Figure 2-1A deep, fertile soil on alluvium ofthe Lerderderg River nats, BacchusMarsh.This area is intensively cultivatedto produoe vegetables and fruit forthe Melbourne markets. A line ofsprinklers is irrigating crops in thebackground. By contrast, thehillslopes on the lell have beenovergrazed and are prone to soilerosion. (photograph by .w.Schleiger).

Figure 2-2A fossil soil in the bank ofJacksons Creek, near Bulla.It may be thought that rocks areold and soils are young. However,examples can be found of soilsformed in past geological ages. Inthis photograph, the geologicalpick is lying on an old clayey soildeveloped on a weat hered lavanow. The soil has been covered bya later basalt flow, which is notvery weathered. It is called a fossilsoil, because it was formed in apast geological period.(Photograph by N.W. Schleiger).

Soils 45

Chapter 2

SOILS

Soils form a very small pan of the Earth's crust compared with the space occupiedby rocks and sediments. Nevertheless soils vary more in their characteristics thanrocks do. Consequently they are more difficult to classify. The composition of asoil depends not only on its parent rocks and minerals, but also on the interplayof various geomorphological, climatic and biological factors.

It is also important to recognise that soils are continually changing with thepassage of time. This means that a soil profile may gain or lose material over theyears. It can also be continually modified by various chemical and biologicalprocesses. Rocks, by contrast, tend to remain unchanged over long periods ofgeological time, apart from the weathering and erosion that takes place close to thesurface of the land.

The best soils for agriculture and forestry are deep. well- trucrured loams withadequate humus and nutrient contents. Such fertile soils only occur on geologically

46 Chapter 2

Soil formation

SOIL UTRIENTSGood growth in crops, vegetablesand other plants depends on theavailability of nutrients in soils.The following eighteen chemicalelements are known to be essentialfor plant growth:

Carbon, oxygen and hydrogencome from the air and water. Theother elements mainly come fromthe soil. A plant may grow poorlyif the soil is deficient in one ormore of the above elements or ifan element is present but notreadily available to the plant.

very large areas in some overseas countries. Examples are the alluvium forming theriver flats and deltas of some of the world's largest rivers, such as the ile and Ganges,and the glacial deposits on the prairies of North America. [n China, there areimportant soils on loess, which is dust originating in glacial deposits. However, inVictoria, first-rate soils are restricted to small areas of river flats and to volcanicash deposits. The largest river flats occur in Gipp land beside the Thomson,Macalister and Avon rivers near Maffra, the Mitchell River near Lindenow and theSnowy River near Orbost. Young volcanic ash deposits mixed with colluvium occuras aprons around volcanic cones in the Western District.·A large area near Koroitis a particularly productive agricultural area.

There are larger areas of soils, which have favourable physical properties butare less fertile. These include the following soils:• red, friable, clayey soils developed from colluvium on hilly areas of basaltic rocks,

e.g. near Ballarat, Trentham, Warragul and Monbulk;• red, brown and yellow friable soils developed from colluvium on mountain slopes

with moist southerly and easterly aspects in the East Victorian Uplands and inthe Otway and Strzelecki ranges;

• red and brown loamy soils developed from windblown (aeolian) deposits on theMallee plains;

• well-structured grey clays on the Wimmera Plain.

By contrast, there are large areas of poor soils in Victoria. These include sands inwestern Victoria (e.g. in the Big Desert and Little Desert), hard-setting soils in thenorth of the State and shallow, stony soils in the drier hills and mountains of theCentral Victorian Uplands.

There are many types of soil in Victoria. This is caused by the great variations inthe soil-forming factors - climate, parent material, landform, organisms (flora andfauna) and age - across the State.

Climate varies from arid in the far north-west of the State to humid in the mountains.It reaches alpine conditions on the highest peaks where snow lies for many months.With increasing rainfall and decreasing temperature in the mountains, there arechanges such as increased humu , improved structure and increased acidity(i.e. lower pH values). Under these conditions, salts are dissolved out of the soilsand carried away in streams.

Older soils in the drier areas are red because the dryness produces a red varietyof iron oxide; this coats the sand grains in the soils.

