The use of the color criteria in the Paleudalfs, and not in the Paleudults goes all the way back to the 7th approximation. The pale concept emerged from the geomorphic studies carried out in the 1960’s, particularly the Desert Project in the west and the Coastal Plain project in the southeast. In general, the intent is to group soils of significant age (older than Holocene) with evidence of well developed horizons of some sort (ie thick argillics, petrocalcics, abrupt texture change, etc.). The criteria have always varied a bit within the orders for the pale definitions. These definitions were developed to reflect the properties of the soils that they wanted to be grouped into the pale great groups. You can get a feel for this if you go to the “Guy Smith Interviews”
ftp://ftp-fc.sc.egov.usda.gov/NSSC/Soil_Taxonomy/Rationale.pdf and search for “pale”. Here is one excerpt from Guy Smith Interviews, page 82 that mentions the red colors in Paleudalfs:
“The red hues that enter into the definition of some of the "pale" groups are there in the definition simply because all the soils we knew that we wanted in that group did have the red hue or they had the mottles which are not indicative of wetness. They had one or the other in the definition. The mottles have very high chromas compared to the mottles in the wet soils of the Midwest.”
So you can tell from Guy’s statement that they knew which soils they wanted to group into the Paleudalfs, and since the red color seemed to be a common property that would help differentiate these Udalfs on old surfaces from younger, less developed ones, it was included in the definition. I suspect there was also a belief that the red color itself was generally a reflection of age and weathering in these soils.
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Paleudalfs vs. Hapludalfs
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