Considering the body of knowledge on which Soil Taxonomy is based, it is surprising there are so few problems in classifying tephra-influenced soils. However, there are four problem areas.
With time, NaF pH has come into wide use as a field-testing criterion for recognition of such soils. Its use as a criterion for recognition of amorphous material, however, is unduly limited by the requirement that it be used only for soils containing more than 20 dag kg−1 of water at 1.5 MPa.
The bulk density range is the most limiting of the criteria for andic subgroups and for amorphous material. Because it is one of the most expensive measurements made for these soils, they have been classified on the basis of other properties, largely field-determined. Consequently, many series included in andic subgroups and medial families have bulk densities of as much as 1.15 Mg m"3.
Because the criterion for volcanic glass content of ashy families and of Vitrandepts is higher than that which occurs in some tephras, mapping of soils formed in tephras has been more dependent on laboratory grain counts than would have been necessary had the criterion been set low enough to have included all tephras.
The definition of amorphous material and the instructions for replacing names of particle-size and mineralogy classes in soils that do not disperse well produce some uncertainty in placement of soils into medial families. This is primarily a result of lack of clarity in the definition of medial families.
Finally, the properties selected in Soil Taxonomy as criteria for differentiating among suborder, great group, subgroup, and family taxa of tephra-influenced soils have worked very well. With use, there has been some apparent migration of class boundaries, but the original set of properties is still the most practical one to use.