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INTRODUCTION

As noted in previous chapters of this manual, the quality of irrigation

water is an important factor in determining sustainability of agriculture

on salt-impaired lands. For a number of reasons, the availability of low-

salinity irrigation supplies has led to (1) an interest in using alternative

supplies, such as recycled wastewaters, and (2) innovative plant and water

management strategies to mitigate the adverse effects of salt and specific-

ion stresses these poor-quality waters may impose on plant growth, yield,

and quality. A second motivating factor is the lack of suitable drainage

outlets in many agricultural areas of the world. Drainage of irrigated

lands is one of the requisites for sustaining agricultural productivity in a

given region over the long term. Adequate drainage not only allows for

better aeration in the crop rootzone but provides a means by which salin-

ity and toxic elements can be managed and controlled. Reuse of drainage

water for irrigation is one way of expanding the useable water supply

while at the same time reducing drainage volume. This chapter provides

a management perspective on (1) how plants respond to salinity and toxic

elements (e.g., Na



, Cl



, and B); (2) crop salt tolerance and the various fac-

tors that influence plant response to salinity; (3) the extent to which salin-

ity affects crop yields and quality; and (4) management strategies to opti-

mize yields by controlling soil salinity. It is not our intent to provide a

comprehensive review of physiological effects of salinity crops. That topic

is covered in detail in Chapter 6.

CHAPTER 13

PLANT SALT TOLERANCE

Catherine M. Grieve, Stephen R. Grattan, and Eugene V. Maas

405

Agricultural Salinity Assessment and Management

2246

Grieve, C.M., S.R. Grattan and E.V. Maas. 2012. Plant salt tolerance. In: W.W. Wallender and

K.K. Tanji (eds.) ASCE Manual and Reports on Engineering Practice No. 71 Agricultural Salinity

Assessment and Management (2nd Edition). ASCE, Reston, VA. Chapter 13 pp:405-459.

SOURCE AND CAUSES OF SOIL SALINITY

Salts found in soils and waters originate from parent rock material that

has undergone geochemical weathering. Over geologic time, primary

minerals have reacted with water, oxygen, and carbon dioxide to form

secondary minerals and salts that were transported by water to oceans or

depressions in the landscape. Inundation of large land masses by saline

seas deposited sedimentary materials that have become the major source

of salt in arid regions.

In coastal or delta regions, salination of soils may occur predominantly

by salts that contaminate freshwater supplies by seawater intrusion. Sea-

water intrusion can impair groundwater quality when wells are pumped

to the extent that they overdraft aquifers near coastal areas. Salinity in

freshwater channels near deltas is affected by the tide and can increase

dramatically during high tides when stream flows are low. Coastal agri-

culture may also be subjected to cyclic salts where saline aerosols are pro-

duced by violent wave activity during storms or high winds on the sea.

These salts can move inland considerable distances, but the most harmful

effects occur on vegetation or crops grown close to the shore.

Salts, however, can be found in groundwater at relatively high concen-

trations without originating from the sea. The concentration and composi-

tion of groundwater is largely dependent on the hydrological and geo-

chemical environment that the infiltrating water encounters en route to

the groundwater. This is particularly true in irrigated soils formed from

marine sediments. Salts contained in irrigation water, regardless of their

source, can salinize agricultural land if the mass of salts that moves out of

the rootzone is less than the mass of salts entering the rootzone for an

extended period of time. A favorable salt balance within the rootzone

must be maintained by adequate leaching.

In closed hydrologic basins, salts may have been present in the soil long

before irrigation was introduced to a region. Upon irrigation, saline water

tables can develop in poorly drained areas in relatively short time periods

(i.e., years). Even if good-quality water is used for irrigation, salination

may occur from capillary movement of salts and water to the surface from

rising saline water tables. Rising water tables are a result of excessive deep

percolation and are often associated with inefficient water-management

practices, such as overirrigation or inadequate drainage systems.

The two processes described, (1) salination from irrigation with saline

water, and (2) salination from shallow saline water tables, are the most

common cause of large-scale soil salination in irrigated agriculture. They

are, of course, not mutually exclusive, and highly saline water tables can

often occur from or in association with saline irrigation water. Other

processes of soil salination described in Chapter 1 occur on a smaller scale

and will not be discussed in this section.

406 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

PLANT SALT TOLERANCE

Plant salt tolerance is defined differently depending on the intended

use and value of the plant. For agricultural and horticultural crops, grow-

ers are most concerned with achieving economic yield and quality under

saline conditions. For landscape designers and managers, the ability of

the plant to maintain an aesthetic quality without excessive growth is of

primary concern. And for the ecologist, the interest is most often on plant

survival and species dominance in an environmentally sensitive area

affected by salinity. Therefore, no one definition is appropriate that covers

all interest groups.

Relative Yield–Response Curves for Agronomic and

Horticultural Crops

The salt tolerance of a crop can be described as a complex function of

yield decline across a range of salt concentrations (Maas and Hoffman

1977; Maas and Grattan 1999; van Genuchten and Hoffman 1984). Salt tol-

erance can be adequately described on the basis of two parameters:

threshold, the electrical conductivity (EC

) that is expected to cause the

initial significant reduction in the maximum expected yield, and slope,

the percentage of expected yield reduction per unit increase in salinity

above the threshold value.

There is considerable uncertainty regarding the yield-threshold soil-

salinity values. The salinity coefficients (yield threshold and slope values)

for the piece-wise linear slope-threshold model introduced by Maas and

Hoffman (1977) are now determined by nonlinear least-squares statisti-

cal fitting that determines the slope and threshold values from a particu-

lar experimental dataset. Despite intense control of salinity and all other

important variables related to plant yield in salt tolerance trials, for many

crops the standard errors associated with the threshold values can be

50% to 100% percent of the best-fit threshold value. Salinity studies on

rice grown in northern California, for example, resulted in a threshold

value of 1.9 dS/m of the field water (Grattan et al. 2002) with a 95% con-

fidence limit ranging between 0.6 and 3.2 dS/m (J. Poss, U.S. Salinity

Laboratory, personal communication, 2004). Obviously, very large

ranges of uncertainty exist and additional studies to resolve this theoret-

ical maximum are needed to refine water quality standards to a greater

degree of confidence.

One approach recently described by Steppuhn et al. (2005a,b) substi-

tutes a nonlinear relationship between relative yield and soil salinity sim-

ilar to the nonlinear model introduced by van Genuchten and Hoffman

(1984) for the linear “yield threshold” model. A curvilinear relationship

better describes relative crop yield data than does the yield-threshold

PLANT SALT TOLERANCE 407

Agricultural Salinity Assessment and Management

expression. In this curvilinear relationship, there is no longer a “yield

threshold” but, rather, a continuous decline in yield with increased soil

salinity.

