There are several series that appear to classify in the Aquic Hapludolls but the lower part of what appears to be a mollic
epipedon does not have tha required organic carbon content. Therefore the aquic conditions are not in or immediately
below the mollic epipedon. These series will then classify in either the Pachic or Oxyaquic subgroup. The known soils with this
problem are limited to the sandy families. There has been a proposal made to change the criteria so that these soils will
classify in the Aquic subgroup.
However, I think we need to take a broader look at the present criteria for the Aquic, Pachic and Oxyaquic subgroups. I am
concerned about the make up of these subgroups. There is a lot of overlape. All three subgroups have soils with aquic
conditions within 40 inches. Pachic epipedons occur in both Aquic and Pachic subgroups. The main difference between the
Aquic and Oxyaquic subgroups is that the Oxyaquic subgroup has a horizon a few inches thick immediately below the mollic
epipedon that does not have the redox features required for aquic conditions. Is this significant?
The problem, as I see it, is that the base of the mollic epipedon is used as the reference for aquic conditions. Since the base of
the mollic epipedon can range from 7 to more than 40 inches the depth to aquic conditions can also be quite variable. This
reference point is used no where else except inthe Hapludolls and Argiudolls. The soil surface is the dominant reference point
used throughout the keys. It is also used for for interpertations of wet soils. Should the reference point for the Hapludolls and
Argiudolls be changed? If yes, within what depth should aquic conditions be required, 30, 40 or some other depth?
The present position of the Pachic subgroup in between the Aquic and Oxyaquic subgroups only occurs in the Udolls. This
sequence and present criteria causes series with pachic epipedons to classify in both subgroups. Keying the Pachic ahead of
the Aquic, as in the Ustolls, would keep series with pachic epipedons togather. Wouldn't this be better?
I would like your thoughts as to what changes should be made. Are ther other changes that need to be considered? Should
the changes be limited to the proposal mentioned in the first paragraph? If you prefer to comment to me directly, my e-mail
address is rmayhugh@ftw.nrcs.usda.gov.
