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MAY 2000

Salinity andSodicity

IN NORTH DAKOTA SOILS

B. D. SeeligSoil Scientist

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WHAT IS SALINITY AND SODICITY?Soil salinity and sodicity are related

problems that are commom in NorthDakota. Both eastern and westernNorth Dakota have their share ofsaline and sodic soils. For example, 24and 17 percent of the soils in GrandForks County and Slope County,respectively, are affected by salinityor sodicity (Soil Survey Staff, 1987).Further, soil survey data from 34 ofthe 52 coun-ties in North Dakotashow approximately 1,900,000 acresaffected by sodium and 700,000 acresaffected by salinity. These problemsoils compose about 9 percent of the34-county area.

Salinity and sodicity are often seenas bare ground with whitish crustthat has a scabby appearance. Scab-land, salt-land, alkali, alkaline, andsour-ground are a few of the localterms that describe these areas.Some of these terms are misleading,because they imply problems that arenot necessarily related to salinity orsodicity. For instance, alkaline andsour-ground actually refer to soilacidity. Alkaline soils are basic(pH > 7) and sour-ground is acid(pH < 7). Many soils in North Dako-ta are basic in pH but are not affec-ted by salinity or sodicity. On theother hand few soils in North Dako-ta have high enough acidity to reducecrop yields. Acid soils are occasionallyfound in in in the extreme south-central and southwestern parts ofNorth Dakota. They have formedin acid sediments of the Fox Hillsformation and are generally used asrangeland.

We are concerned with salinityand sodicity levels high enough toaffect production. Clear definitionsof these two problems are necessaryfor proper identification andmanagement.

SOLUBLE SALTS ANDSALINE SOILS

Soils with high amounts of solublesalts are called saline soils. They oftenexhibit a whitish surface crust whendry. The solubility of calcium sulfateor gypsum (CaSO4) is used as thestandard for comparing solubilities ofsalts (Table 1). Salts more solublethan gypsum are considered to besoluble and cause salinity. Examplesare sodium sulfate or Glauber’s salt(Na2SO4) and sodium chloride, ortable-salt (NaCl). Salts less solublethan gypsum are considered insolubleand do not cause salinity. Calciumcarbonate (CaCO3) or lime is anexample of an insoluble salt com-monly found in North Dakota soils.

Salts found in North Dakota soilsare of three types: sulfates (SO4);carbonates (CO3); and chlorides (Cl).Most saline soils in North Dakota arecomposed of sulfate salts (Keller etal., 1984). However, the northernRed River Valley has extensive areasof saline soils that have high amounts

of chloride salts. Soils with highamounts of carbonates of sodiumoccur rarely, and are usually associ-ated with coarse textured materials.

SODIUM AND SODICSOILS

Soils high in sodium (sodic soils)may present physical restrictionsto plant growth. Sodium (Na+) is apositively charged component, orcation, of many salts. Sodium prob-lems are due to its behavior whenattached to clay particles. If 15 per-cent or more of the clay adsorptionsites are occupied by sodium(sodium-clay), poor physical conditionof the soil often restricts root growthand makes tillage difficult.

The forces that hold clay particlestogether are greatly weakened whensodium-clay and water come intocontact. In this condition clay par-ticles are easily detached from largeraggregates, or dispersed (Figure 1).When dried, however, sodium-clayparticles settle out in dense layers

Table 1. Composition and solubility of some common evaporiteminerals (salts).

Mineral Composition Solubility Chemical Name

(moles/liter)

Calcite (lime) CaCO3 0.00014 Calcium CarbonateGypsum CaSO4 • 2H2O 0.0154 Calcium Sulfate---- CaCl2 • 6H2O 7.38 Calcium ChlorideMagnesite MgCO3 0.001 Magnesium CarbonateHexahydrite MgSO4 • 6H2O 4.15 Magnesium SulfateEpsomite MgSO4 • 7H2O 3.07 Magnesium SulfateBischofite MgCl2 • 6H2O 5.84 Magnesium Chloride(Washing soda) Na2CO3 • 10H2O 2.77 Sodium Carbonate(Baking soda) NaHCO3 1.22 Sodium BicarbonateMirabilite Na2SO4 • 10H2O 1.96 Sodium SulfateThenardite NaSO4 3.45 Sodium SulfateHalite NaCl 6.15 Sodium Chloride

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Figure 1. When the exchange complex has greater than 15 % sodium (Na) the clay is dispersed (A) resulting in poor soil structure.When calcium (Ca) replaces enough sodium the clay is flocculated (B); stable soil aggregates are formed that create good soil structure (C).

(After Rengasamy, P., et al., 1984.)

that are impenetrable to plant rootsand seedling emergence.

SODIUMADSORPTION RATIOAND SODICITY

Sodicity can be defined in termsof the sodium adsorption ratio (SAR).The SAR is a calculation fromlaboratory measurements made ona water sample or water extractedfrom a soil. It is based on the concen-tration of sodium (Na+), calcium(Ca2+), and magnesium (Mg2+) in thesample. There is a relationshipbetween the amount of each ion insolution and its relative amountadsorbed on the clay; however, therelationship is not direct. Na+, Ca2+,and Mg2+ are adsorbed to the claywith unequal strength. Na+ is weaklyheld compared to Ca2+ and Mg2+.The SAR computation (equ. 1)accomodates the difference inadsorption strengths.

