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MARDI Res. Bull. (1986) 14(3), (2t9-230)
AGGREGATE STABILITY AND SUSCEPTIBILITY TO EROSION OFSOME HIGHLY WEATHERED SOILS IN PENINSULAR MALAYSIA
H. GHULAM MOHAMMED*
Keywords: Aggregate, Structure, Erosion, Dispersion.
RINGKASAN
Walaupun tanah terluluhawa di rantau tropika lazimnya dianggap sebagai tanah yang beragregatbaik, terdapat bcberapa perbezaan penting dalam ciri-ciri agregat tanah tersebut. Sifat-sifat dankestabi lan agrcgat mempengaruhi kcupayaan tanah untuk menahan hakisan. Keputusan kaj ianmengutarakan dua faktor pcnt ing untuk kestabi lan agrcgat . ia i tu kandungan l iat yang mcncukupi dannisbah kandungan bahan organik dan kandungan l iat vang t inggi .
Empat s i r i tanah diu j i dengan hujan t i ruan selama satu jam dcngan intensi t i 35 mm scjam.Semuanya mengalami hakisan pcrc ik. Tetapi hakisan tersebut lebih t inggi bagi tanah yang kurang stabi l ,ia i tu s i r i Munchong I dan s i r i Bungor. Hujan t i ruern bcrturut- turut mcningkatk i in hakisan perc ik bagitanah vang kurang stabi l , tetapi t idak member i scbarang kesan pada tanah yang stabi l , sepert i s i r iRengam dan s i r i Kuantan. Tanah vang stabi l in i t idak mengalami perceraian agregat scmasa kaj ian in id i ia lankan.
INTRODUCTION
The soil erosion process is made up ofthree important phases: detachment,transportation and deposition (Er-l lsoN,l94l). The init ial and essential step,detachment, depends greatly on theinherent properties of the soil aggregates,specifically on their stabil ity against theforce of raindrops and of running water(GneeNrnNo, 1977). In addi t ion, theproperty of the soil to swell and slake whenwetted also affects its abil ity to resistdetachment by raindrops or by runoff. Thisis especially important when the soil issubjected to several rainstorms within a fewdays.
The stabil ity of soils againstdetachment is particularly relevant to theMalaysian agricultural scene. This isbecause of the continuing soil erosionproblems being exper ienced. Erosion inPeninsular Malaysia is attributed to twomain factors. Firstly, the tropical rainfallregime with high annual total, high intensityand frequent occurrence of wet spells(Dare, 1960; NreuwoI-r , 1981) is h ighlyerosive. Secondly. eros ion is accelerated bythe continuing replacement of the forest
vegetation with agricultural crops and byrapid urban development.
The most serious erosion affects thesedentary soils which are mostly found inthe inland areas, where slopes are ac()mmon topographic fealure. They arehighly weathered and leached, and belongto the Ul t iso l and Oxisol orders of Soi lTaxonomy. The highly weathered soils ofthe tropics have often been described ashaving very stable aggregation associatedwith high contents of clay and the hydrousoxides of iron and aluminium. However. amore detailed check wil l reveal that, amongthis group of soils, there are some which areunstable. This may be due to one or more ofthc binding substances being present insmall amounts only. A study of theaggregate stabil ity of nine soil types bySoouc (1913) revealed that some topsoils,for example the topsoi l (0-15 cm) of theSerdang series containing l1ok clay, aresignificantly less stable than others such asthe topsoils of Kuantan, Segamat and Prangseries with clay contents ranging from 447cto 64 per cent. Subsoils (15 - 30 cm) werefound to be less stable than topsoils (0 - 15cm) because of lower organic matter andhvdrous oxide contents. Areas where the
Ccntral Rcscarch l -a i rorator ics Div is ion. N{ARDI. Scrc l i rnq. Selangor. Malavsia
219
subsoil is exposed due to cultivation andother causes, are observed to suffer severesoil erosion.
