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CSIXOAUSTRALIA
Division of WaterResources
SeekingSolutions
Water ResourcesSeries No. 4
Understanding Salt and Sodium inSoils, Irrigation Water and
ShallowGroundwaters
A companion to the software program,SWAGMAN® - Whatif
C W Robbins, W S Meyer, S A Prathapar andR j G White
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AUSTRALIA
Division of WaterResources
SeekingSolutions
Water ResourcesSeries No. 4
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•_ .
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Understanding Salt and Sodium inSoils, Irrigation Water and
ShallowGroundwaters
A companion to the software program,SWAGMAN® - Whatif
C W Robbins, W S Meyer, S A Prathapar andR J G White
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UNDERSTANDING SALT AND SODIUMIN SOILS, IRRIGATION WATER AND
SHALLOW
GROUNDWATERS
A companion to the software program,SWAGMAN®-Whatif
by
C.W. RobbinsUnited States Department of Agriculture
and
W.S. Meyer, S.A. Prathapar and R.J.G WhiteDivision of Water
Resources, Griffith Laboratory
CSIRD Water Resources Series No. 4
1991
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National Library of Australia Cataloguing-in-Publication
Entry
Understanding salt and sodium in sods,irrigation water and
shallow groundwaters.
ISBN 0 643 05221 6.
1. Soils, Sails in - Australia. 2. Soilsalinization - Control -
Australia. 3. Irrigatirewater - Pollution - Australia. L Robbins,
CW.(Chuck W.). IL C:S1RO Division of WaterResouroas. III. Title
SWAGMAN-Whatif(Computer Program). (Series : CSIRO waterresources
series; no. 4).
631.4160994
All photographs in this report have been taken byour Divisional
Photographer, Bill van Aken.
Cover
Now do we sustain irrigated agriculture?Where do we go from
here?Peter Fawcett, farmer, Griffith.
GPO Box 1666Canberra ACT 2601 Australiaph. (06) 246 5717fax (06)
246 5800
Publication enquiries to:
Divisional Editor, CSIRO Division of WaterResources
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This booklet is part of the Land and Water Care Program of
CSIRO
SWAGMAN® is a registered trademark of CSIRO Australia
About the authors
Dr Chuck Robbins (BSc, MSc, PhD) is a Soil Chemist at the Soil
and WaterManagement Research Unit, United States • Department of
Agriculture,Agricultural Research Service (USDA-ARS).-
Dr Wayne Meyer (BAgrSc, PhD) is Assistant Chief of the Griffith
Laboratory'of the CSIRO Division of Water Resources. Dr Meyer is
leader of theresearch program 'Water and Salinity Management in
Irrigated Areas'.
Dr Sanmugam Prathapar (BSc(Hons), MS(AgEng), PhD) is a Senior
ResearchScientist, working on groundwater modelling, with CSIRO at
Griffith".
Mr Robert White, (BAppSci,GDCompApp) is an Experimental
Scientist at theGriffith Laboratory'.
USDA-ARSSoil and Water Management Research Unit3793 N 3600 E
KimberlyIdaho 83341USA
CSIRO Division of Water ResourcesGriffith LaboratoryPrivate Mail
Bag 3Griffith NSW 2680Australia
FEBRUARY 1991
Acknowledgment. The contribution of Ms Kathi Eland in editing
this bookletis gratefully acknowledged.
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CONTENTS
PREFACEINTRODUCTIONSALTS AND IONS IN SOIL AND WATER
What are Salts and Ions?SaltsSoluble ionsExchangeable
cations
Salt and Ion Effects on Plants and SoilsThe osmotic
effectOsmosis and osmotic pressureSpecific ion effectEffects on
physical properties of soil
Sources of Soil Salts
SALINITY CLASSIFICATION OF SOILS AND IRRIGATION WATERSSoils
NomenclatureCategories
Classifying Saltiness of Irrigation WaterCriteriaCategories
SAMPLING AND ANALYSING SOILS AND WATERProper Sample Collection
Methods
SoilsVisual selection of sampling locationsCollecting the soil
samples
WaterCollecting water samples
Soil and Water AnalysisTests
SoilsWater
Interpreting the results
PAGE
122233333445
7777888
101010101111111313131313
MANAGEMENT TO REMOVE OR MINIMISE SOLUBLE SALT PROBLEMS
15Soils 15Water 16Choice of Crops 17
Management for Seedlings 17Summary of management
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APPENDICES1 Units and Conversion Factors for Salinity Terms 192
Relative Yield with Increasing Electrical Conductivity
(Salinity) in the Root Zone 20
GLOSSARY
21
FURTHER READING
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PREFACE
Understanding Salt and Sodium in Soils, Irrigation Water and
ShallowGroundwaters is a companion booklet to SWAGMANe-Whatif, a
computermodel that lets you see how salts, soils, water and water
tables interact.SWAGMANkWhatif also lets you assess the effects of
managementpractices that you might undertake in a particular
area.
This booklet gives background information to help you understand
salts,sodium and their interactions with water and soils. It
explains wheresodium and salts come from, how to identify
salt-affected soils, and givesinstructions on taking soil and water
samples for analysis. It also givessuggestions on how to reduce the
harmful effects of salts and sodium, andtells you where to get
advice in making reclamation and managementdecisions for each
situation.
Managing salt and sodium affected soils, together with waters
used forirrigation, is complex. It is not possible to cover all
technical aspects orpossible treatment approaches in this booklet.
Instead, we have given asimple overview of the major principles
involved in diagnosing andmanaging salt and sodium affected soils
and irrigation waters.
It is difficult to summarise salt and sodium effects on soils
and plantswithout using some technical terms, so a comprehensive
glossary has beenincluded.
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Salt crystals an tree trunk
Introduction
Soils in almost all of Australia hold vastamounts of salt. In
many situations this saltis harmless, because it remains below
theroot zone of the plants. However, in somenatural situations, and
increasingly in clearedand cultivated areas, irrigation waters
andrising groundwaters have carried salts intothe zones of plant
growth, devastating eventhe most fertile soils. In Australia, more
than30 million hectares of land is salt-affected,resulting in lost
production which mayexceed one billion dollars annually.
Salts, in particular sodium salts, turnproductive soils into
toxic, structurelesswastelands. Until recently, our approach
tomanaging soils for salt has been hamperedby a lack of
understanding. Now, however,with a greater appreciation of the
interactionof soils, salts and water, as well as moreaccurate
diagnostic methods that haveenabled us to calculate well-defined
criticallimits, our approach to management can becomprehensive.
Not only do we now have the informationneeded to manage our
soils against theoccurrence of salinity, but we also can takesteps
to reclaim the vast amounts of soil thatsalinity has rendered
useless in recent years.Such efforts can only succeed with
thecooperation of all those involved inmanaging any particular
area. One person'slack of understanding in managing his or herland
can waste the efforts of the rest. This isthe reason for the
production of this booklet.It is an attempt to make widely
available apublication that gives a basic explanation ofthe
principles of managing our soils andirrigation waters against the
salting of ourland.
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Salts and Ions in Soil and Water
What Are Salts and Ions?
Salts
The solid part of soil is made up of particlesof silicon, clay,
organic matter and varioussalts. There are many different salts
that areformed when acids and bases are mixed.
Examples of reactions of acids with bases toproduce salts.
If baking soda, which is sodium bicarbonate(NaHCO3), is
neutralised with hydrochloricacid (HQ) (muriatic acid used for
soldering),common table salt, (NaCI) (sodium chloride),carbon
dioxide gas (CO2), and water, (H20),are formed.
NaHCO3NaCI + CO2 + H2O
Neutralising sulfuric acid, (H2SO4), (batteryacid) with calcium
oxide, (CaO), (quicklime,used in making brick mortar) produces
theslightly soluble salt, gypsum, (CaSO4) andwater.
