3 Soil and subsoil constraints - VRO | Agriculture...

DAV00056 Scoping Study- Final Report

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3 Soil and subsoil constraintsA range of processes has influenced soil profile development and subsoil transformations in theHRZ. These include leaching of mobile ions (leading to topsoil acidification), precipitation of otherless mobile chemicals in the subsoil (e.g. formation of CaCO3, and within perched water tables,ferro-manganous nodules as ‘buckshot’) and concretion of these into hard layers. Over millenniaclay has accumulated in the subsoils. Where these clay minerals exhibit high-shrink swellbehaviour, subsoils have been affected by upheaval and churning resulting in formation of Gilgaiwith high subsoil variability. In some landscapes, such as the Victorian Volcanic Plains, the gilgaifeatures are not visible at the land surface due to accumulation, over thousands of years, of finesand which has a levelling effect, masking the subsoil undulations. Accumulation or retention ofsodium as an exchangeable cation has led to sodicity over large areas of the landscape and subsoilsare massive or are characterised by large prismatic and columnar structural units, offering onlylimited opportunities for root growth.

Figure 7. Example (near Winchelsea) of Victorian Volcanic Plains soil with subsurface gilgai featuresmasked by fine sand accumulation.

The majority of soils1 in the HRZ are texture-contrast soils (Isbell 2002) formerly classified asDuplex soils (Northcote 1979). Leached topsoils and upper subsoil horizons (A2 or E horizons) arecommon and permeability of the subsoil is imperfect or poor. The physical properties (texture andstructure) of the soils described have well defined classical relationships with the amount of wateravailable for crop growth, the amount lost as drainage or runoff and reduction in rate of drainagesufficient to induce waterlogging.

Subsoil is often described as a function of depth, for example “below 20 cm”(Price 2004). Since thedepth of the A1, A2 and B horizons can change markedly with topography, the B-horizon provides

1 We have adopted the soil classification nomenclature from the Atlas of Australian Soils data for discussion here andfor the presentation of graphs. This nomenclature follows, for the most part, the Factual Key of Northcote (1979). Thecurrently accepted Australian Soil Classification (ASC) is that of Isbell (2002) but there are no direct equivalents thatcan be inferred, particularly for the texture contrast or Duplex soils. In the Factual Key, divisions are based on textureprofiles (e.g. Duplex) and subdivisions are based on colour. In the ASC, orders (the equivalent level of divisions) arebased on texture profile and chemistry, the sub-orders on colour. The criteria for colour classes in the ASC are notthe same as that of the Factual Key, in particular for the sub-division and sub-order terms ‘brown’ and ‘yellow’.Consequently, many soils classified as ‘Yellow Duplex’ in the Factual Key could be ‘Yellow’ or ‘Brown’ Chromosols,Sodosols or Kurosols (the order name depending on chemistry) in the ASC. For the purposes of this report this is nota significant impediment to interpreting the subsoil constraints, although knowledge of the distribution of sodic soils(Sodosols) and strongly acid soils (Kurosols) would add value to the assessment of limitations to root growth in theHRZ.

Clay subsoil with largeprism or columns

Fine sandy (‘spewy’)A2 horizon

3 – 5 metres


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a standard approach to identification of subsoil (NLWRA 2005). Distinction between ‘topsoil’ and‘subsoil’ may be pedological, relating to different properties of the ‘A’ and ‘B’ horizons, but alsostems from consideration of the impact of tillage. Traditional cultivation practices involvedisturbance of the soil mass, often with mixing or inversion, to depths of 10–20 cm, hence thedepth criterion for distinguishing between topsoil and subsoil. However, with the increasinguptake of minimum or zero cultivation, a distinction based on depth is less relevant whereas adistinction based on changes in soil properties with depth is more significant. Consequently, thesubsoil may be best considered as representing virtually the entire profile affecting the rootzone.In particular the A2 (or E) horizons mentioned above and the boundaries between these and theunderlying B horizon material are significant in controlling water availability, aeration, soilstrength and root growth.

The subsoil environment is optimal for crop growth when aeration and water storage aremaintained at a level to support root growth and nutrient uptake. Improved interaction of the cropwith the subsoil and root water extraction optimised with consistent rainfall and low seasonalevaporation offers the opportunity for increased crop production in the HRZ.