The aim of this paper is to providefurther information on the aggregation ofsome sedentary soils in Peninsular Malaysia.Although it is generally known that soils oflower stabil ity are relatively more erodible,certain characteristics of soil aggregates andsoil moisture content modify this relationship.Some of the main points arising from alaboratory study on aggregate stability andsoil erosion are discussed.
SOILS
The soils of Peninsular Malaysia can bebroadly classified into three groups r.e.,sedentary, reworked and alluvial. The alluvialsoils occupy the coastal plains and river floodplains, whereas the sedentary soils aregenerally found in the inland areas. Sincethere are genetic differences between thesegroups, their physical characteristics alsodiffer. This paper focusses on a number of thesedentary soils.
The sedentary soils are developed bythe in-siru weathering of a variety of parentmaterials, including various types of igneous,metamorphic and sedimentary rocks. Theintense weathering under the humid tropicalclimate of Peninsular Malaysia results in basesbeing leached, silica solubilised, and iron,aluminium and manganese preferentiallyretained. Thus, the soil profiles are normally{ow in bases and silica, but rich in iron andaluminium. The quantities of these substancespresent depend on the composition of theparent rock. For example, Kuantan seriessoil, derived from basalt, is very rich in bothiron and aluminium, whereas Serdang series,derived from sandstone, has relatively loweramounts of those substances.
Humid tropical weathering conditionsand time have brought about mineralogicalchanges through physical disintegration andchemical reaction resulting in the formation ofclays. Thus, because of the advanced stage of
weathering, the clay content tends to be high.However, this property is sometimes alteredby terrain and the nature of the parentmaterial. Soils derived from sandstone orgranite, for example, tend to contain rela-tively higher amounts of quartz which isnormally of sand size. Those located on steepslopes tend to lose a larger proportion of thefiner material through erosion.
The soils used in this study are somesedentary soils of Peninsular Malaysia such asBungor, Kuantan, Munchong, Rengam andBukit Temiang. These soils show differencesin parent material, texture and pedogeneticfeatures as expressed by their classificationaccording to Soil Taxonomy (Table 1). Thetopsoil layer (0- 15 cm), the layer mostlulnerable to erosion, was investigated on.
METHODS
Mechanical Analysis
The soil sample which had passedthrough a 2-mm sieve, was used for thedetermination of particle size distribution.Organic matter was oxidised by hydrogenperoxide. A mixture of sodium hexameta-phosphate and sodium carbonate was used asdispersant, and the samples were physicallyagitated. Clay and silt were determined by thepipette method (Blecx, EvaNS, ENSIratNGeR,WHnE and Clenx, 1965).
Organic Matter Content and pH
The organic matter contents of the soilswere determined by the Walkley and Blackmethod. The values of soil pH in water weredetermined in a 1:5 soil to water ratio.
Iron and Aluminium Contents
'Total' iron and'total' aluminium wereextracted by aquaregia (CorreNIe, VeRLoo,VElcur, and KIBxr,Ns, 1979).'Free' iron and'free' aluminium were extracted by thedithionite-citrate-bicarbonate method(MEHna and JncrsoN, 1960). Iron andaluminium in solution were determined byatomic absorption spectrophotometry.
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Table 1. Sampling location, classification, particle size distribution, pH and organic mattercontent of the topsoil layer (0-15 cm)
Soi lseries
ClassificationSampling accordingsrte to Soi l
Taxonomv
Part ic le s ize dist r ibut ion (%)Soi l
Clay Si l t F ine Coarse pH<2pm 2-20 sand sand ( in water)
pm 20-200 >2(npm pm
OM OM:Clavcontent rat lo
(vo)
Bungor
Kuantan
Munchong I+
Munchong I I+
Rengam
Buki t Temiang
Serdang,Selangor
Buki t
Goh ,Pahang
Serdang,Selangor
Serdang,Sclangor
Sg . Bu loh ,
Selangor
ChangkatBruasEstate,Perak
TypicPalaeudul t
Hapl icAcrorthox
TropepticHaplorthox
TropepticHaplorthox
TypicPalaeudul t
OrthoxicTropudul t
2.80 0.066
4.53 0 .071
2.8',7 0.040
3..18 0.052
3.68 0 .074
4 2 . 1 l . 7 3 3 . 3
63.5 25 .3 6 .5
7 1 . 3 6 . 8 1 1 . 1
6 7 . 1 1 3 . 4 1 1 . 5
49.6 6 . I 7 .7
22.9
1 0 . 5
8 . 0
36.6
4 . 8
A <
5 . 6
.{.5
22.3 10 .9 14 .U -52 .0 ,1 .8 1 . 9 2 0 . 0 8 6+Munchong I and Munchong II were collccted from two different sites. Some earth-moving activity appeared to have
taken place at the s i te of Munchong I . I t is expected that Munchong I I would be more l ikely to exhib i t thecharacter is t ics of the topsoi l of a benchmark Munchong.