Parent material has a major effect on texture and nutrient contents. For e.xample,granites give rise to coarse sandy soils and basalts to clays of high fertility. Aeolianmaterials in the north contain considerable lime (calcium carbonate) and solublesalts. These tend to remain in the soils because the rainfall is too low for themto be dissolved.

Landform differences influence soils greatly, both within and between districts. Forexample in hilly terrain, upper slopes tend to have shallow, stony soils becauseof natural erosion. Red colours also occur because of the good drainage. On theother hand, soils of the lower slopes are deeper and browner because run-{)ff waterpromotes denser vegetation and thus more humus. In the Mallee, in one particularpaddock there can be sand on the dunes and loams on the intervening flats. Thetwo soil types should be treated differently during farming.

Flora and fauna: plants are broken down by animals and organisms such as fungiand bacteria to produce humus. Humic acids hasten the liberation of nutrientsfrom rocks. Roots produce pores through which water can penetrate freely. Deeproots bring nutrients to the surface. Plants also protect soils from erosion. Animalsimprove the structure and nutrient status of soils by adding their excreta. Evensmall creatures such as worms are important because they burrow through soils,thus making the soil porous. All these effects are greater in humid, mountainousregions than in drier areas.

Soils 47

Bedrock

Plateau with reSidual soil malerialsI

,Old alluvium or ...............

. . ' .. : Sheef erosion·'.

~.... '. '.. :-. . .. . ,"[Bedrock outcrop

',,£ inlerspersed_Gully erOSIon •,,;,=- with lh,n soils

I' -,l'!..._--,11~"",""'''''--ColIuvlum overlying"~5:---- reSidua~1materials

I -

::::::::::;;;;§?~<- CollUVium __

Raw malenaldeposited ,from gullies _

The age of a soil depends mainly on its position in a tream catchment. The youngestsoils are on river flats, where alluvium is still being deposited at intervals. Thesesoils are fertile because there has been insufficient time for nutrients to be leachedout from the sediments. On older alluvial terraces, soils with loam topsoils andclay subsoils are common. They are particularly widespread on the plains ofnorthern Victoria, where much of the State's irrigation is carried oul. They alsooccupy large areas in the south-west of the State and in Gippsland.

The oldest soils occur on gentle drainage divides that have been protected fromerosion. Some are more than a million years old. They have very dense clays, whichare strongly mottled with red, grey and off-white colours. These are scatteredthroughout the State, the largest single area being on the Dundas Tableland, westof The Grampians.

II1

002

'Very much smaller amounts ofsodium and chlorine are actuallyneeded, but plants usually take upmuch more salt (sodium chloride)than they need simply because it isin the soil. (From CSIRO Divisionof Soils)

As an exampie, a heal thy 2kilogram cabbage should containthe following elements (approximate):

Figure 2-3bThe origin of soil parent materialsin a landscape.

Various phases of weathering anderosion have produced materials inwhich soils have formed. Thedeepest soils occur on alluvialplains, colluvial slopes andplateau~ Soil development is weakon younger alluvium, so theprofiles are uni form, soft, porousand fertile. There is stronger soildevelopment on older alluviumproducing duplex soils, that arehard-selling and poorly-drained. Insome areas, e.g. the broad RiverinePlain of northern Victoria, olderalluvium is blanketed by dustblown eastwards from the Mallee.Present-day sheet and gully erosionare removing some earlier-formedsoils.

Figure 2-3a (above right)Gully erosion near DarraweitGuim.The area on Palaeozoicsedimentary rocks is notablygullied. Here a gully has developedin sodic duplex soils on valleyalluvium.

01 classificatio Soils are classified on the basis of differences in their profiles (see Chapter I). Thedifferences in any area are detected largely by in pecling sample from road cuttingsor holes drilled with a hand auger to depths of usually not much more than onemetre. The drill holes need not be located on a systematic grid pattern. Rather, thesites are selecled wherever there appear to be differences in the slope of the ground,the type of vegetation or other natural factors. Aerial photographs are used to pinpointwhere each sample site is located. The boundaries between different soil types cansometimes be seen on the photographs.

48 Chapter 2

In Victoria, four broad classes of soil profiles have been recognised, based ona system introduced by the CSIRO Division of Soils (Figure 2-4):• organic soils• uniform soils• gradational soils• duplex soils.