Using nonlinear models, the numerically most reliable curve-fitting

parameter seems to be the value at which yield is reduced by 50% (C

The C

value can still be estimated when too few data points exist to pro-

vide reliable information on the threshold and slope. The set of equations

developed by van Genuchten and Hoffman (1984) takes advantage of the

stability of C

. The C

value, together with a response curve steepness

constant (p), may be obtained by fitting the appropriate function (van

Genuchten 1983) to observed salt tolerance response data (van Genuchten

and Gupta 1993). This approach has been used to develop a salt tolerance

index (ST-index) as a revised indicator of the inherent salinity tolerance or

resistance of agricultural crops to rootzone salinity (Steppuhn et al.

2005a,b).

Both nonlinear models as well as the piece-wise linear fit (Maas and

Hoffman 1977) describe the data extremely well (r

 0.9) (Steppuhn et al.

2005a,b). Water quality regulators prefer the latter since the concept of a

“threshold” value provides them with a regulatory limit to impose on

wastewater dischargers. The former, however, may best describe plant

response from a physiological perspective, but there remains some uncer-

tainty regarding the method that best describes the data in the relative

yield range of 100% to 80%, the range of interest to most users and regula-

tors. Since both curve-fitting methods describe the data well, we choose to

report the most comprehensive and historically familiar list of salinity

coefficients for the Maas-Hoffman model since this chapter is written as a

user’s manual.

Herbaceous crops

Table 13-1 lists threshold and slope values generated by the Maas-

Hoffman model for 81 crops in terms of seasonal average EC

in the crop

rootzone. Most of the data were obtained where crops were grown under

conditions simulating recommended cultural and management practices

for commercial production in the location tested. Consequently, the data

indicate relative tolerances of different crops grown under different con-

ditions and not under some standardized set of conditions. Furthermore,

the data apply only where crops are exposed to fairly uniform salinities

from the late seedling stage to maturity. Plants are likely to be more sensi-

tive to salinity than the tables indicate should crops be planted in initially

high-salinity conditions. Where crops have particularly sensitive stages,

the tolerance limits are given in the footnotes.

The data in Table 13-1 apply to soils where chloride (Cl) is the predom-

inant anion. Because of the dissolution of CaSO

when preparing satu-

rated-soil extracts, the EC

of gypsiferous, (nonsodic, low Mg

2

) soils will

408 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

409

TABLE 13-1. Salt Tolerance of Herbaceous Crops

Crop Salt-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name

Based On: (EC

) (dS/m) (% per dS/m) Rating

(1) (2) (3) (4) (5) (6)

Fiber, grain, and special crops

Artichoke, Jerusalem Helianthus tuberosus L. Tuber yield 0.4 9.6 MS

Barley

Hordeum vulgare L. Grain yield 8.0 5.0 T

Canola or rapeseed Brassica campestris L. 9.7 14 T

[syn. B. rapa L.] Seed yield

Canola or rapeseed B. napus L. Seed yield 11.0 13 T

Chick pea Cicer arietinum L. Seed yield — — MS

Corn

Zea mays L. Ear FW 1.7 12 MS

Cotton Gossypium hirsutum L. Seed yield 7.7 5.2 T

Crambe Crambe abyssinica Hochst. ex R.E. Fries Seed yield 2.0 6.5 MS

Flax Linum usitatissimum L. Seed yield 1.7 12 MS

Guar Cyamopsis tetragonoloba (L). Taub. Seed yield 8.8 17 T

Kenaf Hibiscus cannabinus L. Stem DW 8.1 11.6 T

Lesquerella Lesquerella fenderli (Gray) S. Wats. Seed yield 6.1 19 MT

Millet, channel Echinochloa turnerana (Domin) J.M. Black Grain yield — — T

Oats Avena sativa L. Grain yield — — T

Peanut Arachis hypogaea L. Seed yield 3.2 29 MS

Rice, paddy Oryza sativa L. Grain yield 3.0

Roselle Hibiscus sabdariffa L. Stem DW — — MT

Rye Secale cereale L. Grain yield 11.4 10.8 T

Safflower Carthamus tinctorius L. Seed yield — — MT

Sesame

Sesamum indicum L. Pod DW — — S

Sorghum Sorghum bicolor (L.) Moench Grain yield 6.8 16 MT

Soybean Glycine max (L.) Merrrill Seed yield 5.0 20 MT

Sugarbeet

Beta vulgaris L. Storage root 7.0 5.9 T

(continued)

Agricultural Salinity Assessment and Management

TABLE 13-1. Salt Tolerance of Herbaceous Crops

(Continued)

Crop Salt-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name

Based On: (EC

) (dS/m) (% per dS/m) Rating

(1) (2) (3) (4) (5) (6)

Sugarcane Saccharum officinarum L. Shoot DW 1.7 5.9 MS

Sunflower Helianthus annuus L. Seed yield 4.8 5.0 MT

Triticale X Triticosecale Wittmack Grain yield 6.1 2.5 T

Wheat Triticum aestivum L. Grain yield 6.0 7.1 MT

Wheat (semidwarf)

f,a

T. aestivum L. Grain yield 8.6 3.0 T

Wheat, Durum T. turgidum L. var. durum Desf. Grain yield 5.9 3.8 T

Grasses and forage crops

Alfalfa Medicago sativa L. Shoot DW 2.0 7.3 MS

Alkaligrass, Nuttall Puccinellia airoides (Nutt.) Wats. & Coult. Shoot DW — — T*

Alkali sacaton Sporobolus airoides Torr. Shoot DW — — T*

Barley (forage) Hordeum vulgare L. Shoot DW 6.0 7.1 MT

Bentgrass, creeping Agrostis stolonifera L. Shoot DW — — MS

Bermudagrass

Cynodon dactylon (L.) Pers. Shoot DW 6.9 6.4 T

Bluestem, Angleton Dichanthium aristatum (Poir.) C.E. Hubb. Shoot DW — — MS*

[syn. Andropogon nodosus (Willem.) Nash]

Broadbean Vicia faba L. Shoot DW 1.6 9.6 MS

Brome, mountain Bromus marginatus Nees ex Steud. Shoot DW — — MT*

Brome, smooth B. inermis Leyss Shoot DW — — MT

Buffelgrass Pennisetum ciliare (L). Link. — — MS*

[syn. Cenchrus ciliaris] Shoot DW

Burnet Poterium sanguisorba L. Shoot DW — — MS*

Canarygrass, reed Phalaris arundinacea L. Shoot DW — — MT

Clover, alsike Trifolium hybridum L. Shoot DW 1.5 12 MS

Clover, Berseem T. alexandrinum L. Shoot DW 1.5 5.7 MS

410

Agricultural Salinity Assessment and Management

Clover, Hubam Melilotus alba Dest. var. annua H.S.Coe Shoot DW — — MT*

Clover, ladino Trifolium repens L. Shoot DW 1.5 12 MS

Clover, Persian T. resupinatum L. Shoot DW — — MS*

Clover, red T. pratense L. Shoot DW 1.5 12 MS

Clover, strawberry T. fragiferum L. Shoot DW 1.5 12 MS

Clover, sweet Melilotus sp. Mill. Shoot DW — — MT*

Clover, white Dutch Trifolium repens L. Shoot DW — — MS*

Corn (forage)

Zea mays L. Shoot DW 1.8 7.4 MS

Cowpea (forage) Vigna unguiculata (L.) Walp. Shoot DW 2.5 11 MS

Dallisgrass Paspalum dilatatum Poir. Shoot DW — — MS*

Dhaincha Sesbania bispinosa (Linn.) W.F. Wight Shoot DW — — MT

[syn. Sesbania aculeata (Willd.) Poir]

Fescue, tall Festuca elatior L. Shoot DW 3.9 5.3 MT

Fescue, meadow Festuca pratensis Huds. Shoot DW — — MT*

Foxtail, meadow Alopecurus pratensis L. Shoot DW 1.5 9.6 MS

Glycine Neonotonia wightii [syn. Glycine wightii or Shoot DW — — MS

javanica]

Gram, black Vigna mungo (L.) Hepper [syn. Phaseolus Shoot DW — — S

or Urd bean

mungo L.]