H2O + CaO H2SO4 CaSO4.2H20
The presence of excessive amounts of salts,particularly those
containing sodium, willadversely affect soil structure and
impairplant growth.
The extent to which various salts interactwith soil particles
and plant functionsdepends largely on their solubilities - howwell
they dissolve in water. Sodium andcalcium chloride salts are very
soluble; saltslike gypsum are only slightly soluble, andsalts like
calcium carbonate, CaCO 3, (lintel )are even less soluble.
Figure 1. The adsorption of cations (positively charged) on the
negatively charged surface of a platyclay mineral. Some of these
cations will be replaced with Na + as the soil becomes
salinised.
l ln general use the term lime may also be used to mean calcium
oxide or calcium hydroxide, Ca(OH)2, (also known as slakedlime).
When talking about soil components, only calcium carbonate
(sometimes called free lime) is meant. The other twocompounds do
not exist in soil as they would react with the carbon dioxide that
is always present, and are converted to othercompounds. Similarly,
in general agriculture, the term lime is often used for any calcium
compound that is applied toimprove soils.
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Soluble ions Salt and Ion Effects on Plantsand SoilsWhen a salt
dissolves in water, it dissociates,
or separates, into cations and anions.Cations carry a positive
electrical charge andanions carry a negative electrical charge.
Thecations of most concern in salt-affected soilsare calcium
(Ca2+), magnesium (.4g2+),sodium (Nat), and occasionally,
potassium(K+). The anions of concern are chloride(0), sulfate
(50421, carbonate (CO321, andbicarbonate (HCO31.
Because of the water present in soils, the saltsthat interest us
most are usually found asions. It is the effects of these ions on
bothgrowing plants and the soil itself thatconcern us most.
Exchangeable cations
In addition to soluble cations, anothercategory of cations is of
concern in soils.These are the exchangeable cations.
Thesepositively charged ions are generallyattracted to and attached
onto clays andorganic matter, which carry a negativeelectrical
charge. This negative charge mustbe satisfied by an equal quantity
of positivelycharged ions. In salt-affected soils, thischarge is
satisfied by an excess of sodiumand, sometimes, magnesium cations.
Innormal soils, the charge is satisfied mainly bycalcium and
magnesium ions, although bothsodium and potassium cations will
still bepresent.
In soils with a pH of less than 7.0 (acid soils),hydrogen ions
(Fe), and aluminium ions(Alf), also make up part of the
exchangeablecations. The cations are very tightly held bythe
negative electrical charges. These arereferred to as exchangeable
cations becausethey can only be removed from the chargedsurface by
being exchanged with anothercation from the soil solution.
The osmotic effect
Osmotic potentials develop when any salt orsugar dissolves in
water. This can beillustrated by visualising a cylinder with
asemi-permeable membrane bottom throughwhich water can pass but
solutes cannot. Thecylinder is placed in a tank of distilled
water(see fig. 2). If the tank and cylinder are filledwith water
such that both compartments haveequal water levels, and salt or
sugar is thenadded to the cylinder, water will movethrough the
membrane from the pure waterside into the higher solutes side.
Thedifference in the two water levels is equal tothe difference in
the osmotic potentials. Thisprocess of water movement in response
tosolute concentration differences is calledosmosis. The greater
the difference in thesolute concentrations across the membrane,the
greater the energy or osmotic potentialdifference.
Osmosis and osmotic pressure
Plant roots are semi-permeable membranes.The sap of plant roots
contains sugars andsalts that create a potential differencebetween
the root sap and the soil water. Thisenables water to move readily
from the soilinto the roots of a plant that is growing inmoist,
non-salty soil. As the soil dries, itsremaining water is held more
tightly to thesoil particle surfaces and the saltconcentration in
the soil solution increases.The soil water suction increases,
causing therate of water flow into the plant to decrease.If no more
water is added to the soil, a pointin the drying process is reached
where theroots can no longer take up enough water tomeet the plant
needs, and plant growth stopsand the plant eventually dies. Thus,
the lessdissolved salt there is in the soil solutionphase, the
drier the soil can become beforewater uptake by the roots becomes
limited.Conversely, the higher the salt concentration,the less
available the soil water is to theplant. All soluble salts
contribute to theosmotic effect
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(a)
Tube
Water andsolute
(b) (c)
Figure 2. (a) The tube contains a solution; the beaker contains
distilled water. (b) Thesemipermeable membrane permits the passage
of water but not solute. The movement of water intothe solution
causes the solution to rise in the tube until the osmotic pressure,
resulting from thetendency of water to move into a region of lower
water concentration, is counterbalanced by theheight, h, and
density of the column of solution. (c) The force that must be
applied to the piston tooppose the rise of the solution in the tube
is a measure of the osmotic potential. It is proportionalto the
height and density of the solution in the tube.
In summary, the lower the salt concentration is inthe soil, the
more available the water that ispresent is to the plants.
Specific Ion Effect
Most ions found in soils are needed forhealthy plant growth.
However, some ionsare needed only in small quantities, andhigher
concentrations can be toxic.
The specific ion effect is the adverse or toxiceffect on plant
growth that is peculiar to eachion, in addition to its osmotic
effect. Someplants are very sensitive to chloride andsodium ions
and show signs of leaf margin ortip burn, leaf bronzing or necrotic
(dead)spots. Other plants are quite tolerant tothese ions. Some
crops show sensitivity tohigh carbonate and bicarbonate
ionconcentrations in the soil solution whichinhibits iron uptake by
many plants, causingthe plants to be pate greento yellow. This
isoften referred to as linte-induced chlorosis.High potassium
concentration in the soil caninhibit some crops, especially
grasses, fromtaking up the normal amounts ofmagnesium.
There are also correlations between saltinjury and soil nitrate
levels. Many crops aremore sensitive to high salt
concentrationswhen the soil nitrate levels are below thoserequired
for optimum growth rate. Undercertain conditions, higher than usual
nitrateapplications will partially offset salinity-induced yield
reductions.
Boron concentration above 2 ppm in the soilsolution is toxic to
most crops. In a fewareas, boron or borate ion damage to plantsis a
problem associated with salt-affectedsoils.
Effects on Physical Properties of Soil
The stability of soil aggregates depends onthe electrostatic
forces on the soil particlesand the ions in the soil solution. When
soilor clay particles are surrounded mostly byG12+ ions they are
held quite tightlytogether. Aggregates of these soils tend tostay
together, even in water. However, if theclay particles are
surrounded mostly by Na +ions, the binding of the particles is
weaker.When water is added to these soils, the watermolecules force
their way between
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the clay particles and cause them to fallapart. Thus the soil
disperses on wetting andhas a poor physical structure. Plants find
ithard to survive and grow well in these soils.
If the sodium adsorption ratio (SARe) of asaturation paste
extract is greater than 13(SARIS greater than 5 for a 1:5
soil:waterextract) or the exchangeable sodiumpercentage (ESP) is
greater than 15, the soilmay become dispersed. This is
especiallytrue when the total soluble salts are low(electrical
conductivity - ECe - less than4 dSm-1 ). Under these conditions,
the soilparticles disperse, the soil surface may sealover (crust),
and restrictive layers maydevelop within the soil profile.
Theseconditions impede air movement and waterinfiltration into, and
through, the soil. One ofthe most serious problems in
reclaimingsodic soils (see page 15, Management toRemove or Minimise
Soluble SaltProblems - Sadie Soils) is getting water tomove through
the soil so that undesirablesalts can be leached out and
exchangeablesodium can be replaced with calcium.
