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
AUSTRALIA
Division of WaterResources
SeekingSolutions
Water ResourcesSeries No. 4
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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
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
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
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.
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
17
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
24
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.
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.
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
4
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
5
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
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.
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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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