Soil

SOIL

SOIL is a mixture of weathered rocks and minerals, organic matter, water, and air in varying proportions. Soils differ significantly from place to place because the original parent material differed in chemical composition, depth, and texture (from coarse sand to fine clay), and because each soil shows the effects of environmental factors including climate, vegetation, macro-and microorganisms, the relief of the land, and time since the soil began forming. The result of these factors is a dynamic, living soil with complex structure and multiple layers (horizons). Soils have regional patterns, and also differ substantially over short distances. These differences have shaped local and regional land use patterns throughout history. Because of this, historians have studied soil for clues about how people lived and for explanations of historical events and patterns.

Soil Classification and Mapping

The basis of the modern understanding of soil formation is attributed largely to work in the 1870s by the Russian V. V. Dokuchaev and colleagues. The Russians classified soil based on the presumed genesis of the soils and described the broadest soil categories. Simultaneously but separately, soil scientists in the United States were mapping and classifying soils based on measurable characteristics and focused on the lowest and most specific level of the taxonomy—the soil series. The Russian concepts did not reach the United States until K. D. Glinka translated them into German in 1914, and the American C. F. Marbut incorporated Glinka's ideas into his work. The U.S. system of soil classification that eventually developed considers the genetic origins of soils but defines categories by measurable soil features. Soils are divided into 12 soil orders based on soil characteristics that indicate major soil-forming processes. For example, Andisols is an order defined by the presence of specific minerals that indicate the soils' volcanic origin. At the other end of the taxonomic hierarchy, over 19,000 soil series are recognized in the United States. Research data and land management information are typically associated with the soil series.

Some U.S. soils were mapped as early as 1886, but the official program to map and publish soil surveys started in 1899 by the U.S. Department of Agriculture (USDA) Division of Soils, led by Milton Whitney. The effort was accelerated in 1953 when the Secretary of Agriculture created the National Cooperative Soil Survey, a collaborative effort of states, local governments, and universities led by the USDA Natural Resources Conservation Service. As of 2000, mapping was complete for 76 percent of the contiguous United States, including 94 percent of private lands.

Soil Fertility

Ancient writings demonstrate awareness of the positive effect of manure and certain crops on soil productivity. Modern agricultural chemistry began in eighteenth-century England, France, and Germany, and was dominated by scientists from these countries through the nineteenth century. In the 1840s, the German scientist Justus von Liebig identified essential plant nutrients and the importance of supplying all of them in soil, but this led to a concept of soil as a more or less static storage bin of nutrients and failed to reflect the dynamic nature of soil in relation to plants.

In 1862, state agricultural colleges were established by the Morrill Act, and the USDA was created. The Hatch Act of 1888 created experiment stations associated with the colleges. These developments led to the expansion of research plots that established the value of fertilizer in crop production and defined the variations in soil management requirements across the country.

Soil fertility can change because agriculture and other human activities affect erosion rates, soil organic matter levels, pH, nutrient levels, and other soil characteristics. An example of this is the change in distribution of soil nutrients across the country. In the early twentieth century, animal feed was typically grown locally and manure was spread on fields, returning many of the nutrients originally taken from the soil with the crop. Since farms became larger and more specialized toward the end of the twentieth century, feed is commonly grown far from the animals and manure cannot be returned to the land where the feed was grown. Thus, nutrients are concentrated near animal lots and can be a pollution problem, while soil fertility may be adversely affected where feed crops are grown.

Technology and Soil Management

Soil characteristics influence human activity, and conversely, human land use changes soil characteristics. Many technologies have changed how people use soil and have changed the quality of U.S. soils. The plow is one of these technologies. In 1794, Thomas Jefferson calculated the shape of the plow that offered the least resistance. Charles Newbold patented the cast iron plow in 1796. John Deere's steel plow, invented in 1837, made it possible for settlers to penetrate the dense mesh of roots in the rich prairies, and led to extensive plowing. Aeration of soil by plowing leads to organic matter decomposition, and within decades as much as 50 percent of the original soil organic matter was lost from agricultural lands. Until about 1950, plowing and other land use activities accounted for more annual carbon dioxide emissions than that emitted by the burning of fossil fuels. Fossil fuel emissions have grown exponentially since then, while net emissions from land use held steady and have declined recently.

Soil drainage systems expanded rapidly across the country in the early twentieth century in response to technological advances and government support. Drainage made it possible to farm rich lands in the Midwest that were previously too wet to support crops, and it allowed the use of irrigation in arid lands where irrigated soils quickly became saline when salts were not flushed away. The extensive drainage systems radically changed the flow of water through soil and altered the ability of land to control floodwater and to filter contaminants out of water.