Calcium is the most desirable ion to have asthe dominant soluble
and exchangeablecation. Ideally, calcium should make upabout 60% of
the soluble cations and 80% ofthe exchangeable cations, when
magnesiumis also present. Keep in mind that 'hardwater makes soft
soils and soft water makeshard soils'. This means that irrigation
watercontaining predominantly calcium andmagnesium salts (low SAR)
tends to promotemore friable soil conditions. Waters with
lowcalcium and high sodium ratios (highSAR) tend to cause soils to
disperse, formcrusts, become compacted, and have verylow
infiltration rates and poor air movementproperties.
Sources of Soil Salts
Most soluble salts and exchangeable cationsin soils come from
weathering of rocks,sediments and minerals that served as thesoil
parent materials. Salts can also be addedto the soil surface
as_wind blown mineralsfrom salt plains, from sea mist, from
flood-transported salt laden sediments, from rainand from
irrigation water. Naturalweathering processes such as stream
bedgrinding, dissolution by water and acidsfrom rain water and
plant roots, oxidation by
air and water, and alternating freezingand thawing bring ions
into solution. In highrainfall areas, water leaches the salts from
thesoil as they form. In and and semi-aridareas, annual evaporation
is greater than theannual precipitation, and the salts are
notalways leached from the soil as fast as theyare released. With
time, they accumulate inthe root zone at concentration levels
thataffect plant growth.
Salts often accumulate in soils above shallowwater tables. The
water table may benaturally occurring, it may have beeninduced by
irrigation of poorly drainedareas, by irrigating up-slope from low
lyingareas, by vegetation changes, by removal ofdeep-rooted plants
up slope fromimpervious geological layer outcrops, or
byconstruction of roads or channels that blocknatural surface or
subsurface lateraldrainage. As water moves from the watertable to
the soil surface by capillary rise, orwicking, and evaporates from
the soil surface,salts carried by the water are left on or nearthe
surface. Over time, the salts becomesufficiently concentrated to
inhibit plantgrowth. This kind of salt problem is usuallyfound in
low lying, flat landscapes and alongslow moving streams, drains,
and marshes.
All irrigation waters contain at least somedissolved salt. In
many areas, good qualitywater containing low concentrations
ofdissolved salts is not available for irrigation,and the water
that is used contains more saltthan is desirable. If a sufficient
quantity ofwater does not move through the soil tocarry (leach) the
salts below the root zone,salts from the irrigation water
willaccumulate in the root zone. The amount ofwater needed to leach
salts from the rootzone will depend on the water quality andamount
of salt present. Less water is neededif it is of high quality.
There is often a concern about fertiliser interms of adding
salts. If the fertiliser ormanure is uniformly spread over the
soil, thesalinity effect is usually not measurable.Soluble
fertilisers such as muriate of potash,KO, (potassium chloride) or
ammoniumnitrate, (NH4N0?), applied uniformly at340 kg ha-I, will
initially raise the EC byabout 0.3 dSm-1. This will have very
littleeffect on most crops and would be of shortduration.
Irrigation or rain will quicklyremove the effect. If, however, the
fertiliser is
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banded near seeds or small plants, thesalinity, or osmotic,
effect on the individualplants can be severe. The less
solublefertilisers such as phosphates will have muchless effect.
High concentrations ofammonium ions, (NH4 )+ , from
nitrogenfertiliser or manure, on the other hand, canbe toxic to
germinating seeds and seedlings(a specific ion effect), and may be
confusedwith a salt effect (an osmotic effect). Mostmanure
application rates will not producemeasurable salt effects; however,
somefeedlot manures may contain high sodiumchloride concentrations.
If sufficiently heavy
applications of high sodium chloride manureare applied to a
slightly sodic soil, infiltrationrates may be reduced.
Salt spills or intentional dumping of saltsolutions from mines,
cheese factories, foodprocessing plants, municipal sewage
water,power plant cooling tower water, heavywood ash applications
or other industrialactivities often cause salt or sodiumproblems.
Soil reclamation is very difficultwhen salts are added in high
concentrationsto soils that are normally low in salts,especially
soils in the lower rainfall areas.
Salinity in irrigation area - Lake Wyangan, Griffith
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d1111.11.y DOUSand Irrigation Waters
Soils
Nomenclature
Soils can be grouped, according to howaffected they are by salt,
as (a) normal,(b) saline, (c) saline-sodic or (d) sodic soils.These
are the currently accepted names usedin classification. Other
terms, such as alkali,white alkali, black alkali, and salty also
haveoften been used to describe these soils;however, they do not
mean the same thing toall people, and often cause
considerableconfusion.
Categories
Normal soils do not contain sufficient solublesalts to reduce
the yields of most crops, nor dothey contain sufficient
exchangeable sodium toaffect soil structure. The upper limit
ofelectrical conductivity in the saturationpaste extract (ECe) of
these soils is around4 d5m-1 and the exchangeable sodiumpercentage
(ESP) upper limit is around 5 forAustralian soils.
These upper limits are indicative values only,as certain
salt-sensitive crops would havereduced yields even at these upper
limits.For example, if crops such as beans, apples,pears, citrus,
many ornamentals, small fruitsor berries were grown on soils with
an ECeof 3.5 d5m-1, a significant yield reductionwould be expected
(Appendix 2). Also,irrigating most soils from a large
volumesprinkler system with water containing highlevels of sodium -
an adjusted SAR(SARadiLd more) (see page 14) of mo than 12 -would
produce serious runoff problems, dueto the adverse sodium effect on
soil structure.
A normal soil, then, is one where solublesalts or exchangeable
sodium do notadversely affect yield or quality of the moresalt
tolerant crops.
Saline soils contain sufficient soluble salts (ECegreater than 4
dSm-1) in the upper roof zone toreduce yields of most cultioated
crops andornamental plants. Sodium makes up less than15% of the
exchangeable cations (ESP lessthan L5).
Water entry and movement through thesesoils is not inhibited by
sodium. In the pastthese soils have been called white alkali,
saltyor Solonchak soils. The predominant cationsare caldum,
magnesium, and in a few cases,potassium. The predominant anions
arechloride and sulfate. Bicarbonate may bepresent to a lesser
extent in high magnesiumor potassium soils.
In very severe cases, saline areas may appearas white crusts, or
as white or tan areas witha floury dusty surface when dry if
thepredominant anions are chloride. Infurrowed areas, there may be
white or saltystripes along the furrow edge or between
thefurrows.
Osmotic effects and chloride toxicity are thepredominant causes
of yield reduction andplant injury.
Saline-sodic soils are similar to saline soils inthat the ECe is
also greater than 4 dSm-1 .Saline-sodic soils differ from saline
soils in thatmore than 15% of the exchangeable cations aresodium
and the saturation paste extract SAReis greater than 13.
The anions are predominantly chloride andsulfate with some
bicarbonate when the pHis greater than about 75. As long as the
ECeremains above 4 dSrri l , infiltration rates andhydraulic
conductivities are generally ashigh as in normal or saline soils.
On leachingwith good quality, low calcium irrigationwater, unless
these soils contain gypsum,they will change to sodic soils because
theECe will decrease without the ESPdecreasing. When this happens,
theundesirable properties of sodic soils will beexpressed.
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High osmotic and specific ion effects are thepredominant causes
of plant growthreduction in these soils.
Sodic soils are lower in soluble salts than aresaline-sodic or
saline soils. The EC e is less than 4and often less than 2 dSm -1 .
The pH of a 1:5soil.water extinct is usually at least 1 pH
unitgreater than the saturation paste pH. The ESPis greater than 15
and saturation paste extractSAR (SARe) is greater than 13.