Grama, blue Bouteloua gracilis (HBK) Lag. ex Steud. Shoot DW — — MS*

Guinea grass Panicum maximum Jacq. Shoot DW — — MT

Hardinggrass Phalaris tuberosa L. var. stenoptera (Hack) Shoot DW 4.6 7.6 MT

A. S. Hitchc.

Kallargrass Leptochloa fusca (L.) Kunth [syn. Diplachne Shoot DW — — T

fusca Beauv.]

Kikuyugrass Pennisetum clandestinum L. Shoot DW 8.0 T

Lablab bean Lablab purpureus (L.) Sweet [syn. Dolichos Shoot DW — — MS

lablab L.]

Lovegrass

Eragrostis sp. N. M. Wolf Shoot DW 2.0 8.4 MS

Milkvetch, Cicer Astragalus cicer L. Shoot DW — — MS*

Millet, Foxtail Setaria italica (L.) Beauvois Dry matter — — MS

411

(continued)

Agricultural Salinity Assessment and Management

TABLE 13-1. Salt Tolerance of Herbaceous Crops

(Continued)

Crop Salt-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name

Based On: (EC

) (dS/m) (% per dS/m) Rating

(1) (2) (3) (4) (5) (6)

Oatgrass, tall Arrhenatherum elatius (L.) Beauvois ex Shoot DW — — MS*

J. Presl & K. Presl

Oats (forage) Avena sativa L. Straw DW — — T

Orchardgrass Dactylis glomerata L. Shoot DW 1.5 6.2 MS

Panicum, Blue Panicum antidotale Retz. Shoot DW — — MS*

Pea, Pigeon Cajanus cajan (L.) Huth [syn. C. indicus Shoot DW — — S

(K.) Spreng.]

Rape (forage) Brassica napus L. — — MT*

Rescuegrass Bromus unioloides HBK Shoot DW — — MT*

Rhodesgrass Chloris Gayana Kunth. Shoot DW — — MT

Rye (forage) Secale cereale L. Shoot DW 7.6 4.9 T

Ryegrass, Italian Lolium multiflorum Lam. Shoot DW — — MT*

Ryegrass, perennial Lolium perenne L. Shoot DW 5.6 7.6 MT

Ryegrass, Wimmera L. rigidum Gaud. — — MT*

Salt grass, desert Distichlis spicta L. var. stricta (Torr.) Bettle Shoot DW — — T*

Sesbania Sesbania exaltata (Raf.) V.L. Cory Shoot DW 2.3 7.0 MS

Sirato Macroptilium atropurpureum (DC.) Urb. Shoot DW — — MS

Sphaerophysa Sphaerophysa salsula (Pall.) DC Shoot DW 2.2 7.0 MS

Sudangrass Sorghum sudanense (Piper) Stapf Shoot DW 2.8 4.3 MT

Timothy Phleum pratense L. Shoot DW — — MS*

Trefoil, big Lotus pedunculatus Cav. Shoot DW 2.3 19 MS

Trefoil, narrowleaf L. corniculatus var tenuifolium L. Shoot DW 5.0 10 MT

birdsfoot

Trefoil, broadleaf L. corniculatus L. var arvenis (Schkuhr) Shoot DW — — MS

birdsfoot Ser. ex DC

412

Agricultural Salinity Assessment and Management

Vetch, common Vicia angustifolia L. Shoot DW 3.0 11 MS

Wheat (forage)

Triticum aestivum L. Shoot DW 4.5 2.6 MT

Wheat, Durum (forage) T. turgidum L. var durum Desf. Shoot DW 2.1 2.5 MT

Wheatgrass, standard Agropyron sibiricum (Willd.) Beauvois Shoot DW 3.5 4.0 MT

crested

Wheatgrass, fairway A. cristatum (L.) Gaertn. Shoot DW 7.5 6.9 T

crested

Wheatgrass, intermediate A. intermedium (Host) Beauvois Shoot DW — — MT*

Wheatgrass, slender A. trachycaulum (Link) Malte Shoot DW — — MT

Wheatgrass, tall A. elongatum (Hort) Beauvois Shoot DW 7.5 4.2 T

Wheatgrass, western A. smithii Rydb. Shoot DW — — MT*

Wildrye, Altai Elymus angustus Trin. Shoot DW — — T

Wildrye, beardless E. triticoides Buckl. Shoot DW 2.7 6.0 MT

Wildrye, Canadian E. canadensis L. Shoot DW — — MT*

Wildrye, Russian E. junceus Fisch. Shoot DW — — T

Vegetable and fruit crops

Artichoke Cynara scolymus L. Bud yield 6.1 11.5 MT

Asparagus Asparagus officinalis L. Spear yield 4.1 2.0 T

Bean, common Phaseolus vulgaris L. Seed yield 1.0 19 S

Bean, lima P. lunatus L. Seed yield — — MT

Bean, mung Vigna radiata (L.) R. Wilcz. Seed yield 1.8 20.7 S

Cassava Manihot esculenta Crantz Tuber yield — — MS

Beet, red

Beta vulgaris L. Storage root 4.0 9.0 MT

Broccoli Brassica oleracea L. (Botrytis Group) Head FW 1.3 15.8 MT

Brussels Sprouts B. oleracea L. (Gemmifera Group) — — MS*

Cabbage B. oleracea L. (Capitata Group) Head FW 1.8 9.7 MS

Carrot Daucus carota L. Storage root 1.0 14 S

Cauliflower Brassica oleracea L. (Botrytis Group) 1.5 14.4 MS*

Celery Apium graveolens L. var dulce (Mill.) Pers. Petiole FW 1.8 6.2 MT

Corn, sweet Zea mays L. Ear FW 1.7 12 MS

Cowpea Vigna unguiculata (L.) Walp. Seed yield 4.9 12 MT

413

(continued)

Agricultural Salinity Assessment and Management

TABLE 13-1. Salt Tolerance of Herbaceous Crops

(Continued)

Crop Salt-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name

Based On: (EC

) (dS/m) (% per dS/m) Rating

(1) (2) (3) (4) (5) (6)

Cucumber Cucumis sativus L. Fruit yield 2.5 13 MS

Eggplant Solanum melongena L. var esculentum Nees. Fruit yield 1.1 6.9 MS

Fennel Foeniculum vulgare Mill. Bulb yield 1.4 16 S

Garlic Allium sativum L. Bulb yield 3.9 14.3 MS

Gram, black Vigna mungo (L.) Hepper [syn. Phaseolus Shoot DW — — S

or Urd bean mungo L.]