3.1 Major soils in the proposed HRZThe dominant soils2 in the proposed southern HRZ are as follows: Yellow Duplex > Red Duplex >Massive Earths > Sands > Brown Duplex > Loams > Cracking Clays > Black Duplex > Non-crackingClay > Organic (Figure 8).

Massive earths

Red duplex

Yellow duplex

Brow n duplex

Black duplex

Cracking clay

Non-cracking clay

Organic

Loams

Sands

Figure 8. Dominant soils in the southern HRZ based on the Atlas of Australian Soils (Northcote1960, 1962, 1966).

Whilst soil type is not the only discriminating factor, the dominant soil types within each of thenew HRAEZs provide a direct indication of the problems likely to be encountered, and some cluesas to where common management regimes may be effective. For example, it can be seen thatcentral Victoria, southern Victoria, Southern Highlands and Millicent coast are dominated byyellow duplex soils. Red duplex soils dominate the Riverine plains and Central west NSW whilst amixture of soil types is represented in Tasmania (Figure 9). Millicent Coast HRAEZ is alsodistinguished by having a larger proportion of sandy soils and cracking clays than any of the otherHRAEZs. The distribution of different soils is influenced by terrain, geology and climate. Theremay be some associations between terrain and soil type that would preclude cropping but in the

2 See note 1.


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current study we have not been able to carry out further landscape analysis that would distinguishthe potential cropping land from non-arable land in each zone.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

CentralVictoria

CentralWest NSW

MillicentCoast

RiverinePlains and

Slopes

SouthernHighlands

SouthernVictoria

Tasmania

HRAEZ

Sands

Loams

Massive earths

Red duplex

Yellow duplex

Brow n duplex

Black duplex

Cracking clay

Non-cracking clay

Organic

Figure 9. Relative occurrence of dominant soils in each of the HRAEZs based on the Atlas ofAustralian Soils (Northcote 1960, 1962, 1966).

Figure 7 indicates the relative occurrence of individual soils across the HRAEZs. This showswhere common approaches to subsoil constraints can be adopted. For example, subsoil issuesassociated with Yellow Duplex soils will be common to all of the HRAEZs, whereas issuesassociated with sands will be common to isolated areas on the Millicent Coast, Riverine Plans andSlopes, Southern Victoria and Tasmania.

The proportional data presented in Figure 8 indicate that constraints associated with clays willimpact throughout the HRZ, as Yellow Duplex, Red Duplex and Massive Earths account for 79%of the total area of the HRZ (42%, 21% and 16%, respectively).

0%

20%

40%

60%

80%

100%

Sands

Loam

s

Massiv

e eart

hs

Red du

plex

Yellow

duplex

Brown d

uplex

Black d

uplex

Crackin

g clay

Non-cr

ackin

g clay

Organic

Soil Group

% o

f HR

AE

Z

Central Victoria

Central West NSW

Millicent Coast

Riverine Plains and Slopes

Southern Highlands

Southern Victoria

Tasmania

Figure 10. Relative occurrence (percent) of each HRAEZ with respect to dominant soils of the HRZ.(Data from the National Land and Water Resources Audit).


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3.2 Principal subsoil constraints in the HRZ of SE AustraliaPhysical ConstraintsPhysical constraints to root growth are water availability, mechanical impedance (soil strength)and soil aeration. All of these are affected by four soil properties: soil texture, bulk density,porosity and soil water content. Furthermore there are important relationships between soiltexture, bulk density and porosity that compound with water content to create constraints to rootgrowth, these are summarised in Table 5 and described below. The relationships betweenmechanical impedance, water content, aeration and porosity combine to affect the least limitingwater range (LLWR) for root growth.

Table 5. Physical subsoil constraints are related to four soil factors.

Factors

Soil Texture Bulk Density Porosity Water Content

Ava

ilabl

eW

ater

Pore size distribution,particularly theproportions ofresidual and storagepores.

Total pore space andtherefore waterstorage.

Total volume fractionof storage pores.

Water content abovewilting point inaerobic soil.

