Soil and landform features of mountainous terrain in South Thailand

Geoderma, 18(1977) 207--225 © Elsevier Scientific Publishing Company -- Printed in The Netherlands

207

SOIL AND LANDFORM FEATURES OF MOUNTAINOUS TERRAIN IN SOUTH THAILAND

K.J. VIRGO and D.A. HOLMES

Hunting Technical Services Ltd, Boreham Wood, Hefts., WD6 1SB (Great Britain) (Received August 27, 1976; January 7, 1977)

ABSTRACT Virgo, K.J. and Holmes, D.A., 1977. Soil and landform features of mountainous terrain in south Thailand. Geoderma, 18: 207--225. The results of reconnaissance surveys over 2,000 km 2 are used to describe mountainous terrain on granite, gneiss and pelite. Detailed morphological and laboratory analytical data are presented for soils derived from each of these rock types. The soils, which are classed as Paleudults, are acid, kaolinite-rich and deficient in nutrients. Soils formed on granite and gneiss have high proportions of coarse quartz particles (2.0--50.0 ram); the dilution effect of this inert material further reduces their effective nutrient status. The degree of weathering, as indicated by the silt:silt+clay ratio, increases with profile depth. Mean gradients recorded on the granite and pelite hillslopes are 27 ° and 29 °, respectively. A greater uniformity of slopes was observed on pelites than on granites; the 'missing angle' technique was employed to identify 'natural' slope classes. Gradients tend to increase downslope, indicating a rapid removal of erosion products at the footslope and a rejuvenation of the valleys. A significant association was recorded between soil depth and slope gradient on pelite but not on granite or gneiss landforms.

INTRODUCTION

In recent years a considerable amount of soil survey has been conducted in South Thailand (Dent, 1967, 1968, 1973; Hunting Technical Services, 1968; SSAD .1, 1973). Vijarnsorn and Fehrenbacher (1973) have described the pedology of granite-derived soils in Narathiwat Province. However, these surveys were all concentrated on the lowlands, which have relatively subdued relief, and therefore possess the greatest agricultural potential in the region. The extensive mountainous areas have been largely ignored in the pedological sense and mapped simply as 'steepland' or 'slope complex'. Although these mountainous and mainly forested areas have very limited agricultural potential, they recently have assumed an increasing importance as an outlet for the population pressure of the lowlands, as discussed by ,i Soil Survey and Analysis Division, Bangkok.

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209 Berkoff {1976), and as a source of commercial timber. To provide information for the rational planning of settlement, in 1972 the Royal Thai Government commissioned Hunting Technical Services to carry o u t reconnaissance soil surveys in the region. These investigations covered 220,000 hectares scheduled as the Southern Land Settlements (SLS), which lie in mountainous terrain in the extreme south of the c o u n t r y (Fig.l). The prime objective of this paper is to describe the soil and landform features of these upland areas. The descriptions refer specifically to the Prinyor Settlement b u t reference is made to data obtained over the whole area (Redecon Ltd, 1974). A subsidiary objective is to test the relevance to Thai conditions of some of the principles of soil-slope relationships recently developed in similar regions of West Malaysia (Swan, 1970; Morgan, 1973). GENERAL DESCRIPTION OF THE AREA

Geology and landforms The regional geology has been described and mapped by J u m c h e t (1970) and G o b b e t t (1972). A sketch map of the geology of the SLS, revised from information obtained in the present study, is shown in Fig.2. The region is dominated by the series of intruded granite plutons which form the main mountain ranges of the peninsula. The rest of the area is underlain by a series of pelitic rocks (argillaceous metasediments), sandstones and, locally, dolomitic limestones. These rocks have been metamorphosed to varying degrees by the subsequent intrusions, resulting in a zone of gneiss along the granite boundary. The higher mountains, in which individual summits attain 1,500 m elevation, are formed on granite; gneiss forms mountains of lower elevation. The resistant sandstones and quartzites form long steep-sided strike ridges, whereas the softer pelites usually form low-elevation hills with an intricate pattern of steep-sided branching ridges. Limestones form vertical-sided 'buttes'.

