Mohr - Climate and Soils in Indonesia

8/6/2019 Mohr - Climate and Soils in Indonesia

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CLIMATE AND SOIL IN THE NETHERLANDS INDIES

By Dr. E. C. J. MOHRPoints on which the Netherlands Indies differ intrinsicallyfrom both Europe and the United States of America are climate

and soil. To say that the Netherlands Indies are located in thetropics, is to put the ease too vaguely. This expression - thetropics - covers the whole broad belt between the tropicsof Cancer and Capricorn, or between 23 N. lat. and 23 S. lat.But within this vast belt itself great differences of climate


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an d shines for the same length of time, or very nearly, it isclear that th e daily variations of temperature will be practicallythe same at every point in the archipelago. Particnlarly ho t orparticnlarly cold days differ bu t a few degrees from the average.But curiously enough it seems to be the very slightness of thisvariation which makes human beings so sensitive to such changesof temperature as do occur. As a matter of fact, in the NetherlandsIndies the thermometer very seldom registers what one thinksof as tropical heat, bu t then we must reflect that teniperatureis bu t one of several factors in respect to atmospheric conditionsas affecting the human senses. The maximum temperature atsea-level hardly ever exceeds 33 Centigrade there, while in Europeor the U.S.A. it often reaches 38 or 40 C., and in Arabia andSouthern Persia even 45 or 50 C. On the other hand thetemperature at night shows a smaller drop inside the zonereferred to than outside it , and seldom falls below 23 or perhaps21 C. The daily margin being so slight and the temperaturemoving practically between the same limits every day, it issmall wonder that even at the depth of only one metre thesoil shows no variation in temperature at all, remaining constantat 25 or 26 C. (at sea level). In Europe a constant soil-temperature conld only be expected at about ten times this depth.

There are large stretches of low-lying, flat country in theNetherlands Indies, but the archipelago also includes highlandswhere considerable altitudes are reached. The higher one goes,the cooler it gets. Every 160 metres makes a difference of 1 C.;at a height of 1600 meters, or almost 5000 feet, the averagetemperature is, therefore, IS to 16 C., rising to perhaps 21or 22 C. in the course of the day and dropping at night to 10or IIo C. - a delightfnl c limate, this, as regards temperature;one in which we humans can enjoy life and feel perfectly comfortable. At 2400 meters the average temperature is about 10 C.This same figure is well known as the mean annual temperatureof many places in Western Europe, but it wonld be a mistaketo conclude from this fact that therefore a community ofEuropeans wonld find equally appropriate conditions for livingin both regions. For though the average temperature is thesame in both, there is one great difference which must no t beforgotten. In Western Europe the average winter temperatureis 0 C. and" the average summer temperature 20 C.; in the

tropics the temperature in these seasons varies hardly 1 insteadof 20 C., which, for the people who live there, means whatErnst Haeckel, the great German biologist of the nineteenthcentury, called "the everlasting sameness", and which certainlydoes no t produce a stimnlating effect on the human organism- not to mention such extremely important factors as theinfluence of a practically constant temperature on the vegetationitself and on th e activities of micro-organisms in the soil. Theonly variation the weather undergoes is due to changes of moisture,cloudiness"or rainfall. And these changes ma y be traced to oneof two causes, one local in origin, the other general.

The local cause usually arises on or near th e slopes of highmountains or mountain ranges in the various islands. During themorning hours the sun shines on these and on the coastal areasaud warms them more than it does the adjacent sea. The ai r abovethe land also gets warmer than that which is above the water;it becomes lighter, rises and is replaced by air coming in fromthe sea. This movement causes a current of very damp airflowing from the sea to the mountain slopes; as it rises, th e ai rcools, the vapour condenses to form heavy cumnlous cloudsand heavy rain, often accompanied by a sharp thunder-storm,until during th e afternoon eqnilibrium is established once more.Towards sundown and during the night, the land gets coolerthan the sea. Consequently the cooler ai r from the mountainsdescends to the sea and thereby becomes slightly warmer again,while remaining clear under th e clear, starlit sky. Next da ythe same series of changes recurs - c le ar mo rn in g, then clouds,rain, clear weather once more; unless, indeed, monsoon windsproduce a change in the usual sequence.

