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Soil Landscape Mapping Program – Soil
and Landscape Qualities and Limitations
Soil Qualities and
Limitations
Soil qualities and limitations are properties that can be assessed on an
individual soil material basis and can affect the viability and sustainability of land
uses. The effect of any particular soil quality or limitation may be positive or negative
and will vary with site conditions and land use. The following qualities and limitations
are listed for each soil material in tables within the soil landscape report for each soil
landscape description. Where soil qualities or limitations are not widespread throughout
the landscape, a localised qualifier is used.
Acidification
Hazard.
Poorly buffered soils, especially those with expected buffering capacities
of <30 kmol(+) ha/10 cm/pH unit show rapid increases in acidity. Soils can become more
acid under land management systems that have net acid input, such as nitrogen leaching.
Poorly buffered soils are usually sandy. In these soils, it is very important to maintain
high organic matter levels. See Acidity.
Acidity.
Extremely and strongly acid soils with laboratory measured pH values of
<5.5 (1:5 soil:water) often give rise to acid soil infertility. Associated problems
include toxic levels of aluminium and/or manganese and deficiencies of most nutrients
(especially calcium and molybdenum). While many native plants in eastern NSW have adapted
to acid soil conditions, susceptible species may require heavy applications of lime or
dolomite and often fertiliser to raise the pH (and nutrient supply) to a satisfactory
level. Acid soils may corrode untreated underground metal installations.
Acid Sulfate
Soils Potential.
Acid sulfate soils are clays, muds and sometimes sands associated with
pyrite-rich marine sediments. They may also occur in association with some sulfidic ore
bodies and sulfur-rich parent materials (e.g., some coals). These soils become extremely
acid following exposure or drainage as sulfur compounds are oxidised and converted to
sulfuric acid. This makes them corrosive to iron, steel, aluminium alloys and concrete.
Underground services should be avoided or rust-proofed. Actual acid sulfate soils are too
acid for most plant species and are difficult to vegetate. Very acid drainage waters from
these soils can profoundly disturb aquatic ecosystems. 1:25 000 acid
sulfate soil risk maps and an accompanying report are available for all low lying
coastal areas in NSW from the Department.
Alkalinity.
Alkaline soils have laboratory measured pH (1:5 soil:water) values of
>8.5. Alkalinity may inhibit the growth of plants. High levels of carbonate or
bicarbonate may impair the uptake of iron, manganese, copper and zinc. These soils are
frequently sodic or saline.
Aluminium
Toxicity Potential.
High levels of soluble aluminium are often toxic to non-native plants such
as some pasture, crop and ornamental species. Toxicity can be expected when exchangeable
aluminium levels are >5% and soils are strongly acid. Many native plant species are
tolerant of soils with high concentrations of soluble aluminium. Lime or dolomite can be
applied to raise soil pH and thus reduce exchangeable aluminium.
Erodibility.
Soil erodibility is the susceptibility of a soil to erosion. It is based
solely on soil properties. For sheet and rill erodibility, the USLE K factor (Wischmeier & Smith 1978) of >0.04 is
considered to be highly erodible. In some cases, sheet and rill erodibility is modified
according to field assessment of factors such as existing indications of previous sheet
erosion, fabric and consistence that are out of range or not taken into account by the
USLE. Rankings for USLE K factors in the Soil Test Results (Appendix 7.2 of the soil
landscape reports) are very low (<0.01), low (0.01 - 0.02), moderate (0.02 - 0.04),
high (0.04 - 0.06) and very high (>0.06). For the purposes of tables in chapter 4 of
the soil landscape reports, these are more broadly defined as low (<0.01 - 0.02),
moderate (0.02 - 0.04) and high (>0.04).
Landscape properties such as slope gradient, slope length, landform
element and rainfall characteristics are not included in the assessment. Disturbance
should be minimised on erodible soils, and disturbed areas should be protected by ground
cover as soon as possible.
USLE K factors cannot be used alone to determine soil propensity to
erosion by concentrated water flows. Dispersible/sodic soils, soils with weak or unstable
fabrics such as slaking, or self-mulching or very sandy soils are also prone to erosion by
concentrated flows.
Wind erodibility is related to the size and coherence of soil clods and
only applies to dry and disturbed (or cultivated) soils. Wind erodibility is assessed in
terms of percentage of fragments that are finer than 0.85 mm (USDA 1983).
Fertility.
Soils with poor chemical fertility usually require the application of
chemical fertilisers, seasoned manure or compost to achieve permanent plant cover. Some
soils do not respond well to normal applications of fertiliser. For example, soils with
high aluminium or iron oxide contents readily "lock up" phosphate, making it
unavailable to plants. Soil physical conditions often reduce plant growth.