Climate The area has a humid tropical climate; it is influenced by the NE Monsoon from November to March and by the SW Monsoon from May to September. Climate records are sparse; at Narathiwat (near sea level) the m o n t h l y mean minimum and maximum temperatures are 22°C and 33°C, respectively. The annual mean rainfall in the area is around 2,000 mm (Table I). Mean monthly rainfall exceeds 100 mm in most months b u t there is a rather dry season from February to April. Most rainfall has a high or very high intensity. The p2/p index (where p is the mean rainfall of the wettest month and P is the mean annual rainfall), an indication of the seasonality and aggressiveness of rainfall (Fournier, 1960) increases towards the northeast (Table I), continu-

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211 TABLE I Precipitation means for principal stations Station

Rainfall (mm) annual

p2/p driest

index

month

(February) Narathiwat Town Yala Town Bannang Sata Tarnto

2,645 2,043 1,924 1,928

61 30 50 33

138 68 39 40

ing the trend reported b y Morgan (1974) in West Malaysia. Extrapolating from the Malaysian data used b y Morgan, annual erosivity values of 15,000 to 20,000 J m -2 show the area to be one of high erosion risk.

Vegetation and landuse The SLS lie within the tropical 'moist forest' ecological life-zone as defined by Holdridge et al. (1971). The natural forest contains a great diversity of evergreen tree species, belonging mainly to the Dipterocarpaceae and Leguminosae families. Primary forest still covers some 88,000 ha (40%) of the area b u t is being actively exploited for timber and is currently being destroyed at the rate of at least 1,500 ha annually. In the main valleys and on the lower slopes smaU-scale farming based on rubber has n o w largely replaced the traditional shifting cultivation system of hill rice, bananas and other annual crops. In recent years the increased population, due mainly to formal settlement of the SLS, has forced cultivation onto the steeper slopes. Results from erosion test plots in Tarnto Settlement, operated b y the Department of Land Development, show the annual rates of soil loss under clean cultivated coffee to be 25 to 30 t per ha on a 12 ° slope. Even greater rates can be expected as cultivation is extended to steeper slopes. METHODS The p o o r accessibility and great e x t e n t of the area to be covered necessitated considerable reliance on the use of aerial-photograph interpretation (API). The value of API for soil survey is greatly reduced in forested areas of the humid tropics, because the density of the vegetation masks minor topographic detail and causes underestimation of Slope and because, under well drained conditions, there is rarely any detectable soil-vegetation relationship (Moss, 1975). However in the SLS these inherent limitations were to some extent offset b y the strong relief, which facilitated the identification of physiographic divisions.

21 ?,

Units of land with similar inferred lithology, landform and relative eleva tion were delineated. Field surveys were conducted to characterise the ground features of these API land units. Inspection sites were locat~ed to cover as wide a range of topographic variations as possible within each land unit. At a total of 1,500 sites slopes were measured and soils were examined, enabling the land units to be redefined in terms of their constituent soils and relief. Map series were compiled showing slopes, soils and land capability at scales ranging from 1:25,000 to 1:100,000, according to the density of field survey. Soil samples were collected from representative profiles in Yala Gulong and Prinyor. Samples were air-dried and sieved to pass a 2 mm screen: the residue of coarse separates was weighed. The fine earth fractions were analysed for particle size distribution, using sodium hexametaphosphate as dispersing agent. Exchangeable Ca, Mg, K and Na were determined by atomic absorption after extraction with ammonium acetate; exchangeable hydrogen was determined using Woodruff's Reagent and pH depression; the cation exchange capacity (CEC) was measured by the triethanolamine--barium chloride method. The pH was determined at 1:2.5 dilution in both water and CaC12. Organic carbon was measured using the potassium dichromate--sulphuric acid digestion method. SURVEY

RESULTS

The survey results are related specifically to mountainous terrain in Prinyor Settlement, which exemplifies the range of major soil and landform features typical of the SLS (Southern Land Settlements).