The U.S. Salinity Laboratory Staff(1954) determined a SAR of 13 froma saturated soil extract is comparableto 15 percent of the adsorption sitesbeing occupied by Na+. Because ofthe difficulty and laboratory expenseof analyzing the amount of actualNa+, Ca2+, and Mg2+ adsorbed on the

clay, the SAR determination hasbecome the standard measure forsodicity.

To demonstrate the SAR calcula-tion and the importance of the ratioof ions as opposed to the totalconcentration of sodium, let’s lookat the following two examples:

SAR = [Na+] / {([Ca2+] + [Mg2+]) / 2}1/2 [1]where: [ ] = concentration in milliequivalents/liter (meq./l)

Example 1

[Na+] = 25 meq/l

[Ca2+] = 60 meq/l

[Mg2+] = 30 meq/l

SAR = 25 / {(60 + 30) / 2}1/2 = 25 / (45)1/2 = 25 / 6.7 = 3.7

Example 2

[Na+] = 25 meq/l

[Ca2+] = 4 meq/l

[Mg2+] = 2 meq/l

SAR = 25 / {(4 + 2) / 2}1/2 = 25 / (3)1/2 = 25 / 1.7 = 14.7

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The extracts from these twosamples contain the same amount ofsodium. However, because theamount of calcium and magnesium isdifferent, only example 2 is from asoil that would have a sodicity prob-lem. This points out the fact thatsome soils may have high amounts ofsodium but are not sodic. The highamounts of calcium and magnesiumcounter-balance the sodium.

U.S. SALINITYLABORATORYCLASSIFICATION

The U.S. Salinity Laboratory Staff(1954) established a system for clas-sifying saline and sodic soils in threebroad categories.

Saline SoilsSaline soils are defined as having an

electrical conductivity (EC) greaterthan 4 deciSiemens/meter (dS/m)and sodium adsorption ratio (SAR)of less than 13 in their saturationextract.

Saline-Sodic SoilsSaline-sodic soils have an EC

greater than 4 dS/m and a SAR great-er than 13 in their saturation extract.

Sodic SoilsSodic soils have an EC of less than

4 dS/m and a SAR greater than 13 intheir saturation extract.

NATURAL RESOURCES CONSERVATIONSERVICE, USDA CLASSIFICATION

Saline and sodic soils are recog-nized and shown on soils maps madeby the USDA Natural ResourcesConservation Service (NRCS).Soils shown on maps are classifiedusing a system that depends on bothobservable and laboratory properties.Although the NRCS has incorporatedmuch of the U. S. Salinity Laboratory

system into its classification scheme(Table 2), it relies heavily upon soilcharacteristics that can be observedin the field as opposed to laboratorydeterminations. Field observations ofsoil properties such as structure andsalt crystals are used regularly, butlaboratory data is determined onrelatively few samples.

Table 2. NRCS classification of saline and sodic soils.(Soils Survey Staff, 1993)

Saline Soils

class Non-saline Very slightly to Stronglymoderately saline saline

criteria ^S.E. EC < S.E. EC = S.E. EC >2 dS/m 2–16 dS/m 16 dS/m

Natric (Sodic) Soils

class glossic typic or udic leptic aquollsubgroups subgroups subgroups suborder

subsoilcriteria *SAR > 13 *SAR > 13 *SAR > 13 *SAR > 13

weak strong claypan claypanclaypan claypan high poorly

salinity drained

*Most natric soils meet this chemical requirement; however, some soils are alsoconsidered natric with SAR < 13 under certain conditions.

^Saturated extract (S.E.) electrical conductivity from the topsoil or the average of thesoil profile, whichever is greater.

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EFFECTS OF SALINITY AND SODICITYOSMOTIC EFFECT

High salinity (high EC) causesdehydration of plant cells. Reducedplant growth and often death is theresult. Dissolved salts cause plant celldehydration by decreasing the os-motic potential of soil water. Waterflows from high potential (low salts)to a low potential (high salt) (Figure2). When a soil solution has a lowerosmotic potential than plant cells,plants cannot extract water from thesoil. The effect on a plant is similarto drought stress.

Crop Yield and Soil ECYields of many crops are reduced

noticeably when the soil extract ECreaches 4 dS/m (U.S. Salinity Labora-tory Staff, 1954). Yields will declineproportionately as EC levels (salinity)increase above 4 dS/m. Some crops,such as sugar beets, are quite toler-ant to EC between 4 and 8 dS/m. Atan EC of 16 dS/m the growth andyields of nearly all crops are severelyaffected. The effect of salts on plantgrowth is the basis for the NRCS soilsalinity categories (Table 2).

Crop Symptoms of SalinityCrop symptoms from high salinity

are generally the same as symptomsof moisture stress from dry condi-tions. Plants are stunted with cuppedleaves to reduce water loss throughthe stomata. Initially some plants takeon a deep blue-green color fromexcessive accumulation of wax asan attempt to reduce water loss.Eventually leaves become brown andbrit-tle on the tips and margins asstress continues.

