Soil Classification in the 2 ha Forest Dynamics Plot in Primary Forest,
Bukit Timah Nature Reserve, Singapore
Benjamin L. Turner
Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Republic of Panama
Overview
Soil classification in the 2 ha forest dynamics plot at Bukit Timah was undertaken as part of the HSBC
Soil Carbon Inventory. The plot is described in detail elsewhere (Lum et al., 2004).
Location:
Geology:
Climate:
Elevation:
Topography:
Vegetation:
Bukit Timah Nature Reserve, Singapore (1° 21' 14'' N, 103° 46' 42'' E).
Bukit Timah Granite; mid-Triassic.
Equatorial aseasonal; mean annual precipitation 2473 mm and no clear dry season,
although evapotranspiration exceeds rainfall for 1–3 months around February; mean
annual temperature 26.9°C.
The plot varies from ~74 to 119 m a.s.l.
Steep slopes running north and south from a central ridge.
Primary lowland tropical forest; coastal hill forest dominated by members of the
Dipterocarpaceae; in the most recent census (2003) the plot contained 329 species of
trees with stems ≥ 1 cm diameter at breast height among ~12,000 stems; basal area is
34.6 m2 ha-1 and aboveground biomass is 335 Mg ha-1 (Ngo, 2012).
Previous soils research in Bukit Timah Nature Reserve
In a soil survey of Singapore, Ives (1977) classified Bukit Timah soils as being in the Rengam series.
There were described as well drained soils formed on granite, with 10–20 cm of dark grayish brown
topsoil over yellowish brown to brownish yellow subsoil, becoming yellowish red at > 1.5 m. Texture
ranged from sandy loam topsoil to fine sandy clam loam, to coarse sandy clay in deeper subsoils. A red
variant was noted to occur where erosion had removed the overlying topsoil.
Grubb et al. (1994) reported data from chemical analysis of the upper 40 cm of Bukit Timah soils.
Values are in agreement with results reported here, with soils being acidic (pH 3.5–4.2) and very low in
total phosphorus (50–80 mg P kg-1 for the surface 10 cm and 30–50 mg P kg-1 for 10 to 40 cm). The low
phosphorus concentrations were shown to limit growth of seedlings by Burslem et al. (1994).
Description of fieldwork
Fieldwork was conducted in April 2008 and completed in November 2008 with the assistance of Dr
Shawn Lum and Kang Min Ngo (National Institute of Education, Nanyang Technological University,
Singapore). A profile pit was excavated outside the southwest corner of the plot. The profile was
described according to USDA Soil Taxonomy (Soil Survey Staff, 1999) and samples taken by genetic
horizon for laboratory analysis and determination of bulk density (see below). Further samples were taken
by augering through the pit floor. Soils were air-dried and returned to the Smithsonian Tropical Research
Institute in the Republic of Panama for physical and chemical analyses.
1 Laboratory procedures
Bulk density was determined by the excavation method (Grossman and Reinsch, 2002), removing ~1 L of
soil and using water to measure the volume of the plastic-lined cavity. Soil pH was determined in both
deionized water and 10 mM CaCl2 in a 1:2 soil to solution ratio using a glass electrode. Particle size was
determined by the pipette method following pretreatment to remove soluble salts, organic matter, and iron
oxides (Gee and Or, 2002). Total carbon and nitrogen were determined by automated combustion and gas
chromatography with thermal conductivity detection using a Thermo Flash 1112 analyzer (CE Elantech,
Lakewood, NJ, USA). Total phosphorus was determined by ignition (550°C, 1 h) and extraction in 1 M
H2SO4 (16 h, 1:50 soil to solution ratio), with phosphate detection by automated neutralization and
molybdate colorimetry on a Lachat Quickchem 8500 (Hach Ltd, Loveland, CO, USA). Exchangeable
cations were determined by extraction in 0.1 M BaCl2 (2 h, 1:30 soil to solution ratio), with detection by
inductively-coupled plasma optical-emission spectrometry on an Optima 7300 DV (Perkin-Elmer Ltd,
Shelton, CT, USA) (Hendershot et al., 2008). Total exchangeable bases (TEB) were calculated as the sum
of Ca, K, Mg, and Na; effective cation exchange capacity (ECEC) was calculated as the sum of Al, Ca,
Fe, K, Mg, Mn, and Na; base saturation was calculated as (TEB ÷ ECEC) × 100.
General features of the soil
The soils are very acidic and contain low concentrations of exchangeable base cations (calcium,
potassium, magnesium); most of the cation exchange capacity is aluminum. Total phosphorus
concentrations are also low, being among the lowest in the CTFS plot network (similar concentrations
occur in the Tawar soils of the Pasoh plot in Peninsula Malaysia). There is clear evidence of clay
movement through the profile, with a clay-rich (argillic) B horizon and a decline in clay concentration
below this. The low cation exchange capacity of this horizon classifies it as kandic. The soil is gritty
below the kandic horizon, due to ~20% angular quartz grains slightly greater than 2 mm diameter. The
sandy nature of the soil is therefore not reflected in the particle-size analysis, which indicates a more clay
rich soil. The soil is similar to slope soils of the Khao Chong plot in Thailand (I.C. Baillie, pers. comm.).