In the following discussion, the four broad classes are further subdivided intogroups that are widespread in particular regions of the State, as shown in Figure 2-5.It is important to realise that these groups still represent broad levels of classification.Each group contains many soil types that differ in appearance and performance.

Figure 24The four kinds of soils.Soil profiles like these can be seenin road cuttings and buildingexcavations. They usually varyfrom less than one metre to twometres in depth and are often darkat the top as a result of theaccumulation of organic matter.Soil profiles in each group vary indetail, but still conform to thebasic type.

Mainly plant remains

;:......"1:.:.:=:.:,:::~"~.~.:;:.:!";;.,;:~....~~:.~;:..::.,,;... _lf;~::.,I with some sand or clay

~~~ORGANIC UNIFORM

....".;.;:::~:

.- _-._: ++i::+!-::~:;~-.

. .... :: 1++: ;1+!::H'!::....~ .....;:;_: ...~~~~~~.:..;,..:t'

GRADATIONAL

Sandy

Soil particles becomegradually finerwith depth

Clayey

:::;:,»,:",,::::,:,::::::::: Sand or loam topsoil-:-:-_-_.:.~.:.:_:.:~. Sharp break-------

Clay subsoil

DUPLEX

Figure 2-5Common soils in Victoria.

Parent rock - mtry be treshor weathered bedrock,sedimentary deposits orremnants of older soils

Class Group Commonly used names Occurrences

Organic Peats Peals Coastal salt marshessoils Freshwater swamps beside rivers

High Plains

Uniform Sands Podsols when pale and acidic. Coastal plainssoils Sandy Malice soils for inland Malice duncrield

sands containing abundanl lime.

Clays Soils of heavy texture Wimmera Plain, volcanic plainsMurray Valley nood plains

Looms (deep) Alluvial soils Gippsland rlood plains

Shallow stony loams Skeletal soils Central Victorian Upland:.

Gradational Shallow stony eanhs Stony soils of the hills Cenlral Victorian Uplandssoils Friable earths Krasnozems when red Hills on basalt near Melbourne

Moulllain soils when brown East Victorian UplandsCalcareous earths M:l.llee ..oils Malice dune field

Duplex Red calcareou .. Red brown earlhs Riverine Plain of northern Victoria.soils duplex soils Wimmcra Plain. Werribcc volcanic t,lain

(alkaline)

Sodie duplex ..oils Solodic ...oil.. Genller ...Iopc~ of We"", Victorian Uiliands(neutral) West Victorian Volcanic Plain...

South Victorian Riverine r'lains

Acidic duplex ...oih Podsolic soil~ Lower ... Iopes of East Victorian Uplimds(acidic) L'ltcritic p(Kholic ...oih Many scallered areas 011 gelltle catdllllt~1I1

dividc~

Large areas on Dundas Tilblcland andcoastal rlain~

Figure 2-6A disturbed alpine bog, north ofMouot Cope, in the East VictorianUplands.In the background is a sphagnummoss bog with a shallow organicsoil in the High Plains country.Excessive trampling of this area bycattle led to compaction anddrainage of parts of the bog. Thepeaty soil was eroded, leaving agravelly surface. (Photograph byN.J. Rosengren).

Soils 49

ORGANIC SOILSThese soils are dominated by black to dark brown decaying plant matter in the upper30 centimetres or more. They also contain sand and clay in varying proportions.Peat is a typical organic soil. It consists mainly of plant matter that is saturatedwith water over long periods.

Organic soils form in poorly-drained areas where dead plant materialaccumulates. They can occur at any level from the highest plateaus to the lowestcoastal marshes. Environments, where organic soils can be found, include thefollowing:

1. Salt marshes, e.g. near Queenscliff and around the margins of Western Pon, CornerInlet and Andersons Inlet on the South Gippsland coast.

2. Swamps formed where streams were blocked by either lava flows (e.g. Lake Condahnear Macarthur in western Victoria) or sand drifts (e.g. swamps behind coastaldunes).

3. Deltas and sections of river valley, where drainage has been impeded by eitherfaulting or hard rock bars, e.g. near Heywood, Carrum, Koo- wee-rup and TarwinLower, in parts of the Latrobe River valley and on the Snowy River flats.

4. Valley boltoms and lower slopes where drainage waters accumulate on the HighPlains in the East Victorian Uplands.

Some of the less acid organic soils have important land uses, particularly wherethere is a high clay content. For example, Koo-wee-rup Swamp, to the south-eastof Melbourne at the head of Western Pon, was originally a waterlogged, swampyarea. It was progressively drained between 1876 and 1920. Since then it has becomean important area for market gardening, supplying Melbourne with much of itsvegelable . However, wilh the spread of house and factories into this area in recentyears, some high quality horticultural land has been lost.