Kale Brassica oleracea L. (Acephala Group) — — MS*

Kohlrabi Brassica oleracea L. (Gongylodes Group) — — MS*

Lettuce Lactuca sativa L. Top FW 1.3 13 MS

Muskmelon Cucumis melo L. (Reticulatus Group) Fruit yield 1.0 8.4 MS

Okra Abelmoschus esculentus (L.) Moench Pod yield — — MS

Onion (bulb) Allium cepa L. Bulb yield 1.2 16 S

Onion (seed) Seed yield 1.0 8.0 MS

Parsnip Pastinaca sativa L. — — S*

Pea Pisum sativum L. Seed FW 3.4 10.6 MS

Pepper Capsicum annuum L. Fruit yield 1.5 14 MS

Pigeon pea Cajanus cajan (L.) Huth [syn. C. indicus Shoot DW — — S

(K.) Spreng.]

Potato Solanum tuberosum L. Tuber yield 1.7 12 MS

Pumpkin Cucurbita pepo L. var Pepo ——MS*

Purslane Portulaca oleracea L. Shoot FW 6.3 9.6 MT

Radish Raphanus sativus L. Storage root 1.2 13 MS

Spinach Spinacia oleracea L. Top FW 2.0 7.6 MS

Squash, scallop Cucurbita pepo L. var melopepo (L.) Alef. Fruit yield 3.2 16 MS

Squash, zucchini C. pepo L. var melopepo (L.) Alef. Fruit yield 4.9 10.5 MT

414

Agricultural Salinity Assessment and Management

Strawberry Fragaria x Ananassa Duch. Fruit yield 1.0 33 S

Sweet potato Ipomoea batatas (L.) Lam. Fleshy root 1.5 11 MS

Swiss chard Beta vulgaris L. Top FW 7.0 5.7 T

Tepary bean Phaseolus acutifolius Gray — — MS

Tomato Lycopersicon lycopersicum (L.) Karst. ex Farw. Fruit yield 2.5 9.9 MS

[syn. Lycopersicon esculentum Mill.]

Tomato, cherry L. lycopersicum var. Cerasiforme (Dunal) Alef. Fruit yield 1.7 9.1 MS

Turnip Brassica rapa L. Storage root 0.9 9.0 MS

Turnip (greens) (Rapifera Group)

Top FW 3.3 4.3 MT

Watermelon Citrullus lanatus (Thunb.) Matsum. & Nakai Fruit yield — — MS*

Winged bean Psophocarpus tetragonolobus L. DC Shoot DW — — MT

These data serve only as a guideline to relative tolerances among crops. Absolute tolerances vary, depending on climate, soil conditions, and cultural

practices.

Botanical and common names follow the convention of Hortus Third (Liberty Hyde Bailey Hortorium Staff, 1976) where possible.

In gypsiferous soils, plants will tolerate EC

of about 2 dS/m higher than indicated.

Ratings with an * are estimates.

Less tolerant during seedling stage; EC

at this stage should not exceed 4 or 5 dS/m.

Unpublished U. S. Salinity Laboratory data

Grain and forage yields of DeKalb XL-75 grown on an organic muck soil decreased about 26% per dS/m above a threshold of 1.9 dS/m.

Because paddy rice is grown under flooded conditions, values refer to the electrical conductivity of the soil-water while the plants are submerged.

Less tolerant during seedling stage.

Sesame cultivars Sesaco 7 and 8 may be more tolerant than indicated by the S rating.

Sensitive during germination and emergence; EC

should not exceed 3 dS/m.

Data from one cultivar, Probred.

Average of several varieties. Suwannee and Coastal are about 20% more tolerant, and common and Greenfield are about 20% less tolerant than the

average.

Average for Boer, Wilman, Sand, and Weeping cultivars. Lehmann seems about 50% more tolerant.

*Estimated.

DW, dry weight; FW, fresh weight; S, sensitive; MS, moderately sensitive; MT, moderately tolerant; T, tolerant.

Adapted from Maas and Grattan (1999).

415

Agricultural Salinity Assessment and Management

range from 1 to 3 dS m

1

higher than that of nongypsiferous soils having

a similar soil water conductivity value at field capacity (Bernstein 1962).

The extent of this dissolution depends on the exchangeable ion composi-

tion, cation exchange capacity, and solution composition. Therefore,

plants grown on gypsiferous soils will tolerate EC

s approximately 2 dS

1

higher than those listed in Table 13-1. The last column provides a

qualitative salt tolerance rating that is useful in categorizing crops in gen-

eral terms. The limits of these categories are illustrated in Fig. 13-1. Some

crops are listed with only a qualitative rating, because experimental data

are inadequate to calculate the threshold and slope.

The salt tolerance parameters shown in Table 13-1 are given in terms of

the relative, rather than absolute, yield response under salinity, and for

that reason may be somewhat misleading for growers in selecting crops

for maximum yield and profitability given specific saline field conditions.

Comparison of the relative and absolute yields of alfalfa, a high-value,

high-quality leguminous forage, and tall wheatgrass, a forage of moder-

ate value and quality, provides a good illustration. The salt tolerance

threshold and slope, expressed on a relative yield basis, for alfalfa are 2 dS

1

and 7.3%, respectively; the crop is rated as moderately salt-sensitive.

Tall wheatgrass, conversely, is considerably more tolerant to salinity;

threshold-slope values are 7.5 dS m

1

and 4.2%, respectively (Table 13-1).

416 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

FIGURE 13-1. Divisions for classifying crop tolerance to salinity. From Maas

and Hoffman (1977).

Agricultural Salinity Assessment and Management

A year-long greenhouse sand culture experiment was conducted at the

U.S. Salinity Laboratory in which both crops were irrigated with waters at

two salinity levels: 15 and 25 dS m

1

(Grattan et al. 2004a). In this system,

irrigation water salinity (EC

) is equivalent to that of the sand water

(EC

) which, in turn, is approximately 2.2 times the EC of the saturated-

soil extract (EC

). The soil-water dynamics of the sand are similar to those

found in field soils (Wang 2002). The salinity treatments were, therefore,

estimated at 7.0 and 11.7 dS m

1

expressed as EC

. The threshold-slope

model predicts that at the lower salinity (7 dS m

1

), the relative yield of

tall wheatgrass would not be reduced at all, but that alfalfa yield would

be reduced more than 40%. However, annual absolute biomass produc-

tion for alfalfa irrigated with 15-dS m

1

waters was 28 t ha

1

, whereas tall

wheatgrass produced 20 t ha

1

in the same treatment. As irrigation water

salinity increased to 25 dS m

1

, the relative salt sensitivity of alfalfa

became obvious inasmuch as absolute biomass was reduced by nearly

50% (15 t ha

1

). In contrast, biomass of tall wheatgrass was reduced only

15% in response to the higher salinity. It is likely that if salinity increased

even further (e.g., to EC

 30 dS m

1

) the survivability of alfalfa would be

in question, although tall wheatgrass would, in all probability, maintain

reasonable biomass production. Therefore, both salt tolerance and absolute

biomass production must be considered in crop selection. Clearly the

grower must have a priori knowledge of the overall crop production

potential in order to make an appropriate crop selection for anticipated

saline field conditions.

