Agriculture Reference
In-Depth Information
The Unsubstantiated Change in the Definition of the “iso” Limit
The isohypothermic, isothermic, isomesic, and isofrigid soil temperature regimes were intro-
duced in the Ýrst edition of Soil Taxonomy (Soil Survey Staff, 1975). Their common characteristic
was mean summer and mean winter soil temperatures that differ by less than 5
C at a depth of 50
cm. This value was selected because the area having such temperature Þuctuations corresponds
closely to the intertropical region between the Tropic of Capricorn and the Tropic of Cancer.
In the second edition of Soil Taxonomy (Soil Survey Staff, 1999), the limit for the ÑisoÒ
condition was increased to 6
ο
C. No rationale or reasons to substantiate the change were provided.
Apparently, the change was made to accommodate small areas with ustic or aridic soil moisture
regimes on the islands of Hawaii and Maui in the State of Hawaii, so that all of both islands could
be considered Ñtropical.Ò Classifying these areas as Ñnon-isoÒ would have indicated their different
soil temperature attributes and would have served the same purpose for use and management. But
this was not considered. Although there was no widely circulated proposal, the change was accepted
by the classiÝcation team of the NRCS and incorporated into the system.
We analyzed the climatic data for 55 stations in Hawaii and Puerto Rico, and calculated the
difference between mean summer and winter air temperatures. The delta T values ranged from 1.7
to 4.0
ο
C. The amplitude in soils is normally smaller, and in the Newhall (1972) model, a factor of
0.6 is used to reduce the delta T in the soil. As the islands of the State of Hawaii are located near
the Tropic of Cancer, there may well exist small areas where delta T exceeds 5
ο
C. It appears,
ο
however, that extrapolating this situation to the rest of the world is not warranted.
The
C break to differentiate the iso or tropical conditions from the non-iso was based on
general land use conditions prevailing in the tropics. In the tropics, though winter months could
become cool (at transition zones to the high latitudes), there is no frost to affect plant growth.
Although the change has worldwide implications, its effect has not been documented and there
appear to be no factual global data to substantiate the need for change. However, we understand
that the issue is now being reexamined by the NRCS to study the impact and implication of the
new deÝnition. Unless extensive testing produces convincing evidence that the original 5
∆
T 5
ο
C limit
ο
was Þawed, and that the new delta T of 6
C constitutes a signiÝcant improvement, we consider the
change unnecessary and indeed unfortunate. We further believe that changes in Soil Taxonomy of
such magnitude and consequence should not be made in an impromptu fashion on the basis of
isolated,
ο
ad hoc
information.
The Flawed Definition of Spodic Materials
Spodic materials, the indispensable characteristic of the spodic horizon and Spodosols, are
deÝned in Soil Taxonomy (Soil Survey Staff, 1999) as Ñmineral soil materials [that] are dominated
by active amorphous materials that are illuvial and composed of organic matter and aluminum,
with or without iron.Ò Yet a soil material may qualify for spodic solely on the basis of pH, organic
carbon content, and color. In the northern coastal plain of Puerto Rico, there are clayey deposits
that are overlain by almost pure quartz sand sediments of varying thickness. At the abrupt litho-
logical discontinuity, a black horizon has formed, likely due to lateral seepage from the surrounding
lagoons with which they are geographically associated. This horizon qualiÝes as a spodic horizon
by current deÝnition, and the soils are therefore Spodosols.
Similar soils are extensive elsewhere in the tropics, notably in Southeast Asia, in the southern
part of the Congo Basin and in the Amazon Basin of South America. A major river draining the
basin, the Rio Negro, is loaded with dissolved organic matter making the water black (hence the
name). Along some coastal plains of Southeast Asia, there is a back-swamp separating the inland
from the raised coastal dune sands. Peat or muck occurs in the swamp and the dissolved organic
matter laterally permeates the sandy beach deposits. A transect from the swamp to the sea shows
that the horizon of accumulation reduces in thickness toward the sea. Above this horizon is a layer
