Soil Profiles: Bekenu
Bekenu Series
The Bekenu Series is a member of the Bekenu Family which is fine loamy, siliceous, isohyperthermic, red-yellow to yellow Tipik Tualemkuts. It typifies the family and is developed over mixed sedimentary rocks. Soils of the Bekenu Series are redefined here as being characterized by their deep, well drained profiles with brownish yellow to yellow subsoil colours dominating the subsoil. These soils have an argillic horizon with fine sandy clay loam textures and a ECECclay of less than 24 cmol (+) kg–1 clay in all subhorizons between 25 to 100 cm depth. Structures are weak medium to coarse subangular blocky and consistence is friable. Patchy clayskins occur on ped faces.
Type Location
Soils of the Bekenu Series were first described by Andriesse (1972) during the reconnaissance soil survey of West Sarawak. The above pedon was described by Lim Chin Pang for the CLAMATROPS Tour II organised by the Malaysian Society of Soil Science in 1977 at the 24th milestone Bau-Lundu Road in West Sarawak. The pedon was located on rolling (12–24% or 6–12° slopes) terrain at an elevation of 50 metres (150 feet). The vegetation was logged lowland Dipterocarp Forest. Location: Topographic Sheet 1/109/8, 1°33’20” N, 109°54’00” E (Grid Reference 1/109/8 – 892715 m).
Range in Characteristics
In the past in Sarawak, soils of the Bekenu Series were mapped as Red-Yellow Podzolic soils which can either have a cambic, kandic or argillic horizon with hues of 10YR or 2.5Y with any CECclay value. They are redefined here as soils having only an argillic horizon with red-yellow to yellow colour class and a CECclay of less than 24 cmol in all subhorizons between 25 cm to 100 cm depth. Little is known about the range in characteristics of the Bekenu Series. They generally occur on rolling, hilly to steep terrain. On the steeper terrain the profiles become moderately deep and are no longer the Bekenu Series. Textures in this soil are uniformly fine sandy clay loam and colours are brownish yellow, yellow to olive yellow (10YR6/6–6/8, 7/6–7/8, 8/6–8/8; 2.5Y6/6–6/8, 7/6–7/8, 8/6–8/8). Structure are generally weak medium to coarse subangular blocky and consistence friable. Patchy clayskins are often present. Only the deep soils are retained as belonging to the Bekenu Series in this redefinition. CECclay values are higher than 16 but less than 24 cmol (+) kg–1 clay. In some profiles a few subhorizons may have a CECclay of less than 16 cmol in the 25 to 100 cm depth.
Competing Soils and Their Differences
Soils of the Bekenu Series can easily be confused with other soils developed over mixed sedimentary rocks. Where the clay contents approach 35% they can be confused with soils of the Merit or Bedup Series in Sarawak, the Bungor Series in Peninsular Malaysia and the Kumansi/deep Series in Sabah but all of these soils have more than 35% clay while the Bekenu Series has less. Where the clay content is close to 18% soils of the Bekenu Series can be confused with soils of the Nyalau Series in Sarawak, the Malau Series in Peninsular Malaysia and Kapilit Series in Sabah all of which have less than 18% clay. Other deep soils with fine sandy clay loam textures include the Tukau Series over non-accreting alluvium and the Biawak Series with 2.5Y Hues throughout over metamorphosed sedimentary rocks in Sarawak, the Serdang Series in Peninsular Malaysia and the Tanjong Lipat Series in Sabah. In the proposed redefinition here, the Serdang Series has a kandic horizon (CECclay < 16 cmol) while the Tanjong Lipat Series has an argillic horizon with an CECclay of more than 24 cmol. The Tukau Series like soils of the Rasau Series developed over non-accreting alluvia. The Biawak Series could possibly be correlated to the Bekenu Series yellow variant and this variant could then be deleted from the list of soil series. More data is required.
Setting
Soils of the Bekenu Series typically occur on rolling and hilly terrain (slopes of 12–38% or 6–20°) at elevations of less than 1,000 metres (3,300 feet).
Principal Associated Soils
In the lowlands, soils of the Bekenu Series to-date have been mapped in association with soils of the Nyalau and Merit Series. On the steeper slopes moderately deep and shallow equivalents have also been mapped. These soils are distinguished using the particle-size class and the diagnostic horizon.
Drainage and Permeability
Soils of the Bekenu Series are well drained soils with a good permeability.
Use and Vegetation
Soils of the Bekenu Series have been used extensively for agriculture. They have been used for shifting cultivation, rubber, oil palm and pepper. On the steeper slopes these soils are retained under forest.
Distribution and Extent
To-date soils of the Bekenu Series have only been mapped in Sarawak. The actual extent of these soils is not fully known.
Series Established
This soil was first established by Andriesse (1972) during the reconnaissance soil survey of West Sarawak. The source of name is not known.
Remarks on Classification
The Bekenu Series as redefined here is classified here according to the Malaysian Soil Taxonony – Second Approximation (Paramananthan 1998) as a member of the fine loamy, siliceous, isohyperthermic, red-yellow to yellow family of Tipik Tualemkuts over sedimentary rocks. They are classified here as soils that have a deep argillic horizon with a CECclay of less than 24 cmol (+) kg–1 clay in all subhorizons between 25 to 100 cm depth and with brownish yellow colours. In the Keys to Soil Taxonomy – Eighth Edition (Soil Survey Staff 1998) these soils would probably be Typic Paleudults because these soils have a deep argillic horizon. In the FAO/UNESCO Soil Map of the World – Revised Legend (FAO 1990) the Bekenu Series would probably be classified as Haplic Acrisols due to their CECclay values of less than 24 cmol and the low base saturation.
Suitability for Agriculture
The low fertility status and the terrain on which these soils occur are the main limiting factors for the use of these soils. With proper fertilisation and soil conservation measures a wide range of crops can be cultivated on these soils. These include oil palm, rubber, pepper, fruit trees and upland rice.