Salinity in the field is usually char-acterized by broad transitions thatrun gradually from high to low salin-ity. The pattern of crop response canalso be a gradual change from normalplants to no growth at all. However,in some cultivated species, the plantis most sensitive to damage duringgermination or early growth stages.

Figure 2. Plants can extract water easily from soils with high total water (A). This is a conditionencountered in moist non-saline soils. Dry conditions and dissolved salts decrease the total soil

water potential, thus retarding or stopping plant uptake of water (B).

This may result in a pattern of sharpchange from normal plants to bareground where seeds failed togerminate. Often plants are able toflourish during later growth stages atsalinity levels that would not haveallowed germination or growthduring earlier stages.

Plant IndicatorsPlants can be used as indicators of

saline and sodic conditions, becausetolerance varies with plant species.All plants attempt to adjust toosmotic moisture stress caused bysalinity. The osmotic adjustment re-quires energy that would otherwise

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be used for growth and production.As a result, when plants attemptto adjust, growth and yields arereduced.

HALOPHYTIC (SALT LOVING) PLANTSPlants tolerant to salinity are

known as halophytes. Technically,plants that can survive in soils thathave greater than 0.5 percent (byweight) soluble salts are consideredto be halophytes. Some halophyteshave developed sophisticated systemsof taking in saline water, extractingthe salts, and then excreting the salts.Halophytes common to our areathat can be used as indicators of highsalinity include salt grass (Distichlisspicata), alkali grass (Puccinellia nuttal-liana), cordgrass (Spartina gracilis),spearscale (Atriplex subspicata), salt-wort (Salicornia rubra), and sea blite(Suaeda depressa). Other less reliableindicator plants are wild barley (Hor-deum jubatum) and kochia (Kochiascoparia). Although they often occuron saline soils, they are also regularlyfound in non-saline environments.

VEGETATION PATTERNSAND SALINITY

A study of vegetation and salineseeps in northwestern North Dakotashowed a consistent pattern of salin-ity and vegetation (Figure 3). Wildbarley was the dominant plant alongseep margins with low salinity. Kochiawas the dominant plant on theinterior of the seep with high salinity.This is a common pattern seen onsalt affected soils in North Dakota.Salt grass was the dominant vegeta-tion on sodic soils with high levels ofsurface salinity on pastureland incentral North Dakota (Seelig et al.,1990).

SPECIFIC ION EFFECT –PLANT TOXICITIES AND DEFICIENCIES

Saline soils may also have specificion effects on plant growth. Higherthan normal concentrations of someions can hinder or block nutrientuptake and certain physiological pro-cesses. Few instances of specific ioneffects have been reported in NorthDakota saline soils. Of the threecommon cations (Ca2+, Mg2+, andNa+) responsible for saline soils inNorth Dakota, no toxic ion effectshave been reported. Toxic ion effectsrelated to sodium (Na+) have beenobserved in some plants in otherparts of the U.S.

Under rare circumstances, boronhas been reported to cause toxic ioneffects. Boron toxicity is a potentialproblem when concentrations in thesaturation extract are greater than1 milligram/liter (mg/l). This level ofboron may occur in some saline soilsin the northern Red River Valley.However, almost without exception,the osmotic effect plays the most im-portant role in limiting plant growthand yield on saline soils in NorthDakota.

Neither cloride (Cl-) nor sulfate(SO4

2-) have been shown to causeplant toxicities in saline soils. Planttoxicities from excessive bicarbonate(HCO3

-) have been reported in otherareas; however, few saline soils inNorth Dakota have high levels ofsoluble HCO3

-. Many soils in NorthDakota do have large accumulationsof calcium carbonate (CaCO3) thatis quite insoluble. Both phosphorusand micronutrient deficiencies arepos-sible in high lime soils. It may benecessary to increase additions ofphosphorus and micronutrients toovercome these problems.

HIGH pH ANDSODIC SOILS

High pH (7.8 to 8.5) generallyoccurs in sodic soils. Extremely highpH (greater than 8.5) occurs in sodicsoils when soda (NaHCO3) or sodi-um carbonate (Na2CO3) is present.Sodium-clay, bicarbonate (HCO3

-),and carbonate (CO3

2-) react withwater to form hydroxyl (OH-) ionsthat cause high pH [equations 2, 3,

Figure 3. The dominance of plant species in saline seeps is shown by weight % versus EC.(After Worcester and Seelig, 1976.)

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and 4]. Soda and sodium carbonateare relatively soluble; therefore, highamounts of soluble carbonate is avail-able to react with water to produceOH-. For this reason, sodic soils withsoda and sodium carbonate are likelyto have a pH greater than 8.5.

Na-clay + H2O → H-clay + Na+ + OH- [2]

HCO3- + H2O → H2CO3 + OH- [3]

CO32- + H2O → HCO3

- + OH- [4]

Extremely high pHs (greater than8.5) are generally not encounteredin sodic soils of North Dakota,because soda and sodium carbonateare rarely present. When solublecarbonate salts are found, the soilsgenerally have coarse textures.When the soil pH is greater than9.5 live plant roots are vulnerableto deterioration.