Woody crops

The salt tolerance of trees, vines, and other woody crops is complicated

because of additional detrimental effects caused by specific-ion toxicities.

Many perennial woody species are susceptible to foliar injury caused by

the toxic accumulation of Cl



and/or Na



in the leaves. Because different

cultivars and rootstocks absorb and transport Cl



and Na



at different

rates, considerable variation in tolerance may occur within an individual

species. Tolerances to these specific ions are discussed in the following.

In the absence of specific-ion effects, the salt tolerance of woody crops,

like that of herbaceous crops, can be expressed as a function of the con-

centration of total soluble salts or osmotic potential of the soil solution.

One could expect this response to be obtained for those cultivars and root-

stocks that restrict the uptake of Cl



and Na



. The salt tolerance data

given in Table 13-2 for woody crops are believed to be reasonably accu-

rate in the absence of specific-ion toxicities. Because of the cost and time

required to obtain fruit yields for extended periods of time (i.e., multiple

years), particularly for alternate-bearing trees, tolerances of woody crops

have been determined for vegetative growth only. In contrast to other

PLANT SALT TOLERANCE 417

Agricultural Salinity Assessment and Management

418

TABLE 13-2. Salt Tolerance of Woody Crops

Crop Salt-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name

Based On: (EC

) (dS/m) (% per dS/m) Rating

(1) (2) (3) (4) (5) (6)

Almond Prunus duclis (Mill.) D.A. Webb Shoot growth 1.5 19 S

Apple Malus sylvestris Mill. — — S

Apricot Prunus armeniaca L. Shoot growth 1.6 24 S

Avocado Persea americana Mill. Shoot growth — — S

Banana Musa acuminata Colla Fruit yield — — S

Blackberry Rubus macropetalus Doug. ex Hook Fruit yield 1.5 22 S

Boysenberry Rubus ursinus Cham. and Schlechtend Fruit yield 1.5 22 S

Castorbean Ricinus communis L. — — MS

Cherimoya Annona cherimola Mill. Foliar injury — — S

Cherry, sweet Prunus avium L. Foliar injury — — S

Cherry, sand Prunus besseyi L., H. Baley Foliar injury, stem growth — — S

Coconut Cocos nucifera L. — — MT

Currant Ribes sp. L. Foliar injury, stem growth — — S

Date palm Phoenix dactylifera L. Fruit yield 4.0 3.6 T

Fig Ficus carica L. Plant DW — — MT

Gooseberry Ribes sp. L. — — S

Grape Vitis vinifera L. Shoot growth 1.5 9.6 MS

Grapefruit Citrus x paradisi Macfady Fruit yield 1.2 13.5 S

Guava Psidium guajava L. Shoot and root growth 4.7 9.8 MT

Guayule Parthenium argentatum A. Gray Shoot DW 8.7 11.6 T

Rubber yield 7.8 10.8 T

Jambolan plum Syzygium cumini L. Shoot growth — — MT

Jojoba Simmondsia chinensis (Link) C. K. Schneid Shoot growth — — T

Jujube, Indian Ziziphus mauritiana Lam. Fruit yield — — MT

Lemon Citrus limon (L.) Burm. f. Fruit yield 1.5 12.8 S

Lime Citrus aurantiifolia (Christm.) Swingle — — S

Loquat Eriobotrya japonica (Thunb). Lindl. Foliar injury — — S

Agricultural Salinity Assessment and Management

Macadamia Macadamia integrifolia Maiden & Betche Seedling growth — — MS

Mandarin orange; tangerine Citrus reticulata Blanco Shoot growth — — S

Mango Mangifera indica L. Foliar injury — — S

Natal plum Carissa grandiflora (E.H. Mey.) A. DC. Shoot growth — — T

Olive Olea europaea L. Seedling growth, Fruit yield — — MT

Orange Citrus sinensis (L.) Osbeck Fruit yield 1.3 13.1 S

Papaya Carica papaya L. Seedling growth, foliar injury — — MS

Passion fruit Passiflora edulis Sims — — S

Peach Prunus persica (L.) Batsch Shoot growth, Fruit yield 1.7 21 S

Pear Pyrus communis L. — — S

Pecan Carya illinoinensis (Wangenh.) C. Koch Nut yield, trunk growth — — MS

Persimmon Diospyros virginiana L. — — S

Pineapple Ananas comosus (L.) Merrill Shoot DW — — MT

Pistachio Pistacia vera L. Shoot growth — — MS

Plum; Prune Prunus domestica L. Fruit yield 2.6 31 MS

Pomegranate Punica granatum L. Shoot growth — — MS

Popinac, white Leucaena leucocephala (Lam.) de Wit Shoot DW — — MS

[syn. Leucaena glauca Benth.]

Pummelo Citrus maxima (Burm.) Foliar injury — — S

Raspberry Rubus idaeus L. Fruit yield — — S

Rose apple Syzygium jambos (L.) Alston Foliar injury — — S

Sapote, white Casimiroa edulis Llave Foliar injury — — S

Scarlet wisteria Sesbania grandiflora Shoot DW — — MT

Tamarugo Prosopis tamarugo Phil Observation — — T

Walnut Juglans spp. Foliar injury — — S

These data serve only as a guideline to relative tolerances among crops. Absolute tolerances vary, depending on climate, soil conditions, and cultural

practices. The data are applicable when rootstocks are used that do not accumulate Na



or Cl



rapidly or when these ions do not predominate in the

soil.

Botanical and common names follow the convention of Hortus Third (Liberty Hyde Bailey Hortorium Staff, 1976) where possible.

In gypsiferous soils, plants will tolerate EC

about 2 dS/m higher than indicated.

Ratings with an * are estimates.

*Estimated.

DW, dry weight; FW, fresh weight; S, sensitive; MS, moderately sensitive; MT, moderately tolerant; T, tolerant.

Adapted from Maas and Grattan (1999).

419

Agricultural Salinity Assessment and Management

crop groups, most woody fruit and nut crops tend to be salt-sensitive,

even in the absence of specific-ion effects. Only the date palm is rated as

relatively salt-tolerant. Olives, pistachios, and a few others are believed to

be tolerant to moderately high salinity, at least during the first few years

of growth. As time under exposure to salts increased, however, tolerance

may decline due to progressive toxic levels of salts accumulated in leaves

or woody tissues. The long-term effects of olives grown under field con-

ditions provide a striking example. Two years after planting and imposi-

tion of salt stress, the olive cultivar Arbequina was rated in 1999 as salt-

tolerant with a threshold (EC

) of 6.7 dS m

1

. In 2000, the threshold

decreased to 4.7 dS m

1

. By 2001, Arbequina was rated as moderately salt-

sensitive as the threshold declined to 3.0 dS m

1

(Aragüés et al. 2005).

