Of all the possible ways that exist to map the earth, a perhaps underappreciated endeavor is that of the soil survey. Before the 20th century, soil mapping in the United States at its core was traditionally focused on the potential of land for agricultural productivity, with most soil maps containing broad regional generalizations about soil qualities. Inspired by European advances in soil science by such soil luminaries as Friedrich Fallou and Vasily Dokuchaev, an effort in the United States to better organize and classify soils for varied uses took root around the turn of the century.
The first major step undertaken in pursuit of a more advanced system of soil science was the Soil Survey, created by the United States Department of Agriculture’s Division of Soils. Milton Whitney was named the first Chief of the Division of Agricultural Soils in 1894. Whitney’s studies often focused on the texture of the soil itself, which he believed had a major effect on the soil’s potential for agricultural productivity.
The initial soil surveys produced by the Bureau largely focused on localized agricultural areas: the first four soil surveys undertaken by the new Bureau were Cecil County, Maryland, the Connecticut Valley, the Salt Lake Valley, and Pecos River in New Mexico. By 1913, the map below illustrates the spread of soil surveys themselves, as detailed progress was made to understand soil conditions in locations across the country.
Under Whitney, the primary concern was the utility of soil for agricultural production. Soil maps of this period were primarily created with the view that the earth possessed natural regions which could be identified in discrete geographic bounded areas through human observation, and the maps themselves were typically easy to read and understand. These early 20th century soil maps often used descriptive terminology that was intuitive to most map readers that described textures, such as “fine glaciated soil” (as seen on the map below, from 1920):
In the decades after the formation of the Bureau of Soils, soil surveys expanded, with a focus that shifted to include the composition, structure, and use of soil. Terms used to describe soil evolved as well, moving from general terms to more scientific terms as the properties of soil were better understood and more meticulously described. New levels of understanding also allowed for more differentiation to be built into the maps. Collaborative soil surveys also expanded, as the federal government worked with states and agricultural colleges to accomplish soil mapping while addressing very localized needs.
The Dust Bowl and widespread soil erosion issues in American agricultural lands in the 1920s and 1930s saw the formation of the Soil Conservation Service in 1935. Land-grant universities with agricultural experiment stations were partners in soil research, and graduates of agricultural colleges enriched the background of many who worked on soil surveys. During this period, it became more common for soil to be discussed as part of a wider ecological ecosystem, with more scientists and geographers expanding their terrain of thought to consider that a series of relational interactions formed the geographic conditions of a given area, rather than the condition or quality of the soil being an essential characteristic of a place. For example, Curtis Marbut (who worked on the map above), established that soil classifications are not simply a reflection of geological weathering, but instead a result of multiple related interactions. Marbut contributed to the 1935 “Atlas of American Agriculture: Soils of the United States,” which helped define the classification system used by the National Cooperative Soil Survey.
In 1965, the Department of Agriculture introduced a system of soil taxonomy (often referred to as the “Seventh Approximation”) which categorized soils based on their inherent chemical, biological, and physical properties. This new system was adopted as the classification of the National Cooperative Soil Survey and invited spirited debate among geographers and soil scientists, as the easy to understand soil maps of prior decades were replaced without the clear bounded regions of the past, replaced instead with a more complex system of soil classification which sought to understand the soil from a precise scientific basis. The soil taxonomy helped integrate soil properties with their full geographic context (such as climate, topography, and geology). As the 20th century continued, soil science increasingly became understood within the context of wider environmental concerns, such as climate, water quality, ecosystem management, and urban planning.
By the end of the 20th century, geographic information system and digital data transformed soil mapping. Of note is the Soil Survey Geographic Database (SSURGO), which contain detailed survey data for US counties as collected by the National Cooperative Soil Survey, a huge landmark in the dissemination of soil data.
Further Reading:
For a glimpse into the debates over “Seventh Approximation,” I recommend “Soil Taxonomy and Mapping” by Philip J. Gersmehl in Annals of the Association of American Geographers, Vol. 67, No. 3 (Sep., 1977), pp. 419-428.