Land forms Prinyor Settlement extends eastwards from the watershed formed by the main granite massif at 500 to 1,000 m down towards the Sai Buri river at less than 50 m elevation, crossing at right, angles to the regional strike, The cross-profile (Fig.3) illustrates the division of the area into mountainous terrain in the west and undulating to rolling lowland terrain in the east. This distinct division between lowlands and steeplands (at an elevation of 100 to 150 m) was observed throughout the SLS and broadly divides the agricultural from the currently non-agricultural lands. A similar topographic boundary has been reported as being a significant geomorphological feature of the Malay Peninsula (Leamy and Panton, 1966). The lowlands comprise a series of erosional 'terraces' which are tentatively correlated with the 'low', 'middle' and 'high' terraces reported by Dent (1968) in the western province of Trang. The moderately extensive low and middle terraces have undulating relief whereas the high terrace is limited in extent, more dissected and usually separated from the middle terrace by a pronounced bluff.

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The mountainous areas are characterised by very strong relief. The granite forms bold irregular mountains with broad convex summits and a low density of drainage. They are coarse grained rocks, consisting of more than 70% free quartz, that weather to produce several metres of gravelly material containing boulders. The slopes on the granite mountains have great variety and appear to bear little relation to position in the topographic sequence. Areas of :moder. ate relief are distributed at various elevations, interspersed with very steep gradients or even free faces comprising smooth rock outcrops. Most slopes in the granite hills lie in the range 20 ° to 35 ° . At several localities in the steep granite hills there exist small areas of undulating land, situated slightly upstream of waterfalls or 'nick-points'. They are seldom more than 30 ha in extent and typically occur along the boundary with the pelites. They are thought to represent remnants of formerly more extensive erosion surfaces: the resistant nature o f the granites and in some cases sandstones, has protected these upland valleys from incision by the subsequent advance of headward erosion in the softer pelites, resulting in the distinct nick-points at the geological boundary. Further evidence of an older erosion surface is indicated by the concordant summits of pelite hills in Yala Gulong Settlement, which lie at 300 to 400 m elevation. The landscape on gneiss is very similar to that on granite but the hills are generally of lower elevation. Typically the gneiss forms a broad belt of hills alongside the granite. Hill-forms on pelite exhibit a distinctive pattern of ridge and ravine terrain, in which the ridges have very steep rectilinear side slopes and rounded crests: short lateral spurs occur alternately along each side of the main ridge. The steepest gradients c o m m o n l y occur on the lower slopes, which form an angular break with the level terrain of the narrow valley floor. A similar steepening of gradients has been observed in the south of the Malay Peninsula (Young, 1972; Morgan, 1973), where a regional lowering of the base level of erosion has resulted in a very rapid removal of weathered debris on the lower slope elements which prevents the formation of concave colluvial footslopes. The rocks are argillaceous (shales, phyllites, schists) and weather to produce a clay-textured shaley soil parent material, the depth of weathering being usually less than one metre. Drainage patterns are sub-dendritic and comparatively dense, reflecting the impermeability of the pelite regolith.

Slope features In the mountainous terrain gentle slopes are confined to the narrow valley floors and the rounded hill crests, which between them comprise a very small proportion of the total area in terms of land surface. Future agricultural development will necessarily be located mainly on the steep land. "In an a t t e m p t to assist mapping of gradients an analysis was made of the slope records for 340 routine survey sites located on the main hill slopes on both granite and pelite landforms. Data for valley floors and hill crests were excluded.

215

Individual slope measurements were compiled as frequency occurrence histograms for each of the land units (Fig.4). The granites and pelites are seen to have a similar range of major slopes; there is a significant (P = 5.0 per cent) difference between the mean slope values, which are 27 ° and 29 °, respectively. Although the coefficient of variance for pelites (20.2) is only slightly less than for granites (21.6), the landforms on pelites show a greater uniformity of slope with a pronounced peak frequency occurring at the median value of 30.1 °. Whereas the different slopes in the granite areas were likely to occur at any point in the topographic sequence, field traverses on pelites showed a closer correlation between gradient and topographic position. The pred o m i n a n t gradient on the rectilinear mid-slopes is 30--32 ° , which probably