These monsoon winds are no t local in origin an d constitutethe general cause referred to above. They arise in the followingmanner: In June the su n is fa r to the North. At this season itis ho t in Southern Asia and comparatively cold in Australia.Over Asia cyclonic conditions prevail, in other words, in S.E.Asia the prevailing wind is from the S.W.; this changes it scourse an d becomes South Easterly in so far as it comes fromSouth of the equator in the Netherlands Indies. Over Australiaan anti-cyclonic condition prevails, causing a South East windacross Northern Australia, which arrives in the NetherlandsIndies as a dr y wind from S.E. to E. This dr y East monsoon


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blows from May to September, or even a little longer, especiallyin the Sonthern portion of the archipelago. The further Northor West it gets, the less strongly it is felt.In December the snn stands far to the Sonth, above thecentral part of Anstralia. This means that the cyclone is noWthere, and that conseqnently the prevailing wind blowing acrossNorthern Anstralia and South of the archipelago is from SonthWest to ,West. At this season it is cold in Asia, which fact 9ausesa North East wind that, on crossing the equator, turn's intoa North West wind. This wind is noted everywhere throughoutthe Indies as a wet monsoon, especially when it blows conjointlywith the South Westerly Ocean breeze caused by Australia.Wherever it meets highland conntry or mountains on it s course,it causes heavy rains, especially on the West coast of Sumatra,in Java, Borneo and Celebes.Rain or dry weather in the plains is a matter depending chiefly onthe dry monsoon; bu t on the sides of the mountains the rainfallis ouly increased or decreased by this wind. Hence there areregions where it rains practically every day of the year, becausethe dry monsoon does no t penetrate there. The portion of Sumatranear the equator, almost the whole of Borneo and large portionsof New Guinea are cases in point; on the other hand we findregions, such as the Northern coastal part of East Java and theSmall Sunda Islands, where the East monsoon is very dry andlasts very long. Between these two extremes are all sorts ofintermediate conditions. But there is hardly a spot in the wholeof the Netherlands Indies where it does not rain hard for atleast three or four months of the yea;r.

The heavy rainfall of between I to 7 metres per year, whichis particularly characteristic of these tropical areas, stronglyaffects the soil and consequently the vegetation. For theabnndance of rain-water no t only wets the soil, bu t most distinctlyleaches it at the, same time. All substances that are solublein water, however slight the solubility may be, are dissolvedin the long rnn an d finally carried away into the depths of th eearth to springs, and thence to rivers and to the sea. This processalso takes place in th e very damp portions of the temperatezones, bu t there it works much more slowly, firstly, because

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rainfall is .jess, an d secondly because the temperature isower - c.Ircumstance which greatly decreases solubilityA very sIgmficant point is the fact that among these s o l u b l ~ :b::,ances are t h o ~ e which serve to feed the vegetation. Hence. say t ~ a t ill all tropical regions the soil is constantlyb e ~ g =povenshed, is everywhere tending to a final condition:nhich would make all vegetable growth impossible, becauseIt would mean that plant food was entirely lackiog. Fortnnatelythere are a number of factors which greatly in

tl' some cases verygrea y, retard the process towards this fatal end19l t ' ,orevenveryar e y preven Its accomplishment. Human welfare demands

that. we should know exactly what these factors are and i fpossIble, learn to promote their action '. I t is obvious that leaching decreases ~ v i t h the rainfall. Hencem areas ,:here the dry East monsoon is felt, the soil is generallycomparat;vely fertile, and what is more, retains it s fertilityc o m p : r r ~ ~ I v e : y 10Ilg. This rule applies all over the globe. Allold cIvilizati?ns which have been able to hold their own formany centunes have ha d little rain. Cases in point are MexicoPeru" C a r t h ~ g e , Egypt, Palestine, Mesopotamia, India, C ~ a : The most highly developed cultures that have appeared in the~ e t h e r l a n d Indies flourished in Middle Java East J d thIslands of Bali d L , ava , n e. an ombok, that is, in areas where the totalyearly rarnfall was no t more than two metres and the Eastmonsoon was strongly felt.T r J h ~ lower the temperature, the less i n t e n ~ i v e the leachings IS .why often the leaching of the soil on the sides of t h ~ mountams and on ~ h e plateaux is less advanced than at thefoot of the mountams and in the lowlands Thi .ouly h ld d . s compansonsinill 0 : goo when the areas compared have an equal andar rainfall. Examples drawn from the Netherlands Indiesare s e l ~ o m really ter,rmg, because the rains that fall there onm o ~ n t a r n slopes at high altitudes are often very heavy. In suchrlegIons the greater quantity of rain-water compensates theower temperature.T ~ : s e o r ; : p ~ v e rocks d i ~ i n t e g r a t e and decompose, soil is produced.of b . ways contams clay an d this clay possesses the facn1tya s.orbmg plant-food from the water circn1ating in the soiland discharges it I I Thi' 'hi very s ow y. s absorptIon counteracts theeac ng process and retards it. Humus performs a more or less