In chapter 4 tables of the soil landscape reports, each soil material is
ranked according to its suitability for soil conservation purposes of revegetation and
topdressing. It is an assessment of the soil material as a growth medium and incorporates
soil factors that are detrimental as well as beneficial to plant growth. Rankings used are
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Low. Not generally or only marginally suitable as a revegetation
medium. Will have toxicity or extremely poor chemical fertility, and/or significant
structural or water storage problems that will be expensive to overcome. Regular
maintenance including applications of fertiliser and attention to soil moisture supply
will be essential. Applications of ameliorants may also be necessary.
Moderate. Moderately suitable as a revegetation medium. Will have limited
structural and/or water storage problems, and/or poor chemical fertility.
High. Desirable revegetation medium. Will have modest to good physical
and chemical fertility with limited or no structural or water storage problems.
Fire Hazard.
Highly organic soils such as peats or litter build-up can be ignited by
vegetation fires during drought. They may smoulder for months and are very difficult to
control. Peat fires lower ground levels, sterilise the soil and in some instances, leave
the ground bare for subsequent erosion.
Hardsetting
Surfaces.
Hardsetting soils become hard, massive and compact when dry. They do not
readily absorb rainwater and cause high runoff with consequent soil erosion. They do not
offer favourable environments for seed germination and require careful water management.
Regular cultivation should be avoided, although some cultivation may be necessary to break
up the hard layer for successful germination.
Non-cohesive Soils.
Loose, sandy soils can be subject to severe wind erosion, gully erosion
and batter failure. Batters steeper than 25% should be supported with retaining walls.
Batters with slopes less than 25% should be revegetated quickly.
Organic Soils.
Soils with large amounts of organic carbon (generally >12% (Isbell 1996)) such as peats and sandy peats are
generally unsuitable for use as engineering materials because they have low wet bearing
strength and their physical properties may be subject to change through decay. They are
generally well-structured for plant growth and have high waterholding capacities; however,
they are often very acid and may require large quantities of lime and nitrogen as well as
other nutrients and trace elements for optimum plant growth. Most topsoils contain
sufficient organic matter to be unsuitable for engineering purposes. Also, highly organic
soil materials located in swampy areas tend to suffer significant structural decline when
drained.
Periodically
Frozen Soil (Frost Action Potential).
Frost action potential is a rating for the susceptibility of the soil to
upward or lateral movement by the formation of segregated ice lenses. It rates the
potential for frost heave and the subsequent rapid loss of soil strength when the ground
thaws and the ice crystals and lenses within the soil melt. Unequal heaving and subsidence
upon thawing can crack or tip concrete slabs. In Australia, this hazard is generally
recognised by the presence of large ice crystals in topsoils. Although most soils in
Australia have zero frost action potential, a few colder areas may exhibit low frost
action potential where damage to buildings and roads is unlikely, but still a possibility
(Soil Survey Staff 1993).
Permeability
(High).
Soils that drain water quickly are highly permeable. They usually have
coarse textures (sands) and many interconnecting pores. They are not suitable for
absorbing effluent from septic systems because liquid drains rapidly into the groundwater
where it can cause pollution and potential health problems elsewhere. Soils with high
permeability often have low waterholding capacities. Seedlings and newly established
plants require regular, light irrigation.
Permeability
(Low).
Soils of low permeability usually have very slow drainage and are likely
to pond water for long periods. They usually have clayey textures and mottled or greyish
colours. They are not suitable for absorbing effluent. Special drainage may be required.
They may also be sodic and have low wet bearing strengths.
Plant Available
Waterholding Capacity (PAWC).
Soil materials with low available waterholding capacity can store only
limited amounts of water that can be extracted by plants. Plants growing in these soils
require small and frequent applications of water for optimum growth. PAWC is of greatest
importance in areas with seasonal rather than regular or highly unreliable rainfall. PAWC
ratings in Soil Test Results (included in Appendix 7.2 of each soil landscape report) are
very low (<5), low (5 - 10), moderate (10 - 15), high (15 - 20) and very high (>20).
For the purposes of chapter 4 tables, these are more broadly defined as low (<5 - 10),
moderate (10 - 15) and high (>15).
Plasticity.
Plastic state occurs at water contents where soils deform or change shape
without change in volume. It occurs between the semi-solid (crumbly) and liquid state and
is defined as the difference between the plastic and liquid Atterberg limits (Hicks 1991). A soil with high plasticity has
plastic properties over a wide range of moisture contents.
Highly plastic soils are typically high in clay content and deform easily
when mechanically stressed in the moist to saturated state. They are often tough and hard
when dry, do not support loads well and have poor trafficability when wet. Soils with no
or low plasticity change from solid to liquid with little change in moisture content and
may be prone to mass movement (Hazelton & Murphy
1992). Highly plastic soils can be very sticky, are unsuitable for foundations and
usually have low wet bearing strengths and high shrink-swell potential.