Aggregate Stability
Aggregate stabil ity was init iallyassessed by subjecting aggregates to pressurebetween the thumb and forefinger. Then, itwas determined by two other methods,namely dry-and-wet sieving (De LeeNHrrnand De Boonr, 1961) and prolongedimmersion in water.
One hundred aggregates in the sizerange of 2 mm to 8 mm were subjected tomaximum pressure between the thumb andforefinger. The number of aggregates brokenwas noted to provide a preliminary assess-ment .
In the dry-and-wet sieving method,1 500 g of air-dried soil (diameter < 8 mm)was sieved to obtain four size fractions. i.e..4.76-8.00, 2.83-4.76, 2.00-2.83 and <2.00 mill imetres. From here the aggregationpercentage, that is the percentage of soilaggregates > 2.00 mm diameter, was thencalculated. For the wet sieving, 100 g of thefract ions of 2.00-2.83, 2.83-4.76 and
4.76-8.00 mm in the same proportion asobtained in dry sieving, were used. Each ofthe three fractions was placed in a nickel cupand was wetted by large water drops fall ingfrom a height of 50 centimetres. Sufficientnumbers of drops were added to bring thesoil moisture content to field capacity. Thewetted soil aggregates were kept in a high-humidity chamber for 24 hours.
The aggregates were then sieved underwater for f ive minutes. Sieves with thefollowing aperture sizes: 4.J6, 2.83, 2.00,1.00, 0.50 and 0.297 mm were used. Theamount of soil retained on each sieve wasdried and weighed.
The mean weight diameter (MWD)was calculated according to the formula ofDE LeBNneER and DE Booor (1967\.
MWD _ t % weig.ht mean diameter of
L/ ot soll x
the size fraction
The instabil ity index is given by thedifference between the MWD values of the
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dry aggregate distribution and the wet
aggregate d is t r ibut ion.
To complement the results of the dry-
and-wet sieving method in characterizing
soil aggregate stabil ity, the extent of disper-
sion and slaking of the soils was also
assessed.
For each soil, about 300 aggregates in
the size range of 3 mm to 5 mm were
immersed in water for one week. It was
observed earlier that the largest amount of
stable aggregates was in this size range.
Slaking and dipersion in water were allowed
to take place without any disturbance. This
was followed by drying according to theprocedure described by GnpENE-KEt-t-v(re73).
The water was carefully replaced with
a solution of an organic solvent, methanol.The concentration of the methanol solution
used was progressively increased from25Vo,
50%,75Vo to 100 per cent . The solut ion was
changed every24 hours. The 1fi)7o methanol
solution was replaced with ether which was
allowed to evaporate giving d.y soi-
aggregates.
The methanol conferred hardness to
the aggregates, allowing size separation by
sieving. The fractions of sizes obtained were
0 . 3 - 0 . 5 , 0 . 5 - 1 . 0 . 1 . 0 - 2 . 0 , 2 . 0 - 2 . 8 , a n d2.8-4.7 mi l l imetres.
Soil Erosion and Runoff
In order to measure soil loss and
runoff, simulated rainfall was applied to
aggregates <8 in diameter. Four of the soils,namely, Munchong I , Bungor, Kuantan andRengam were selected for this investigation.These soils were chosen to represent a widerange of aggregate stabil ity. The MunchongI was preferred over Munchong II because itis much less stable than Kuantan, Bungorand Rengam. The Bukit Temiang was leftout so that the influence of high amounts ofcoarse sand and gravel in the erosionprocess was excluded in th is s tudy.