References
Burslem, D.F.R.P., Turner, I.M., Grubb, P.J., 1994. Mineral nutrient status of coastal hill dipterocarp forest and adinandra
belukar in Singapore: bioassays of nutrient limitation. Journal of Tropical Ecology 10, 579–599.
Gee, G.W., Or, D., 2002. Particle size analysis. In: Methods of Soil Analysis, Part 4: Physical Methods, edited by J.H. Dane and
C. Topp. Soil Science Society of America: Madison, WI, USA, pp. 255–293.
Grossman, R.B., Reinsch, T.G., 2002. The solid phase. In: Methods of Soil Analysis, Part 4: Physical Methods, edited by J.H.
Dane and C. Topp. Soil Society of America, Madison, WI, USA, pp. 201–293.
Grubb, P.J., Turner, I.M., Burslem, D.F.R.P., 1994. Mineral nutrient status of coastal hill dipterocarp forest and adinandra
belukar in Singapore: analysis of soil, leaves and litter. Journal of Tropical Ecology 10, 559–577.
Hendershot, W.H., Lalande, H., Duquette, M., 2008. Ion exchange and exchangeable cations. In: Soil Sampling and Methods of
Analysis, edited by M.R. Carter and E. Gregorich. Canadian Society of Soil Science and CRC Press, Boca Raton, FL, USA,
pp. 173–178.
Ives, D.W., 1977. Soils of the Republic of Singapore. New Zealand Soil Survey Report 36. New Zealand Soil Bureau,
Department of Scientific and Industrial Research, Lower Hutt, New Zealand.
Lum, S.K.Y, Lee, S.K., LaFrankie, J.V., 2004. Chapter 25. Bukit Timah forest dynamics plot, Singapore. In: Tropical Forest
Diversity and Dynamism: Findings from a Large-Scale Plot Network, edited by E.C. Losos and E.G. Leigh, Jr. The
University of Chicago Press, Chicago, IL, USA, pp. 464–473.
Ngo, K.M., 2012. Quantification of carbon in Bukit Timah Nature Reserve, Singapore. M.Sc. thesis, National Institute of
Education, Nanyang Technological University, Singapore.
Soil Survey Staff, 1999. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. United
States Department of Agriculture–Natural Resources Conservation Service, Lincoln, NE, USA.
2 Profile Description
Classification:
Profile location:
Date and season:
Elevation:
Slope and site position:
Soil moisture regime:
Soil temperature regime:
Parent material:
Vegetation:
Drainage:
Surface features:
Faunal activity:
Rooting depth:
Notes:
Diagnostic features:
ULTISOL: Fine, kaolinitic, isohyperthermic, Typic Kanhapludult
~20 m south of the forest dynamics plot in primary forest; ~40 m south of
junction of Rock Path and Catchment Path
8–11 April and 29 November 2008
~100 m a.s.l.
Upper slope, convex shoulder, ~25% facing southeast, dropping linear radially
Udic; 2473 mm annual rainfall
Isohyperthermic; mean annual temperature 26.9°C
Bukit Timah Granite
Lowland dipterocarp hill forest
Well drained
100% cover of leaf litter
None observed
Roots throughout the profile to 5.5 m
Horizons > 50 cm contained many angular quartz fragments slightly larger than
coarse sand, so the clay decrease was more pronounced than evident from the
particle-size analysis on the fine earth fraction. Color values are for moist soil
and textures are field textures.
(i) Kandic horizon from 20 to 120 cm with a clay decrease below
(ii) Udic moisture regime
(iii) > 35% clay in the particle-size control section (kandic horizon)
A—0 to 10 cm; very dark grayish brown (10YR 3/2; 10YR 5/3 dry) sandy clay loam; moderate coarse subangular
blocky structure, breaking to medium blocky; slightly sticky and very plastic; many very fine, fine, and
medium, and common coarse roots; few quartz granules; clear smooth boundary.
E—10 to 20 cm; brown (10YR 4/3) slightly gritty sandy clay loam; weak to moderate coarse subangular blocky
structure, breaking to fine and very fine subangular blocky; sticky and plastic; very few fine, few medium
and common coarse roots; gradual smooth boundary.
Bt1—20 to 50 cm; yellowish brown (10YR 5/6; 10YR 7/6 dry) gritty sandy clay; moderate coarse subangular
blocky structure, breaking to fine and very fine subangular blocky; sticky and plastic; common medium and
coarse roots; common organically-enriched patches formed from decaying roots; diffuse smooth boundary.