Peat bogs on the High Plains are important in regulating stream flows. Theyabsorb a great deal of rainwater and release it slowly. This decreases flooding anderosion after heavy rains and prolongs stream flows in long dry periods. They arevery acidic and have low fertility.

UNIFORM SOILSUniform soils have no distinct texture boundaries and only minor lexture differencesIhrough their profiles. They are mostly clays and sands. There are also small areasof uniform loams, but uniform silt soils are extremely rare in Victoria.

1. Uniform clays occur over parts of the flood plains of the Murray River, itstributaries and many rivers in southern Victoria. They are also found on theWimmera Plain and on the low-lying parts of the volcanic plain of westernVictoria. Clays on the Wimmera Plain are particularly important for growingwheat, barley and oats. Both the flood plain and volcanic plain clays are subjectto waterlogging. However, they can be used for dairying and grazing where theyhave been drained.

Extensive red gum forests grow on these soils on the Murray River nood plainnear Barmah and Cohuna. The forests are commercially important because thewood is durable and water-resistant. It is used for railway sleepers, fence postsin the country and house stumps in urban areas.

50 Chapter 2

2. Uniform sands are typical of sand dunes. There are two kinds:• siliceous dunes made up of quartz grains. These are common in the Little

Desert, Big Desert and Sunset Country in the north-west of the State and alongparts of the coast;

• calcareous dunes that are mostly made up of calcium carbonate (lime) derivedfrom shell fragments washed or blown from the sea-floor. They form someof the youngest coastal dunes. Calcareous dunes also occur inland in the Malleebut there the lime appears to have been deposited from groundwaters.Uniform sands are typically pale, except the top few centimetres, which are

darkened by organic matter. These soils are vulnerable to wind erosion and havelow productivity when farmed. Natural vegetation on them should not becleared.

Coarse uniform sands also occur on colluvium in granitic country, e.g. onthe slopes of Mount Alexander near Harcourt and the You Yangs near Geelong.

3. Uniform loams can be either deep or shallow. Deep loams are restricted to smallareas on deposits such as young river alluvium.

Poor, shallow, loamy soils, less than 15 centimetres thick, occur between rockoutcrops on the steepest hillcrests, particularly in the Central Victorian Uplands.These soils have a low capacity for storing water. They are called shallow stonyloams or skeleral soils.

GRADATIONAL SOILSThese are soils that become progressively more clayey with depth, but each horizongrades into the next without an obvious change. They are commonly known as 'earrhsand the main occurrences are as follow:I. Shallow stony earths occupy the steeper hillslopes in the drier parts of the Central

Victorian Uplands. They are typically about half a metre deep and used mainlyfor sheep grazing. Even though the subsoils are clayey, they are quite porous andthus water can easily pass through them. This is important for groundwarerrecharge, that is the addition of water to natural underground storages (see Chapter6). However, if the country is cleared of forests, the levels of water tables mayrise and possibly cause waterlogging. This is because forests absorb large quantitiesof water. Rising water tables also bring soluble salts into the upper soil layers.The salts may eventually kill pastures and native vegetation.

2. Friable earths have favourable physical characteristics and reasonable chemicalqualities. The most fertile are red clayey soils found on colluvium in many hillyareas of basaltic rocks, e pecially near Ballarat, Trentham, Warragul, Thorpdaleand Monbulk. They are called krasnozems, a Russian word for red soils. Theyare excellent agricultural soils, that are used intensively for dairying and producingpotatoes, vegetables, berries and other fruits.

There are widespread red, brown and yellow friable earths developed fromcolluvium on humid mountain slopes in the East Victorian Uplands and the Otwayand Strzelecki ranges. This country is used for timber production. arureconservation is important in forested area where magnificent eucalypts, such asmountain ash and alpine ash, grow to height of up to 100 metre. Cleared areasare used mainly for dairying.

3. Calcareous earths have developed on dust deposits on the Mallee plains. Theyare used mainly for wheat growing and heep grazing but yields are limited bythe dry climate, wind erosion and soil salting. Several irrigated area are renownedfor their grapes, oranges and grapefruit, but again there are problems with salinity.

