As Native Americans had done before them, early European farmers opened patches of forest by killing trees and planting corn or wheat amidst the stumps. Early travelers reported back to Europe and the eastern U.S. with scarcely believable tales of corn harvests on virgin soils of 100 bushels per acre. (Varying accounts put the harvests as high as 120 bushels and as low as 50-80 bushels).
The natural fertility of such fields was exhausted after a few years and the exhausted fields usually were abandoned. This cycle has been widely denigrated as "slash-and-burn agriculture," but Native Americans rested their plots, allowed woody growth to overtake them, and then burned it off. The result was, some researchers argue, a sophisticated crop rotation system in which nutrients were first drawn to the surface by plants from deep soils and then converted into fertilizer by fire.
As native populations had learned, the soil's riches were easily spent, even on the prairies. The prairies were what scientists would later describe as nitrogen-constrained ecosystems. The big grasses are heavy feeders of nitrogen, and left little of it free in the soil. By the 1870s yields on even good prairie land had slipped to 30 bushels. Soil nitrogen has to be listed among Illinois' natural resources that were exploited to the point of depletion in the past century.
Presettlement horticulture was in many ways as complex, accomplished, and productive as that of today. Native American techniques of planting beans, squash, and corn in the same field and using seed "hills" that required little tilling, left the ground covered and protected against erosion, and yielded crops that could be harvested through the season.
The arrival of numbers of experienced European farmers in the 1820s and 1830s began what some see as the golden age of Illinois agriculture. Over the next century farming moved out of the woods onto the prairie. Because animals figured as prominently in farm economics as grain crops, much of the land was kept in grass-like hay and pasture, which provided soil cover. The animals provided, in turn, a regular source of manure. In the view of agricultural historian Carl Sauer, this conservative plow-and-animal husbandry was a self-sustaining ecological system that differed from earlier systems insofar as no stage of it exploited soil fertility to the point of exhaustion.
Like the early hill agriculture had done, the mixed grain- and-livestock farms of Illinois left little land exposed to weather. Anecdotal accounts describe Illinois streams as running clear most seasons of the year well into the 20th century, in spite of the fact that much of their watersheds had been intensively farmed for decades. It was not farming but a particular kind of farming--exclusive row-crop cultivation --that resulted in soil erosion so ubiquitous as to turn most Illinois streams the color of cafe au lait all year around.
Illinois soils remain among the richest in the world in terms of their ability to retain moisture, their store of trace minerals, and their benign pH. Best of all, they are watered and warmed by a climate particularly clement for agriculture. Nevertheless, while Illinois soil is often described as "most fertile," perhaps it would be more accurate to describe it as "most productive." Soil organic matter has been extensively depleted, its decomposition accelerated by annual cropping. This has lowered the capacity of most soils to recycle, accumulate, and store nutrients. To meet the high demand for nitrogen by crops such as corn, Illinois farmers apply inorganic fertilizers at one of the highest rates in the Midwest.
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