Bt2—50 to 70 cm; brownish yellow (10YR 6/6) very gritty sandy clay; moderate coarse subangular blocky
structure, not breaking; plastic, sticky to very sticky; common organically-enriched patches formed from
decaying roots; common medium and coarse roots; diffuse smooth boundary.
Bt3—70 to 120 cm; reddish yellow (7.5YR 6/8; 7.5YR 7/6 dry) very gritty sandy clay; moderate coarse subangular
blocky structure; sticky and plastic; common large quartz grains; common medium, coarse, and very coarse
roots; gradual smooth boundary.
BC—120 to 275 cm; yellowish-red (5YR 5/8; 5YR 7/6 dry) very gritty sandy clay; weak to moderate coarse
subangular blocky structure; many large angular quartz grains; sticky; few medium and coarse roots; gradual
smooth boundary.
C1—275 to 420 cm; red (2.5YR 4/8; 2.5YR 6/6 dry) very gritty sandy clay; many large angular quartz grains and
other coarse angular fragments of weathered granite with mixed colors; very few medium roots; diffuse
smooth boundary.
C2—420 to 485 cm; yellowish red (5YR 5/8; 5YR 4/6 dry) very gritty sandy clay; fine and medium granular
structure; many large angular quartz grains; diffuse smooth boundary.
C3—485 to 550+ cm; reddish yellow (7.5YR 6/8; 7.5YR 7/6 dry) very gritty sandy clay; many large angular quartz
grains and large pieces of well-weathered granite breaking with moderate pressure; very few fine and
medium roots.
3 Laboratory Analysis
Stones (>2 mm) Sand Silt Clay —%— —%— —%— —%— 0.2 53.5 11.7 34.8 sandy clay loam 1.36 0.4 50.5 35.2 14.3 loam Bt1 1.20 0.7 50.6 9.9 39.6 sandy clay Bt2 1.42 5.8 54.5 8.3 37.3 sandy clay 70–120 cm Bt3 1.45 18.3 45.5 12.7 41.8 sandy clay 120–275 cm BC 1.65 37.8 48.9 11.7 39.4 sandy clay 275–420 cm C1 – 15.7 52.0 18.2 29.8 sandy clay loam 8 420–485 cm C2 – 20.0 50.2 25.4 24.3 sandy clay loam 9 485–550 cm C3 – 22.1 46.7 24.2 29.1 sandy clay loam Total N C:N Total P —%— Horizon Depth Designation BD —cm— —g cm —
1 0–10 cm A 0.89 2 10–20 cm E 3 20–50 cm 4 50–70 cm 5 6 7 Horizon ‐3
——————Soil pH—————— Water CaCl2 BaCl2 Total C —%— mg P kg
1 3.78 3.35 3.48 4.04 0.203 19.8 55.6 2 3.78 3.36 3.73 1.64 0.104 15.7 39.7 3 3.96 3.55 3.91 0.98 0.074 13.3 16.9 4 4.01 3.66 4.05 0.44 0.042 10.5 6.8 5 4.04 3.68 4.05 0.35 0.038 9.2 5.0 6 4.19 3.76 4.06 0.10 <0.01 19.7 5.1 7 4.46 3.96 4.20 0.06 <0.005 28.9 5.2 8 4.47 3.97 4.22 0.05 <0.005 23.0 7.3 9 4.48 3.99 4.28 0.05 <0.005 21.7 3.6 Horizon Al Ca Fe K Mg Mn Na TEB ECEC ‐1
————————————————cmolc (+) kg —————————————————— Textural class ‐1 Base saturation —%— 1 5.66 0.26 0.14 0.19 0.17 0.01 0.05 0.67 6.49 10.4 2 5.60 0.09 0.10 0.11 0.07 0.01 0.13 0.40 6.11 6.6 3 4.92 0.05 0.02 0.09 0.04 <0.01 0.04 0.21 5.15 4.0 4 3.66 0.03 0.01 0.06 0.02 <0.01 0.03 0.14 3.82 3.7 5 3.56 0.01 0.01 0.06 0.02 <0.01 0.02 0.11 3.67 2.9 6 3.42 0.03 <0.01 0.03 0.02 <0.01 0.06 0.15 3.56 4.2 7 2.31 0.01 <0.01 0.06 0.02 <0.01 0.02 0.11 2.42 4.7 8 2.53 0.03 <0.01 0.05 0.03 <0.01 0.02 0.12 2.64 4.7 9 3.15 0.03 <0.01 0.05 0.03 <0.01 0.06 0.17 3.32 5.2 4 Figure 1. The soil profile close to the 2 ha forest dynamics plot at Bukit Timah Nature Reserve,
Singapore, showing an organically-enriched A horizon above a yellow argillic (clay-enriched) B horizon
and a red gritty C horizon. Photo: B. Turner.
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