The air-dried soil sample was gently
packed into a soil pan (30 x 30 x 5 cm) and
tilted to a slope of 33 per cent. Slopes of
such steepness are considered as seriouslimitations to crop growth and are commonin the inland areas.
Free subsurface drainage from the soilpan was facilitated by a fritted false bottom
underneath the soil sample. The density of
packing was 1.1 g per cubic centimetre.
Simulated rain was aPPlied at anintensity of 35 mm/h for an hour using a
ra infa l l s imulator which had been descr ibedby Gasnle ls , DE Booor and MIN:nuw(1973). Runoff, splash and percolation were
collected at regular intervals.
Runoff was collected at the end of atriangular-shaped runoff concentratorattached to the front of the soil pan. Soilmaterial splashed onto the sides of the pan
was collected by placing metal splash boardsat about 6 cm away from each side of thepan. Percolation was collected throughopenings at the bottom of the Pan.
Rainfall intensity was calculated basedon readings of f ie ld ra in gauges.
RESULTS AND DISCUSSION
General Characteristics of the Soils
Particle size analysis, pH, organicmatter content and other information aregiven in Table I. Contents of iron and
af uminium are given in Table 2. Brief notes
on the soils are given in Appendix L
Characteristics of Aggregate Stability
The values of mean weight diameter(MWD), instabil ity index and aggregationpercentage are shown in Table 3. Thenarrow range in MWD (dry) indicates a
close similarity in the distribution of soil
aggregates in the air-dried state. A rela-
tively large proportion of these aggregates,ranging from 34.9Vo to 54.IVo are larger
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Table 2. Iron and aluminium contents of the soils under study
Soil series
Iron content (7o)
Aquaregia extracl DCB extract
Aluminium content (70)
Aquaregia extract DCB extract
BungorKuantanMunchong IMunchong IIRengamBuki t Temiang
3 . 1 019.265 . 5 84.791 . 6 10.69
3 . 1 811.245.003.931 . 5 70.62
2.65t0.246 . 1 25. ' .713.2'73.5'7
0 .873. i l1 . 3 8t . 7 10.500 . 1 9
DCB : Dithionite-citrate-bicarbonate
Table 3. Aggregate stabil ity data obtained by dry-and-wet sieving
Soil series Aggregation (7o) Mean wt. diameterInstability
indexDry We1
BungorKuantan
Munchong IMunchong I IRengamBuki t Temiang
45.4334.9040. l052 .7049.1054. l0
4 . 4 14.294.40/ ^ 1
4 . 5 14.60
2.603.071 .802.78J . L J
2.82
1 . 8 11 . 2 22.601 . 6 41 . 2 81 . 7 8
Soi l ser ies
Table 4. Distribution of 3-5 mm aggregates after immersion in water for one week
Distribution (7o wlw) of 6 aggregate sizes (mm)
2.8-4 . '7 2 .0-2 .8 1 .0 -2 .0 0 . 5 - 1 . 0 0.3-0.5 <0.3
BungorKuantanMunchong IMunchong IIRengamBuki t Temiang
48.368.530.856.750.51 8 . 4
1 5 . 510.21 1 . 510.3I l . )
7 . 9
1 0 . 86 . 8
15.412.114.124.0
9 . 12. '76 .89 . 25 . 4
l 0 . l
8 . 76 . 1
1 9 . 86 .1
l 1 . tl - ) - - )
'7.6
5 . 7l f . /
5 .3o . 4
1 6 . 3
than two mill imetres. This is a favourablesoil structural characteristic.
The stability of the soils against theforces employed in the sieving method, thatis water drop impact and agitation in water,is shown by the values of MWD (wet) andinstabil ity index. These values show morevariation than the values of MWD (dry).Kuantan series was shown to be very stable,whereas Munchong I series was relativelyunstable.