The pH trend reneets the extent towhich rainwater has removedsoluble substances from the soils.

Figure 2-7Soil pH and c1imale.

Category

alkalineneutralacidic

pH

>77

<7

Climate

semi-aridsub-hwnid

humid

DUPLEX SOILSThere are marked contrasts in the textures and colours of the A (topsoil) and B(subsoil) horizon of duplex soils. The transition i often quite sharp and, at most,occurs over an interval of 10 centimetres. The A horizons are sandy or loamydepending largely on the parent material. There is often a subdivision into an upper,brown, A I horizon containing humus and a lower, pale-coloured, A2 horizon. Clayand iron oxide are leached from the A and concentrated in the B horizon. Calciumcarbonate may be present in the lower part of the B horizon. The clay horizons arecoloured in variations of yellow, red, brown and grey. Sometimes ironstone nodulesoccur near the boundary of the A and B horizons.

Duplex oils are by rar the commonest soils in VictOria. They occur everywhereexcept on the higher mountains. The profiles vary in detail from one area to anotherdepending on their hi tory of formation. This variation has an important bearingon their agricultural value and stability. They are subdivided into three broadcategorie according to the pH of the deeper subsoils, as in Figure 2-7.

Figure 2-8A common distribution of soils insouthern Victorian landscapes.The sand soils ncar the coast havelow fertility and low water-holdingcapacity and thus are not suited toagriculture or forestry. Koo-wee·rupSwamp has been drained and thepealy soils are highly prized forvegelable growing. The olderalluvium further inland has yellowduplex soils which are seasonallywaterlogged and used mainly forgrazing. The mountains remainforested; here the soils haveravourable physical characteristicsexcept that they arc shallow on thesteep nOrlh- and west-facing slopes.

Soils 51

1. A IkaJine duplex soils.

The most widespread soils in this category are red calcareous duplex soils, alsoknown as red brown earths. Calcium carbonate is visible in the subsoils as whiteflecks and nodules.

These soils have developed mainly from alluvium and aeolian material on theRiverine Plain of northern Victoria, the Wimmera Plain and the Werribee basalticplain. Crops and pastures are grown on these soils under dryland and irrigatedagriculture. Agricultural productivity is limited by the poor structure of both theA and B horiwns. This leads to hard-setting surfaces, waterlogging, wind erosionand salting. Plains with these soils have been almost completely cleared of theoriginal vegetation.

2. Neutral duplex soils.

These soils are known as sadie duplex soils or solodic soils. The term 'sodie'indicates the presence of excess sodium, which gives the soils undesirablecharacteristics.

Subsoils swell on wetting, blocking off soil pores and thus limiting funher entryof water and causing waterlogging. Rainwater runs off the surface of hilly landcausing erosion of the topsoils. The sodic nature of the subsoils causes them todisperse, that is to form a suspension in water. Thus the subsoils also are readilyeroded by moving water. The water becomes murky and causes silting up of streamsand water storages.

Because of the poor drainage, subsoils are usually yellow and often mottledwith grey. Upon drying, the A horiwns set hard. After heavy rains they becomesaturated for long periods, lose strength and it i impossible to drive vehicles acrossthem.

The main occurrences of neutral duplex soils are in the Central VictorianUplands, (particularly the we tern part where the rocks are mainly Palaeozoicsandstones and mudstones), the West Victorian Volcanic Plains and the SouthVictorian Riverine Plains in both the eastern and western parts of the State. Thesesoils are mainly used for grazing. However, the poor physical nature of the soilslimits agricultural productivity and promotes erosion, particularly in the WestVictorian Uplands. Salting and waterlogging are also problems.

3. Acidic duplex soils.Acidic duplex soils are often known as podsolic soils. Podsol i a Russian termmeaning "ash-like below": it refers to the pale subsoil horiwns. The B horizonclays are fairly porous, so that profiles are not very prone to waterlogging anderosion. Nearby streams tend to run clear. The A horizons have moderately goodstructure and do not set very hard when dry.

The main occurrences are on lower slopes in the humid East Victorian Uplands,which remain largely forested. They are quite productive when cleared for pasture.This is especially the case when the levels of nutrients, particularly phosphorusand nitrogen, are rai ed and maintained.