A third critical soil technology was the development of manufactured fertilizers. During World War I (1914– 1918), the German chemist Fritz Haber developed a process to form ammonia fertilizer. Nitrogen is commonly the most limiting nutrient for intensive crop production. Phosphorus, another important limiting nutrient in some soils, became readily available as fertilizer in the 1930s. The use of these and other manufactured fertilizers made it possible to grow profitable crops on previously undesirable lands, and made farmers less dependent on crop rotations and nitrogen-fixing plants to maintain soil productivity.

A fourth technology was the development of herbicides beginning after World War II (1939–1945), combined with the refinement of"no-till" farm machinery in the 1970s. No-till is a method of crop farming that eliminates plowing and leaves plant residue from the previous crop on the soil surface. This residue protects the soil and can dramatically reduce erosion rates. The system also requires less fuel and labor than conventional tillage and thus allows a single farmer to manage more acres. The result has been a substantial reduction in erosion rates around the country and an increase in the amount of organic matter stored in the soil. The organic matter and associated biological activity improve productivity and reflect the sequestration of carbon dioxide from the atmosphere into the soil.

Erosion and Conservation

Soil degradation can take many forms, including loss of organic matter, poor biological activity, contamination with pollutants, compaction, and salinization. The most prominent form of land degradation is erosion by wind or water. Erosion is a natural process that is accelerated by over grazing and cultivation. In Conquest of the Land Through 7,000 Years (1999), W. C. Lowdermilk attributed the loss of numerous civilizations to unsustainable agricultural practices that caused erosion, resulting in silting of irrigation systems and loss of land productivity.

The first English colonists in America faced heavily forested lands but gradually cleared the land of trees and planted tobacco, cotton, and grain year after year in the same fields. In the eighteenth century there were references to worn-out land, and by 1800 much farm acreage along the coast had been abandoned. In 1748 Jared Eliot, a Connecticut minister and physician, published a book of essays documenting his observation of the connection between muddy water running from bare, sloping fields and the loss of fertility. John Taylor, a gentleman farmer of Virginia, wrote and was widely read after the Revolution (1775–1783) on the need to care for the soil. Perhaps the best known of this group of pre–Civil War (1861– 1865) reformers was Edmund Ruffin of Virginia. Clean-cultivated row crops, corn and cotton, according to Ruffin, were the greatest direct cause of erosion. He urged liming the soil and planting clover or cowpeas as a cover crop. His writings and demonstrations were credited with restoring fertility and stopping erosion on large areas of Southern land.

After the Civil War farmers moved west, subjecting vast areas to erosion, although interest in the problem seemed to decline. In 1927, Hugh Hammond Bennett of the U.S. Department of Agriculture urged, in Soil Erosion: A National Menace, that the situation should be of concern to the entire nation. In 1929, congress appropriated funds for soil erosion research.

The depression of the early 1930s led to programs to encourage conservation. The Soil Erosion Service and the Civilian Conservation Corps began soil conservation programs in 1933 with work relief funds. The dust bowl dust storms of 1934 and 1935 influenced Congress in 1935 to establish the Soil Conservation Service (SCS). Within a few years the service was giving technical assistance to farmers who were organized into soil conservation districts. These districts, governed by local committees, worked with the SCS to determine the practices to be adopted, including contour cultivation, strip farming, terracing, drainage, and, later, installing small water facilities. By 1973, more than 90 percent of the nation's farmland was included in soil conservation districts. The SCS was renamed the Natural Resources Conservation Service in 1994.

According to USDA Natural Resources Inventory data, erosion rates declined significantly during the 1980s, largely due to widespread adoption of reduced tillage practices. In the mid-1990s, erosion rates leveled off to about 1.9 billion tons of soil per year.

BIBLIOGRAPHY

Brady, Nyle C. The Nature and Properties of Soils. Upper Saddle River, N.J.: Prentice Hall, 2001.

Helms, Douglas. "Soil and Southern History." Agricultural History 74, no. 4 (2000): 723–758.

History of the Natural Resources Conservation Service. Available at http://www.nrcs.usda.gov/about/history/

Lowdermilk, W. C. Conquest of the Land Through 7,000 Years. Agriculture Information Bulletin No. 99. USDA Natural Resources Conservation Service, 1999.

Simms, D. Harper. The Soil Conservation Service. New York: Praeger, 1970.

U.S. Department of Agriculture, Yearbook (1938, 1957, 1958).

Ann Lewandowski