indahnya Pitas dan Cerita tanah-tani di Sabah

jalan-jalan kat Pitas

Sambil tengok pemandangan, apa kata kita imbau sejarah penyiasatan tanah di Sabah, seperti yang dinyatakan dalal Soil of Sabah (Vol 1 -5).

hujung Pitas...

Climate is dominant factor in soil formation in Sabah, both directly through the element of rainfall and temperature and indirectly through its control over vegetation zones. This is seen in the association of Dipterocarp forest forest with the Tropical Rainy Climate over most of Sabah and the change to Heath Montane Forest where altitude causes a change to a Warm Temperature Rainy Climate.

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Intense weathering over long periods of time or in iron-rich parent materials leads to the process of ferralisation in which bases and silica are removed by leaching to give a relative accumulation of aluminium and iron oxides accompanied by the formation of 1:1 lattice clays of the koalinite group. A horizon in which most of the clays have been weathered to kaolinite and sesquioxides, and which as a result has very low cation exchange capacity, is called an oxic horizon

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The physical processes that result in the movement of particles through soil profiles include leaching and clay translation. Leaching is intense under high rainfall conditions and leads to the formation of soils of low base status; soils with high base status are usually formed on base-rich parent materials. The only counteracting process is the uptake of nutrients by plants and the subsequent deposition at the surface in organic matter. The majority of soil profiles show that the available nutrients are greatest per unit weight of soil at the surface and this is reflected in the values for base saturation. Intensive leaching of highly siliceous parent materials, such as sandstone and coarse-textured alluvium associated with surface layers of acid humus result in the process of podzolisation, in which the removal of bases and sesquioxides leads to an increase in the proportion of silica in the profile and the formation of bleached or albic horizons ; humus and sesquioxides accumulate at depth to form spodic horizons.

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Clay translocation leads to the formation of argillic horizons, which are recognized by an increase in the fine clay fraction with depth and the presence of cutans on ped surfaces and in pores. An increase in the fine clay fraction may also be due to clay formation in the soil profile, but it is often not possible to ascertain in the field whether the clays are illuvial or formed in situ and studies of thin sections are usually required. The argillic horizon forms below an eluvial horizon, but it may be at the surface if the soil has been truncated by erosion ; in this situation there is often little evidence of a change of texture with depth.

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Vegetation acts as a soil-forming factor chiefly by supplying organic matter to the soil, where it accumulates as organic horizons. Plant remains fall onto and are largely decomposed at surface, so that there is usually a decrease in soil organic matter content with depth. Organic matter however, is also present within many profiles as concentrations in root and animal channels of decayed plant roots or material carried down by fauna, notably worms and termites. In addition organic matter is carried downwards by soil water and is redeposited as coatings on pores and structure faces.

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It is well established that under dipterocarp forests below about 900 m (3 000 ft) the decomposition of wood, bark and leaves by bacteria is so rapid that it keeps pace with the continuous addition of plant materials to the surface. As a result surface accumulations of organic matter are generally very shallow.

They comprise thin layers of leaf litter, chopped leaves and mixed humus and mineral particles. These shallow surface layers defined as ochric horizons average 3-5cm (1-2 in) in thickness. Organic carbon contents in general average 2-4% but range from 1% upwards. Carbon/nitrogen ratios are also generally low, and percentage base saturation values of ochric epipedons are extremely variable.

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Surface horizons which comprise humus intimately incorporated with mineral matter, and which have dark colours, high base saturation and well development structure are defined as mollic horizons. Under hydromorphic conditions, which result mainly from the temperature at altitude greater than about 900 m (3 000 ft), the decomposition of plant remains is retarded and partly decomposed organic matter accumulates at the surface.

The surface horizons which result are referred to as histic O horizons. Deep accumulations of such organic matter are known as peat. The degree of humification of organic matter accumulating under hydromorphic conditions varies from being only slightly humified and consisting essentially of non-humified plant remains to very strongly humified organic matter lacking visible plant remains.

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Surface horizons consisting of fresh and/or partly decomposed organic matter accumulated under predominantly aerobic conditions and having similar organic carbon contents to histic horizons are referred to simply as O horizons. They accumulate on sites where the bacterial decay of plant remains is curbed by condition other than very poor drainage. Such conditions are satisfied for example under heath vegetation where extreme acidity of parent materials, very low nutrient levels and periodic conditions of extreme dryness retard plant decay. Under such conditions the O horizon comprises distinct layers of leaf litter, partly decomposed litter in which decomposition is proceeding, and wholly humified organic matter sharply separated from the under-lying mineral soil.