Acidic duplex soils with deep mottled clay subsoils occupy gentle catchmentdivides, the South Victorian Coastal Plains and the Dundas Tableland in westernVictoria. The mottling is striking, with large patches of red, grey and whitish clays.Pieces of ironstone frequently occur in the upper horizons and scattered on thesurface. These profiles are commonly called lateritic podsols. The deep clay storesoluble salts. When the salts are dissolved by groundwater, they can cause soilsalting at downhill sites and increased salinity in streams.

-§!>;t---

LANDFORM COASTAL SWAMP e g ALLUVIAL MOUNTAINSDUNEFIELD KOO-WEE -RUP PLAIN

PARENT MATERIAL SAN!) PEAT ALLUVIUM COLLUVIUM FROM- BEDROCK e gSandSlOne MudstoneGranite

BROAD SOIL CLASS UNIFORM ORGANIC DUPLEX GRADATIONAL-

52 Chapter 2

Human impacton soils

Figure 2-9Sheet and rill ero ion by runningwater on cropland in norlhernVictoria.Cultivation of the land had left itbare in preparation for sowing acrop later in Ihe year. Soils arevery vulnerable to running waterunder these condition . An area ofcultivated soil has been completelyremo\'ed and many rill have alsoformed afler water has rusheddownhill from the top right tolower left of Ihe pholOgraph.

Soils gradually change with the passing of geological time, because there arecontinuous changes in climatic factors and in the weathering and erosion of parentrocks. If soils and weathered rocks were not eroded naturally, most sediments, thatlater formed sedimentary rocks, would not have been produoed.

When the first settlers, the Aborigines, came to Victoria, soil erosion increasedand soil fertility decreased because the people regularly set fire to the bush. Theseand other adverse changes accelerated after European settlement began. Great pressurehas been plaoed on the land where it has been cleared for settlements, cultivationand grazing. Smaller areas have been affected by road construction, mining, timberharvesting and other activities.

Agriculture has had the greatest impact. The native Victorian vegetation wasadapted to the natural low fertility of most of the soils. But harvesting of new cropsand grazing soon exhausted the small reserves of plant nutrients. Productivity wasrestored, however, by the addition of new chemical substances as fertilisers. The mostimportant addition has been phosphorus as superphosphate. Other elements appliedin fertilisers include sulfur, potassium and traoe elements such as molybdenum, copperand zinc. A further improvement has been the introduction from overseas of grasses,clovers and other legumes, which have raised the nitrogen content and restored humuslevels in soils. On well-managed farms, soil fertility can now be higher than it wasbefore the land was used for agriculture. However, fertility has declined in intensively­cropped areas.

In addition to the problem of declining soil fertility, there are four otherprocesses of soil degradation. These processes are:• soil erosion: the permanent loss of soil because it is washed or blown away;• sa/ination: the addition of harmful salts, especially sodium chloride, to a soil;• acidification: the decrease in pH of a soil;• compaction: a process that packs soil particles tighter and impedes drainage,

aeration and the spread of roots.

SOIL EROSIONSoil erosion mainly occurs after:• vegetation is removed to prepare land for crops;• grasses are eaten down to the roots by livestock and pest animal, particularly

rabbits.These processes leave the urface of a soil wholly or partly bare. In this State, soilcan easily be removed by running water or wind.

Water erosion occurs in everal ways:I. Sheet erosion - where rain water remove the urface of a hillside by the impact

of raindrops, sheet flow and tlow along mall channel a few centimetres deep(rills) (Figure 2-9).

2. Gully erosion - where a Slream ClltS a channel imo a oil, often more than ametre deep.

3. Tunnelling - where waters find passages underground and excavale cavern .

Figure 2-10Wind erosion in (heM.Uee.Coarse soil particles haveaccumulated along a north-southfence after westerly winds everelyeroded paddocks. The finerparticles were blown away in duststorms.

Figure 2-11Soil erosion prevention works oncropland in northern Victoria.Soil erosion by running water afterrain i prevented on this propertybecause the movement of waterdown the slopes i slowed byvarious works. Contour banks havebeen constructed around thehUiside; the e faU gently tOwardsthe long grassed strip near themiddle of the photograph. Thegentle slopes and the grass coverlimit erosion. The dam also help10 slow the movement of water. Atthe foot of the hill the water is fedsafely to a local creek. By contrastthe hills in the background areaffected by heet and gully erosiondue to exces ive clearing of nalivevegetation.