Mec

hani

cal

Impe

danc

e

Particle packing andinter-particle pore sizeaffects soil strength.Inter-particle pore sizeis negativelycorrelated withstrength.

Density and strengthare positivelycorrelated–dense soilsare stronger.

Transmission poresoffer unimpeded pathsfor root growth.

Water content andstrength arenegatively correlated–wetter soil is weakersoil.

Con

stra

ints

Aer

atio

n

Structuralarrangement ofparticles intoaggregates affects porecontinuity andtransmission pores.

Total pore space,macropore volumeand pore continuityare affected by thebulk packing of soil.High soil density isusually negativelycorrelated with soilaeration porosity.

Transmission poresneed to constitute 10–25% of total soilvolume to ensuredrainage and oxygenreplacement.

Water content andaeration are negativelycorrelated–morewater, less air.

Water availability is affected by porosity, bulk density, moisture content and soil texture

Total pore space is inversely related to bulk density–the greater the bulk density, the smaller thetotal pore space (Figure 11).

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

Bulk Density t m-3

0.2

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

Tota

l por

osity

Porosity =2.65 - bulk density

2.65

Figure 11. Relationship between total porosity and soil bulk density, assuming particle density of2.65 t m-3.


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The pore space can be simply characterised into three size ranges with different functions.Transmission pores greater than 50–100 micron diameter are normally gas–filled when the soil isat field capacity. These pores are essential for drainage and aeration and their volume fractiondepends on soil structure (aggregation, arrangement, cracks and biopores).

Storage pores with diameters between 0.2 microns (permanent wilting point) and 50–100 microns(field capacity) control the available water capacity. The volume fraction of the smaller storagepores is controlled by texture, whilst the volume fraction of the larger storage pores is affected bysoil structure (aggregation and bulk density).

Pores smaller than 0.2 microns in diameter are referred to as ‘residual pores’, they hold water tootightly for plant use, and their number and volume depend on soil texture. In fine-textured soils(clays) a large fraction of the total pore volume is made up of residual pores.

So, a simple relationship exists between pore size distribution, bulk density and soil texture. Theworst case scenario for root growth is a high density clay as this will have a low total pore volumemuch of which will be made up of residual pores. The volume fraction for available water storagewill be restricted and the soil is also likely to have insufficient transmission pores for aeration andfor unimpeded root growth. Our analysis of the regional soil data for the HRAEZs reveals thatboth these factors, high clay content (Figure 12) and high bulk density (Figure 14), are common inthe subsoils.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

CentralVictoria

CentralWest NSW

MillicentCoast

RiverinePlains and

Slopes

SouthernHighlands

SouthernVictoria

Tasmania

Per

cent

of t

otal

are

a

Sands / Sandy Loams / Loams Clay Loams / Light Clays Clays

Figure 12. Subsoil texture in each of the HRAEZs (Data from the National Land and Water ResourcesAudit).

For example:

Clay percentages in soils sampled for Southern Farming Systems Inc. (Southern Victoria HRAEZ)are around 50% in the subsoils (Figure 13a). The relationship between measured wilting pointsand clay percentage for soils in the Southern Victorian HRAEZ is shown in Figure 13b. Wiltingpoints are generally greater than 25% (w/w) for samples with more than 50% clay.

For a bulk density of 1.5 t m-3 the moisture retained at wilting point occupies 37.5% of the total soilvolume (θv = ρθg , where: θv is the volumetric water content, v/v; ρ is dry soil bulk density, w/v; andθg is gravimetric water content, w/w). This is equivalent to 86% of the total porosity and thereforeresults in very little capacity for storage of available water or for aeration.


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(a) 0 10 20 30 40 50 60 70 80 90

Percentage Clay (<2 micron)

5

4

3

2

1

0

Dep

th In

terv

al1 = topsoils, 0-10 cm2 = A2 or B21 horizons, 10-45 cm3 = B21 horizons, 20-50 cm4 = B horizons, 40-80 cm5 = B horizons, 80+ cm

95% confidence interval and range (n)

(19)

(12)

(23)

(15)

(14)

(b)

0 10 20 30 40 50 60 70 80 90

Percentage clay

0

10

20

30

40

Perm

anen

t wilt

ing

poin

t (θ g

g 1

00g-1

)

r2 = 0.8

PWP = (-0.0027 x %clay + 0.594) x %clay + 0.883

Western Plains SW Victoria

Figure 13. Soil properties for Southern Victorian HRAEZ, (a) Clay profiles for raised bed croppingsites, (b) Relationship between permanent wilting point and clay percentage for Western Plains, SWVictoria.