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Landform affects soil development directly the nature of relief and its effect on erosion, deposition and soil moisture regimes. Erosional forces create the relief and slope of landforms and in turn slope affects the soil-forming processes active within the weathering mantle. The depth of soil on any slope represents the balance between the rate of weathering and the rate of removal by forces due to gravity including mass movement, surface wash and gully erosion.

These forces are most active on steep slopes and the depth of soil on such slopes is in general less than on gentle slopes. Mass movement occurs on very steep slopes and is seen in the form of terracettes, screes and landslides. Slope wash results from surface runoff and takes place on slopes greater than 5º, being most active on steeper slopes. On steep slopes, mass movement and slope wash produce unstable stony deposits in which profile development is periodically interrupted by further movement. The weakly developed horizons in these deposits are defined as cambic horizons and they also occur in alluvial deposits in the early stages of soil formation.

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Slope is the main determinant of soil drainage in terms of removal of water by surface runoff, downward percolation into the soil and the presence of water tables . Excessively drained soils occur on very steep slopes or on deep coarse-textured deposits unaffected by groundwater fluctuations.

Well drained and moderately well drained soils occupy the majority of middle and upper slopes, while imperfectly drained soils occur on lower slopes affected by groundwater fluctuations. Imperfectly and poorly drained soils occur on valley floors, floodplains, low terraces and beaches where groundwater is near the surface.

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Poorly and very poorly drained soils occur in back swamps and coastal swamps with high watertables. Fluctuating watertables result in alternate periods of reduction and oxidation in the soil horizons above the permanent groundwater level and ferric iron compounds, released by weathering, are reduced to ferrous compounds and change from red valley or yellow to grey. The process is known as gleying and is seen in the profile in the form of grey and rust mottles. Horizons in which are mainly grey in colour are referred to as gleyic horizons.

All iron compounds, however are not gleyed when the soil drainage is poor, true gley colours perhaps being masked by residual colours of unreactive compounds. Iron compounds in the form of rust mottles may in time develop into concretions which, should they persist, act as nuclei for further iron concentration. In extreme cases the accumulation of iron compounds leads to the formation of plinthite , which on drying hardens irreversibly in the form of ironstone.

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In the early stages of soil formation the nature of the parent material usually determines soil properties and soils lack diagnostic horizons ; recent alluvial deposits in tidal swamps, meander belts and valley floors and recent beach deposits are examples of such parent materials. On steep slopes where there is active erosion profiles do not develop beyond the stage of shallow surface horizons overlying hard rock.

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With time, soil forming processes become active and this is seen in the development of soils with cambic horizons. They occur on alluvial deposits and on a wide range of unstable colluvial deposits on steep slopes, where the normal processes of clay eluviation and illuviation are interrupted. The parent materials are in all cases young alluvial or colluvial deposits and the age of the geological formation from which they are derived is irrelevant.

segarnya enveronment

Soils with gleyic horizons form irrespective of the age and nature of parent materials because the gleyic process is dependent on site drainage. However, the majority are formed on alluvial deposits where watertables are continually high. In sites with fluctuating watertables cambic horizons are common and argillic horizons develop with time.

Soils with argillic horizons are the most widespread soils in the survey area; they are pedologically older than soils with cambic horizons, because they occur on more stable slopes, on which the normal processes of eluviation and illuviation can operate without interruption. Such soils occur on a wide range of parent materials of varyingage including alluvium on floodplains and terraces, igneous rocks, sandstone, mudstone, limestone and tuffaceous rocks.

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In pedological term the only old soils which occur in the area are those with spodic and oxic horizons ; in both cases the processes of soil formation have been accelerated by particular factors other than the age of the deposits on which they are developed.

Soils with spodic horizons are formed on materials including comparatively recent beach sands, old terrace alluvia and stable slopes of sandstone. In all cases the presence of coarse-textured, largely siliceous deposits, has favoured the establishment of heath forest with associated accumulations of raw humus followed rapidly by podzolisation.

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Soils with oxic horizons are normally associated with old land surfaces, which have undergone a complete cycle of weathering. In the survey area, however the majority occur on steep slopes on colluvial deposits derived from ultrabasic rocks, in which ferralization has been rapid. It is thus the nature of the parent material rather than its age which has given rise to these pedologically old soils.

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tekstur

The climate is hot and humid throughout the year and although Sabah lies outside the typhoon belt the areas are occasionally affected by severe tropical storms. The average annual rainfall ranges from about 1 780 mm (70 in) to about 3 800mm (150 in). The highest rainfall is in the south-west ( Beaufort and Labuan) and the lowest in the interior (Tenom, Keningau and Tambunan) and at Tawau on the south-east coast.