Higher carbonate and hydroxide ionconcentrations exist in these
soils than inother soils, and that causes the calcium toprecipitate
out of solution as CaCO3, or lime.The combination of high ESP and
pH andlow E; causes the clay and organic matterto disperse. This
dispersion of soil particlesdestroys the soil structure and causes
thesoils to 'run together' and form 'slick spots'when wet. These
spots have extremely lowrates of water intake, and if they are in
lowor flat areas, water will stand for extendedperiods without
soaking into the soil. Thedry soil often has a black greasy or
oily-looking surface and no vegetation growingon it.
It is not uncommon to have a mix of two ormore kinds of
salt-affected soil within asingle field. Salt-affected soil
characteristicsare usually highly variable from one part of afield
to another.
The four definitions are summarised inTable 1.
Classifying Saltiness ofIrrigation Water
Criteria
Irrigation water quality is based on threecriteria: total salt
concentration (MIA),sodium adsorption ratio (SARw) andadjusted
sodium adsorption ratio (SARadj).
Categories
Low salinity irrigation water has an ECGbetween 0 and 0.7 dSnr i
(Total SolubleSalts TSS, 0-420 mg La).
All crops can be grown with this saltconcentration in the water
as long as periodicleaching takes place. On moderately to
well-drained soils, salts in the soil will notincrease and may even
decrease with timeunder these conditions.
Moderately saline irrigation water has an ECGbetween 0.7 and L3
dSm-1 (TSS, 420-800mg L-1).
Very salt sensitive crops require specialisedpractices to avoid
salt injury. Moderatelytolerant crops can be grown if
sufficientleaching is allowed to prevent salt buildup inthe root
zone.
Highly saline irrigation water has an ECwbetween 1.3 and 3.0
d5nta (TSS, 800-1800 mg
Table 1. Chemical characteristics of salt and sodium affected
soilsfor Australian conditions.
Soil salinity class EC.-
ESP 1 SAR.,
SARI.
_Normal soil 5
8
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This water should only be used on welldrained soils with high
infiltration rates andno shallow water table. Only salt
tolerantcrops can be successfully grown. Sprinklerirrigation during
hot weather is notadvisable. Excess water must be applied forsalt
leaching. Adverse degradation ofunderlying aquifers will be a
concern.
Very highly saline water has an ECw of 3.0 to5.0 eiSm-1
(TS5,1800-3200 mg vi-Y.
Water in this salinity range is acceptable onlyunder conditions
of extremely porous, welldrained soils and very salt tolerant
crops.A lower salinity water may be needed forseedling germination.
Degradation ofsubsurface water supplies is likely underlands
irrigated with this quality of water.
Water with an ECG, in excess of 5.0 dSm-1OM, 3200 mg 1..-1)
should not be consideredfor irrigation under any conditions.
The SAR of an irrigation water should beconsidered along with
the EC,, indetermining the ultimate suitability of awater for an
irrigation. The higher theSARw, the greater the probability
thatinfiltration rates and water flow through thesoil will become a
problem. The effect on soilof sodium in the irrigation water will
bemodified by bicarbonate and carbonateconcentrations. A correction
to the value ofSARI" can be made to account for this, andwill be
discussed later (see page 14).
The four definitions are summarised inTable 2.
Table 2. Chemical characteristics of salt-affected irrigation
watersfor Australian conditions.
Water salinityclass
EC. range TSS
Low salinity q - 0.7 o- 420Moderately saline 0.7 -1.3 420 -
800Highly saline 1.3 - 3.0 800 - 1800Very highly saline 3.0 - 5.0
1800 - 3200
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Sampling and Analysing Soilsand Water
Proper Sample CollectingMethods
Soils
Visual selection of sampling locations
The locations of soil sample collection shouldinitially be based
on visual observations inthe field. The categories of soil types
givenpreviously (see page 7, SalinityClassification of Soils and
IrrigationWaters) include some descriptions ofthe appearance of
various salt-affected soils.
If the land has not been recently cultivated oris in native
vegetation, the vegetation willgive a good indication of where the
saline orsodic areas are. Plants vary in their salinitytolerance;
and the presence of certain speciesis indicative of soil salinity
conditions.
Plants that can tolerate salinity up to anelectrical
conductivity of about 3 dSnfl in asaturated paste extract (ECe)n or
0.6 dSm4 ina 1:5 extract, include
Hill wallaby grass (Danthonia eriantha) andWimmera rye grass
(1.oliu gn rigidum).
Moderate soil salinity levels (ECe of up toabout 7 dSni i , or
1.4 dSm-1 in a 15 extract)can be tolerated by plants such as
Saltmarsh grass (Puccinellia stricta)Sea barley grass (Hordeum
marinum)Couch grass (Cynodon dactylon)Tall wheat grass (Agropynm
elongation)Windmill grass (Chloris truncata)Spiny rush (Junco
acutus)Toad rush (Juncos bufonius)Buck's horn plantain (Plantago
coronopus)Coast sand spurrey (Spergularia media)Salt angianthus
(Angianthus preissianus)Strawberry clover (Trifolium
fragiferum)
Swamp weed (Selliem redicans)Swamp paperbark (Melaleuca
ericifolia).
Other species2 which may be present
Zoysia macranthaSporobolus virginicusSporobolusEnigroStis
pergmcilisEnzgrostis dielsiiEmgrostis australasicaMaireana
aphyllaChenopodium nitrariaceumChenopodium auricomumDiplachne
*seaPhragmites australiaAtriplex vesicariaAtriplex
nummulariaRhagodia spinescensBaunwa junceaGahnia trifidaTypha
domingensis.
Some species will only grow in moderatelysaline soils and do not
do well in less salinesoils. These include
Annual beard grass (Polypogon monspeliensis)Australian salt
grass (Distichlis distichiphylla)Curly rye grass (Parapholis
incurua)Slender barb grass (Panipholis strigosa)Creeping brookweed
(Samolus repens)Ice plant (Mesembryanthemum crystallinum)Water
buttons (Cotula coronopifolia).
Other species include
Hainardia cylindricaSamolus eremaeusGunniopsis spp.Trianthema
spp.Mollugo spp.Puccinellia spp.Cyperus gymnocaulosCrams
laevigatusBolboschoenus caldwelliiMuehlenbeckia coccoloboides.
2We are indebted to Mr Geoff Saitrty (Sainty and Associates),
and Dr Surrey Jacobs (Royal liotanical Gardens, Sydiley) for
thisinformation.
10
-
Severely salt-affected areas (ECe of 7 to20 dSm-1 , or lA to 35
riSm-i in 1:5 soilextracts) will usually have only limited
plantcover. If the salinity has recently increased,dead trees and
shrubs will be present in thearea. Plants that will tolerate these
salinitylevels include
Beaded glasswort (Saw:vomit; quinueflom)Round-leaf pigface
(Disphyma clavellatum)Sea blite (Suaeda spp.) andSamphire
(Hallosarcia).
Other species include
Pachycornia triandraSolerostegia spp.Gunniopsis quadrifigia.
These species will seldom be found on nonsaline soils and are a
good indicator of highsoil salinity levels.
Crop height and colour can help identifysaline or sodic areas in
cultivated fields.Some crops are more salt or sodium tolerantthan
others, and the degree of injury willvary with crop and management
practices(Appendix 2). Crops such as beans orpotatoes will show
greater salt injury thanpeas, onions, corn, or wheat, while barley
orlucerne show the least salt damage.
Collecting the soil samples
• Strategic samplingWith the visual variability in vegetation
andsoil surface features in mind, samples shouldbe taken to cover
the different soil situations,within the limits of the number of
samples tobe collected. This may be the first place thatoutside
help should be considered - keeping -in mind who is going to pay
the chemicalanalysis bill. A few, strategically locatedsample sites
will give maximum informationat a minimum cost.
Soil samples should include a few samplesfrom the best part of
each field as a reference.Take at least one or two samples from
thepoorest areas, some from spots with verypoor growth,
intermediate looking are% andsome from the better areas.