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216

represents the equilibrium slope for pelites under forest cover. Slopes in excess of 33 ° occur chiefly on steepened footslopes. A recent analysis of slope values in Selangor State, West Malaysia (Olofin~ 1974), using a systematic point-sampling procedure, indicated the absence or paucity of particular gradients when the data were compiled as a histogram. Olofin interpreted the incidence of low or zero recordings of specific gradients as representing 'natural' slope boundaries; at these gradients, he suggested, the soils are inherently unstable, especially susceptible to erosion and are therefore seldom represented in the clinosequence. Olofin utilised the data to develop slope class divisions, defined by the missing angles, for use in agricultural land classification studies. Although Olofin's data referred to the whole range of topographic positions and did n o t distinguish between lithologies, the principles are applicable to the SLS data, which relate to specific slope segments and uniform rock types. Several missing angles are apparent in Fig.4, occurring at 18 °, 24 °, 28 °, 34 ° and 38 ° for granites, whereas on pelites the major breaks occur at 18 ° to 20 ° and 39 °, with subsidiary divisions at 27 ° and 29 ° . The results indicate that the system of slope classification (Sheng, 1971), originally adopted in the SLS survey, with divisions at 2 ° , 7 °, 15 ° , 20 ° and 30 °, should be modified to conform with the apparent natural boundaries on the main hill slopes. During the survey the lower boundary of the steepest mapping class was raised from 30 ° to 34 ° and this produced a more rational delineation of landscape features. It is anticipated that the adoption of other divisions, as indicated by the missing angle technique and based on random site sampling, would result in a still more logical mapping of slope classes in the region.

Soils The soils are described as broad associations defined by the landform unit within which they occur and their distribution can be inferred from the geological map (Fig.2). The USDA system (Soil Survey Staff, 1975) is used to classify the soils: they are tentatively placed in the sub-order Udults, mainly as Paleudults on the basis of predicted low contents of weatherable minerals in the sand and coarse silt fractions.

Morphology In the mountainous areas the regoliths are predominantly residual, being derived from the weathering products of the underlying/n situ rocks; many profiles are shallow .1 and are described as being skeletal due to their high contents of particles coarser than 2.0 ram.

*~ Depth ranges: 0--25 cm, very shallow; 25--50 cm, shallow; 50--100 c m moderately deep; 100 c m +, deep.

217

(a) Soils of the pelite hills (PS) These have reddish-yellow, well drained profiles with clay loam to clay textures and range in depth from very shallow to deep. The deep profiles are classified as Typic Paleudults (Soft Survey Staff, 1975}. PS softs occur at all positions in the topographic sequence on pelite hills, at elevations ranging from 100 to 650 m. The representative profile was described on a convex ridge-crest with sideslopes of 29 °, situated in Yala Gulong Settlement (6°18'40"N 101°22'00"E) at 180 m above sea level. The profile is well drained. Rubber, undersown with Centrosema sp., had recently been established on the site after clearance of the forest. A1

O-- I0 cm

Brown (7.5 YR 5/4) fine sandy loam; moderate medium to fine subangular blocky structure; slightly hard dry; many fine tubular pores and termite burrows; many fine fibrous roots; pH 4.7; clear smooth boundary.

A3

10-- 30 cm

Yellowish-red (5 YR 5/6) fine sandy clay loam; moderate medium subangular blocky; firm moist; few fine pores; c o m m o n fine roots; pH 4.7; clear smooth boundary.

B2

30-- 75 cm

Yellowish-red (5 YR 5/7} fine sandy clay; moderate medium to fine subangular blocky; friable moist; few termite burrows and fine pores; thin cutans to pores and some peds; few fine and medium roots; pH 5.1; clear smooth boundary.

B3

75--130 cm

Red (2.5 YR 5/6) stony clay; weak fine angular blocky; firm moist; few thin cutans; c o m m o n fine tubular pores; 40% by volume of subangular stones and gravel of schist and quartz; no roots; clear wavy boundary.

