Soils in Peninsular Malaysia are developed from different parent materials, topography and geomorphology. Generally, soils in Peninsular Malaysia are divided into six major groups consist of marine, riverine, sedentary, reworked, Beach Ridges Interspersed with Swales (BRIS) and organic soils (Table 2).

Marine alluvial soils consist of marine, estuarine and brackish water deposits. They are usually found along the coastal plain which covers the total area of 870,531 hectares. The nature of the deposits and the landscape on the West Coast differ from the East Coast. On the West Coast, the presence of Sumatra results in calm seas and consequently clayey material are deposited. On the other hand, the open sea of East Coast results in strong waves and hence a series of beach ridges occur. Beach ridges soils stretched along the East Coast from Kelantan to Johor which covers the total area of 160,090 hectares (Figure 1).

Alluvial soils occur extensively both along the east and West Coast of Peninsular Malaysia. In Peninsular Malaysia, two major groups of alluvial deposits have been identified as riverine deposits and the marine, estuarine and brackish deposits. The riverine deposits which include colluvial and hill wash sediments are further divided into three groups which is known as Older Alluvium, Sub-recent and Recent Alluvial. A total area of 1,514,412 hectares have been map as riverine alluvial soils through out Peninsular Malaysia.

A large variety of sedimentary and metamorphic rocks occur along Titiwangsa Range. A total area of 8,909,919 hectares which covers the largest part of Peninsular Malaysia are limestones, sandstones, shales and granites.

Lateritic soils with profile characteristics having close resemblance to the pediments. They are common soils occur on many part of Peninsular Malaysia but are mainly found in northern Kedah. This soils cover 511,759 hectares. Other groups are known as miscellaneous units consist of mine land, urban land and water bodies which cover 502,876 hectares.

Table 2: Subdivision of Soils in Peninsular Malaysia

Sedentary Soils

The sedentary soils are soils developed in-situ over weathering parent materials or rocks and occur on undulating to steep terrains. These in-situ soils are developed from weathering of igneous, sedimentary or metamorphic rocks. This group of soils occupies major portions of Peninsular Malaysia, especially in areas along the Main Range. Sedentary soils are placed and divided into two tables:

1. Table 1 for soils developed on igneous and high-grade metamorphic rocks.
2. Table 2 for soils developed on sedimentary and low-grade metamorphic rocks.

The main criteria used to differentiate the soils in Table 1 and 2 into soil series are diagnostic horizon, soil depth, colour, texture and base saturation. Sedentary soils are variable in term of textures, colour and soil depth, since they are developed on various types of rocks and are influenced by the nature of slope, climate and geomorphological processes.

However, soils developed on limestone and other calcareous materials are not placed in Table 2 but in Table 12 due to differences in the formation and position on the landscape. Limestone soils normally have high pH, high base saturation, manganese nodules in their profile and are found on flat to undulating terrain.

Reworked Soils

Reworked soils are developed on materials, which have been moved around in their environment by various geomorphic forces, especially pedimentation. In the pedimentation process, the soils and their parent materials are moved from their original sites and redeposited elsewhere on truncated landscapes as evidenced by the exposure of the underlying in-situ saprolite of various parent materials. Many cycles of pedimentation can take place moving the reworked materials back and forth in the environment. Reworked soils are divided into the older alluvium and lateritic soils.

Older Alluvial Soils

Older alluvial soils refer to soils that are formed on reworked granitic materials. The solum overlies saprolitic materials which are not their parent materials. If present, stonelines consisting of quartz gravels, stones and pebbles normally occur just above the saprolite. These soils occur on undulating to hilly topography. The main criteria used to differentiate the soils in this group into soil series are depth to saprolite, colour, plinthite, textural class, and stonelines.

Lateritic Soils

Lateritic soils are formed on pediplains developed on iron-rich shale parent materials. These soils occur mainly on undulating to rolling terrain. By definition, a lateritic soil has a lateritic band that is at least 25 cm thick, consisting of indurated iron rich gravels with its upper boundary occurring within 75 cm of the soil surface. The shapes of lateritic gravels are either subangular, subrounded or rounded depending on the frequency and intensity of reworking the materials have undergone. The main criteria used to differentiate the soils in this group into soil series are depth to saprolite, colour, textural class and types of lateritic gravels.

Riverine Alluvial Soils

Riverine alluvial soils are formed along the river and they are grouped into two tables – Table 5 for soils developed on recent alluvial and Table 6 for sub-recent alluvial soils.

Recent Alluvium Soil

Recent alluvial deposits form a sequence of terraces along the banks of present-day large rivers. These soils occur on generally level to gently undulating landscapes with a relief amplitude of less than two metres, and often have a high silt/clay ratio and weatherable minerals especially mica flakes. The drainage status of the soil varies from very poorly drained to excessively drained. The soil textures vary from sandy loams, sandy clay loams, clays and heavy clays. The main criteria used to differentiate the soils in this group into soil series are texture and drainage class.

