Hawaii and the Pacific Islands - U.S. Army Corps of Engineers

ERDC/EL TR-12-5 34redox concentrations may be absent or difficult to see, particularly in darkcoloredsoils. It may be necessary to let the soil dry to a moist state (5 to30 minutes or more) for the iron or manganese to oxidize and redoxfeatures to become visible.Particular attention should be paid to changes in microtopography overshort distances. Small changes in elevation may result in repetitivesequences of hydric/non-hydric soils, making the delineation of individualareas of hydric and non-hydric soils difficult. Often the dominant condition(hydric or non-hydric) is the only reliable interpretation (also see thesection on Wetland/Non-Wetland Mosaics in Chapter 5).Use of existing soil dataSoil surveysSoil surveys are available for most of the region and can provide usefulinformation regarding soil properties and soil moisture conditions for anarea. A list of available soil surveys is located at http://soils.usda.gov/survey/online_surveys/ and soil maps and data are available online from the Web SoilSurvey at http://websoilsurvey.nrcs.usda.gov/. Soil survey maps divide the landscapeinto areas called map units. Map units usually contain more than one soiltype or component. They often contain several minor components orinclusions of soils with properties that may be similar to or quite differentfrom the major component. Those soils that are hydric are noted in theHydric Soils List published separately from the soil survey report. Soilsurvey information can be valuable for planning purposes, but it is notsite-specific and is generally less detailed for areas at higher elevations inthe islands than at lower elevations, due to differences in topography andaccess. Soil survey information does not preclude the need for an on-siteinvestigation.Hydric soils listsHydric Soils Lists are developed for each detailed soil survey. Using criteriaapproved by the NTCHS, these lists rate each soil component as eitherhydric or non-hydric based on soil property data. If the soil is rated ashydric, information is provided regarding which hydric criteria are met andon what landform the soil typically occurs. Hydric Soils Lists are useful asgeneral background information for an on-site delineation. However, not allareas within a mapping unit or polygon identified as having hydric soils may