C/R

130--170 cm

In situ weathered schist with foliated rock structure; crumbles to fine sandy clay.

(b) Soils of the granite hills (GR) The GR softs are well drained with pale brown, yellowish-brown or brown colours and sandy clay loam to gritty *~ clay loam textures. The profiles are usually moderately deep b u t they contain erratically distributed rounded boulders. Classed as skeletal Typic Paleudults, the softs are mostly in residium. The GR softs were mapped on all b u t the steepest slopes, where bare rock outcrops, at elevations ranging from 100 to 1,200 m above sea level. .1 G r i t t y refers t o t h e fine gravel f r a c t i o n ( 2 . 0 - - 5 . 0 m m d i a m e t e r ) .

218 The type profile was recorded at an elevation of 200 m in Yala Gulong Settlement (6°18'00"N 101°22'25"E) on the convex upper slope of a granite ridge. The soil is well drained and the site slopes at 24 °. Rubber had recently been planted in a dense ground cover of Imperata cylindrica. A1

0-- 10 cm

B21

10-- 30 cm

B22

30-- 47 cm

B3

47--170 cm

C

170--270 cm

Very pale brown (10 YR 7/3) fine sandy loam; hard dry; weak coarse subangular blocky; few cracks and m a n y pores; c o m m o n termite burrows and charcoal fragments; m a n y roots; pH 4.8; diffuse boundary. Yellow (10 YR 7/6) gritty clay loam; moderate medium subangular blocky; hard dry; m a n y medium pores and termite burrows; weak cutans; many roots; pH 4.7; diffuse boundary. Reddish-yellow to yellow (7.5 YR--10 YR 7/6) gritty clay loam; moderate medium subangular blocky; hard dry; few fine pores; thin cutans; few roots and slight faunal activity; pH 4.9; gradual smooth boundary. Reddish-yellow (5 YR 7/6) with few medium distinct pale yellow mottles; very gritty clay loam; very firm moist; weak medium subangular blocky; many fine pores with weak cutans; rare fine roots; sub-rounded quartz grit comprising 40% of horizon volume; pH 4.8; diffuse boundary. Variegated reddish-yellow and light grey to pale yellow (7.5 YR 7/6--2.5 Y 7/3); same material as previous but more micaceous.

(c) Soils of the gneiss hills (GN) These are well drained, deep to moderately deep soils. Yellowish-brown sandy clay loam A horizons overlie brownish-yellow to yellowish-red gritty and gravelly sandy clay. The softs are classed mostly as skeletal Typic Paleudults, but include some Orthoxic Tropudults where contents of weatherable felspars are high. They are mainly formed in residuum. They were mapped on the whole range of slopes on the gneiss hills from elevations of 100 mm up to 1,400 m. The representative profile was described on a lower slope position in Prinyor Settlement (6°27'15"N--101°27'20"E). The site lies at 70 m above sea level and slopes at 5 ° to the east. The land was under rubber with sparse ground cover.

219 A1

0-- 15 cm

Very dark greyish brown (10 YR 3/2) sandy loam; moderate medium subangular blocky; friable moist; common faunal activity and many fine fibrous roots; common fine pores; pH 5.4; clear smooth boundary. Yellowish-brown (10 YR 5/4) gritty sandy clay loam; moderate medium to fine angular blocky; firm moist; weak cutans; few fine pores; many subangular quartz grit and gravel; few fine roots; pH 5.2; gradual smooth boundary.

B21

15-- 60 cm

B3

60--100 cm

Light yellowish-brown (10 YR 6/4) with few faint fine pale-brown mottles; gravelly sandy clay; weak medium angular blocky; extremely firm moist; few fine pores with cutans; many angular quartz grit and gravels; no roots; pH 5.5; gradual smooth boundary.

C

100--150 cm

Light grey to reddish-yellow (10 YR 7/1--7.5 YR 6/4) with many prominent medium-red reticulate mottles; gritty sandy clay; weak medium angular blocky; very firm moist; many angular gravels and grit of quartz and felspar; no roots; pH 5.6.