Porosity, bulk density, moisture content and soil texture affect mechanical impedance.

The transmission pores provide space in the soil for mechanically unimpeded root growth. Fineroots (<0.6 mm diameter) may comprise as much as 70% of the root architecture (Zobel 2005) andwill grow readily into these pores and subsequently enlarge them through radial expansion andcompression of the surrounding soil. The presence, volume and distribution of continuoustransmission pores are therefore critical in determining unimpeded root growth. In subsoils of theHRAEZs transmission pores are not common and are often widely spaced, principally occurringas fissures between large structural units.

Roots will also grow into the soil matrix if moisture is available. Compression of the soil in a pre-existing pore or penetration of the soil matrix by roots requires that the root can overcome themechanical strength of the soil. Soil strength depends on moisture content, soil texture and bulkdensity. Dry soil is stronger than moist or wet soil, but in clay soils with high bulk densitymechanical strength can be high even when most of the pore space is filled with water.


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Bulk density affects the mechanical strength of soil directly through the packing and arrangementof soil particles. Compact soils are more closely packed and more energy is therefore required todisplace particles than in less compact soils. Texture can influence the mechanical strength of soilthrough its effect on pore size distribution for the soil matrix, finer textured soil matrices (clays)will have predominantly much smaller pores than coarser textured matrices (loams and sands).The Growth–Limiting Bulk Density (GLBD) at which root growth is mechanically impeded,therefore varies with texture (Daddow and Warrington 1983; Jones 1983). Indicative GLBDs afterUDSA (1999) are shown in Table 6.

Table 6. Soil bulk density and texture relationships with respect to root growth (USDA 1999).

Soil texture Ideal bulk densities Bulk densities that mayaffect root growth

Bulk densities thatrestrict root growth

t m-3

sands, loamy sands < 1.60 1.69 > 1.80

sandy loams, loams < 1.40 1.63 > 1.80

sandy clay loams, loams,clay loams

< 1.40 1.60 > 1.75

silts, silt loams < 1.30 1.60 > 1.75

silt loams, silty clay loams < 1.40 1.55 > 1.65

sandy clays, silty clays, someclay loams (35–45% clay)

< 1.10 1.49 > 1.58

clays (> 45% clay) < 1.10 1.39 > 1.47

The NLWRA data indicate that average bulk densities in the HRAEZs range from 1.1 to >2.0 t m-3

with a mean of 1.5 t m-3 (Figure 14). The density of the soil matrix (i.e. excluding macropores) willexceed these values and it is therefore likely that subsoil strength is a major constraint to rootgrowth, particularly in the clay soils, although roots will grow into pores and fissures.

0 0.5 1 1.5 2 2.5

Tasmania

Southern Victoria

Southern Highlands


Millicent Coast

Central West NSW

Central Victoria

HRAE

Z

Bulk density (t m-3)

Figure 14. Bulk density (area-weighted means and data range) for subsoils in each HRAEZ (Datafrom the National Land and Water Resources Audit).

Aeration is affected by porosity, bulk density, texture and moisture content.

Aeration depends on the presence and continuity of transmission pores. In dense or compactedsoils these pores comprise a smaller fraction of the total pore volume than in less dense soils–theyare the first pores to be sacrificed when soils become compacted. Aeration porosity requirements


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may also vary with texture (Pierce et al. 1983) and root growth limiting air-pore space values havebeen proposed for different textures (Table 7).

Table 7. Root growth limiting air-pore space values by soil texture (From Pierce et al. 1983).