• Sampling depthsSampling depth and number of depths to betaken
presents an additional choice. Hereagain cost becomes a factor. If
one depth isused, the sample should probably be fromthe surface
down to 0.25 to 035 m. If twosample depths are used, the
uppersample should probably be from the surfacedown to 020 or 0.30
in, and the secondshould be from 0.20 to 0.40, or 0.30 to 0.60
m,depending on soil condition. Sampling bysoil horizons is most
desirable, such as fromthe surface down to the bottom of the
ploughlayer, and from the bottom of the ploughlayer down to the
bottom of the next horizon.Occasionally, a 5 to 10 mm thick sample
ofediting soil crusts or salt layers right at thetop of the ground
surface is desirable.
• Composite samplesThe best soil samples are composites.A
composite sample is obtained from anumber of samples taken from the
same soildepth, over an area that appears to beuniformly
salt-affected. These smallersamples are thoroughly mixed together
and asingle sub-sample, the composite sample, istaken from the mix
for chemical analysis.
• Sample volume and storageOne litre (or 1 kg) of soil is
usually adequatefor each sample. Record sampling date,depth,
relative crop growth and appearance,previous and current or next
crop, locationby field and within the field. Samples shouldbe air
dried (do not dry in an oven),thoroughly mixed, and sticks and
stoneslarger than 10 mm should be removed andthe samples stored in
sealed containers. Anydean, durable container that is easilyhandled
can be used. The samples should bestored in a dry, cool location
until they aredelivered to the testing laboratory.
Water
Collecting water samples
• When to sampleWater samples from bores (wells) should betaken
only after the pumps have run for atleast half an hour, so that
water standing in
11
-
the bore casing and the area next to the boreis removed and a
representative sample isobtained. Usually, bore water quality
willnot change throughout the growing season.In cases where an
aquifer is consistentlybeing lowered by pumping, water qualitymay
change with time. In this case, it wouldbe wise to sample the bores
over time.
Irrigation water quality in large river systemswith large
storage reservoirs will usually notchange over the season, but
water in smallstorage systems and stream systems withfluctuating
flows may change as the flowchanges. Water samples should be
takenonly during the irrigation season and shouldalso be taken if
'new' volumes of water moveinto the water supply.
• Sample volume and storageOnce the bore or stream water quality
hasbeen established, it will probably not benecessary to sample
every year unlesschanges occurred that could cause waterquality
changes.
Water samples of 250 mL are sufficient formost irrigation water
quality analysis.Sample containers should be clean and freefrom
oil, salts, or chemical contaminants.Rinse each container with the
water to be
sampled before saving the sample. Use tightclosures and record
the sample date, time,place, water flow (approximate),
irrigationmethod and crops to be grown. Refrigerate(do not freeze)
the samples until analysedand analyse as soon as practical.
Indicatewhich water samples go with which soilsample when more than
one water source isavailable. Both water quality data and
soilsalinity status are needed to make propermanagement
decisions.
• Sampling from a water tableWhen a shallow water table is
suspected,make bore holes down into the water tablenear each corner
of the field of concern.Water samples should be taken from
eachhole, and the depth to the water surfaceshould be measured once
the water hasstopped rising in each hole. If the water tablesurface
elevations from a fixed referencelevel are measured at the four
points, thewater table flow direction can also bedetermined. These
sampling proceduresshould be carried out at the beginning andend of
the irrigation season. This will give anindication of irrigation
and seasonal effectson the water table depth and quality.
Thesewater samples should be collected andanalysed by the same
procedures as theirrigation water samples.
Figure 3. Determination of water table depth and direction of
flow.
Unsaturated soilWater table
Direction ofgroundwater flow
-
Soil and Water Analysis
Tests
Once the samples are collected and labelled,take them to either
a private or a stategovernment soil testing laboratory. Samplesto
be tested for salinity and sodium arehandled differently than
samples collectedfor fertiliser analysis and recommendations.When
salinity or high sodium is a concern,the following tests should be
requested.
Sails
1. Saturation paste (not extract) pH.
2. Saturation paste extract analysis. Theextract should be
analysed for calcium,magnesium, sodium and electricalconductivity
(ECe). For some areas,potassium should be requested.
3. Carbonate, bicarbonate, chloride, andsulfate should be run on
enough saturationpaste extracts to get an idea of which anionsare
dominant.
4. If the pH is greater than 8.5 and the ECe isless than 4.0
d.Sm-1, or the calculated sodiumadsorption ratio (SARe) is greater
than 10,the exchangeable sodium percentage (ESP)should be obtained
for these samples. Thecation exchange capacity (CEO is requiredto
calculate ESP, but need not be run on morethan 4 samples per field
as it is a relativelyfixed value. It does not need to be
obtained
again because it will not change significantlywith time or
treatment.
Some laboratories would rather use a 1:1 or15 soil:water extract
than a saturation pasteextract. Information from saturation
pasteextracts takes longer to get but is moreaccurate in describing
the salinity status ofthe soi13. Soil:water extracts cannot
beinterpreted as reliably.
Water
Irrigation and groundwater analysis shouldinclude ECw, calcium,
magnesium, sodium,chloride, carbonate, bicarbonate and sulfate,and,
occasionally, potassium. In areas ofknown boron toxicity, boron
should also bedetermined.
Be sure that your samples are analysed bythe correct methods,
otherwise the results areimpossible to interpret relative to
knownstandards.
Interpreting the Results
Laboratory results may have to be convertedfrom one set of units
to another in order touse the commonly recommended
standards.Saturation percentage, pH, boronconcentration,
exchangeable sodiumpercentage IESP), sodium adsorption ratio(SAM,
percentage lime and percentagegypsum data usually do not need to
bechanged. Electrical conductivity (EC),
3Note For any soil sample with the same SARe, regardless of soil
type, the SARs calculated from other types of extracts willvary
greatly and non-uniformly. The reason for this is apparent from the
formula shown in the glossary; when calculatingSAR from diluted
solutions the SAR. is calculated from the diluted Na value, but
from the square root of the diluted Ca andMg values. Thus, as you
dilute the extract the SAR decrease' with the effect being greater
for lower saturation percentagesand sandier soils. The following
table illustrates this.
•
Saturation Percentage Saturation pasteExtract SAE.
2d. ExtractSAR
13 ExtractSAX
12.5 (Sandy loam) 14.1 5.0 2225 (Silt loam) 14.1 7.1 3.250 (Clay
loam) 14.1 10.1 4.575 (Clay soil) 14.1 12.3 5.5
100 (Clay subsoil) 14.1 14.1 6.3. ,
-
cation exchange capacity (CEO, and thecation and anion
concentrations may be inone of several units and should be
convertedto standard metric system units. These unitsand their
conversion factors are shown inAppendix 1.
If the SAR has not been calculated, it can bederived from the
cation concentrations (theglossary shows how this is done.
If water analysis gives a value for SAR, itshould be adjusted
SAR (SARadi). Often italso is given, incorrectly, for soil
analysis.
SARad should only be used for irrigationIts ts calculation takes
into
consideration the fact that the water willundergo chemical
reactions that will changethe effective SAR of the water
movingthrough the soil. The final SAR of soil incontact with water
is affected by the valuesfor pH, carbonate and bicarbonate in
theirrigation water. Depending on these values,sometimes CaCO3, or
lime, will dissolvefrom the soil and lower the calculated SAR.In
other situations, lime will precipitate fromthe soil solution, and
the calculated SAR willincrease.
-
Management to Remove or MinimiseSoluble Salt Problems
Wetland
Once the salinity source and types of saltshave been identified,
a management plan canbe developed to make the best use of
theavailable resources.
