and Missouri are one such example. Other notable regions of the world include the Palouse of the Northwestern United States and the Loess Plateau of China. Though most loess deposits across the planet are not outwardly obvious, their influence on soils can be quite profound.

Many soils formed from loess are amazingly productive. One simple reason is due to their texture. The high silt content of loess-derived soils allows water to drain freely through the profile, insuring that plant roots receive constant supplies of oxygen and do not become water logged. Water that does stay within the soil is held loosely so plant roots can access it with minimal effort. The texture of silt also enables roots to colonize soil profiles without restriction.

Though productive, loess soils have their share of challenges. A dominant concern: they are highly vulnerable to wind and water erosion. In fact, one heavy rainstorm can remove several tons of soil per acre if the ground is not adequately protected. The severity of erosion is most intense on lands with steep slopes, like in the Loess Hills of Iowa.

To use loess soils sustainably, managers must control wind and water erosion. Two key ways to do this are keeping the soil covered and reducing the speed at which water flows across the landscape. Farmers grow cover crops during fallow periods, use intercropping – growing alternate rows of different crops. They also leave plant residue on the field post-harvest, and limit soil tillage. Installing terraces and grass waterways helps to slow water down and cover the most vulnerable land areas. Some non-agronomic strategies to reduce erosion include creating retaining walls, protecting temporarily exposed soils with siltation fencing, and installing rain gardens in drainageways. The steepest areas may be better left undeveloped, or used as grazing land, recreation, and/or conservation land.

Whether thick or thin, loess deposits have a tremendous economic, environmental, and cultural significance. Areas of deep loess like the Loess Hills of Iowa are stunningly beautiful. They support unique assemblages of plants and animals, and provide an interesting playground for people who are drawn to the outdoors. Loess soils also comprise a significant percentage of the world’s most arable land. If used sustainably, these soils will continue to support life in the ground and across the planet for generations to come.

Source: SSSA


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