Soils 53

4. Stream bank erosion - where creeks and rivers undercut their banks and theoverlying material eventually collapses.

Gullies and tunnels can join to destroy large areas of land.

Wind erosion mainly causes loose soil to blow away (Figure 2-10). The finerparticles can be carried off in dust storms, sometimes travelling hundreds ofkilometres. The coarser particles remain, often forming sand dunes. Wind erosionis most severe in northern Victoria, especially the Mallee region.

The net result of these various forms of soil ero ion over 150 years is that hugeamounts of soil, particularly the humus-rich layers, have been blown away or washedinto streams. The soil carried away in stream has either ilted up rivers and damsor been carried into the sea.

After the Second World War, most State governments formed soil conservationauthorities. These have helped considerably to develop improved land use methodsthat protect soils. One technique is to reduce ploughing to a minimum, so that theland is rarely left bare. Stubbles from previous crops and pastures are retained insteadof being ploughed under.

Many erosion gullies have been filled in and stabilised with vegetation.Revegetation has been encouraged in sheet-eroded area. Water run-off down hillsideshas been further reduced by the introduction of practices such as contour ploughingand the construction of contour banks. The e all ensure that rainwater drains slowlyalong channels around a hillside instead of rUShing downhill cau ing maximumerosion (Figure 2-11).

Overgrazing has also been reduced by better control of livestock numbers andpe t animals, notably rabbits.

54 Chapter 2

Figure 2-12Soil salination and erosion on theWestern District volcanic plain.A severe salinity problemdeveloped in the soils and allvegetation died. Once the soilbecame bare, sheet, rill and gullyerosion occurred. All three formsof erosion are clearly seen to bewidespread. The hills in thebackground are volcanic cones.

Plants vary greatly in theirsensitivity to soil pH. For example,camellias corne from acid soils inthe Himalayas; they grow poorly ifsoil pH exceeds 7. By contrast,lucerne evolved on alkaline soils inthe Mediterranean region andcannot thrive on acid soils. A fewvegetables are very tolerant to acidsoils (down to a pH of 5) these arepotatoes, rhubarb, shallots andwaterlnelons.Most plants prefer soils with a pHin the range 6.0 (slightly acid) to8.0 (slightly alkaline). Most treesand crops tend to prefer slightlymore acid soils than do flowersand vegetables. A few examples ofrecommended pH ranges in soilsfor common plants are givenbelow:

Tre

( c

SALINATIONThis problem was slower to develop than erosion. This is because it has involvedthe slow rise over a long period of underground water containing harmful salts. Whensaline water approaches within a few metres of the surface, it slows down the growthof deep-rooted crops, pastures and trees. If the groundwater rises even higher to withinone to two metres of the surface, the water evaporates and salts crystallise out. Thesekill the vegetation, leaving bare salt-encrusted ground. The largest areas affected arethe drier north-western and northern parts of the State, but there are manyoccurrences elsewhere (Figure 2-12).

The salts naturally present in the drier landscapes have been redistributed bymoving groundwaters. More water moves around now under crops and pasturesbecause there is less transpiration by deep-rooted native vegetation. Another causeof salination has been the excessive addition of water to the ground by flood irrigation.(See Chapter 6 for further discussion on salination).

ACIDIFICATIONThe acidity of a soil is measured by the concentration of hydrogen ions, expressedas pH. Most plants prefer soils to be about neutral. There are some modern farmingpractices which benefit soils because they provide nutrients, but due to variouschemical reactions, they also increase acidity. This has the counter effect of loweringproductivity. Examples are the use of dovers and nitrogenous fertilisers. Agriculturallime (finely ground limestone) can be used to neutralise the excess acid.

COMPACTIONIn the drier regions of Victoria, particularly in the north, some soils are naturallycompact and therefore their productivity is low. However, the problem of compactionhas been increased on these and other soils by both overcropping and overgrazingand an associated dedine in humus content. Productivity declines because rootscannot penetrate the hard layers, and heavy rainwaters tend to run off ralher thanenter the soils. Thus less moisture is available for planl growth.