However, in interpreting these results,one must also consider the different texturalcomposition of these soils. High gravel andcoarse sand content, as in Bukit Temiangseries, tends to increase the values of MWDas well as aggregation percentage. Thus, the
high MWD values and low instabil ity indexobtained for Bukit Temiang, contradict thefindings during an init ial assessment ofstability by pressing air-dried aggregatesbetween the thumb and forefinger(Appendix 2). Therefore, a more completeassessment of the aggregate stability has tobe made by also taking into account theextent of aggregate dispersion that occurredafter the air-dried aggregates wereimmersed in water for a week.
Table 4 shows the percentages of 3-5mm aggregates which have broken downinto the various size fractions. Soils withhigher percentages of smaller-sized aggre-gates are considered to be less stable thanthose with lower percentages of such aggre-gates. A careful examination of the data
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reveals that Bukit Temiang and Munchong I
are relatively less stable, whereas Kuantanis the most stable (Appendix 2).
A categorization of these soilsaccording to aggregate stability is carriedout. This categorization takes note of boththe results of dry-and-wet sieving andprolonged immersion in water of 3-5 mmaggregates, the init ial stabil ity assessmentby pressing air-dried aggregates as well asthe textural differences of the soils. Ascoring scheme was devised for this purpose(Appendix 2). The aggregate stabilitycategories were:
Very stable - KuantanStable - Munchong II, Bungor,
RengamModerately - Munchong I
stableUnstable - Bukit Temiang
The process of aggregation depends onthe existence of clays. If the bathing l iquidhas a sufficiently high ionic concentrationand a large proportion of divalent ions suchas Ca*r, clays tend to flocculate (Russall,1973). The aggregation process is init iatedwhen the clays flocculate and cluster to formdomains. Substances which are said to binddomains together include organic matter
and sesquioxides. Thus, the high stabil ity ofKuantan series may be attributed to the highcontent of these substances (Tables I and 2).These constituents interact with each otherand are strongly held together by a com-bination of electrostatic, Van der Waals andhydrogen bonds, as suggested by Et--Swntrv( 1980).
The other soils, namely Rengam,Munchong II and Bungor, are also stablebut relatively less so than Kuantan. Thebinding of soil contituents in these soils isprobably less intense because of the rela-tively lower contents of the mentionedconstituents (Table 2).
However, high amounts of clay andsesquioxides alone do not necessarily ensurehigh aggregate stabil ity. The relative ease of
dispersion of Munchong I seems to point toa low organic matter: clay ratio (unpublishedinformation). It appears that its high claycontent (717a) requires a relatively highorganic matter content to aid in the bindingof all the clay domains. This indicates thatthe role of organic matter is very importantin stabil ising aggregates.
However, it is evident from the l itera-ture (Bnven, GanoNr,R and GanoNeR,1972; Russen, 7973) and from this studythat a prerequisite for the formation ofstable aggregates is the presence of clayparticles. Their active surfaces enable themto react with ions and the binding agents aswell as with each other. Thus, one finds thesoil with low clay content, Bukit Temiang,exhibit ing the lowest aggregate stabil ity.
However, in the case of BukitTemiang series, a small proportion of itslarge aggregates was found to exhibit astabil ity similar to that of the other soils,when pressed between the fingers. Whenthese large and stable aggregates were
specially selected and analysed for texturalcomposition, the clay content was found tobe reasonably high compared with that ofthe whole sample. Table 5 gives a com-parison between the clay contents ofselected 5-8 mm aggregates and those of
the whole samples. This substantiates theabove comment that a considerably high
clay content is a prerequisite for stableaggregation.
