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The 19% is taken from the USDA.
http://www.nrcs.usda.gov/technical/land/nri02/nri02lu.html
The Soil Survey Handbook describes how land catagories are determined.
soils.usda.gov/technical/manual/">http://soils.usda.gov/technical/manual/
Here is a short excerpt (truly short compared to the complete
definition) from the definition of farm land.
"Soil surveys in agricultural areas identify the soil characteristics
that determine the suitability and potential of soils for farming.
Interpretations for farming involve placement of the soils in
management groups (land capability system) and identification of the
important soil properties that pertain to crop production, application
of conservation practices, and other aspects of agriculture. The other
aspects of agriculture include yield potential, susceptibility to
erosion, depth to layers that restrict roots, available water capacity,
saturated hydraulic conductivity, the annual pattern of soil-water
states (including soil drainage class, inundation, and free water
occurrence), qualities that describe tilth, limitations to use of
equipment (including slope gradient and complexity, rock fragments,
outcrops of bedrock, and extremes in consistence), salinity and sodium
adsorption ratio, presence of toxic substances, deficiency of plant
nutrients, capacity to retain and release plant nutrients, capacity to
retain soluble substances that may cause pollution of ground water,
capacity to absorb or deactivate pesticides, and pH as related to plant
growth and the need for liming."
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