Nutrient Availability andHigh pH

High pH affects plant growth byreducing availability of some plantnutrients. Phosphates are most avail-able to the plant at a pH between6 and 7. Micronutrients such as iron,manganese, zinc, copper, and cobaltare all much less available at a pHgreater than 7.

RESTRICTIVE SUBSOILS AND CRUSTINGPROBLEMS

Slowly PermeableClaypans

Soil structure in sodic soils is poorand permeability is low. Permeabilityis the ease of air and water move-ment through soils. Good permeabil-ity allows water and gasses (oxygenand carbon dioxide) to circulateeas-ily to and from plant roots. This isneeded for good plant growth.

A dense layer of clay occurs ator near surface of sodic soils. Thisnatural layer, often called a claypan,is a barrier to roots. Most roots arerestricted to the topsoil above theclaypan, because movement of water,nutrients, and gases is too slow in theclaypan. A dry claypan can be hardenough to physically restrict rootpenetration. The overall effect onplant growth is one of stress similarto that caused by extremely dry orsaline conditions.

UNDULATING OR SPOTTY PLANTGROWTH ON SODIC SOILS

Both claypan depth and rootrestriction vary considerably withina few feet. Depth to saline materialis also highly variable. This variation insoil properties creates extremedifferences in plant available water,plant rooting depth, and osmoticpotential. Crop response to theseextremes is an undulating or spottypattern of growth that is characteris-tic of sodic soils.

Soil CrustingSurface crusting is a common

problem with cultivated sodic soils.Plowing mixes sodium-clay from theclaypan with topsoil or surfacematerial. Surface material with highamounts of sodium-clay is highlyerodible, because it has low aggregatestability and is easily detached by raindrop impact. When dry, the surface

forms a hard crust that is a barrier toseedling emergence.

Salinity andMetal Corrosion

Saline soils are corrosive withrespect to certain materials com-monly used for construction. Whenmetal is placed in the soil, it is ex-posed to electrochemical reactionsthat change its physical properties;iron rusting is a good example. Soilshigh in soluble salts enhance electro-chemical corrosion. Electrochemicalcorrosion can be reduced by coatingthe metal with organic materials, suchas tar or pitch. Metal pipelines areoften protected by passing anelectrical current through them(cathodic protection) to replace theelectrons lost to chemical reactions.

Sulfates andConcrete Corrosion

Soil solutions high in sulfate(SO4

2-) are corrosive to concretestructures. The sulfate solutionpenetrates concrete and reacts withcalcium in the cement to form CaSO4that precipitates within the pores.This reaction destroys the integrity ofconcrete in two ways. The cementingagent is changed to a non-cementingmaterial (CaSO4) and large crystalsof CaSO4 are formed within voids,causing physical disruption of theconcrete.

Much of the damage caused bysulfate solutions can be avoided byusing sulfate-resistant concrete. TypeI (standard Portland cement) has littleresistance to the corrosive action ofsulfatic solutions. Type II has mediumresistance to sulfates, and Type V hashigh resistance.

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NATURALLYOCCURING SALINESOILS

Saline and sodic soils generallyoccur on landscape positions wheregroundwater discharges by evapo-transpiration from a shallow watertable. In these soils, the water tablewill flucuate seasonally, but generallywill be within 6 feet of the surface fora large part of the growing season.Because soil texture affects the flowof water through soil, it also affectsthe depth of the water table neces-sary to cause salinization. Coarsetextured soils allow upward capillarymovement of water to occur onlyover a short distance. Therefore ahigher water table is required forsalinization of coarse textured soils,such as sands or sandy loams,compared to finer textured soils,such as loams or clays. However,because the large pores in coarsetextured soils offer much lessresistance to water flow compared tothe smaller pores in fine texturedsoils, salinization can occur muchquicker in coarse textured soils.

As groundwater is dischargedthrough the soil surface, dissolvedsalts precipitate and accumulate.Accumulations generally occur nearor at the surface of the soil; however,they may occur anywhere in the soilwhere water is extracted by plantroots.

Discharge as evapotranspiration isonly one part of the water balance ata given landscape position. Seasonalrain causes downward percolationof water that leaches salts from theupper parts of the soil.

Salinization occurs in those soilsthat favor discharge (evapotranspira-tion loss) over leaching. Not all soilswith a water balance favoring dis-charge are saline. Salinization alsodepends on groundwater quality. If

the groundwater has a only a smallamount of soluble salts, salinization isvery slow or may not occur at all.

Many combinations of ground-water discharge, water quality, andlandscape position exist in NorthDakota. As a result, a variety of dif-ferent types of both saline and sodicsoils have developed. Saline and sodicsoils with similar properties can oftenbe located by geographical area.

NORTHERN RED RIVER VALLEYSaline soils in the Grand Forks

County area (Figure 4) are the resultof regional discharge of artesiangroundwater from the Dakota Sand-stone formation (Benz et al., 1961).The chemistry of these saline soils isuncommon for the northern GreatPlains because of the accumulation ofchloride salts. Sulfate salts generallydominate saline soils in NorthDakota.