Erosion and Runoff Resulting fromConsecutive Rainstorms
Table 6 gives the erosion and runoffdata obtained after applying simulated rainof intensity 35 mm/h and of one-hourduration for three consecutive days oninit ially air-dried soil samples. During eachstorm, soil splash occurred almoslimmediately. The measured side splasherosion (SSE) generally increased as a rain-storm progressed (Figure I). Runoffoccurred only in the Munchong I andBungor series soils, starting on the first day
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Table 5. Particle size distribution of 5-8 mm aggregates and clay content of whole samplesPart ic le s ize dist r ibut ion (% wlw)
Soil series 5-8 mm aggregates Whole sample
Clay< 2 p m
S i l t2-20 pm
Fine sand20-20O pm
Coarse sand> 200 pm
Clay< 2 p m
BungorKuantanMunchong IMunchong IIRengamBuki t Temiang
+ J . 4
63.270.769.164.943.1
6 . 426.86 . 5
12.48 . 2
12.6
30.5) . 1
12.01 1 . 89 . 0
12.9
19.74 . 9
1 0 . 86 . 4
1 1 . 93 l . l
4 2 . 163.51 1 . 367.149.6l l . - )
Table 6. Erosion and runoff from three rainstorms applied on consecutive daysVariable Bungor Kuantan Munchong I Rengam
Side splash erosion (g)
I st rainstorm2nd rainstorm3rd rainstormTotal
Soi l wash (g)
1st rainstorm2nd rainstorm3rd rainstormTotal
Runoff (cmr)
l st rainstorm2nd rainstorm3rd rainstormTotal
3 . 1 67.30
12.7223 .18
t . ,
0 . 1 00 . 7 80.88
0.791 . 1 9I .843.82
1.041'7.782 2 . 1 746.99
1 . 1 32 . 5 63 . 1 86.87
0 . l 60 .220.2-s0 .63
U40
558598
36804
1 2782 1 1 t t
000U
0t,,00
3rd darl 4
1 2
I (.)
o
6
3 1' a
e . . .
f ; ' so 1 6
' a 1 a
6= , ^
.? 108
6
1
2
Figure L Increase in side splash erosion withthe progress of rainstorms for Bungor and
Munchons I series soils.
for Munchong I and on the second day forBungor.
' l 'he time to incipient runoff
decreased on each successive day. ForMunchong I, the times taken were 48, 5 and3 minutes for the first, second and third dayrespectively. For Bungor, the durationswere 50 and 5 minutes for the second andth i rd day respecl . ive ly .
Under natural conditions in manyareas in the humid tropics, the peak inten-sity of rainstorms may greatly exceed 35mm/hour. For example, 28Vo of the rain-storms recorded between July 1978 andAugust 1980 in Serdang, Selangor hadmaximum 30-minute intensity (136) values of40 mm/h or more (GHur-ev MonavlrEoand ZAKr,1981). The peak in tensi ty va lueswould be much higher. The rainfall recordsalso show that, during rainy periods, stormsoccur on a number of days successively.Thus, it is common for high-intensity rain tofall on wet ground and, increase runoff anderosion on soils of relativelv low stabil itv.
0 l 0 2 0 3 0 4 0 5 0 6 0
Time (min)
225
-. 4i)
c -
! l o= - v
- 1 0
The results also show that soil splashtakes place on all four soils, indicating thatunder natural conditions this phenomenonis experienced on soils over a wide range ofaggregate stability, if they are not protectedby vegetation or mulch.
The Effect of Initial Moisture Content onRunoff and Erosion
The effect of initial moisture contenton splash erosion is shown in Figure 2. Theinit ial moisture content increases on eachsuccessive day because of the moisturereceived from earlier rainstorms. Table 6shows the increases in runoff and erosion oneach successive day.
. - ls t dav
r - 2nd day
o 3rd day
B BungorK - Kuantan
MI MunchongR - Rengam
20 30
Side splash erosion (g)
Figure 2. Effects of initial moisture contenton side splash erosion.
When init ially dry aggregates aresuddenly wetted, water enters the pores andcauses 'air explosion' (Bolr and KoeNlcs,1972) due to trapped air escaping from thepores. This results in either weakening orslaking of the aggregates. The extent ofweakening and the possibil i ty of slakingtaking place, depend on how strongly thesoil particles are held together by theaggregating agents. In soils which aresusceptible to this weakening process,erosion wil l take place more readily whenthey are further subjected to the impact ofraindrops. This behaviour is shown byMunchong I and Bungor.
