LOCATION ISAAC VI+PR

Inactive Series
Rev. BCD
02/2018

ISAAC SERIES

The Isaac series consists of moderately deep to very deep, well drained, moderately permeable on dissected volcanic uplands. They formed in weathered extrusive volcanic rocks. Slopes range from 5 to 40 percent. The mean annual precipitation is about 45 inches and the mean annual temperature is about 79 degrees F.

TAXONOMIC CLASS: Fine, mixed, active, isohyperthermic Typic Argiustolls

TYPICAL PEDON: Isaac gravelly clay loam. (Colors for moist conditions unless otherwise stated.)

Ap--0 to 8 inches; dark reddish brown (5YR 3/3) gravelly clay loam; weak fine granular structure; friable, slightly sticky, slightly plastic; many fine roots; about 30 percent of this horizon consists of hard angular volcanic gravels; slightly acid; clear smooth boundary. (6 to 10 inches thick)

AB--8 to 11 inches; dark reddish brown (5YR 3/3) gravelly heavy clay loam; weak fine subangular blocky structure; friable, slightly sticky, slightly plastic; common fine roots; about 30 percent of this horizon is hard volcanic gravels; slightly acid; clear smooth boundary. (3 to 6 inches thick)

Bt1--11 to 19 inches; red (2.5YR 4/6) clay; moderate medium subangular blocky structure; firm, slightly sticky, plastic; few fine angular volcanic fragments; few faint clay films on ped surfaces; few fine roots; neutral, gradual smooth boundary. (6 to 10 inches thick)

Bt2--19 to 24 inches; yellowish red (5YR 4/6) clay loam; weak medium subangular blocky structure; friable, slightly sticky, slightly plastic; common fine and medium volcanic fragments; very few faint clay films on ped surfaces; very few fine roots; neutral; gradual smooth boundary. (3 to 7 inches thick)

C--24 to 36 inches; yellowish red (5YR 4/6) light clay loam; massive; very friable, nonsticky, slightly plastic; original rock structure is visible; neutral; clear smooth boundary. (8 to 14 inches thick)

R--36 plus inches; semi-consolidated, bedded extrusive volcanic rock.

TYPE LOCATION: St. Croix, Virgin Islands, 1,000 feet northeast of St. Croix Beach Hotel, Photo No. 1337-7-319.

RANGE OF CHARACTERISTICS: Thickness of the solum varies from 18 to 33 inches and depth to hard rock ranges from 20 to 72 inches plus. Thickness of the underlying saprolite ranges from 10 to 60 inches. Reaction of the profile ranges from medium acid to neutral (pH 5.5 to 7.3).

The A horizon has hue of 7.5YR or 10YR, value of 2 or 3 and chroma of 2 to 4. Texture ranges from clay loam to gravelly clay loam with the amount of gravel ranging from 5 to 50 percent of the soil mass.

The Bt horizon is dominantly clay and has colors with values of 3 or 4, chromas of 4 to 8 in hues of 5YR or 2.5YR. Structure ranges from weak medium to moderate coarse, subangular blocky. Strong vertical cleavage and pressure faces are evident when soil is dry.

COMPETING SERIES: These include the Coamo and Parasol series, all in the same subgroup and family. The Coamo soils are browner (hues of 10YR and 7.5YR) and have calcic horizons within 40 inches of the surface and in addiiton have stratified gravelly underlying horizons. The Parasol soils are yellower (hues of 10YR), more friable and in addition are underlain by highly weathered and fractured intrusive rocks.

GEOGRAPHIC SETTING: These soils occur on moderately sloping to moderately steep topography on side slopes and foot slopes of dissected volcanic uplands with slope gradients of 5 to 40 percent. The regolith consists of partially weathered basic extrusive volcanic rocks. The climate is semi-arid tropical with annual precipitation ranging from 35 to 55 inches and mean annual temperature ranging from 78 to 80 degrees F.

GEOGRAPHICALLY ASSOCIATED: These include the Cramer, Dorothea and Victory. The Cramer soils are underlain by rock sufficiently hard to qualify as lithic at 20 inches or less. The Dorothea soils, are yellower in color (in hues of 7.5YR and 10YR) and lack the dark colored (mollic) surface layers. The Victory soils are yellower in color (hues of 7.5YR and 10YR) have weakly developed cambic horizons. All these associated soils occur in similar topographic positions in the dissected volcanic uplands.

DRAINAGE AND PERMEABILITY: Well drained with medium runoff and moderate permeability.

USE AND VEGETATION: Most areas are in brush or brushy forest. Very few areas are being used for pasture or for cultivated crops. Some areas have been subdivided for housing developments.

DISTRIBUTION AND EXTENT: American Virgin Islands. Series is of very minor extent.

SOIL SURVEY REGIONAL OFFICE (SSRO) RESPONSIBLE: RALEIGH, NORTH CAROLINA

SERIES ESTABLISHED: St. Croix, U.S.V.I.; 1972.

REMARKS: The classification was updated with the 4/91 draft from Fine, mixed, isohyperthermic Udic Argiustolls to Fine, mixed, isohyperthermic Typic Argiustolls. The present definition in Soil Taxonomy does not adequately define Udic-ustic soil moisture regimes for the tropics. The previous OSED date was 6/65.

The Isaac soils were mapped as Descalabrado in the 1932 Reconnaissance soil survey.

Diagnostic horizons and features recognized in this pedon:

Mollic epipedon - zone from 0 to 11 inches (Ap and AB horizon)

Argillic horizon - zone from 11 to 24 inches (Bt horizons)

Lithic contact - zone at 24 inches (R layer)

National Cooperative Soil Survey
U.S.A.