SoilsNormal soils irrigated with good qualityirrigation water
should produce most cropswithout any salinity or drainage
problems.Poor irrigation methods and inadequatedrainage will
inevitably cause soildegradation as water tables rise, salts
aredeposited in the root zone and good physicalstructure is
destroyed. These are no longer`normal' sons.
Saline soils, in the absence of a water tableand carefully
irrigated with good qualitywater, will usually reclaim themselves
assalts are leached below the root zone.Initially, the rate of
reclamation will dependon the amount of water travelling throughthe
profile (the leaching fraction). After that,soil salinity will also
be a function of thewater quality and mineral weathering withinthe
soil.
If the salts have come from a shallow watertable, the water
table must be lowered, by
providing drainage or intercepting theincoming water, before
reclamation can beaccomplished. In some situations, it may notbe
economical to lower a water table, and analternative land use might
be a better choice.
Once the water table is lowered, all that isgenerally needed is
leaching of the solublesalts with good quality water. Additions
ofgypsum, sulfur, soil amendments or othercalcium salt materials do
not help reclaimsaline soils.
Saline-sodic soils irrigated with goodquality water, in the
absence of a shallowwater table, have the potential of
developinginto sodic soils. This will occur if the solublesalts are
leached out of the profile withoutcalcium being added to replace
theexchangeable sodium. In such a situation theEC, decreases, while
the SAR A remains high.
The exception to this is when naturallyoccurring gypsum is
present in the profilenear enough to the surface that ploughingcan
mix the gypsum with the surface soil.
If the salinity and sodium are coming from ashallow water table,
reclamation must
-
include drainage or intercepting thegroundwater. As the salts
are leached fromthe soil, calcium can be added as gypsum orcalcium
chloride, or if the soil contains limenear the surface, sulfur or
iron (ferrous)sulfate can be added to dissolve lime as ameans of
making calcium available in the soilsolution. Sulfuric acid has
also beensuccessfully added to these soils as a meansof dissolving
lime and making calciumavailable for reclamation. Adding
theseamendments is of little value unless leachingalso takes
place.
Sodic soils irrigated with good quality waternearly always
present infiltration andleaching problems because they are
generallysufficiently compacted and dispersed thatwater
infiltration rates are very low.
If a high water table is part of the problem, itmust be lowered
as the first step in thereclamation process.
Reclaiming a sodic soil requires the reductionof the ESP to
below a value that will dependon the soil texture and irrigation
method, butwhich will fall in the range from 6 to 12.Such a
reduction can be achieved byincreasing the exchangeable
calciumconcentration or by increasing the EC toabove 4 dSm- '. When
saline watercontaining high amounts of calcium isavailable, it can
be used to increase theinfiltration rate by increasing the
solublecalcium and the EC. Then, as the sodium isreplaced, better
quality water can graduallybe used.
If gypsum is used for sodic soil reclamation,the gypsum
requirement is calculated todetermine the amount of gypsum needed
toreclaim the soil to a particular depth. Thecalculation for gypsum
requirement is givenin the glossary.Other choices include adding
calciumchloride or sulfur, sulfuric acid or ferroussulfate as a
means of dissolving soil lime tosupply the needed calcium. Sulfur
does verylittle good on the soil surface and must beincorporated to
aid reclamation. Coarseorganic matter such as straw, corn stalks,
orsawdust or wood shavings used for animalbedding, that decomposes
slowly, can helpopen up sodic soils when used with otherreclamation
practices. Heavy manure or old
lucerne hay applications that are worked intothe soil dissolve
lime and release calcium asthey decompose.
Sodic soils do not contain natural gypsum inthe surface,
otherwise they would be saline-sodic. Sodic soils are usually the
mostexpensive type of salt-affected soils toreclaim and under many
conditions theymay not be economical to reclaim.
Water
Irrigation water is a source of salt. If salinityproblems have
developed from salts andminerals in the irrigation water, there
areonly a few options available. The mostdesirable option would be
to use betterquality irrigation water (lower salt and/orsodium). If
this is not a valid choice, it maybe possible to leach salts from
the soil duringnon-cropping periods. In areas withoutshallow water
tables, it is often possible toirrigate late in the autumn so that
the soil iswet going into the winter. The winterprecipitation will
then be more effective inmoving salts below the root zone. When
thetotal salt load in the irrigation water is low,but the SAR or
SARadi is high, its use willincrease the exchangeable sodium in the
soil.However, gypsum added to this water canlower the SARadj and
overcome anotherwise undesirable cation ratio in thewater. Low ECw,
high SAR irrigation watertreated with sulfuric acid can also be
helpfulwhen used on soils containing lime.
It is not uncommon for shallow water tablesto develop from
excessive application ofirrigation water over an entire
irrigationarea. Soil salts gradually become a problemas the water
evaporates from the soil surface.If one fanner in an area applies
less water,his problem increases faster than hisneighbour who
continues to irrigateexcessively, because more salts move upfrom
the water table below his soil. Underthese conditions, it may
become mandatoryto require all irrigators to use less waterbefore
the overall problem can be resolved.There may be legal problems
inimplementing this kind of an approach, eventhough it would be in
everyone's bestinterest
-
Choice of Crops
Choosing the right crops and bestmanagement practice will
increase thechances for successful crop production andsoil
reclamation. Each crop and plant specieshas its own tolerance to
high pH, soilsalinity, and drought. Soil water content alsohas a
strong influence on a plant's reactionsto high pH and salts
contained in the soil.Appendix 2 shows a sample of available
datathat can be used to help choose crops orornamentals on the
basis of soil salinity.Tables are also available for pH, boron,
ESPand water quality sensitivity for differentcrops.
Management for Seedlings
Most seedlings are more sensitive to salteffects than older
plants. This is due mostlyto the seedling roots being in the upper
partof the soil profile, which is often saltier anddrier than
deeper in the profile. Seedlingsrequire time to produce sufficient
sugars inthe sap to offset the osmotic effect of the saltsin the
soil solution. The seedling's greatersusceptibility to salt injury
can often beminimised by preplant irrigation which bothincreases
the soil water content and flushessome of the salt deeper into the
soil.Additional light irrigations are often helpfulafter planting
or emergence to allow thetender seedlings time to become
established.Increasing the soil water content dilutes mostsalts,
thus decreasing the osmotic effect onplants. This dilution, in
combination withhigher water content, makes it easier for theplants
to extract water from the soil. Anirrigator may have a choice
between two ormore waters of different quality. Whenpossible, the
less salty water should be usedto establish the seedlings and then
the poorerquality water can be used on more mature ormore
salt-tolerant crops.
Summary of Soil Management
To remove the solubksalts from the soilthree things have to
happen:
1. Less silt must be added to the soil than isremoved;-
2. Salts have to be leached downwardthrough the soil and;
3. Water moving salts upward from shallowwater tables must be
removed orintercepted to avoid the accumulation ofsalts in the root
zone. In sodic and saline-sodic soils, the exchangeable sodium
mustalso be replaced with another cation,preferably calcium and the
sodium mustbe leached from the root zone.
Soil amendments (sulfur, gypsum, ironsulfate, and sulfuric acid)
are only beneficialon sodic and saline sodic (with no gypsum)soils
and only when leaching takes place.These materials are added to
replace thesodium so it can be leached from the soil. Ifhigh
exchangeable sodium is not a problem,as in normal or saline soils,
these materialswill not be beneficial except when the sulfuris
needed as a plant nutrient. If a soilcontains natural gypsum, even
in a saline-sodic soil, amendments will be of little use.
Getting Advice
Slate agency agronomists can provide ad-ditional help or refer
you to soils specialistswho have experience with saline or sodic
soilproblems. Soil Conservation Servicepersonnel are a good source
of help or theycan direct you to someone who can adviseyou on
management decisions. An on-siteinspection of your particular
situation willallow these specialists to be more helpful.