Analytical characteristics (a) Particle size The softs from granite and gneiss have relatively high proportions of coarse sand and clay and low contents of fine sand and silt (Table II), reflecting the predominance of quartz and felspar in their parent materials. The softs from pelite are derived from argillaceous material and are dominated by fine sand, silt and clay, in similar proportions. The clay fraction can be seen to increase down the profiles, particularly in the GN and GR soils. However, these data are presented in the conventional manner as percentages of the fine earth (<2.0 mm) ignoring the proportion of grit and gravel which increases down the profiles, especially in the GR and GN soils (Table III). Recalculating the analytical data to take account of the total soil mass, rather than the fine earth fraction alone, shows that the apparent changes in values for each of the <2.0 mm fractions are largely explained by changes in the coarse particle fractions. Clay, silt and sand contents in fact decrease with depth in the GN and GR soils, whereas they remain relatively stable in the PS profile (Table IV). Nevertheless, some clay illuviation has occurred as evidenced by the existence of thin clay cutans to the pores and peds in the lower B horizons of the profiles. On the basis of the fine earth proportions, the B horizons qualify as argillic (Soft Survey Staff, 1975), although only marginally so in the GR profile.

45 18 52 41

5 20 41 41

0 0 0

56 37 28 33

33 31 30 26

11 ii 11

18 10 12 9

15 12 13 10

30 27 24

10 7 7 7

14 14 12 11

26 26 22

16 46 53 51

38 43 45 53

33 36 43

Particle size (ram) gravel f'me earth (%) . . . . . . {%) CS FS Si CI

*t p H 1:2.5 soil-water suspension. ,2 p H 1:2.5 soil-CaCl 2 suspension. ,3 Values less than 0.1 indicated.

0"- 15 15-- 60 60--100 100--150

GN

0-- 10 10-- 30 30"- 47 47--100

GR

0-- i0 1 0 - 30 30-- 75

PS

(cm)

Unit depth

Soil analytical results

TABLE II

0.1 ,3 0.1 ,3

0.2 0.1 0.2 0.3

0.3 0.2 ,3

,3 ,3 ,3 0.1

0.1 0.1 0.1 0.2

0.3 0.1 ,3

0.1 0.1 0.1 0.1

0.1 0.1 0.1 0.1

0.1 0.1 0.1

0.1 0.1 0.1 0.1

0.3 0.1 0.1 0.1

0.3 0.1 0.1

4.6 1.7 1.2 1.8

5.8 5.7 5.4 4.1

5.1 6.0 5.1

Exchangeable cations (mequiv./100 g) _ _ Ca 2÷ M g ~+ N a + K + H +

4.9 1.9 1.5 2.0

6.5 6.1 5.9 4.8

6.1 6.5 5.3

CEC (mequiv./ 100 g)

6.1 13.7 20.0 12.5

9.2 5.3 6.8 13.4

15.1 6.8 2.8

Base sat. (%)

5.4 5.2 5.4 5.6

4.8 4.7 4.9 4.8

4.7 4.7 5.1

pH *'

4.4 4.5 4.5 4.3

4.1 4.1 4.1 4.1

3.8 4.0 4.3

,2

1.4 0.7 -

3.1 0.5 --

4.6 0.1 -

Organic carbon (%)

t~

221 TABLE III Percentage coarse separates (2.0--50.0 mm fraction): mean values Horizon

Soil unit PS

GR

GN

A1 A2 B2 B3/C

3.2 4.4 7.5 7.1

8.5 13.8 23.3 33.6

26.3 15.5 35.1 38.7

Profiles sampled

(8)

(3)

(3)