GLACIATED AREAS NORTH AND EASTOF THE MISSOURI RIVER

In glaciated areas, saline soils areassociated with the edges of closeddepressions or broad swales wheredischarge of groundwater can occur.

Saline soils with most salts at thesurface are associated with veryshallow water tables. Saline soils withlower water tables may actually havethe highest levels of salinity deeper inthe soil (Seelig et al., 1987). Leachedsoils with little salt are often foundin depressions within areas of salinesoils.

WEST AND SOUTH OF THE MISSOURIRIVER

In nonglaciated areas of NorthDakota the water table generallyap-proaches the soil surface on lowslopes that gently grade to naturaldrainageways (streams or rivers).Surface water flow and storage iscontrolled by the system of intercon-nected drainageways. This is differentthan the hydrology in glaciated areasthat is controlled by a system ofclosed depressions (potholes).

Some saline and sodic soils west ofthe Missouri River are not neces-sarily related to groundwater dis-charge. These soils inherited saltsfrom sedimentary material (marineshales, sandstones, etc.) in whichthey formed. They may be found onlandscape positions that have verydeep water tables.

LOCATION AND OCCURRENCE OF SALINE AND SODIC SOILS

Figure 4. The geologic cross section of Grand Forks County shows the relationship between theDakota sandstone and the overlying glacial deposits. (After Benz et al., 1961.)

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Naturally Occurring SodicSoils

Groundwater discharge may leadto the development of sodic soils assodium salts are concentrated in thesoil. During evaporation, sodium saltsseparate from calcium salts becauseof different solubilities. The solublesodium salts concentrate high in thesoil profile. As the soil solutionbecomes more concentrated withsodium, the percentage of sodium onthe exchange complex also increases.Eventually there is enough exchange-

able sodium to cause dispersion ofclay and organic matter.

In the unglaciated parts of westernNorth Dakota, sodium affected soilsare quite common and the saline soilsare generally also affected by sodium(Figure 5). Sodic soils are also foundin the glaciated parts of NorthDakota; however, saline soils unaf-fected by sodium are far moreprevalent (Figure 5).

SECONDARYSALINIZATION

Secondary salinization is the termused to describe soils salinized as aconsequence of human activities. InNorth Dakota there are five types ofsecondary salinization:

1) saline seeps;

2) salinization along road ditches;

3) salinization along lagoon margins;

4) salinization from irrigation;

5) salinization from wetland drainage.

Figure 5. The distribution of saline and sodic soils in North Dakota.(Adapted by D. D. Patterson, C. Fanning, and B. D. Seelig from the General Soil Map of North Dakota, Soil Survey Staff, 1961).

Severe problem area

Frequent inclusion in productive land

Frequent inclusion in badlands

Occasional to rare inclusionin productive land

Severe problem area

Frequent inclusion in productive land

Occsional inclusion in productive land

Rare inclusion in productive land

Saline SoilsSodic Soils

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Saline Seep FormationMost saline seeps have developed

recently (post settlement) in thenorthern Great Plains. In the last 20years, investigators have concludedthat saline seep formation is closelyrelated to the practice of summerfallow for moisture conservation.Researchers have determined thatthe soil in the rooting zone undersummer fallow reaches its storagecapacity long before the end of thefallow period. Deep percolationof additional moisture beyond therooting zone is likely to occur inthis situation.

Rate and amount of downwardpercolation is controlled by soiltex-ture. Average pore diameter incoarse textured soils is larger thanin fine textured soils. Faster rates ofwater flow and less water storage aredirectly related to larger porediameters. As a result, deep percola-tion is more likely to occur in coarsetextured soils. In some places rela-tively coarse textured soils overlayimpermeable material of finertex-ture. Under these circumstances,deep percolation leads to lateralwater flow along the surface of the

impermeable material (Deutsch,1977; Seelig, 1978). If the contactbetween the two different materialsapproaches the soil surface along ahill slope, as often happens, the later-ally moving water will create a wetspot that eventually becomes saline asthe water is evaporated (Figure 6).

LOCATION AND TYPES OFSALINE SEEPS

Saline seeps are most commonsouth and west of the Missouri Riverand on the Missouri Coteau. Areasmost prone to saline seeps havesloping stratified geologic materials.They have been classified into sixgeneral groups according to theunderlying materials (Figure 7).Doering and Sandoval (1976) esti-mated that 100,000 acres of westernNorth Dakota cropland are affectedby saline seeps. Although this is asmall percentage of the total acreageof saline and sodic soils, saline seepsare a serious problem. They areresponsible for deterioration of landthat was once productive. Efforts toavoid wet saline spots on tracts ofcultivated land increases costs ofproduction, especially with today’sfarm machinery.

Road Ditch andLagoon Margin Salinity

Saline soils that have developedaround lagoons and drainage ditchesare also controlled by the watersource. Salinity along drainage ditchesis due to lateral movement of waterfrom the ditches to adjacent fields(Figure 8). Extremely flat areas,such as the Red River Valley, areparticularly susceptible to this typeof secondary salinization. Thelow grade on most drainage ditchesallows water to stand for longperiods of time.

