Development of Soil Taxonomy Family Mineralogy Class Criteria.

Families were not included as a category in the 7^th Approximation of Soil Taxonomy (1960). In a 1962 memo, W.S. Ligon listed the early evolution of Dr. Guy Smith’s ideas on mineral classes and the applicable particle size classes. At this time, there was much consideration of using whole soil mineralogy, and keying mineralogy to particle size. A more extensive development of mineralogy classes was issued by the Soil Survey Staff in 1967 (Supplement to the 7^th approximation) that proposed 15 <2-mm and whole soil classes and six clayey classes. In 1970, an unedited key to mineralogy classes was issued, and in Soil Taxonomy (1975), a subsequent key was presented. Five classes of silicate clays and a mixed class were presented for clayey textures, and seven classes (including a mixed) for <2-mm and whole soil were included. The mixed and oxidic classes presented the most problems. Extensive evaluation and testing followed which was facilitated by the development of new methodologies. In joint S-5 and S-9 symposia in 1981 and 1982, problems and suggested changes in class definitions were presented (SSSA #16, 1985, J. Kittrick, ed.). The International Committee for Family criteria (ICOMFAM) was formed in 1986, and in 1994, ICOMFAM issued a final report; “A Review of the Soil Family”. The ICOMFAM recommendations to amend Soil Taxonomy were summarized in a newsletter in 1999. The adopted changes were issued in a Key to Soil Taxonomy and incorporated into the 2^nd ed. (1999) of Soil Taxonomy. A major change was the inclusion of cation exchange activity classes for mixed and siliceous mineralogy classes, and the inclusion of ferrihydritic, smectitic, magnesic, and isotic classes, and para prefixes.