Sub-recent Alluvium Soils

The sub-recent alluvial deposits are terraces of former river which occur on flat to undulating terrain of wider relief amplitude. These soils were probably formed by a much earlier river systems but have been subsequently dissected by the present-day river system or reworked by geomorphic processes. Termite mounds are common on the landscape. This group also includes the colluvial deposits, alluvial fans and pediment soils, which share the same landscape. The soil drainage classes vary from poorly drained in more clayey textured soils to excessively drained in sandy soils. Plinthites are often associated with poorly to imperfectly drained, clayey soils. The main criteria used to differentiate the soils in this group into soil series are texture, drainage class, plinthite, and base saturation.

Marine Alluvial Soils

This group of soils is developed over marine clay and sand, and mixed estuarine deposits. Marine clay deposits are mostly formed in the West Coast, while sandy deposits in the East Coast of Peninsular Malaysia. The sandy deposits are also known as BRIS soils are grouped into Table 7. Marine clay soils are found mostly on the coastal plains, especially at the West Coast of Peninsular Malaysia, and in its natural state are covered with mangrove and related swamps and separated into two tables, i) Table 8 - Acid sulfate soils, with the presence of the sulfuric horizon or sulfidic materials within 100 cm of the soil profile, and ii) Table 9 - Non-acid sulfate marine clay soils.

Marine Sand (BRIS) Soils

The term BRIS means Beach Ridges Interspersed with Swales which form the dominant landscape of coastal areas of eastern part of Peninsular Malaysia. BRIS soils occur extensively along the East Coast of Peninsular Malaysia. The soils on the BRIS landscape are predominantly infertile, podzolized soils with sandy textures. Podzolization is the major soil forming process in these soils. The sandy textures facilitate leaching of humus, iron and aluminium from the surface horizons and the deposition of these materials near the water table. This process results in a bleached or albic horizon overlying a spodic horizon in various stages of hardening. BRIS soils are classified based on the drainage class, soil texture and the depth of the spodic horizon.

Acid Sulfate Soils

Acid sulfate soils are characterized by the presence of a sulfuric horizon with a high acidity (low pH) and aluminum concentrations resulting from the chemical and biological oxidation of pyrite upon drainage. The main factors influencing the formation and accumulation of pyrites (FeS2 ) are the supply of sulfur in the sea water, presence of organic matter an iron compounds and the redox environment. Upon drainage, the sulfuric horizon develops. The sulfuric horizon has a pH (water) value of 3.5 or less and is at least 15 cm thick, and there must be evidence to show that the low pH is due to sulfuric acid such as the presence of straw yellow jarosite mottles (with the colour around 2.5Y 8/4) in the profile, sulfidic material below it or containing water soluble sulfate of 0.05% or more. The main criteria used to differentiate the soils in this subgroup into soil series are low pH, profile development, depth to the C horizon, “n” value, and the nature of the subsoil.

Non-acid Sulfate Marine Clay Soils

Non-acid Sulfate Marine Clay Soils Non-acid sulfate marine soils are developed mainly on marine deposits, mixed marine and riverine deposits, which are not influenced by acid sulfate conditions. Although formed in the same environment as the acid sulfate soil, these soil do not have sulfuric horizon within the control section of the profile. The main criteria used to differentiate the soils in this group into soil series are the “n” value, profile development, depth to the parent materials and salinity status.

Organic soils

Organic soils Organic soils or commonly known as peat soils have layers of organic soil materials with a thickness 50 cm or more, cumulatively in the top 100 cm of the profile. Organic soils occur both in the highlands and lowlands of Peninsular Malaysia. However, the highland organic soils are not widespread. The lowland peat occurs almost entirely in low-lying, poorly drained depressions or basins (unless they are artificially drained). In their natural state, the lowland peat consists of dark reddish brown to black, loose, partly decomposed leaves, branches, twigs and tree trunks with a low mineral content. The water table is always high and occurs at or near the surface. In drained areas, the peat is transformed into a compact mass consisting of partially and well-decomposed plant remains with large wood fragments and tree trunks embedded in it. The compact nature of the drained peat is due to decomposition, shrinkage and consolidation of the virgin peat.

The common soil profile morphology in the drained peat consists of three distinct layers: sapric-hemicfibric. The upper layer consists of well-decomposed organic materials or sapric type, a middle layer of containing semi-decomposed organic materials belonging to the hemic type and a lower layer of fibric peat which is made up of a large quantity of relatively undecomposed organic materials comprising tree trunks, large wood fragments and branches.

Organic soils are divided into two tables – Table 10 for shallow organic soils and Table 11 is for moderately deep and deep organic soils. A shallow organic soil has an organic layer of 50-100 cm thick; a moderately deep organic soil has an organic layer of 100-300 cm thick; and a deep organic soil, an organic layer more than 300 cm thick. The main criteria used to differentiate the soils in this group into soil series are the thickness and degree of decomposition of organic layer; and the presence of woody materials.