Salinity and IrrigationUnder irrigated agriculture, secon-

dary salinization may occur if wateris not properly managed. All waterfrom sources other than precipitationcontains some dissolved salt.As irrigation water is used by crops,salts are precipitated in the soil. Thisprocess may eventually lead to salineconditions that plants cannot tolerate(Figure 9).

The rate at which salinizationproceeds depends on the amount andquality of water added. Water withhigh amounts of dissolved salts willcause rapid salinization. Poorlydesigned and improperly managedirrigation systems have been respon-sible for salinization of thousands ofacres in the U.S. Before an irrigationsystem is developed, soil and watercompatibility should always be deter-mined by a qualified soil scientist.

Wetland Drainage andSalinity

Drainage of certain types ofwetlands in the northern Great Plainsmay also lead to soil salinization inthe wetland interior. Class 3 and 4wetlands are noted for their non-saline soils but are often surroundedby saline soils on the wetland edge.Hydrologically, many of thesewetlands are called flowthroughFigure 6. A generalized diagram of a saline seep.

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Figure 7. Six classes of saline seeps found in North Dakota. (After Worcester et al., 1975.)

Seep

Seep

Seep

Seep

Slough

Seep

Seep

A

C

E

D

F

B

Loamy Till

Sand

Dense Till

Sandy Loam

SandGravelly

Dense Till

Till

Till

Gravel

Silty Clay / Clay

Loam

Loamy Sand /Sandy Loam

Sandy Loam

LoamLoamSilt Loam

Highly Permeable Layer

Loam

Loam

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wetlands, because water flowslaterally through the wetland soils anddoes not allow accumulation of salts(Figure 10). When the hydrology of aflowthrough wetland is disrupted bydrainage, evaporative discharge canoccur through the drained wetlandsoil. Under the undrained condition,evaporative discharge is confinedto the wetland edge; soils may beextremely saline at this location.When the area of evaporativedischarge is expanded to the wetlandinterior, saline conditions may beexpanded to the entire wetland(Figure 10). The final result offlowthrough wetland drainage is oftenno gain in crop production, but a lossof flood protection, groundwaterrecharge, and wildlife habitat.

Figure 8. The location of maximum salinity and crop damage along a drainage ditch in the RedRiver Valley. (After Skarie et al., 1986.)

Figure 9. Secondary salinization by irrigation (A) and (B) may be prevented, if the field isadequately drained (C).

Road

Ditch

Maximum salinityand

crop damage

Water Flow

Wat

er F

low

Salts

A

Wat

er

Flo

w

Water Table

Salts

Wat

er

Flo

w

B

Tile Drain

Wat

er F

low

C

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Figure 10. Salinity is confined to the edges of flowthrough wetlands (A), unless the wetlands are drained, and the area ofevaporative discharge is expanded to the wetland basin (B).

SALINE AND SODIC SOIL MANAGEMENTManagement stategies for saline

and sodic soils must be designed withprocesses of salt accumulation inmind. Different management tech-niques may be necessary for soilswith different salt compositions andwater regimes.

Most saline and sodic soils are theresult of evaporation from a shallowwater table. Before effective methodsof management can be designed fora specific parcel of land, the watertable level must be determined. Aslong as a shallow water table exists,potential for salinization and sodifi-cation exists.

SALINE SOILMANAGEMENT

The type of ions in the soil solu-tion does not influence the osmoticpotential to any large degree; it is thetotal of all ionic species that controlsthe osmotic potential. From a prac-tical point of view, management ofsaline soils with different salt compo-sitions is essentially the same.

Plant Tolerance to SalinitySalt tolerant plant species should

be selected on the basis of saltconcentrations found in saline areas.Drought resistant plants such as

grasses and small grains are generallymore tolerant to salinity than rowcrops, trees, shrubs, and vegetables(U.S. Salinity Laboratory Staff, 1954).If an accurate estimate of field salinityis known, crop tolerance informationcan be used to determine the eco-nomic feasibility of growing certaincrops.

Crop tolerance to salinity hasbeen tested for some of the majorcrops in North Dakota (Figure 11). Itis noteworthy that sunflower is moreresistant to salinity than experiencefrom other production areas wouldindicate.

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Many cultivated crops are mostsusceptible to salinity during germina-tion and early growth stages. If plant-ing coincides with periods shortlyafter salts have been flushed fromthe surface, plant germination andseedling survival is more likely to besuccessful.

Reduction ofEvaporative Discharge

Control of salinity must includemanagement practices that reduceevaporative discharge. Subsurfacedrainage is an effective means oflowering the water table; however,it may not be economically feasible.Evaporative discharge is also affectedby the condition of the soil surfaceand the plants growing on it. Surfacemulches can be used to reduceevapotranspiration and salt accumula-tion.

SODIC SOILMANAGEMENT

Soils affected by sodium must bemanaged similarly to saline soils withrespect to drainage. Management forthese soils includes drought tolerantplant species. Chemical amendmentsand physical disruption of the claypanmay help to reduce the restrictivenature of these soils.