Besides the increase in erosionrecorded on each successive day (Figure 2and Table 4), the phenomenon is also shownin Figure 1 and Table 7 where increases inerosion and runoff are observed during theprogress of each individual storm.
Table 7. Side splash erosion durirtg theprogress of rainstorms for the relatively
more stable soils, Kuantan andRengam ser ies
Time af ter Cumulat ive s ide splash erosion (g)
i n i t i a t i u n o f -ls t dav 2nd day 3rd daY
ralnstorm I mtn I
Kuantan
l 02030405060
Rengam
l 02030405060
0.040.070 . 1 30.360 .510.79
0.020.040.070 . 1 00 . 1 30 . l 5
0.250.420.600.'720.891 . 1 9
0.030.070 .120 . 150 . 1 90.22
0.370.'t40.98t . L - )
1 A 1
1 .84
0.020.050.090 .160-220.25
The Influence of Aggregate Stability onErodibility
The two soils with relatively loweraggregate stabil ity, Bungor and MunchongI, suffered runoff and soil wash (sheeterosion) in addition to splash erosion,whereas the other two soils. Kuantan andRengam suffered only minimal splash erosion(Table 6). The difference in soil erosion ofthese two pairs of soils is much larger thantheir differences in instabil ity index wouldsuggest. Thus, soils which deteriorate uponbeing subjected to rain, rapidly lose theirstabil ity when further stresses are applied.Those which are init ially resistant, maintaintheir stabil ity, as exemplif ied by Kuantanand Rengam.
A plot of instabil ity index againstsplash erosion suggests a l inear relationship(Figure 3). However, an anomaly occurswith respect to Rengam. This could be
226
o - Total for 3 stormsr - lst sJorm
B - BungorK - KuantanMI - Munchong IR - Rengam
0 2 0 4 0 6 0Side splash erosion (g)
Figure 3. Relationship between instabilityindex and side solash erosion.
explained by the generally larger aggregatesof Rengam when compared with those ofKuantan (Table 8). Larger aggregates areless easily splashed.
The splashed Kuantan soil are fineaggregates and not dispersed material. Theamount of splash increases slightly withhigher moisture content (Table 7) becausethe finer aggregates are splashed togetherwith the puddled water.
It is clear that aggregate dispersionoccurred only in Munchong I and Bungor.This dispersion led to crust formation andthe onset of runoff.
CONCLUSIONS
As many factors influence soilaggregation and aggregate stability, no onesingle method can completely compare thesoils according to their aggregate charac-teristics. Many factors have to be taken intoaccount when the relative stabilities of thesoils are studied. The factors include texture,instability index obtained by dry-and-wetsieving, and aggregate stability undersubmerged conditions. It was found that,when the clay content was sufficiently high,aggregate stability was influenced by theratio of organic matter content to claycontent.
Aggregate stabil ity is related toerosion resistance, with soils of low stabil itysuffering greater erosion. However, in thehigher range of stabil ity, aggregate size alsoplays a role, there being less splash erosionwhere aggregates are larger.
Aggregates with a high init ial moisturecontent are more easily eroded because theyare less cohesive when wet. However, this isonly true for the relatively less stable soils.The more stable ones have greater cohesionand suffer minimal splash erosion regardlessof the moisture content.
For a high resistance to erosion, thisstudy showed that a prerequisite was a
Table 8. Field description and laboratory data on aggregate size
Structural units described in the field Distribution+ (Vo wlw) of 4 sizes (mm)Soi lseries Type & class Size (mm) Grade 4.76-8.00 2.83-4.76 2.ff i -2.83 < 2.00
Bungor
Kuantan
Munchong I
Rengam
Medium &coarse SAB
Fine &mediumgranules
Fine &mediumSAB
Mediumgranules &fine SAB
10-2020- 50
2-5
5 - 1 010-20
2 -5
5 - 10
Moderate
Strong
Moderate
Strong
15.89
12.08
14.82
20.43
19.86
13.95
1 5 . 1 6
r7 .39
9.68
9.02
t0 .04
1 1 . 0 6
54.58
64.95
59.98
5 1 . 1 3
+Laboratory dataSAB : Subangular blocks
227
sufficiently high clay content. But this aloneis no guarantee. A wide organic matter: clay
AvlNuootN, Y. and EswnuN, H. (1973). Pedologicalstudies on Kuantan series - a representative soilo f Ma lays i a . MARDI Rep . No .11 , l 1pp .Serdang: MARDI Press.