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similar function. Soil rich in clay and humus impoverishesslowly; quartz-sandy soil impoverishes quicldy. Lnckily t ~ e rocks in Java do no t contain much quartz and hence there 1Slittle quartz-sand in the soil there, bu t in many parts of Sumatra,Borneo, Celebes and New Guinea th e soil is rich in that sterilemineral, quartz. This same is true of the Congo and many partsof Europe and America.In the tropics humus is always at a disadvantage, for th ehigher the temperature, the greater the rate at w ~ c h the l J , u m ~ s is decomposed an d mineralized, whenever mOIsture and. ~ I T are plentiful. Fo r this reason th e soil in low-lying areas contamsrelatively little humus, particularly where forests have beenabsent for a long period of time. The higher we go, the coolerit is the richer the soil is in humus. Hence it is on the mountains l o p ~ s that the flourishiog tea and cinchona plantations areto be found; th e best are on newly cleared forest-land.

In the tropics rain almost always comes down in formof a sndden downpour such as in Europe would certainly betermed a cloud-burst. This type of rain-storm produces anothereffect on the soil besides the chemical leaching described above,namely, mechanical surface washiog, or erosion. W h e ~ e v e r th eground slopes even slightly, th e r a i n - w a ~ e r , . strean:mg downswiftly over it s surface, carries away soil, m partIcular th evaluable top-soil containing humus. After a number of suchheavy showers have fallen on an open piece of arabI,: land allthe top-soil is washed away into the brooks and nvers andtransported to the lowlands and the sea, and nothing remainsbu t the naked sub-soil. When this has occurred on an estate,we may say that it has lost most of the capital value representedby the soil, especially where the sub-soil is old, leached o.ut,senile. In such cases it is very difficult to induce new vegetatlOnto grow on this very poor soil. I f the sub-soil is not yet wornou t bu t still juvenile, the task will be easier and the resultsmore promising; bu t even then it can ouly be accomplished bymeans of hard work and much care.But, curiously enough, once this completely exhausted subsoil has come to the surface, only further erosion can save th esituation. Fo r th e layer of senile soil which has come to th etop must be washed away, so as to expose a m o ~ e juv,:nile l a ~ e r as a suitable bearer of a new cycle of vegetatlOn, e1ther wild

or cultivated. Be that as it may, however, as long as there isnatural wood humus left in the soil of the tropics, erosion thereis as great a calamity as i t is in temperate climates (U.S.A.).

Furthermore, in certain parts of the archipelago - on th e SmallSunda Islands, for instance - there are clear signs of winderosion as well as water erosion. This, too, carries away muchof the top-soil at times, when the land has become thoronghlydried out, cracked and crumbly after a long East monsoon. Heavyclay soil which is fairly well able to withstand the action ofrain-water flowing over it s surface, falls a prey to strong windsdnring the dry season and is blown away as dust.