Texture Root limiting % pores normally filled with air

sand 24%

fine sand 21%

sandy loam 19%

fine sandy loam 15%

loam 14%

silt loam 17%

clay loam 11%

clay 13%

Least Limiting Water Range (LLWR)

The Least Limiting Water Range (LLWR) serves as an index of soil structural quality thatintegrates values of soil matrix potential, aeration and soil strength (da Silva et al. 1994).Quantification of LLWR requires knowledge of soil moisture contents for the following fourlimiting states:

1. Wilting point (-1.5 MPa); θwp2. Field capacity (-0.01 MPa); θfc3. Critical air filled porosity (10%); θafp4. Critical soil strength (2 MPa); θsrThe upper limit for the LLWR is equal to θafp if <θfc, otherwise it is equal to θfc. The lower limit forthe LLWR is equal to θsr if >θwp, otherwise it is equal to θwp.

The application of this concept has been the subject of several papers in the last five years (Zou etal. 2000; Benjamin et al. 2003; Wu et al. 2003; Lapen et al. 2004; Leao et al. 2006). Groenevelt et al.(2001) went further and proposed the term ‘integrated water capacity’ (IWC) as a developmentfrom LLWR, introducing overburden pressure as another factor and integrating changes in bulkdensity of swelling soils to allow for better estimation of water availability.

Estimation of LLWR is not possible for HRZ soils without data on field capacity and related air-filled porosity or soil mechanical strength. However, a simple representation of the limiting AFPand WP values for given soil textures at a range of bulk densities indicates the threshold bulkdensities at which water availability becomes excessively restricted. Figure 11 illustrates therelationship between total porosity and bulk density. This has been augmented in Figure 15 toillustrate the limiting factors of wilting point (data from Figure 13b) and minimum gas-filledporosity (approximated from general literature and Table 7 values).


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1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

Bulk Density t m-3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Volu

me

frac

tion

m3 m

-3

Clays θg= 0.25

Loams θg = 0.15

Sands θg= 0.05

Loams εa = 0.15

Sands εa = 0.24

Total Porosity= Saturation at wilting point

= Restricted water use at critical air filled porosity

= Soil water content θg (v/v) at wilting point

= Critical air-filled porosity εa (v/v)

Clay loams and Clays εa = 0.1

Figure 15. Relationship between bulk density, porosity and water content, illustrating critical limitsfor air filled porosity (red lines) and wilting point water contents (blue lines) for sands, loams andclays (Generalised data from various literature sources).

In well-structured soils field capacity moisture content may be less than that at which aerationbecomes limiting, in such soils LLWR would be less than represented in Figure 15. Critical soilstrength moisture content is proportional to bulk density and high bulk density soils are thereforemore likely to have lower limit moisture contents that are wetter than the soil at wilting point.Intersections between wilting point and critical aeration values in Figure 15 indicate growthlimiting bulk densities for the three texture ranges illustrated, and these values compare well withthose given in Table 6 after USDA (1999). The GLBDs for different textures would appear to be auseful indicator for judging the physical subsoil constraints.

GLBD, LLWR and the rhizosphere

Values of bulk density can be used in conjunction with texture to indicate the likely LLWR. LLWRapproaches zero as bulk density increases. A small LLWR is highly likely to induce stressesthroughout the growing season as the window of opportunity for water storage and extraction isvery narrow. Da Silva and Kay (1997) provide evidence for this, showing that a small LLWR soilinduces more stress due to drought or waterlogging than a soil with a larger LLWR.

Whilst the GLBD is a useful indicator of a poorly structured soil with respect to restricted LLWRand low potential for root growth, there are other aspects of structure that should be considered.Bulk density is, as it sounds, a measurement applied to bulk soil and does not reveal aspects ofpore geometry that would affect root growth— number, volume, connectivity and orientation ofmacropores are particularly important in dense soils as they determine air and water movement toa limited rhizosphere. In structured clay soils, roots are usually observed in fissures, interpedalpores and biopores. Whilst the volume occupied by roots in such soils may be small, the physicalenvironment is quite different to that measured in the bulk soil.

Whalley et al.(2005) have demonstrated that water retention and aeration properties of therhizosphere soil are enhanced with respect to root growth compared to the properties of the bulksoil. Passioura (2002) describes the many interactions between root and soil that demonstrate theability of plants to adapt to and survive in stressed situations. Gregory (2006) proposes that more


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integrated research into root-soil interactions in the rhizosphere is required because roots are anessential part of soil.