The quality of soils can be restored by cultivating them as little as possible ­a technique known as minimum IiI/age. A reduction in livestock numbers also helps,because this encourages vigorous pasture growth which in turn improves soil structure.The addition of gypsum also leads to better soil structure.

A problem that is difficult to overcome is direcl compaction by the hooves ofanimals. This is most serious when soils are wet. It is a very serious problem insouthern Victoria where the rain fall is high and the main land use is dairying.

Government and privately-sponsored research is being carried oul 10 solve theseproblems, so that the productivity of soils can be maintained or even improved inthe future.

Soils of theMelbourne subur

Soil geochemistr

Soils 55

Soil type is important not only in farming country but also in urban areas. Soilsaffect human living in many ways. Two aspects are the stability of buildings andthe management of gardens, as shown by the following examples around Melbourne

Most of the hilly areas in the north-eastern and eastern suburbs have sodic duplexsoils on Silurian sedimentary rocks. The B horizons shrink and swell on drying andwetting. Shrinking is particularly great during unusually dry summers. The resultis cracked walls of dwellings, particularly those with rigid brick walls. Nowadaysto overcome this problem, houses are built on concrete slabs (see Chapter 7 for furtherdetails). The A horizons of the duplex soils are poorly structured for gardening, butthey can readily be improved with compost and gypsum. Waterlogging is a problem.

The higher parts of the eastern suburbs are capped by Tertiary sediments. Thesehave acidic duplex soils, which provide stable foundations for houses. They are alsoeasier to cultivate. However, waterlogging can be a problem, often causing the uddendeath of citrus trees.

Clays on the basaltic plains of the western suburbs are particularly prone tosoil movement on wetting and drying. This causes much damage to buildings. Theseclays are too tough for easy cultivation. They can be improved with large amounts ofcompost, gypsum or sometimes sand. Water often disappears down large cracks inthe summer time.

Sands are widespread in some bayside suburbs and they are scattered on theTertiary sediments of the eastern suburbs. They are not prone to move sea onallyand they provide the most stable foundations for buildings. They are not ideal forgardening, however, requiring frequent watering because of their low water-holdingcapacity. They also have low fertility and therefore need heavy dressings of fertilisersand compost. Another problem is that nutrients are easily washed out of these soils.

The best soils for gardening are deep loams on flood plains beside creeks andrivers, ego Maribyrnong River flats. Few houses are built on these soils because ofthe flooding hazard.

So far soils have been discussed largely in terms of their agricultural value However,some soils are of interest to investigators in an entirely different industry - thatof mining and mineral exploration. Until fairly recent times, deposits of mineralscontaining useful metals, (such as lead and copper), could only be found by searchingthe surface of the land. However, many deposits may not reach the surface or theymay be covered by vegetation or soil. To find these hidden resources, geologists usetechniques that depend on the physical and chemical properties of the minerals. Inparticular, geochemical surveys are employed to find exceptionally high concentrationsof metal ions in soils and sediments. Such concentrations may be due to the presencesomewhere nearby of a valuable mineral deposit.

During the weathering of a mineral deposit, metal ions may be carried awayin solid mineral grains or in soluble salts. In a geochemical survey, many samplesof soils or stream sediments are collected over a large area and analysed for certainmetals. Geologists look for the presence of chemical anomalies, that is, concentrationsof metals that are higher than the tiny amounts that normally occur in soils andsediments. In soil investigations, it is important that geologists should recognise thetypes of soils present. Metal anomalies are most useful where they are found in soilsderived from underlying rocks. If anomalie are found in soils formed on transportedparent materials, it may be difficult to determine their source.

Figure 2-13Loss of soil caused by a landslide near Leongalha.Land slumps of this kind arc prone (0 occur in the Strzelecki and Olway ranges where theoriginal forests have been removed.

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