T h e r a t i o s o f silt t o clay+silt s h o w a c o n s i s t e n t decline w i t h d e p t h in each p r o f i l e (Table III). S t e w a r t e t al. ( 1 9 7 0 ) suggested t h a t , in softs o f u n i f o r m a n d u n c o n t a m i n a t e d p a r e n t materials, a decline in this r a t i o reflects an increasing d e g r e e o f w e a t h e r i n g . T h e i n f e r e n c e t h a t the degree o f w e a t h e r i n g in t h e s e softs is g r e a t e r at d e p t h t h a n a t t h e s u r f a c e is s o m e w h a t c o n t r a d i c t o r y , b u t similar t r e n d s in t h e silt:silt+clay r a t i o are a p p a r e n t f r o m the d a t a o f V i j a r n s o r n a n d F e h r e n b a c h e r ( 1 9 7 3 ) a n d h a v e b e e n o b s e r v e d e l s e w h e r e in the humid tropics (Ogunwale and Ashaye, 1975). The explanation may be t h a t the clay f r a c t i o n in t h e s u r f a c e h o r i z o n is selectively r e m o v e d b y overl a n d f l o w o f w a t e r ; R o o s e ( 1 9 7 0 ) states t h a t t h e e r o s i o n a n d lateral m o v e m e n t o f fine particles f r o m t h e s u r f a c e is o n e o f t h e p r i n c i p a l p e d o l o g i c a l f e a t u r e s o f similar ferrallitic f o r e s t softs in I v o r y Coast.

TABLE IV Particle size: values expressed as proportion of total soil mass Soil unit

Depth (cm)

Particle size (%) gravel sand silt

clay

Silt: silt+clay

PS

0-- 10 10-- 30 30-- 75

0 0 0

41 38 35

26 26 22

33 36 43

0.44 0.42 0.34

GR

0-- 10 10-- 30 30-- 47 47--100

5 20 41 41

46 35 26 21

13 11 7 6

36 34 26 32

0.27 0.24 0.21 0.16

GN

0-- 15 15-- 60 60--100 100--150

45 18 52 41

41 38 20 25

5 6 3 4

9 38 25 30

0.36 0.14 0.11 0.12

222

(b) Soil chemistry The levels of exchangeable bases are very low in each soil profile. The highest values are present in the surface horizons, reflecting the contribution to the total cation exchange capacity (CEC) by organic matter, which ranges from 2.5 to 8.0% in the A1 horizons of the profiles sampled, decreasing rapidly in the lower horizons. Calcium is the dominant exchangeable base but all values are so low as to preclude meaningful comparisons of ionic ratios. The balance of the exchange capacity comprises mainly hydrogen. Vijarnsorn and Fehrenbacher (1973) detected exchangeable aluminium at levels of 0.6 to 2.8 mequiv. /100 g in granite-derived softs. The low pH values recorded, especially in the PS soils (3.8 to 4.3, CaC12 suspension), would suggest that aluminium is present on the exchange complex of the SLS soils, but the close similarity between the sum of exchangeable cations (Ca, Mg, K, Na and H) and the measured CEC indicates that the levels are probably relatively low. The derived values for CEC of the clay decrease down each profile: the low levels, especially in the GN soils, are consistent with a clay fraction dominated by kaolinite and finely divided particles of iron and aluminium oxides. X-ray diffraction analyses of granite-derived Phuket softs, with values of CEC per 100 g clay similar to those presented here, have shown kaolinite to be practically the sole constituent of the colloid fraction (Vijarnsorn and Fehrenbacher, 1973). These soil chemical characteristics are expressed in relation to only the fine earth fraction, no account being taken of the high proportions of coarser material. Quartz is the principal component of the coarse fraction, especially in the GN and GR softs. This inert material contributes virtually nothing to the chemical activity of the soil but has the effect of diluting the already severely deficient nutrient content.

Relationship between soil depth and slope Considerable evidence has been accumulated in west Malaysia to show that the degree of soil weathering is related to the distance of the site from the crest rather than to the form of slope on which the soil is situated (Swan, 1970; Morgan, 1973). These authors suggest that in landscape evolution in this region, greater rates of erosion, steeper gradients and less weathered soils tend to occur in the lower situations near the local base level of erosion, as compared with crest sites. Similarly, at a given depth in the profile, softs are less weathered on steeper slopes than on gentler slopes, due to their 'younger' stage of weathering (Young, 1972). Further evidence is provided by the detailed clinosequence studies made by Furley (1974) in the Maya Mountains of Belize. Furley states that gradient appears to be of minor importance in the distribution of soil properties as compared with the influence of the distance down slope from the summit.