Subsoil Chemical constraintsChemical constraints to crop production in the southern HRZ occur as a result of environmentalprocesses within the soil matrix. High rainfall research has generally focussed on the impacts andalleviation of soil acidity (e.g. Helyar 1991; Scott et al. 2000) and waterlogging (e.g. Belford et al.1990; Cox and McFarlane 1990; MacEwan et al. 1992; Cox and McFarlane 1995; Sarlistyaningsih etal. 1995; Christy 1996; Malik et al. 2002; Barrett-Lennard 2003; Peries et al. 2004) on soil processesand plant growth responses. This represents a significant effort directed towards these two majorHRZ constraints, however, application of remedies are required and chemical constraints such assalinity, restricted nutrient availability and aluminium toxicity, will also impact on cropproduction in the HRZ.

Acidity

Soil acidity restricts plant growth through impacts on internal metabolism of the plant as aconsequence of nutrient toxicities associated with Al, H and Mn. It also impacts on the availabilityof trace elements such as Mo. Soil acidity is limiting to the production of most crop species atpH < 5.0 (Helyar 1991). Interactions between waterlogging and acidity increase the hazard posedby toxicity from Al and Mn in the HRZ

Surface acidity in the A1 horizon is already a management issue for HRZ producers, particularlyon texture-contrast soils (Helyar 1991; Coventry 1992; Scott 2000). This has resulted from elevatedrainfall, more intensive crop production strategies (eg increased utilisation of N), an environmentmore conducive to nutrient turnover and N mineralisation and the greater opportunity for lossesof excess nutrients through leaching (Helyar 1991; Raij 1991; Coventry 1992; Scott et al. 2000).Current data (NLWRA 2005) suggests that subsoil acidity is at a level that will not inhibit theproduction of most crop species. However, the development of subsoil acidity is an insidious andsignificant threat in HRZ cropping environments as cropping activities expand and increases innitrogen fertiliser application occur in order to meet higher yield expectations (Helyar 1991;Coventry 1992; Scott et al. 2000).

Subsoil acidity in the Southern Highlands and Tasmanian HRAEZs may require furtherinvestigation to determine whether it is actually a problem on arable land. The generalised datapresented in Figure 16 include all soils mapped in the HRAEZ and will include attributes of soil interrain that may be unfit for cropping.

2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Tasmania

Southern Victoria

Southern Highlands


Millicent Coast

Central West NSW

Central Victoria

HRAE

Z

Soil pH (0.01M CaCl2)Figure 16. Subsoil pH (area-weighted means and data range) for subsoil samples in each HRAEZ(Data from the National Land and Water Resources Audit).


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Salinity

Salinity of arable land associated with shallow saline groundwater, although highly publicised, isnot a major feature of the South-east Australian HRZ except for some areas of the Millicent coastHRAEZ and lower lying land in the Victorian Volcanic Plains (Southern Victorian HRAEZ). Morecommonly, transient salinity, associated with poor permeability of subsoils, may occur in the rootzone unrelated to any groundwater influence (Rengasamy 2002). Transient salinity is associatedwith reduced subsoil infiltration common in texture-contrast soils, particularly Sodosols. Whilstthere have been considerable efforts expended in mapping dryland salinity and risk from risinggroundwater, the extent of transient salinity is not known.

Regardless of the causes of salinity, the effects are the same. Whilst there is some considerationgiven to chloride and sodium toxicities, the primary impact of salts in the root zone is to reducewater availability through the osmotic effect on total water potential. Chloride and sodiumtoxicities are more likely to occur in irrigation situations in which water is relatively unlimited. Indryland cropping the water potential experienced by the roots is much more subject to extremesthroughout the season and therefore the osmotic effects are important. Osmotic potential isdirectly proportional to the salt concentration of the soil solution and this concentration increasesas the soil dries. The relationship between osmotic potential and salt concentration is given by thefollowing equation:

Osmotic potential (Pa) = 3.6 x Electrical Conductivity (S m-1) x 105 (Marshall et al. 1996)

The osmotic potential plus the soil matric potential constitute the total suction that plant rootsmust exert to obtain water from the soil. Hence salts reduce the LLWR (least limiting water range).If we apply the concept of growth limiting bulk density (Figure 15) to soils containing salts in theroot zone then a lower value for GLBD should be selected.