So far we have ouly spoken of impoverishment- an d occasionalenrichment - of arable land by the top-soil being carriedaway. But the opposite often occurs too. The wind - andeven more frequently runuing water - supplies new soil hereand there by covering certain areas with sand an d dust or siltand clay. Whether the contribution so obtained improves theland or not, depends on th e quality of the new soil-coveringmaterial an d this again depends on whether the importedelements come from a region where surface erosion prevailsgenerally, i.e., affects the whole surface equally, or from onewhere this erosion takes the form of gulleying. In the former caseth e silt carried by the water will consist chiefly of fertile topsoil; in the latter it is as a rule barren material brought to th esurface from some considerable depth. Where the silt is likelyto be fertile, an effort will be made to promote flooding as muchas possible, while keeping i t entirely under control by meansof technically well-constructed irrigation works; where it consistsof barren material, precautionary measures will be taken toprevent flooding as much as may be, an d irrigation will beresorted to bu t sparingly an d with special care.

All factors mentioned hitherto as contributing to thepostponement or prevention of the complete exhaustion of thewet soil of the tropics are really subsidiary to one radical factorwhich ma y at an y time suddeuly bring about a fundamentalwange in the whole situation, namely, the action of youngvolcanoes. By this we mean the action of volcanoes that areyoung, no t in th e geological sense o{belonging to the quarternary,

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in this case the holocene period, bu t in th e sense that they havebeen active within the historical period, and preferably so youngthat they have been active during the present century - areactive still, in fact.

In th e Indies such volcanoes are chiefly found in Java, bu tthere are also some in Bali and Lombok and on some of theother Small Sunda Islands, in Celebes, and finally, in certainparts of Sumatra. These all belong to th e type which ejectgreat quantities of ashes, sand and stones over the surroundingcountry. This means thorough rejuvenation of the soil in theareas concerned.

At first everything in the immediate neighbourhood of thecentre of eruption, on the slopes of the mountain, is in ruins,buried under all those ejecta. But it is surprising how quicldyth'e new surface becomes covered with a fresh mantle of vegetation.A quarter of a century is often sufficient to bring this about.Tliis fact was noted in connection with the eruption of Krakatauin I883, of Klut in Ig02 an d again in IgIg. I f here is no immediaterecurrence of the eruption, the new soil remains extraordinarilyfertile for centuries, to be finally subjected once more to gradualimpoverishment as a result of leaching by tropical rains.Hence we may fitly apply the old Latin adage to th e soilof the tropics in the more limited sense referred to at the begiuningof this article: "Igne natura renovatur integra." Lacking thatvolcanic fire, the soil would deteriorate completely, whetherslowly or quickly, and the means at man's disposal forcounteracting this process of impoverishment are, after all,ouly makeshifts, important though they be from our humanstandpoint.

Soil conditions actually obtaining in the Netherlands Indiesand, 1mltatis mlltandis, in other tropical areas in Asia and SouthAmerica corroborate the views roughly outlined above.I t is no mere fortuitous circumstance that Java is the most

highly developed of all the islands, bu t the inevitable outcomeof natural conditions. A considerable number of volcanoeswhich have been active within recent centuries, or even decades,have repeatedly brought about complete rejuvenation of th esoil of the surrounding country. Here in Java we find cinchonaand te a plantations - both very exact ing cul tures - on theslopes of the v o l c a n o ~ s , and on the plains at their foot - thanks

to the highly developed irrigation system - rice, sugar andseveral other crops. Bu t those parts of the island which liebeyond the sphere of volcanic influence are obviously severaldegrees less valnable from the agricultural point of view. Fortun-ately such areas are comparatively rare in Java.

In Sumatra the distribution is different. There, fine volcanicareas are in the minority, occurring ouly in the North in theBattaklands and Deli; in the central portion along th e Westerncoast and in the highlands of Padang; and in South Sumatra inthe Palembang highlands; bu t all these added together form bu ta small fraction of this great island. Furthermore in comparingSumatra with Java, we must remember two thiugs: Firstly,that the dry East monsoon ouly touches the Southern portionof Sumatra, an d ouly reaches it when much reduced in streugth.Hence the leaching of th e soil is continuous in these regions,for there is scarcely a spot where an average rainfall lower thanIOO mm. is ever registered for any month of th e year whatever.Secondly, no t all volcanic products are of the same nature.In Sumatra th e ejecta often belong to the more "acid" type,while in Java they are more "basic"; which means that in Sumatrathey contain more silicic acid, in Java more calcium, magnesia,iron and phosphoric acid. Furthermore, in Java potassium isfound in a more easily assimilable form than in Sumatra. Inshort, the ejecta in Java are more fertile an d produce bettersoil for agricultural purposes than those of Sumatra. I f th ereader should be inclined to observe that there are volcanicareas on the latter island not included in the above list, heshould remember that these are the districts where the acidejecta predominate.