However, this decline in salt tolerance over the years was not observed in

plums. The more salt-sensitive Santa Rosa plum (Prunus salicina Lindl) on

Marianna 2624 rootstock (P. cerasifera Ehrh.  P munsoniana Wight and

Hedr.) showed little change in tolerance due to age. At the end of a 6-year

field trial, the salt tolerance parameters (i.e., threshold  2.6 dS m

1

; slope

 31%) based on fruit yield determined after the first 3 years of the trial

(1984–1985) were not significantly different than those obtained for

1987–1989 (Catlin et al. 1993).

Quality of salt-stressed agronomic and horticultural crops

While crop salt tolerance is based solely on yield, salinity adversely

affects the quality of some crops. By decreasing the size and/or quality of

fruits, tubers, or other edible organs, salinity reduces the market value of

many vegetables, such as carrots, celery, cucumbers, peppers, potatoes,

head cabbage and lettuce, artichoke, and yams (Bernstein et al. 1951;

Bernstein 1964; Francois and West 1982; Francois 1991, 1995). Rye grown

on saline soils produces grain with poorer bread-baking quality (Francois

et al. 1989). Salinity appears to have only limited effect on the quality of

citrus fruit (Maas 1993).

Not all the effects of salinity on crop quality are negative, however

(Grieve 2010). Salinity often confers beneficial effects on crops, which may

translate into economic advantages (Pasternak and De Malach 1994). Salin-

ity can increase yields in crops that show a strong competition for photo-

synthates between vegetative and reproductive structures. In certain

crops, salt stress can slow growth of the vegetative parts, allowing the

excess photosynthates to flow to the generative organs. Cotton is a good

example of such a crop. Saline water (EC

 4.4 dS/m) irrigation resulted

in 15% increases in fruit dry matter (g/plant) and number of bolls on fruit-

ing branches as well as a 20% increase in boll number per plant (Pasternak

et al. 1979). Although final internode number was reduced by 11%, reduc-

tion in total dry matter yield and plant height was not significant.

420 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

In addition to the conservation of high-quality waters, the con-

trolled use of degraded waters offers a second benefit by providing a

unique opportunity for the production of value-added crops with health-

promoting constituents (Grieve 2010). Many plants adapt to salt stress by

enhanced biosynthesis of secondary metabolites, such as soluble solids,

sugars, organic acids, proteins, and amino acids (Ashraf and Harris 2004),

which may act as osmolytes or osmoregulators to maintain plant turgor

under salt stress. The presence of these metabolites often greatly increases

the nutritive quality and marketability of fruits and vegetables (Mizrahi

and Pasternak 1985). Beneficial effects include increased sugar concentra-

tion of carrots (Bernstein and Ayers 1953) and asparagus (Francois 1987),

increased total soluble solids in tomatoes (Adams and Ho 1989; Krauss

et al. 2006; Campos et al. 2006), muskmelon (Shannon and Francois 1978;

Botia et al. 2005; Colla et al. 2006), cucumber (Chartzoulakis 1992; Tra-

jkova et al. 2006), mandarin orange (Garcia-Sanchez et al. 2006), and

improved grain quality and protein content of durum (Francois et al.

1986) and bread wheat (Rhoades et al. 1988). Salt-stress may increase

firmness and improve postharvest handling characteristics in eggplants

(Sifola et al. 1995), strawberries (Sarooshi and Cresswell 1994), tomatoes

(Krauss et al. 2006), and melons (Navarro et al. 1999). Onion bulb pun-

gency may be reduced by salt-stress, although the content of flavor pre-

cursors often increases (Chang and Randle 2005). Salinity may also cause

oxidative stress and induce production of reactive oxygen species, which

are damaging to all classes of biomolecules. The primary defensive

plant response to oxidative stress is the biosynthesis of antioxidants

(Bartosz 1997). As a result, salt-stressed plants often contain enhanced

concentrations of antioxidants, such as flavonoids, ascorbate, tocopherols,

carotenoids, and lycopene. With proper management practices, it is likely

that economic losses associated with yield reductions due to salinity may

be offset by production of high-quality food crops that can be marketed

at a premium to meet the changing demands of the market and health-

conscious consumers (Cuarto and Fernández-Muñoz 1999; De Pascale

et al. 2001).

Ornamental and landscape species

Research on the salt tolerance of floriculture species continues to be

largely devoted to providing information that would help commercial

growers maintain crop productivity, quality, and profitability if recycled

waters are used for irrigation. Quality standards for landscape use are far

less stringent than those required by the floriculture industry. For exam-

ple, a major quality determinant for important cut flowers is stem length,

a growth parameter that is generally reduced when the plant is chal-

lenged by salinity. In their drive for high-quality products suitable for

PLANT SALT TOLERANCE 421

Agricultural Salinity Assessment and Management

premium markets, commercial growers would likely use the highest-

quality water available to maximize inflorescence length, flower diame-

ter, and plant height. However, a flowering stalk of stock (Matthiola

incana), a moderately salt-sensitive crop, would still be aesthetically

acceptable for landscape purposes if, compared to a premium-grade stalk,

the flowering stem was 5 cm shorter and the inflorescence contained one

or two fewer florets. Minimal reduction in growth and flowering capacity

should be permissible, provided that the overall health of the plant is not

compromised, the stems are robust, the colors of the leaves and flowers

remain true, and no visible leaf or flower damage due to salt stress is evi-

dent. For landscape purposes, stock is rated as very salt-tolerant.

Applying salt-tolerance criteria derived from the ecophysiological

literature to landscape plants sometimes results in completely mislead-

ing tolerance ratings. The performance of statice (Limonium spp.) under

saline conditions provides a good example. In HALOPH, a database

of salt-tolerant plants of the world, Aronson (1989) lists more than

50 species of Limonium. The commercially important species, L. perezii and

L. sinuatum, are listed among those that will complete their life cycles in

waters more salty than seawater (e.g., EC  50 dS/m). That these species

grow to maturity under highly saline conditions is clearly a halophytic

characteristic. Although one would not expect either species to produce a

high-quality crop under irrigation with hypersaline waters, the question

arose: Could flowers suitable for the commercial market or for landscape

purposes be produced at lower salinities, for example, in the range of

20 to 30 dS m

1

? To answer this question, both statice species were grown

under irrigation with waters ranging from 2 to 30 dS m

1

(Grieve et al.