223

Assuming that for a given parent material a greater degree of weathering and a lower rate of erosion should result in a greater depth of soil, then the deepest soils would be expected to occur at or near the crest of the landform. In order to test this hypothesis an analysis was made of the relation between soil depth and land slope at 776 routine soil survey sites, distributed over each of the three upland soil units. The results of the analysis (Table V) show that only in the pelite landforms is there a significant association (×2 = 42.4, significant at 0.1% level with 10 degrees of freedom) between soil depth and site slope. In this landform there is a strong tendency for deep soils to occupy the gentler slopes, with shallow soils occurring on the steeper gradients. However, the relief on pelite is very regular; the gentler slopes occur mainly on the ridge crests and gradients increase down slope (vide supra). Thus the position of a site relative to the crest can be inferred from the slope angle. It is probable, therefore, that the significant association shown to exist between soil depth and slope could be more correctly ascribed to a relation between depth and distance from the crest, which would be in agreement with the Malaysian morphogenetic theories. This also conforms with the field observations that the deepest soils do tend to occur on the ridge crests, with the shallowest soils being developed on the foot-slopes, a tendency which has been observed in similar terrain in Sarawak (Baillie, 1971). The results for the granite and gneiss landforms are less conclusive, there being no significant association indicated between land slope and soil depth (Table V).

TABLE V

Relationship between soil depth and site slope Slope classes

Frequency occurrences of sites by soil depth ranges (m): pelite granite gneiss 0--0.5

0.5--1.0

1.0+

0--0.5

0.5--1.0

1.0+

0--0.5

0.5--1.0

1.0+

2-- 7 ° 1 7--15 ° 1 15--20 ° 7 20--25 ° 8 2 5 - - 3 4 ° 31 34°+ 37

5 16 14 37 137 47

8 15 14 24 72 22

4 2 2 8 12 6

1 4 2 8 17 8

4 3 11 15 29 7

0 3 1 0 5 1

1 7 6 6 11 6

7 19 19 19 20 6

Total

256

155

34

40

69

10

37

90

85

sites ×2 ,1 S i g n i f i c a n t (P = 0.1%).

42.4 .1

10.1

10.7

224

CONCLUSIONS

The three main landform units described for Prinyor Settlement, namely those formed on pelite, granite and gneiss, are characterised by very steep mountainous terrain. These three units occupy, respectively, about 20, 33 and 25% of the area of the Southern Land Settlements and are thus representative of the dominant land units of the whole upland region. Landforms on pelite are characterised by relatively uniform ridge and ravine topography; the hill slopes have a mean gradiant of 29 °. Irregular landforms with more convex slopes and varied gradients are typical of granite and gneiss; the mean gradient on the granite hillsides is 27 ° . The softs are broadly divisible into two groups: clay-textured Typic Paleudults on pelite and gritty/gravelly sandy clay loams, classed as skeletal Typic Paleudults, on granite and gneiss. The fertility status of the soils has been shown to be very low. The erosion risks involved in establishing crops and the difficulties inherent in executing conservation works increase with increasing slope. The 'missing angle' technique is considered to offer a useful method for defining rational and mappable slope classes, within which specific conservation requirements may be defined. An analysis of the available data shows a significant association between angle of slope and soil depth on pelites but n o t on granite or gneiss. The observed tendency, particularly on pelite landforms, for the Aeepest soils and gentlest gradients to occur on the hill crests indicates that, within individual units of land, agricultural settlement should proceed along the crests rather than along the valley floors, if maximum use is to be derived from the deepest soils. ACKNOWLEDGEMENTS

The authors wish to thank their colleague, F.W. Hasselaar, for his invaluable assistance during the original field surveys, and also Dr T.N. Jewitt and D.E. Parry for their technical advice. An acknowledgement is also due to the Department of Public Welfare, Bangkok, for w h o m the original surveys were conducted, and to the Overseas Development Administration, London, who provided financial assistance for the SLS study. All opinions stated in this paper are solely those of the authors.

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