State agencies that can help are:
NSW
NSW Agriculture and FisheriesPO Box K220HAYMARKET NSW 2773Ph:
(02) 217 6666
Soil Conservation Service,PO Box 1980-1ATSWOOD NSW 2057Ph
(02)413 5555
Department of Water ResourcesPO Box 3720PARRAMATTA NSW 2150Ph:
(02) 895 6211
-
VIC
Department Agriculture & Rural AffairsPO Box 500EAST
MELBOURNE VIC 3002Ph: (03) 651 7011
Rural Water Commission590 Orrong RoadARMADALE VIC 3143Ph: (03)
508 2222
WA
Department of AgricultureBaron-Hay CourtSOUTH PERTH WA 6151Ph:
(09) 368 3333
Conservation and Land Management50 Hayman RoadCOMO WA 6152Ph:
(09) 367 0333
QLD
Department of Primary IndustriesGPO Box 46BRISBANE QLD 4001Ph:
(07) 239 3111
SA .
Department of AgricultureGPO Box 1671ADELAIDE SA 5001Ph: (08)
226 0222
Wayne talks to fanner - Griffith
-
APPENDIX 1
Units and Conversion Factors for Salinity Terms
To convert from Column A units to Column C units, multiply A by
B.Conversely, to convert from Column C units to Column A units,
divide C by B.
Term Column AUnits
Column BConversion
factor A to C
Column CUnits§
CEC me 100 g-1 .10A mmole chargelc'
_
cmole charge kg' 10.0 mmole chargekg4
EC mmhos cm' 1.0 dSm4S m4 10.0 dSm'
mmhos cm' 0.001 dSreEC units 0.001 d5rn4
TSS units (ppm) 0.00167 dSreor Trig 1.4
Ca ppm_
0.025 mmole L'1me L4 0.5- morale L'
Mg ppm 0.041 nunole L4me L4 0.5 mmole L'
Na ppm 0.043 mmole 1.4me L4 1.0 mmole L'
K ppm 0.026 inmate L4me I.4 1.0 mmole L'I
CI ppm 0.028 morale L'me L4 1.0 mmole L'
SO4 ppm 0.010 mmole 1.4me L' 0.5 morale L4
CO3 ppm 0.017 mmole L4me 1.4 0.5 mmole L4
HCO3 ppm 0.016 mmoie L4me L4 1.0 mmole 1,4
...
§ The units in the right hand column are the. currently
preferred SI units.I mmole 1,4 are• equal to mole rn3.Example: To
convert 40 ppm Ca to mmole 1-4, multiply 40 ppm by 0.025 to give
2.0 mmole Ca 1.4.
Abbreviationsme L4:cmole(+) kg':mmhos cm'':S m-1:dSEC
units:TSS:
of Unitsmilliequivalents per litrecentimoles of (positive)
charge per kilogrammillimbos per centimetreSiemens per
metredeciSiemens per metreElectrical Conductivity oohs WS cm')Total
Soluble Salts
-
1.188888888888888888888888888888888888888
Hithlillht
11 - 1 1 8111 43 gg c5
IA
Ca Cs A te*A gin g4
N
g '15 - K A z * °RSEI%AR
g ° A A MRS 4' g t$,z,Z
A *A- A 0 '1" %KA .1a$I gg 253
4AR %°18 e, :2 4 ta8z2 A g rite&Amg c relF.1,1 V411°Z r4
ittAIR * g RKS$rd
8 ZDt g -DrASAA1:2A°°S .0 010.1 sERK *RAzARA
.8. °Rzs g14,7,*8416-,A1 4) :e43 ° °*A %AZ ARRA88r-t
8:°&Jr%*$*8MAM g A g A 12418 A88 u3S8S888
8Av g $4:K88SAMK%*K n *Sisl 88S R §? gg a A
,8, *8. %K2 g ,8, t-0te.Vi%*z tr1 2 g 1 2° .8, 8, 8it c'%88, S8,
8, &!
R88MMKVX8IInt2rKFg RRIkOR*888R gIti,88888S
;
S8z8SERSA*8 .8..8. AAA .8. XArg*E ,84 7:8§1p8a8,.8.„-- 88
1 1 1 111111111111 1111 111111htliiiiii
-
Glossary
Alkali or alkali soil: Old terms that are nolonger used in soil
science because of theirvariable meanings. Soils are now
moreusefully categorised under saline and sodicsoil categories
(page 7).
Acid or acidic soils: Soils that have a pH lessthan 7. Usually
found in sites that are highlyleached.
Anion: A single atom or small group ofatoms with a negative char
e, such aschloride (C11, sulfate (SO441, carbonate(CO32-), or
bicarbonate (HCO3-).
Cation: A single atom or small group ofatoms with a positive
charge, such as calcium(Ca2+), magnesium (Mg2+), sodium
(Na),+potassium (I(+), or ammonium (NFI4+).
Cation exchange: The replacement of acation held on the surface
of a negativelycharged material, such as clay or organicmatter, by
another cation from the soilsolution. See Exchangeable cations
(page 3).
Cation exchange capacity (CEC): The totalquantity of cations
that can readily beexchanged on a unit amount of soil
material,expressed as inilliequivalents per 100 gramsof soil - me
100 g-1; centimoles of charge perkilogram of soil - cmol (positive)
charge kg-1;or, preferably, as rnillimoles of charge perkilogram of
soil - rnmol(+) kg -1 .
Electrical Conductivity (EC): The propertyof a material to
conduct electricity. The easewith which electrical current passes
throughwater is proportional to the saltconcentration in the water.
Consequently,the total salt concentration of a soil solutioncan be
estimated by measuring the EC Thehigher the EC, the greater the
saltconcentration. The value of the EC for aparticular soil sample
will vary according tothe preparation of the sample (EC e
specifiesthe EC of a saturation paste extract). Thepreferred unit
of measurement isdeciSiemens per metre (dSm -1 ).
Evapotranspiration: The loss of water fromplants and the soil
surface to the atmospherein a given time period, through
evaporationas well as transpiration from leaves. Usuallyexpressed
as millimetres of water depth.
Exchangeable sodium percentage (ESP):The percentage of the
cation exchangecapacity neutralised by sodium, that is,
theproportion of the total cation sites on thesurface of a soil
material that are occupied bysodium. It is calculated as:
Exchangeable sodiumCation exchange capacity
x100
Field capacity (field moisture capacity): Themaximum amount of
water that a well-drained soil can hold after any excess hasbeen
allowed to drain, that is, the amount ofwater the soil will hold
against gravitationaldrainage. It is defined as the water
contentremaining in a soil 2 to 3 days after beingsaturated and
then allowed to drain, with noevapotranspiration taking place.
Fieldcapacity of a particular soil layer is usuallyspecified in
millimetres (mm) of water permillimetre of soil depth (volumetric
basis) oras kilogram of water per kilogram of soil(weight
basis).
Gypsum requirement (GIO: The amount ofgypsum needed to lower the
ESP of 10 cm ofsoil to a desired level. It is is expressed
inapproximate tonnes needed per hectare andis calculated as:
GR = (Present ESP minus desired ESP)x CEC x 0.0015
The factor of 0.0015 assumes SO%reclamation efficiency, a
desirable SAR adiin the irrigation water and that CEC is
inInmoles(+) kg-.1 . If the CEC is in me 100 eor crnol(+) kg-1
units, the factor is 0.015.
ESP
-
Infiltration rate: The maximum rate atwhich ponded water can
enter the soil. It isusually given in millimetres per hour or
perday (mm mm c1-1 ).
Leaching: The removal of soluble salts fromthe soil and soil
solution, by the downwardmovement of water.