2005; Carter et al. 2005). These trials confirmed that both species were

halophytic; both flowered and set seed in all treatments. However, nei-

ther species possessed a high degree of salt tolerance as understood by

horticulturists and agronomists whose research focuses on crop yield

and quality. Growth response of statice more closely resembled that of

glycophytic plants. Height of the flowering stalks decreased consistently

as salinity increased. Those plants receiving the 30-dS m

1

treatment

were only one-quarter as tall as those irrigated with nonsaline waters.

The salt tolerance of both species, rated for commercial production on the

basis of stem length, is correctly rated as low (Farnham et al. 1985).

Reduction in stem length should not necessarily be the limiting factor in

species selection for landscape plants, however. Even under severe salt

stress, both ‘American Beauty’ and ‘Blue Seas’ produced acceptable,

healthy plants with attractive foliage and colorful inflorescences on sturdy,

albeit short, stems. For landscape purposes, the species fall in the “very

tolerant” category.

Many examples are available illustrating that the effects of salinity on

landscape plants are not always adverse. Salt-related stress can benefi-

422 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

cially affect quality and disease resistance of plants. If plant aesthetics are

not compromised, salt-stressed landscape plants will be slower growing,

requiring less trimming and maintenance. In some instances, the uptake

and accumulation of salinizing ions stimulates growth. Cabrera (2000) and

Cabrera and Perdomo (2003) observed a positive correlation between rela-

tively high leaf-Cl concentrations (0.45%) and dry weight for container-

grown rose (‘Bridal Pink’ on Rosa manetti rootstock). Yield and quality

were unaffected. Salinity imposed early in the life cycle of some cut-flower

species tends to limit vegetative growth, with favorable results. Salinity-

induced reduction of stem length may be beneficial in species such as

chrysanthemum, where tall, rangy cultivars are treated with growth regu-

lators to keep the plants compact and short. While plant height is often

reduced by moderate salinity, the length of time to maturity and the size of

developing floral buds generally remain unaffected by stress (Lieth and

Burger 1989).

Salt tolerance ratings of selected landscape species (Table 13-3) are

based on aesthetic value and survivability. In some cases, two contrasting

ratings are given. Differences may be due to variety, climatic, or nutri-

tional conditions under which the trials were conducted. In addition,

some of the ratings are derived from data collected from closely related

varieties of horticultural or agronomic value. There are no data, for exam-

ple, on the salt tolerance of ornamental brassicas, such a kale and cabbage,

but it would be reasonable to assume that their salt tolerance would not

differ very sharply from that of the same leafy vegetable crop grown

under field conditions in agricultural settings.

Excellent resources for additional information regarding the salt tol-

erance of landscape plants are the Salt Management Guide (Tanji 2007)

and Abiotic Disorders of Landscape Plants: A Diagnostic Guide (Costello

et al. 2003).

Potential uses of halophytes

A promising approach for the practical use of heavily salinized soils

and waters that are otherwise unsuitable for conventional agriculture is

the use of highly salt-tolerant plant species, (halophytes). Many halo-

phytes are valuable for economic reasons (human food, fodder, oil, fuel)

or for ecological reasons, such as dune stabilization, erosion control, CO

sequestration, reclamation, and desalinization (Koyro 2003). True halo-

phytes are defined as those plants that are able to survive and complete

their life cycles in hypersaline environments and whose maximum

growth occurs at a soil water salinity of ⬃20 dS/m (Salisbury 1995). Halo-

phytes have developed a number of morphological adaptations and

physiological mechanisms to avoid and resist salt stress: salt hairs and salt

glands, waxy cuticles, selective ion uptake, salt exclusion from different

PLANT SALT TOLERANCE 423

Agricultural Salinity Assessment and Management

424 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

TABLE 13-3. Salt Tolerance of Selected Landscape Plants

Botanical Name Common Name Salt Tolerance

(1) (2) (3)

Agapanthus orientalis Lily of the Nile sensitive

Ageratum houstonianum Ageratum moderately sensitive

Alstroemeria hybrids Inca lily, Peruvian lily very sensitive

Amaranthus hypochondriacus Pygmy Torch tolerant

Amaranthus tricolor Love-Lies-Bleeding tolerant

Anthurium andreanum Anthurium very sensitive

Antirrhinum majus Snapdragon tolerant

Artemesia stelleran Dusty Miller moderately sensitive

Begonia bunchii Begonia sensitive

Begonia Rex-cultorum Rex Begonia very sensitive

Begonia ricinifolia Begonia sensitive

Bouvardia longiflora Bouvardia moderately sensitive

Brassica oleracea Ornamental Cabbage sensitive

Brassica oleracea Ornamental Kale sensitive

Calendula officinalis Pot Marigold moderately tolerant

Callistephus chinensis China Aster moderately sensitive

moderately tolerant

Calocephalus brownii Cushion Bush moderately sensitive

Camellia japonica Camellia sensitive

Carathamus tinctorius Safflower moderately tolerant

Catharanthus roseus Vinca sensitive

Celosia argenta cristata Crested Coxcomb moderately sensitive

Celosia argenta cristata Chief Celosia tolerant

Cereus peruviana Apple Cactus moderately sensitive

Chlorophytum comosum St. Bernard’s Lily tolerant

Chrysanthemum morifolium Mum moderately tolerant

Clematis orientalis Clematis very tolerant

Coleus blumei Coleus tolerant

Codiaeum punctatus Croton moderately tolerant

Consolida ambigua Larkspur sensitive

Cosmos bipinnatus Cosmos very sensitive

Coreopsis grandiflora Coreopsis moderately sensitive

Crassula ovata Jade Plant moderately sensitive

Cyclamen persicum Cyclamen sensitive

Cymbidium spp. Orchid very sensitive

Dianthus barbatus Pinks moderately sensitive

Dianthus caryophyllus Carnation moderately tolerant

Dianthus chinensis Carnation moderately tolerant

Eschscholzia californica California Poppy moderately tolerant

Euphorbia pulcherrima Poinsettia ‘Red Sails’ sensitive

Euphorbia pulcherrima Poinsettia ‘Barbara Ecke’ very sensitive

Euryops pectinatus Golden Marguerite sensitive

Eustoma grandiforum Lisianthus moderately sensitive

Agricultural Salinity Assessment and Management

PLANT SALT TOLERANCE 425

TABLE 13-3. Salt Tolerance of Selected Landscape Plants (Continued)

Botanical Name Common Name Salt Tolerance

(1) (2) (3)