Highly plastic (HP) soils have USCS classifications of CH-CL, OH-CL,
CL-OH, Pt, CH-OH, OH-CH, CH and MH. Moderately plastic (MP) soils have USCS class of
CL-CH, CL, and OH. Low plasticity (LP) soils have USCS classes CL-ML, ML-CL, ML, CL and
OL. Non-plastic (NP) soils include all other USCS categories and are sandy or gravelly.
Poor Seedbed
Conditions.
Surface soil materials with properties that create difficulty in preparing
adequate seedbed conditions, may be naturally cloddy, hardsetting, sandy or
sodic/dispersible. Poor seedbed conditions are associated with very low organic matter,
very high or low clay contents, and high silt and fine sand contents.
Salinity
Hazard.
Excessive salt is toxic to most plants. Saline surface soils are usually
bare or have sparse plant cover. These soils have a high erosion hazard and are often
poorly drained. Treatment of saline soils often involves removal of saline water by
drainage and deep ripping as well as establishment of salt-tolerant plant species. Cover
crops, mulches and large applications of nitrogenous fertilisers as well as gypsum are
often required for successful vegetation establishment.
Measures that further reduce concentrations of salts within the plant root
zone such as tree or lucerne plantings in recharge areas may be required to ensure
long-term rehabilitation. Saline soils may be corrosive to untreated underground services.
Shrink-swell
Potential.
Expansive soil materials shrink and swell with changes in moisture
content. Such soil materials have volume expansions of >30% or linear shrinkages of
>17% and characteristics such as slickensides, seasonal cracking and high plasticity.
When soil moisture content changes, shrink-swell soils can damage structures such as
buildings, roads, dams, walls and underground services that are not appropriately
designed. The shrink-swell potential of most soils can be reduced by compaction, the
addition of lime or gypsum, or burial with a stable material. Keeping soil moisture levels
constant can eliminate soil movement.
Sodicity/Dispersion.
Sodic and dispersible soils are often highly erodible and may have low wet
bearing strengths. They are often very hardsetting when dry and often form surface crusts,
restricting water entry and hampering seedling emergence. They are prone to erosion and
structural degradation and require very careful management. Sodic soils may be treated
with additions of lime or gypsum. Sodic/dispersible soils exhibit sodic/dispersible
characteristics in the field. They are either highly dispersible (D% >50 or have an
Emerson Aggregate class of 1, 2 or 3) or have an Exchangeable Sodium Percentage and
sufficient sodium to be considered sodic.
Stoniness.
Gravels, stones and rocks increase the cost and difficulty of excavation
for underground services and increase the difficulty of cultivation. Gravels, stones and
rocks occupy soil volume, reducing plant exploitable moisture and nutrients. Surface
stones can have mixed effects on water infiltration, soil erodibility and moisture loss
through evaporation. Soils that contain more than 20 - 50% coarse fragments are considered
to be stony.
Structural
Decline Hazard.
Through inappropriate management techniques such as overgrazing and/or
excessive cultivation when too wet or dry, soils may become structurally degraded over
time. Structural decline usually involves at least one of the following-- increased soil
bulk density, increased strength and cloudiness, decreased soil organic matter content,
decreased soil porosity, and the formation of hardpans or hardsetting layers at or near
the soil surface. Comparisons between different sites land uses, most notably between
native or pristine areas and disturbed or developed areas, can often vividly demonstrate
soil structural decline.
Management strategies to overcome structural decline include the use of
mulches, appropriate crop and pasture rotation, reduce tillage or direct-drill practices,
herbicide control of weeds, and more appropriate stocking rates.
Water
Repellence.
Water repellence is rated after Roberts
and Carbon (1971) according to the amount of time for a droplet of water to be
absorbed by a dry soil surface.
The effects of water repellence include reduced water infiltration and
poor germination and growth (Handreck & Black
1984) as well as increased runoff and increased erosion.
Wet
Bearing Strength.
Soils with low wet bearing strength are dominated by a limited range of
particle sizes. They are pliable and deform easily under pressure when wet. Quicksand is
an example. If poorly drained, they can be unsuitable for foundations and have poor
trafficability when wet. Low wet bearing strength soils often suffer severe structural
damage if cultivated or mechanically disturbed when wet. Classifications used in chapter 4
tables of each soil landscape report are low (fluid or very soft mud), moderate (totally
unripe to half ripe) and high (nearly ripe or ripe), rated according to their strength
when the soil is not at field capacity (Pons &
Zonnefeld 1965).
Soil Qualities and Limitations