Groenevelt et al.(2004) have modelled water availability in saline soils and this exhibits adependence on texture, particularly at the lower end of the LLWR. They modelled three soils witha low ECse of 2 dS/m. Taking into account the osmotic potential of the soil water, wilting point (-1.5MPa) occurred at a matric potential of -0.95 MPa for a loamy sand, -1.18 MPa for a calcareousloam, and -1.37 MPa for a marine clay.

This theoretical approach of Groenevelt et al.(2004) would suggest that transient salinity atequivalent EC values will have greater impact on the lower limit of LLWR in coarse-textured thanin fine-textured soils. There is a simple explanation for this that can be derived from the moisturecharacteristics for different textures and the corresponding differences in gravimetric moisturecontent for pure water at -1.5 MPa. The values used in Figure 15 can be used as an example: θfc

(g/g) for sand is 0.05 and for clay 0.25. At θfc the sand therefore would have five times theconcentration of salt as the clay for the same ECse. The greater dilution effect in the clay comparedto the sand results in the total water potential reaching -1.5 MPa much earlier in the sand than inthe clay. Transient salinity effects may therefore be greater in A2 horizons than in the clay Bhorizons of the HRZ texture contrast soils, and this is a potential factor that would limit rootextension through the A2 and into the B horizon.

Sodicity

Sodic soils are common throughout the HRZ. The impact of sodicity on crop production has beenextensively reviewed (McKenzie et al. 2002; Rengasamy 2002; Surapaneni et al. 2002). Soil sodicitylimits plant growth indirectly through poor physical conditions and directly as a consequence ofsodium toxicity. The poor physical conditions result from the dispersive behaviour of sodic clayswhich, through blocking of fine pores and densification of the soil matrix, reduces wateravailability and soil aeration. Sodicity generally increases with depth and this is illustrated well bythe graph in Figure 17(a). This has been compiled from data collected during a survey of soilsbeing used for raised bed cropping in south-west Victoria, largely in the Victorian Volcanic Plains.In this landscape sodicity in the subsoil is higher in the east than in the west (Figure 17b) andcalcareous accumulations follow the same trend. This is probably related to the rainfall gradient


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which increases to the west, thus providing better conditions for leaching of the soil profile, but itcould also be related to the type and age of the basalt parent material. These data provide someevidence of general trends in one HRAEZ but also indicate the high regional variance in a singleproperty, such as sodicity, both vertically and laterally.

(a) (b)

0 5 10 15 20 25 30 35

Exchangeable Sodium Percentage

5

4

3

2

1

0

Dep

th In

terv

al

Sodic ( > 6% ESP )

1 = topsoils, 0-10 cm2 = A2 or B21 horizons, 10-45 cm3 = B21 horizons, 20-50 cm4 = B horizons, 40-80 cm5 = B horizons, 80+ cm

95% confidence interval and range (n)

(18)

(12)

(24)

(18)

(10)73 78 83 89 94 99 104

Site ID

0

10

20

30

40

50

60

ESP

SW VIC transect: Subsoil Sodicity = ESP as % (Ca+Mg+K+Na+Al)

ESP > 6% = "sodic"

West East(Colac) (Lara)

Figure 17. (a) Exchangeable Sodium Percentage as a function of depth in soils used for raised bedcropping south-west Victoria. (b) Subsoil ESP in a West to East transect in SW Victoria (sites 80-104are on the Victorian Volcanic Plains).

Nutrient availability

A variety of other chemical constraints are likely to impact on crop production in association withthe more obvious subsoil conditions such as waterlogging and acidity. For example Al, Mn and Htoxicities, and Mo deficiencies are associated with acidic soils (Helyar 1991). Plant available Al andMn has been suggested to be increased in acid soils in the presence of waterlogging (Khabaz-Saberi et al. 2005). In alkaline subsoils Na, B, Al and carbonate toxicities have been identified(Rengasamy 2002). In addition to the major chemical constraints identified above, a number ofsubsoil constraints have been identified. Limited nutrient availability has also been identified as asubsoil constraint to plant growth and root exploration of the soil (Gourley and Robson 1992;Rengasamy 2002).

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