Leaching fraction (LE): That fraction of theinfiltrated
irrigation water that percolatesbelow the root zone:
LF -sleep drainage water
infdtrated irrigation water and rainfall
Milliequivalent (me): A measure of ioniccharge.
Osmotic potential: The pressure exertedacross a semipermeable
cell wall ormembrane as a result of unequal solute(dissolved salts
or sugars) concentrations oneither side of the cell wall or
membrane. Thesolvent will move from the side with thelowest solute
concentration through themembrane into the side with the
highersolute concentration. This process of solventmovement is
known as osmosis.
Parts per million (ppm): Concentrationbased on the number of
parts of solute in amillion parts of solution (the mixture of
thesolvent and the solute), that is, aconcentration of 15 ppm
sodium chloridewould give 15 milligrams of sodium chloridein 1 kg
(approximately) of water.
pH: A measure of the acidity or basicity of amaterial or
solution. A substance with a pHof less than 7 is an acid and more
than 7 is abase, 7 being neutral. The value of the pHfor a
particular soil sample will varyaccording to the preparation of the
sample.
Reclamation efficiency (in relation togypsum requirement): A
fraction obtainedby dividing the theoretical gypsumrequirement by
the actual gypsumapplication rate that is required to lower
theexchangeable sodium percentage (ESP) tothe desired level. The
best reclamationefficiency that can be obtained, with goodquality
(low SARW) irrigation water and
adequate internal drainage, is eighty percent. This means that
an application rate 125times that calculated by the
gypsumrequirement would be needed to achieve thedesired ESP under
optimum conditions.
Saline soil: A soil with an excess of salts (notonly sodium
chloride, NaC1) in it..
Salt-affected soils: Soils that are eitherchemically or
physically changed by highconcentrations of different salts. The
changesare such that some plant growth is adverselyaffeCted.
Saturation paste: A useful paste for soilanalysis, prepared by
mixing distilled waterwith the soil sample. The water content of
asaturation paste is approximately twice thatcontained at field
capacity.
Saturation paste extract: The soluteobtained from a saturation
paste. Thisextract gives the most accurate analysis of thesalinity
status of a soil. In this text, theabbreviations of measurements
obtainedfrom a saturation paste extract aresubscripted with an
'e'.
Saturation percentage: A figure calculatedby dividing the weight
of oven-dry soil bythe weight of water needed to wet the soil
tosaturation, then multiplied by 100 to obtain apercentage.
Sodic soil: A soil with an excess of sodiumions on the soil
exchange complex. Excesssodium will generally cause soil to have
poorphysical structure.
Sodium adsorption ratio (SAR): The SARof the soil solution or
irrigation water is arelationship between Na* and Ca 2+ plusMg2+
concentrations that predicts the Na+status of the soil exchange
complex when theexchange of cations within the soil comesinto
equilibrium with the soil solution orinfiltrating irrigation water.
The value of theSAR for a particular soil sample will varyaccording
to the preparation of the sample(SARe specifies the SAR of a
saturationpaste extract, SARw specifies the SAR ofirrigation water
or groundwater). SAR iscalculated as:
-
SAR = Na$4-Ca + Mg]
where the cation concentrations areexpressed in units of mmol
L-1 or moles m-3.
If the units are in milliequivalents L-1 , thenthe sum of Ca and
Mg is divided by 2. Thatis:
Na
linCa Mg)/2]
SARadj : The SARAi is the SAR of theirrigation water, corrected
for the effect thatthe carbonate and bicarbonate concentrationand
pH of the water will have on the soil incontact with that water.
The effect thatwater, carbonate, bicarbonate and pH haveon soil is
measured through a change in soilESP. Calculating SARadi for soil
extract datagives incorrect informati6n, as it only appliesto
water. For additional information andmethods of calculating SARadi
see jurinak(1990), listed under FURTHER
Soil amendment or ameliorants: Anymaterial such as lime, sulfur,
gypsum,sawdust, sand or straw used to alter thephysical or chemical
properties of a soil.Fertilisers, which are added to supply
plantnutrients, are not soil amendments orarneliorants.
Soil dispersion: The process of soil particlesdisaggregating,
that is, falling apart anddispersing when in contact with
water.
Soil exchange complex: A whole range oforganic and inorganic
particles within soilwhich have some electrical charge. Ions
canmove onto and off these particles.
Soil horizon: A visibly different layer withina soil profile.
Differences between layersmay be caused by differences in
colourand/or texture.
Soil profile: The description of the changesin texture, colour
and composition of the soilwith increasing depth from the soil
surface.
Soil:water extract The solute made byshaking a soil sample with
an excess of purewater usually expressed on a
volume:volumebasis.
Solute: That part of a salt or chemical that isdissolved in
water.
Specific ion effect The effect, usually toxic,that a particular
ion has on plants.
Total Soluble Salts (TSS): The total amountof all salts
dissolved in water, usuallyexpressed in ppm or preferably
milligramsper litre (iftg
Water table: The upper free water surface ofground water; that
is, the level below thesoil surface where water stands in an
openhole in the soil.
MR
-
Further Reading
Boruvka, V., and Matters, J. (1987). FieldGuide to Plants
Associated with Saline Soils.Department of Conservation, Forests
andLands, East Melbourne, Victoria.
Bresler, E., McNeal, B.L., and Carter, D.L.(1982). Saline and
Sodic Soils. (Springer-Verlag, New York.)
Humphreys, E., Muirhead, W.A., andvan der Lelij, A. (eds)
(1990). Management ofSoil Salinity in South-East
Australia.Australian Society of Soil ScienceIncorporated, Riverina
Branch, WaggaWagga, New South Wales.
Jurinak, J.J. (1990). The chemistry of salt-affected soils and
waters. In AgriculturalSalinity Assessment and Management,ed. K.K.
Tanji, American Society of CivilEngineering, New York, pp.
42-63.
Malcolm, C.V. (1962). Plants for salty water.Journal of the
Department of Agriculture,Western Australia, Vol. 3, pp.
793-94.
Mass, E.V. (1990). Crop salt tolerance. InAgricultural Salinity
Assessment andManagement, ed. K.K. Tanji, American Societyof Civil
Engineering, Irrigation and DrainageDivision, New York, pp.
262-304.
Matters, J., and Boron, J. (1989). SpottingSoil Salting: A
Victorian Field Guide to SaltIndicator Plants. Department
ofConservation, Forests and lands, EastMelbourne, Victoria.
Queensland Department of PrimaryIndustries (1987). Landscape,
Soil and WaterSalinity: ?wit-Wings of the BrisbaneRegional Salinity
Workshop, Brisbane.Queensland Department of PrimaryIndustries
Conference and Workshop SeriesNo. QC87003, Brisbane,
Queensland.
Robbins, C.W. (1990). Field and laboratorymeasurements. In
Agricultural SalinityAssessment and Management, ed. K.K.
Tanji,American Society of Civil Engineering,New York, pp.
201-19.
Spurling, M.B. (1962). Water from bores,wells and streams -
suitability for irrigationand household use. Journal of
theDepartment of Agriculture, South Australia,Vol. 65, pp.
492-96.
Tennison, K. (1991). Irrigation SalinityDecision Support System,
Books 1, 2 & 3.NSW Agriculture and Fisheries.
U.S. Salinity Laboratory Staff (1954).Diagnosis and improvement
of Saline andAlkali Soils, ed. L.A. Richards. USDepartment of
Agriculture HandbookNo. 60. US Department of
Agriculture,Washington.
Victorian Irrigation Research and AdvisoryServices Committee
(1980). Quality Aspectsof Farm Water Supplies. Victorian
SoilConservation Authority, Melbourne,Victoria.
Wilcox, L.V. (1959). Determining the Qualityof Irrigation Water.
Agricultural InformationBulletin No. 197, US Department
ofAgriculture, Washington. .
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