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ABSTRACT
Although the highly weathered soils of the humid tropics are generally thought of as well-
aggregated soils, there are impertant differences in aggregate characteristics among them. These
influence their susceptibility to erosion. In the group of soils under study, the results highlighted two
factors which are essential for high stability: a sufficiently high clay content and a relatively high organic
matter to clay ratio.
Simulated rainfall of one-hour duration at 35 mm/h caused splash erosion on all series of soils, but
to a much greater degree on the less stable ones, Munchong I and Bungor. Successive rainstorms
increased soil splash considerably on the less stable soils, but produced negligible increases on the more
stable ones, Kuantan and Rengam series. The stable soils did not suffer dispersion under the conditions
of this experiment.
REFERENCES
228
Appendix 1. Description of the various soil series
Soil series Description
BukitTemiang Coarse-grained granite derivative.Sand and gravel contents are relatively high.Clay fraction comprises kaolinite halloysite, gibbsite and traces of mica.Silt fraction comprises kaolinite, gibbsite, quartz and mica (WoNc, 1976).Shallow (< 100 cm) and situated on upper slopes.
Bungor Derived from interbedded sandstones and shales.Sand content is relatively high with a greater proportion of fine sand.Clay fraction comprises mainly kaolinite, gibbsite and goethite.
Kuantan Derived from olivine basalts of Quartenary age, dark brown in colour, verystable microaggregation and very deep profile.Clay fraction comprises kaolinite and halloysite (357o), gibbsite (40%) andgoethite and magnetite (21Eo).Silt fraction comprises i lmenite, magnetite, kaolinite, quartz and gibbsite(ArrarNuoorN and EswnnRN, 1973).Occurs in a gently undulating landscape. (e.g. Bukit Goh Reserve, BukitGoh Felda Scheme, Jabor Valley and Southern Terengganu).
Munchong Derived from iron-rich shale. Occurs in undulating to roll ing terrain. Clayfraction dominated by kaolinite, gibbsite and goethite.
Rengam Derived from granite. Occurs in undulating, roll ing and hil ly terrain withslopes ranging from 10' to 25'. Relatively high sand content. Clay fractioncomprises kaolinite, halloysite, gibbsite, goethite and quartz (WoNc,1976). Silt fraction comprises kaolinite, gibbsite, quartz and mica (WoNc,| 976)
229
Appendix 2a. Scoring scheme for categorizing soils according to aggregate stabil ity
S c o r e 0 l 2 3
No. of aggregates that yield topressurer( ,2) 80- 100 50-80 20 -50 < 20
Instabi l i ty index2
Aggregate > 1 mm3 (o/a wlw) < 50 50-60 60 _75 75-100
Coarse sanda (7c w/w) > 50 ,10-50 20 -40 < 20rAggregate stabi l i t l - asscssed by pressing ai r -dr ied aggrcgatcs bctwccn the thumb and foref inger.-Oh ta i ned
f r om d rv - l nd -we t s r c r i ng .r? ' s table aggregates af ter prolongcd immersion in water.rTextural factor (Thc presence of coarse part ic lcs contr ibutcs to a lower instabi l i ty index. Thus, a lower scorc is g iven
for a h igher content of coarse sand to of fset the error) .
Appendix 2b. Scores for the soils used in the study
Var iable Buki t Temiang Bungor Kuantan Munchong I Munchong I I Rengam
Yield toD r e s s u r e 0 2 3 1 3 3
Instabi l i tyindex 1
Aggregates) 1 m m 1 2 3 1 3 3
Coarse sandc o n t e n t 0 2 3 3 3 2
Total 2 I 1 1 5 l 0 1 0
1 2 0 r 2
230