How much difference does O horizon thickness make regarding summer soil temperatures?
Soil Taxonomy defines a 50 cm soil summer temperature threshold of 15°C for mineral soils lacking an O horizon, and having a mean annual temperature of less than 8°C. If an O is present, the summer temperature requirement drops to 8°C.
Our office has collected preliminary data suggesting that one of our soil temperature sites in northern Michigan averaged 7.6°C over the past year, and had a summer soil temperature of only 14.8°C. If it wasn’t for the 1 cm O horizon, this would have been a cryic soil temperature regime. In contrast none of Michigan’s coldest soils have ever been mapped as anything other than frigid, not even Isle Royale.
Similarly, a study in the Southern Appalachians (Lietze and McGuire, 1987) suggests a summer soil temperature would be around 12.5°C at the elevation where soils drop below 8°C in annual soil temperature (above 1600 m in GSMNP, despite frigid soils being commonly mapped below 1400 m). But most of these soils do have an O horizon as thick as 8 cm.
The implication is that either cryic soils would occur immediately adjacent to frigid soils wherever processes removed the O horizon, or that soil temperatures would increase to as much as 7 degrees higher in the summer without the O horizon.
If on the other hand, the O horizon has to be a certain thickness, then some of the area currently mapped as frigid soil that also happen to have relatively cooler summers, such as near the Great Lakes or in the southern mountains, should actually be considered cryic.
So what are the intentions behind the requirements for cryic vs. frigid soils?
Lietzke, D.A. and McGuire, G.A., 1987. Characterization and classification of soils with spodic morphology in the southern Appalachians. Soil Science Society of America Journal, 51(1), pp.165-170.
Edited 1 time(s). Last edit at 09/30/2020 01:38PM by greg.schmidt.