From the above it follows - particularly if we consider thatvast areas in Sumatra are entirely outside the range of volcanicinfluence - that this island as a whole will never be as fertileor as intensively cultivated as Java, uuless, indeed, countlessvolcanoes become active there and thus rejuvenate an d improvethe soil by scattering first-class volcanic ash over it , as forinstance Krakatau did all over the Southernmost portions ofSumatra, when it erupted in I883. Sixty-five years ago theLampong Districts were territory in which there was very littledoing; since I883 this region has revived; it is being developedagriculturally; European enterprises flourish there and we find

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immigration from Java to join already prosperous "colonies"of migrants from that island. The impulse that led to all thisactivity was given by the volcano.Celebes differs in many respects from both Java and Sumatra.There, too, we find young volcanic areas, chiefly in the NorthEastern peninsula, or the Minahasa, and these produce fertilesoil. The same ma y be said of the South Western peninsula.As might be expected, these are the most prosperous and mostdensely populated parts of the islaud. The remaining pqrtions,namely, the central part with its two protrusions extendingNorth East and South East respectively, possess no volcanoes,nor any agriculture to speak of - at most coco-nut groves hereand there along the coast - because the soil there does no tencourage agriculture. And the population is much less densehere than in the Minahasa and the S.S.W. portion of the island.

Now if we look at Borneo an d New Gninea - we are consideringouly the Netherlands section of the latter - we find that thesetwo islands are entirely devoid of volcanoes. No need to, searchfor juvenile volcanic soil-types there, for there are none. Nowhereis the soil of such a character that it could be used withoutprevious special preparation to grow food crops for a numberof consecutive years. There are areas that present the necessaryphysical characteristics bu t all the soil has reached an advancedstate of senility as a result of continuous leaching. Rubbertrees and the like demand very little from their surroundingsand will grow on the recently deposited alluvial soil as i t is,bu t in almost all other cases a crop needs manure, either animalor artificial. Under these circumstances ouly crops that furnishhighly valuable products can be made ".to pay, and even thenthere comes a time when previously effective measures provevain. A case in point is the now extinct tobacco-growing industryin British North Borneo. Where at an earlier date the junglewas forced to make room for plantations, the jungle has oncemore made good its claim to the land.

To comment on all the other islands of the archipelago wouldlead us far beyond the scope of this article. We will mentionouly one or two points. I t is interesting to note that in the earlydays the Netherlands United East India Company settled inthe Moluccas, on the islands of Ternate, Tidore, Ambon andBanda. The object was to cultivate valuable spices such as

cloves, nutmeg and mace. Curiously enough, all these islandsare volcanic. Burn, Ceram an d Misool are much larger, bu tno t volcanic, and these the company ignored.

In the Small Sunda Islands group we have Bali dominatedby Mount Batur, Lombok by Mount Rinjani. These two volcanoeshave provided their respective territories with first-rate juvenilesoil, on which has arisen a dense, prosperous and highly cultivatedpopulation. Sumba, on the other hand, has had very littlevolcanic influence, it s dreary, desert-like wastes being composedchiefly of bare, calcareons rocks. All there is of good soil iswashed or blown from th e highlands into the lowlands an dhence we find, besides th e very poor, utterly deserted areasmentioned above, also fairly prosperous plains.

The above may be summarized as follows: I t is true thata fall of rain may be a blessing to an agricultural area parchedfrom drought, bu t it is equally true that in the tropical zonein the narrower sense of the term - the zone in which theNetherlands Indies are situated - too much rain is bad and,further, that abundant rainfall is tile cause of continuallyincreasing impoverishment of th e soil. The only regenerationof the soil that spells radical improvement is that producedby volcanoes. Without active volcanoes the future can oulymean retrogression. But retrogression may be greatly retardedan d counteracted by human action. This last is the splendidtask of the science of agriculture. Much has already been achievedalong this line, and probably much more will be achieved inthe future.

Mohr - Climate and Soils in Indonesia

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