Controversies in the Definition of “Iso” Soil Temperature Regimes

The term iso in soil temperature regimes has been used in Soil Taxonomy to distinguish between tropical and temperate zones. It defines regimes with temperature differences of less than 6°C between summer (June, July, and August) and winter (December, January, and February). In the northern hemisphere, however, these months are not necessarily the warmest and coldest months. The island of Tenerife, which lies between 28 and 29° N near the Tropic of Cancer, is a transition point between the temperate and tropical regions. The trade winds, affecting mainly the northern side of the island, are the main reason for the presence of a tropical zone featuring iso, isomesic, and isothermic temperature regimes. In this zone, the warmest and coldest months are displaced by 2 to 3 mo compared with the months indicated in Soil Taxonomy. We examined the characteristics of “iso” regimes and a number of other possible methods for determining isotivity: the warmest and coldest months, astronomical summer and winter, and the equinoxes. We concluded that the soil temperature regime of the zone most influenced by the trade winds—a zone with rainforest vegetation and where tropical characteristics are most in evidence—would not lose its “iso” character under any of the methods. We suggest using the warmest and coldest months instead of summer and winter for the purposes of defining “iso” status. To facilitate estimations, we also suggest that the temperature of the equinox days be used. Changing the limit from 6 to 5°C would not affect the tropical soils of the island of Tenerife.