Since the early 1960s, public concern has increased in the United States that urbanization might be eclipsing potential farmland. The national perspective has focused on the loss of highly productive farmland as urban development expands into agricultural landscapes. This trend could affect our future capacity to produce food, feed and fiber. Indeed, under the pressure of urban growth, some farmers have relocated to less productive soils or have abandoned the agricultural business.
 
The U.S. concern is also international. The world’s population is estimated to reach 8 billion by the year 2025, a 38 percent increase from its current population. Yet, expansion of cropland has not kept pace with growth. Eighty-eight percent of the projected population increase is in Africa and Asia, where land development has been increasing faster than anywhere else in the world and where food shortages are common, according to the United Nations Food and Agriculture Organization. Cropland acreage is also decreasing rapidly in China, Thailand, India, Indonesia and Vietnam as a result of urbanization.
 
In the United States, most land-use decisions happen on state or county levels. But the overall loss of potential agricultural land is a national issue because it can determine the future capacity of our land for producing our food. Prompting alarm was a 1981 assessment by the National Agricultural Lands Study, led by what is now the U.S. Department of Agriculture’s National Resource Conservation Service and by the White House Council on Environmental Quality. This study estimated that the annual conversion of rural, nonfederal land to urban uses increased from 445,000 hectares between 1958 and 1967, to 850,000 hectares between 1967 and 1975.
 
The National Resource Inventory is currently the main source of national data showing the extent of prime farmland and the extent of that farmland converted to urban uses. It is estimated that 570,000 hectares of rural land was converted to urban uses between 1982 and 1992. The inventory tracks prime farmland, which is a generalized and qualitative indication of soil limitations as related to soil management and does not consider the entire range of soil, landform and climate characteristics of the land.
 
The results of these studies are aerial estimates of rural land or farmland converted to urban uses. But the adequacy of the nation’s land to produce food, feed and fiber can only be measured in terms of potential soil productivity of the land converted. All rural lands, even those presently used as cropland, are not equally productive.
 
We need systems for measuring just how much land containing highly productive soils is being converted to houses, shopping centers, parking lots and other urban uses. The extent of urbanization is traditionally identified and mapped from census data and aerial photography. However, these approaches are time-consuming, expensive, and inappropriate for regional- and conterminous-scale analyses.
 
To make a more comprehensive assessment of how much urban land is eclipsing farmland in the United States, we combined urban landcover data with soil productivity data. Most assessments of urban growth only measure loss of space. But these assessments tell only half the story. Our research aims to measure what kind of land is being lost.
 
To plot the location and extent of urban land, we used the “city lights” maps of the U.S. Air Force Defense Meteorological Satellite Program’s Operational Linescan System (DMSP/OLS). Because these satellites collect visual and infrared images of Earth’s surface during daytime and nighttime hours, they provide maps of urban concentrations based on light emission. The satellites follow a sun-synchronous orbit near the poles at approximately 830 kilometers above Earth and cross any point of the planet twice a day. The OLS, one of the six DMSP satellite sensors, is primarily designed to monitor sunlight and moonlight reflected off clouds, and in the process acquires images of light sources on Earth’s surface at a spatial resolution of 2.7 kilometers. We used “city lights” imagery of stable lights for the conterminous United States, creating a GIS (geographic information systems) layer that showed urban landcover.
 
Against this we plotted soil productivity ratings, using the Soil Rating for Plant Growth Model developed by the U.S. Department of Agriculture’s Natural Resources Conservation Service. The model uses climate, landscape and soil parameters contained in STATSGO (State Soil Geographic), the Department of Agriculture’s national database on soils. We grouped the soils according to high, moderately high, moderate or low productivity. Using ARC/INFO software, we combined the urban land-use map with the soil productivity map to determine what kinds of soils are experiencing urbanization.
 
We found that the most productive soils are concentrated in the Midwest — Illinois, Indiana, Iowa, Minnesota, Missouri, Ohio and Wisconsin — and in two of the Northern Plains states, Nebraska and Kansas. They also appear in delta and river valleys, such as the Mississippi Valley and Delta; in river and coastal alluvial plains, such as the Lower Rio Grande Plain and Western Gulf Coast Plain; and in the Northern Piedmont and other valleys between mountains. Land with “low” soil productivity is primarily in states located in the West Region and in the western part of the Northern Plains, primarily Montana, Wyoming and Colorado. The most productive soils occupy 3 percent of the U.S. landcover, and the second most productive occupy 26 percent.
 
Urban land represents approximately 3 percent of the total U.S. land area, excluding water. More than 80 percent of the total urban land use is in the Midwest, South Central, Eastern and Southeast regions.
 It appears that 83 percent of the land under urbanization consisted of soils with moderately high or moderate productivity. Six percent of the total U.S. urban land, approximately 1 million hectares, occupies areas that originally housed soils of high productivity. In several states, such as Alabama, California, Colorado and North Dakota, the urbanization occurs mostly on land that had either moderately high or moderate soil productivity prior to development. Nebraska is the greatest exception, where 51 percent of the urban land is on areas that once hosted highly productive soils.
 
Overall, the level of urbanization of U.S. land increases with increasing soil productivity: 1 percent, 2 percent, 4 percent and 5 percent for land in the low, moderate, moderately high, and high soil productivity classes, respectively. Of the total land area with “highly” productive soils under urbanization, a little over 50 percent of that land is distributed in four states: Illinois, Indiana, Iowa and Texas. In almost half of the states, the degree of urbanization increases from land in the low to that in the high soil productivity category, thus indicating that land with the most productive soils experiences high urbanization pressure. Developers prefer land with deep, well-drained, and nearly level soils — areas best suited for agricultural production.
 
Results of this study are a first step in determining the current status of soil resources in the United States as affected by development. They should be interpreted according to the scale and resolution of the data sources used. Results can be different on smaller scales, such as the higher resolution soil maps produced by states and counties. Nevertheless, our work is a basis for developing sound management and land-use plans for state and regional agencies. County level and site-specific assessments will need more detailed data sets. They may also yield different results depending on the level of soil productivity and on the development pressures in an area.

Small cities, too In recent work to analyze how much potential farmland is being replaced by urban growth in China, Eric Warner of Penn State and his colleagues found that small really matters.   Studying an area that contains Suzhou, China, a city of 1.5 million people, five cities of populations between 100,000 and 300,000, and 60 cities with populations of 10,000 or fewer, Warner’s team found that about 50 percent of the area’s high-quality soils were taken up by the expansion of the small cities.  “The small, incremental changes eat up a lot of ground,” Warner says. The study assessed urban growth by using Landsat Thematic Mapper data from 1984, 1988 and 1998, and 1994 images provided by the French satellite company SPOT Image. The researchers compared this urban landcover data with a provincial soils map produced by the Chinese government.   Also useful were 1966 photos from the CORONA database, a collection of photographs collected by U.S. reconnaissance satellites during the 1960s. The photographs were declassified in 1995 and are now maintained by the EROS data center. “It helped us establish a baseline,” Warner says. “It gave us a way of confirming what trends were like before China started developing in the late 1980s.” Kristina Bartlett

Petersen is a Distinguished Professor of Land and Soil Resources, Department of Crop and Soil Sciences, and Co-Director of the Office for Remote Sensing of Earth Resources at Penn State.

Warner is a Research Associate for the Penn State Office for Remote Sensing of Earth Resources.

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