The contrasts in regional rainfall reflect the occurrence of 2 main seasons; the north-east monsoon begins in November and lasts until March and it is during this season that the north-east coast experiences its heaviest rainfall ; the south-west monsoon prevails from May until September. The temperature varies little with the season and averages about 27ºC ( 80ºF ) near sea level.

Surface temperatures inland fall at a rate of about 1.5ºC ( 3ºF ) for every 300 m ( 100ft ) increase in altitude so that above about 1 200 m ( 4 000 ft ) there is a change from Tropical Rainy Climate to Warm Temperate Rainy Climate ( Trewartha, 1954 ) ; the latter affects much of the Crocker and Trusmadi Ranges above 1 200 m (4000 ft) . Climatic data are very limited, but records of a reasonable length are available for Labuan, Kota Kinabalu, Kudat, Beaufort,Tenom, Sandakan, Tawau and Tambunan. Climate is discussed at length in Volumes 2, 3 and 4

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Sabah can be divided into 4 main physiographic region, namely the Western Lowlands, the Western Cordillera, the Centrals Uplands and the Eastern Lowlands ( Collenette, 1963 ). The Western Lowland include the foothills, plains and islands to the west of the Crocker Range.

The Western Cordillera comprises a belt of mountainous country about 80 km ( 50 mi ) in width parallel to the west coast. It includes the Crocker, Trusmadi, Witti and Maligan Ranges and associated intermontane plains and valleys. The Crocker range is one of the main geographical features of the country and its culminates in Gunong Kinabalu, which at 4 101 m ( 13 455 ft ) is the highest mountain in sout-east Asia.

The Central Uplands comprise extensive tracts of mountainous country to the east of the Western Cordillera, including the Labuk, Kuamut, Segama and Tawau Highlands. The Eastern Lowlands stretch from the Bengkoka Peninsula in the north to the Semporna Peninsula in the south and include extensive tracts of moderate to low hills, the broad valleys, of the Sugut, Labuk, Kinabatangan and Segama rivers and extensive deltas.

segar

The oldest rocks in Sabah, referred to collectively as Crystalline Basement, include granodiorite, diorite, gabbro, migmatite, amphibolite, hornblende, gneiss, hornfels and schist ; they only occur in the Tawau and Lahad Datu Districts, notably in the Segama catchment.

Intrusive igneous rocks occur in 3 main areas, namely Kinabalu, the Labuk Highlands and in the Tawau and Lahad Datu District. Ultrabasic igneous rocks, composed largely of serpentinised periodotite, are most extensive forming, in particular, the mountain range in the Labuk Highlands extending from Gunong Rara in the south to Suroh Suroh in the north. Bassic igneous rocks such as gabbro, dolerite and diorite occur sporadically in the 3 areas referred to, but acid igneous rocks including granodiorite, adamellite and tonalite are restricted to the Kinabalu area, forming in particular Mount Kinabalu.

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Volcanic rocks occur in the Labuk Highlands and in the Semporna Peninsula. In the former they comprise spilite and basalt lavas and in the latter comprise olivine basalt, dacite lavas, andesite, volcanic breccia, tuff and pyroclastic rocks.

Sedimentary formations are widespread in Sabah ; they range in age from the Eocene to the Pliocene and comprise sandstone, mudstone, shale, clay, and limestone. The Crocker formation is by far the most extensive, forming most of the west coast and the Crocker range and extending to the north-east coast. It comprises interbedded sandstone, shale and mudstone. Limestone form minor parts of many formations, but are dominant in the Gomantong Limestone, Togopi and Timohing Formations.

Sedimentary- volcanic formations are dominant on the east coast, extending from the Labuk estuary across the Kinabatangan and Segama to the Dent and Semporna Peninsulas. They comprise sandstone, mudstone, slump breccia, chert, spilite and various volcanic rocks.

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Terraces of sand and gravel occur in the interior plains, along the main rivers and on the coasts, particularly in the north-east and south-east. Recent alluvial deposits of clay and sand are extensive along the east coast in the lower reaches and estuaries of the Sugut, Labuk, Kinabatangan and Segama, around Cowie Harbour and the south of the Semporna Peninsula and along the west coast, notably on the Klias Peninsula. Peat is associated with the recent alluvial deposits in the Klias Peninsula and the lower reaches of Kinabatangan and Segama.

source:

Acres, D., et.al (1975). Soil of Sabah. Kota Kinabalu.