Although root plaques and associated oxidized root channels are used f
or wetland identification as field indicators of wetland hydrology, li
ttle information is available concerning their reliability as related
to the environmental and biotic factors controlling their formation. T
herefore, this review describes and evaluates the current state of kno
wledge of the factors controlling the formation of iron plaques and re
commends research to address information gaps. Both abiotic and biotic
factors control the presence and degree of iron plaque formation. The
most important abiotic factor is the availability of soil iron. Howev
er, the effect of site variation in soil physico-chemical characterist
ics (e.g., texture, organic matter, pH, Eh, and soil fertility) on iro
n availability and microbial activity can influence the formation and
persistence of root plaques and oxidized root channels. Although the o
xidizing capacity of the plant root is the most important biotic facto
r controlling plaque formation, only a limited number of wetland speci
es have been evaluated for this ability, so species-specific differenc
es are generally unknown. Unlike some of the other hydrologic indicato
rs used in wetland delineation (e.g., water marks on trees or sediment
deposits) root plaques and oxidized root channels indicate soil satur
ation for a sufficient period to produce anaerobic soil conditions. Ad
ditionally, when found in conjunction with a living root, oxidized roo
t channels indicate that the anaerobic conditions occurred within the
life span of the plant root. Therefore, the presence of oxidized root
channels and iron plaque surrounding living roots is a relatively good
indicator of current wetland hydrologic conditions. However, research
is needed to elucidate the relative abilities of different plant spec
ies to produce oxidized root channels, the temporal persistence of the
root iron plaque and the role that soil physico-chemical condition pl
ays in controlling plaque formation. Without a better understanding of
the controls on iron plaque formation and disappearance, the absence
of oxidized root channels, in itself, should not be used to indicate t
he absence of a wetland.