Calcium Amendments forSodic Soils

Sodic soils may be improved byreplacing adsorbed sodium withcalcium. A number of amendmentshave been used with limited successover the years (U.S. Salinity Labora-tory Staff, 1954). Amendments thatrelease high amounts of calciumto the soil solution are the mosteffective. Unfortunately very solubleamendments, such as calcium chloride(CaCl2), are cost prohibitive. Gypsum(CaSO4) is an amendment lesseffective than calcium chloride butpopular in many areas because of

Figure 11. Thresholds of yield reduction due to salinity for some of the major crops in NorthDakota. Salinity was expressed as the electrical conductivity (EC) of the saturated soil extractfrom the plow layer (0 to 6 inch depth). (After Maianu, 1983; 1984; and unpublished data;

Maianu and Lukach, 1985; Nelson, unpublished data.)

cost. Gypsum, however, is oftenineffective on sodic soils in NorthDakota, because they already havehigh amounts of gypsum in them.For those sodic soils in North Dakotathat do not have gypsum in the upperpart of the claypan, CaSO4 may bean effective amendment. Soil inspec-tion and analysis by a qualified soilspecialist before calcium amendmentsare applied to fields with sodic soilsis recommended.

Deep Plowing andSodic Soil Improvement

Deep plowing has improved somesodic soils in western North Dakota(Sandoval, 1978). Deep plowing notonly disrupts the restrictive claypan,but may also mix CaSO4 from deep-er soil layers into the claypan.

Caution should be taken notto plow too deep or too shallow.Plowing too deep may bring excessiveamounts of soluble salts to the sur-face, creating a salinity problem.Plowing too shallow will not disruptthe lower part of the claypan andwill not reach gypsum accumulationsbelow it. The effectiveness of thedeep plowing technique dependson the location of the water table andthe presence of native gypsum in thesoil.

Soil improvement through deepplowing is not expected to be sus-tained on a sodic soil that has a shal-low water table, because sodium saltsare continually introduced to the soilthrough evaporative discharge. If theclaypan already has high amounts of

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gypsum, mixing of gypsum fromdeeper substrata will have no effecton replacement of adsorbed sodium.Consultation with a qualified soilspecialist is recommended beforedeep-plowing is attempted toimprove fields with sodic soils.

SALINE-SODIC SOILMANAGEMENT

Saline-sodic soils have uniqueproblems with respect to manage-ment, because they have both highsalinity and sodicity. Some saline-sodic soils have well aggregatedstructure, although high amounts ofsodium are present. The SAR rangedfrom 18 to 30 and EC ranged from10 to 20 dS/m in a typical saline-sodicsoil from northeastern NorthDakota. High salinity counteracts thedispersive effect of sodium. Attemptsto leach the salts will likely result ina puddled soil, because the counter-active effect against dispersion is lostas the salts are removed.

MANAGEMENT OF SOILS AFFECTED BYSECONDARY SALINIZATION

Saline SeepsRECHARGE AREA MANAGEMENT ANDSALINE SEEP CONTROL

Saline soil improvement oftenincludes managing the water table.In the case of saline seeps, the watertable may be controlled by reducinglocal groundwater recharge. Eliminat-ing summer fallow in upland rechargepositions is generally the mosteffi-cient method of controlling thewater table in a saline seep. Com-pared to native vegetation, croppingsystems may not use moisture effi-ciently, particularly when summerfallow is included in the rotation. Theresult is high water tables that con-tribute to evaporative discharge andsalinization of farmland. Where feas-ible, reduced summer fallow, continu-ous cropping, and inclusion of hay orpasture in long term rotations shouldreduce groundwater recharge inareas prone to saline seeps. Deep-rooted crops such as alfalfa have beenshown to effectively withdrawmoisture from recharge areas andreduce discharge from saline seepsdownslope (Brun and Worcester,1974; 1975).

INTERCEPTION MANAGEMENT ANDSALINE SEEP CONTROL

Other management practices focuson interception of lateral flow withtile drains or bands of deep rootedcrops. These methods have beensuccessful; however, they haveserious disadvantages. Interceptionof saline water with tile drains canbe costly, and the problem of salinewater disposal is created. Intercep-tion of lateral water with deeprooted crops may be short livedif salts build up in the root zone.Interception methods fail to deal withthe source of the problem, rechargefrom locations farther upslope.

Road Ditch and LagoonMargin Management forSalinity

Lateral movement of water tosoils adjacent to road ditches canbe reduced by preventing waterfrom standing in the drainage ditches.Ditches should be designed andmaintained to move water rapidly andminimize standing water. Lagoonsshould be designed to preventleakage that causes salinization ofadjacent soils.

Irrigation Management toPrevent SecondarySalinizationLEACHING AND DRAINAGE

When subsurface drainage isadequate, salt accumulation inirrigated soils can be avoided byapplying more water than is neededby the crop. Excess water dissolvessalts and percolates beyond the rootzone (Figure 9).

The amount of water needed toadequately leach the salts (leachingrequirement) is determined by thequality of the irrigation water. Poorquality water (high salinity) dictates alarger leaching requirement. Propersubsurface drainage is absolutelynecessary to prevent shallow watertables from developing belowirrigated soils. A shallow water tableunder an irrigated soil defeats theapplication of leaching water, becauseleached salts are moved back into therooting zone by capillary action(Figure 9).

