"So we had this map of where the urbanized places are in the U.S. We then wanted to merge it with the soils map of the U.S. created by the United Nations Food and Agricultural Organization (UNFAO)," says Imhoff. The UNFAO regularly samples and tests soils all across America from the tip of Maine to Southern California in an effort to determine the amount and location of productive farmlands throughout this country. They then lay out their results on a map of the U.S., classifying each soil in terms of limiting factors.

"The more limiting factors you have, the more expensive it is to produce agricultural products because those limiting factors have to be overcome," explains Imhoff. One limiting factor may be that the soil is too damp and needs to be drained before it can be farmed. Another limiting factor may be that the soil is too acidic and would have to be limed. Generally, a soil with three or fewer of the less severe limiting factors is considered to be prime soil because very little has to be done with it in order to plant a crop.
 

Imhoff and his group used Geographic Information System (GIS) software tools to merge his map and the UNFAO map point-for-point in terms of latitude and longitude so that the Goddard team could compare the two. When they tallied up the amount and type of soils covered by urbanization, they found that the soils with just one or two limiting factors were being urbanized the most. For the entire United States, nearly five percent of these high-quality soils had been dug up for development. However, soils with no limiting factors came in at fewer than three percent. "What we think is going on across the country is that there might be some preservation of the very best soils, but it’s at the expense of the next best."

As Imhoff suspected, the soils that weren’t being urbanized at all were those with six or more limiting factors—soils with hardly any agricultural value. "These are areas in the desert and in the high mountains. They are generally places where there are severe seasonal limits on biological activity or where there is simply less activity overall. So humans aren’t there in large numbers yet," he says.

To get a clearer picture of the way in which future urbanization was moving, the Goddard team zeroed in on and analyzed the four states that, according to the 1989 census, have the highest market value of agricultural goods–California, Wisconsin, Illinois, and Florida. For these states the team matched up both the urban and peri-urban areas to soil types. They found that the distribution of urban area over soil types was close to the national average. However, in some instances the peri-urban areas—those areas where development was likely to expand—covered more than twenty percent of the higher-grade soils in the state.
 

The most extreme case is California, where there is apparently almost no effort to protect agricultural lands. "Over 16 percent of the best soils in California are already in urban use here. If you look at the peri-urban areas, those are the corridors of development. The trend is pretty clear. Development is following soil resources," Imhoff says. More specifically, if these peri-urban areas are developed in California, more than 50 percent of their best soils could be lost and replaced by the houses and businesses of people who need to eat.

 Saving What We’ve Got Before It’s Gone
 The Aura of Urbanization