FIELD LEVELLINGSecondary salinization can occur

from uneven distribution of irrigationwater due to irregular topography.Microdepressions act as points offocused recharge; salts are leachedfrom the recharge locations (Figure

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12). Adjacent microknolls, however,act as points of focused evaporativedischarge; salts accumulate and maycause salinity problems at thedischarge locations (Figure 12).Levelling irrigated fields allows moreeven water distribution and avoidsconcentration of water and salts atspecific places in the field (Figure 12).

REFERENCESBenz, L. C., R. H. Mickelson, F. M. Sandoval,

and C. W. Carlson. 1961. Ground-waterinvestigations in a saline area of the RedRiver Valley, North Dakota. J. Geo. Res.66:2435-2443.

Brun, L. J. and B. K. Worcester. 1974. Therole of alfalfa in saline seep prevention. ND.Farm Res. 31:5:9-14.

Brun, L. J. and B. K. Worcester. 1975. Soilwater extraction by alfalfa. Agron. J. 67:586-588.

Deutsch, R. L. 1977. Detection and character-ization of saline seeps in western NorthDakota. MS. Thesis, Soil Sci. Dep., NorthDakota State University, Fargo.

Doering, E. J. and F. M. Sandoval. 1976.Hydrology of saline seeps in the northernGreat Plains. Trans. Am. Soc. Agric. Eng.19:856-865.

Keller, L. P., G. J. McCarthy, and J. L.Richardson. 1986. Mineralogy and stability ofsoil evaporites in North Dakota. Soil Sci.Soc. Am. J. 50:1069-1071.

Maianu, A. 1983. Salt tolerance of hard redspring wheat. In proceedings annualAgricultural Short Course and Trade Show,North Dakota Agric. Assoc. 311-312.

Maianu, A. 1984. Salt tolerance of soybeans inthe Red River Valley. In proceedings annualAgricultural Short Course and Trade Show,North Dakota Agric. Assoc. 170-171.

Maianu, A., J. R. Lukach. 1985. Salt toleranceof barley. In proceedings annual AgriculturalShort Course and Trade Show, NorthDakota Agric. Assoc. 102-103.

Rengasamy, P., R. S. B. Greene, and G. W.Ford. 1984. The role of clay fraction in theparticle arrangement and stability of soilaggregates - a review. Clay Res. 3:2:53-67.

Sandoval, F. M. 1978. Deep plowing improvessodic claypan soils. ND. Farm Res. 35:4:15-18.

Seelig, B. D. 1978. Hydrology and stratigraphyof saline seeps. MS. Thesis, Soil Sci. Dep.,North Dakota State University, Fargo.

Seelig, B. D., J. L. Richardson, and W. T.Barker. 1990. Charactersitics and taxonomyof sodic soils as a function of landformposition. Soil Sci. Soc. Am. J. 54:1690-1697.

Figure 12. Secondary salinization in irrigated fields at points of evaporative discharge (A)may be eliminated by even distribution of irrigation water after field levelling (B).

Seelig, B. D., R. Carcoana, and A. Maianu.1987. Identification of critical depth andcritical salinity of the groundwater on NorthDakota high water table cropland: Finalproject report to the North Dakota WaterResources Research Institute, Fargo.

Skarie, R. L., J. L. Richardson, A. Maianu, andG. K. Clambey. 1986. Soil and groundwatersalinity along drainage ditches in easternNorth Dakota. J. Env. Quality 15:335-340.

Soil Survey Staff, North Dakota StateUniversity and Soil Conservation Service.1963. General soil map North Dakota.Agric. Exp. Station, North Dakota StateUniversity, Fargo.

Soil Survey Staff. 1975. Soil taxonomy: A basicsystem of soil classification for making andinterpreting soil surveys. Agric. Handb. No.436, SCS, USDA. U.S. Gov. Print. Off.,Washington, D. C.

Soil Survey Staff. 1987. Sodic, sodic-saline, andsaline soils of North Dakota. Misc. Publ., SoilConservation Service, USDA, Bismarck, ND.

Soil Survey Staff. 1993. Soil Survey Manual,Hndbk 18, USDA, NRCS. U.S. Gov. Print.Off., Washington, D.C.

U.S. Salinity Laboratory Staff, 1954. Diagnosisand improvement of saline and alkali soils.Agric. Hanb. No. 60, USDA. U.S. Gov. Print.Off., Washington, D. C.

Worcester, B. K. and B. D. Seelig. 1976. Plantindicators of saline seep. ND. Farm Res.34:1:18-20.

Worcester, B. K., L. J. Brun, and E. J. Doering.1975. Classification and management ofsaline seeps in western North Dakota. ND.Farm Res. 33:1:3-7.

NDSU Extension Service, North Dakota State University of Agriculture and Applied Science, and U.S. Department of Agriculture cooperating.Sharon D. Anderson, Director, Fargo, North Dakota. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. We offerour programs and facilities to all persons regardless of race, color, sex, religion, age, national origin, or handicap; and are an equal opportunityemployer. This publication will be made available in alternative formats for people with disabilities upon request 701/231-7881.

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