Felicia amelloides Felicia sensitive

Fuchsia hybrida Fuchsia very sensitive

Gardenia augusta Gardenia sensitive

Gazania aurantiacum Gazania moderately tolerant

Gerbera jamesonii Gerbera Daisy moderately sensitive

Gazania spp. Treasure Flower very tolerant

Gladiolus spp. Gladiola sensitive

Gomphrena globosa Globe Amaranth moderately sensitive

Gyposphila paniculata Baby’s Breath moderately tolerant

Helianthus annuus Sunflower moderately tolerant

Helianthus debilis Cucumber Leaf very tolerant

Hibiscus rosa-sinensis Hibiscus sensitive

Hippeastrum hybridum Amaryllis very sensitive

Hymenocallis keyensis Spiderlily moderately tolerant

Impatiens x hawkeri Impatiens sensitive

Kalanchoe spp. Kalanchoe moderately tolerant

Kochia childsii Kochia tolerant

Lathyrus japonica Sweet Pea moderately tolerant

Lilium spp. Asiatic Hybrid Lily sensitive

Lilium spp. Oriental Hybrid Lily sensitive

Limonium spp. Japanese Limonium very tolerant

Limonium latifolium Sea Lavender very tolerant

Limonium perezii Statice Sensitive; very tolerant

Limonium sinuatum Statice Sensitive; very tolerant

Lobularia maritima Sweet Alyssum moderately tolerant

Matthiola incana Stock very tolerant

Narcissus tazetta Paperwhite Narcissus sensitive

Oenthera speciosa Mexican Evening moderately tolerant

Primrose

Ophiopogon jaburan Giant Turf Lily moderately sensitive

Ornithogalum arabicum Arabian Star Flower very sensitive

Pelargonium x hortorum Geranium sensitive

Pelargonium domesticum Geranium tolerant

Pelargonium peltatum Ivy Geranium moderately tolerant

Petunia hybrida Petunia tolerant

Portulaca grandiflora Moss Rose very tolerant

Phalaenopsis hybrid Orchid very sensitive

Protea obtusifolia Protea moderately tolerant

Rhododendron hybrids Azalea moderately sensitive

Rhododendron obtusum Azalea sensitive

Rosa x hybrida Rose sensitive

Stapelia gigantea Starfish Flower moderately tolerant

(continued)

Agricultural Salinity Assessment and Management

426 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

TABLE 13-3. Salt Tolerance of Selected Landscape Plants (Continued)

Botanical Name Common Name Salt Tolerance

(1) (2) (3)

Strelitzia reginae Bird of Paradise very sensitive

Tagetes erecta Marigold moderately tolerant

Tagetes patula Marigold moderately tolerant

Trachelium caeruleum Blue Throatwort sensitive

Tropaeolum majus Nasturtium moderately sensitive

Vinca major Periwinkle moderately tolerant

Vinca minor Myrtle sensitive

Viola x wittrockiana Pansy sensitive

Zinnia elegans Zinnia moderately sensitive

Criteria for assigning salt tolerance: not more than 50% reduction in growth, no visually

observable foliar burn, and maximum permissible ECe (dS m

1

) as follows:

2 very sensitive

2–3 sensitive

3–4 moderately sensitive

4–5 moderately tolerant

5–6 tolerant

6 very tolerant

Adapted from Grieve et al. (2007) and Tanji (2007).

plant organs (root, stem, leaf or fruit), salt sequestration in vacuoles or in

senescent leaves, succulence, dilution of plant salt concentration by

increased growth, osmotic adjustment, compatible osmotic solutes, root

excretion of salts, and root molecular sieves (Ungar 1998).

Halophytes may also be of value in water treatment. Improvement of

water quality through the use of natural or constructed wetlands is a rel-

atively new concept for treating effluents from agricultural operations,

such as dairies, livestock feedlots (Ibekwe et al. 2003; Ibekwe et al. 2007;

Ray and Inouye 2007), and nurseries (Arnold et al. 2003). Wastewaters

from agricultural operations are generally brackish and typically con-

tain high levels of nutrients and other pollutants. Vegetation plays a sig-

nificant role in wastewater purification by reducing nitrogen and the

biochemical oxygen demand and removal of suspended solids (Gers-

berg et al. 1986). Certain aquatic plant species possess unique anatomi-

cal and morphological features that together with their pollutant

uptake capacity and survivability, make them of prime importance in

wetlands ecosystems. Wetland species improve water quality by direct

uptake of nutrients and also by reducing water velocity, which allows

suspended particles to settle (Ray and Inouye 2007). Ecologically valuable

species for these purposes include bulrush (Scirpus validus), common

tule (S. acutus), rush (Juncus balticus), spike rush (Eleocharis palustris),

Agricultural Salinity Assessment and Management

common reed (Phragmites communis), cattail (Typha latifolia), and carex

(Carex nebrascensis).

Additional information concerning ecologically important species

may be found in HALOPH (Aronson 1989). This compilation lists halo-

phytic crops by plant family and gives maximum reported salinity toler-

ance, geographical distribution, and potential economic uses for many

species. Another valuable resource is Cash Crop Halophytes (Lieth and

Mochtchenko 2003), which addresses topics, such as ecophysiological

research on salt tolerance of plants, halophyte utilization for reclama-

tion of soils, and sustainable systems under irrigation with seawater. A

CD-ROM accompanies the text and gives taxonomic classification and

highest reported salinity tolerance for more than 2,000 species.

SALINITY AND NUTRITIONAL IMBALANCE

Salinity can induce elemental nutrient deficiencies or imbalances in

plants depending on the ionic composition of the external solution.

These specific effects vary among species and even among varieties of a

given crop. The optimal concentration range for a particular nutrient ele-

ment in the soil solution depends on many factors, including salt concen-

tration and composition (Grattan and Grieve 1994). This is not surprising

since salinity affects nutrient ion activities and produces extreme ion

ratios (e.g., Na



/Ca

2

, Na



, Cl



/NO



) in the soil solution. Nutrient

imbalances in the plant may result from the effect of salinity on (1) nutri-

ent availability, (2) the uptake and/or distribution of a nutrient within

the plant, and/or (3) increasing the internal plant requirement for a

nutrient element resulting from physiological inactivation (Grattan and

Grieve 1999).

A substantial body of information in the literature indicates that nutri-

ent element acquisition by crops is reduced in saline environments,

depending, of course, on the nutrient element in question and the com-

position of the salinizing solution. The activity of a nutrient element in

the soil solution decreases as salinity increases, unless the nutrient in

question is part of the salinizing salts (e.g., Ca

2

, Mg

2

, or SO

2

). For

example, phosphate (P) availability is reduced in saline soils not only

because the ionic-strength effect reduces the activity of phosphate, but

also because its concentration is controlled by sorption processes and by

the precipitation of Ca-P minerals. Therefore, P concentrations in many

full-grown agronomic crops decrease as salinity increases (Sharpley et al.

1992). Soil salinity can affect nutrient acquisition by severely reducing

root growth. Reductions of 40% to 50% have been reported in root

weight and lengths of citrus and tomato (Zekri and Parsons 1990; Snapp

and Shennan 1992).

PLANT SALT TOLERANCE 427

Agricultural Salinity Assessment and Management

Other evidence indicates that salinity may cause some physiological

inactivation of P, thereby increasing the plant’s internal requirement for

this element (Awad et al. 1990). These investigators found that when

NaCl concentrations were increased from 10 to 100 mM, P concentration

in the youngest mature tomato leaf necessary to achieve 50% yield nearly

doubled. Moreover, at any given leaf P concentration, foliar symptoms of

P deficiency increased with increased NaCl salinity. It would not be sur-

prising to find similar relationships involving other crops or even other

nutrients.

Nutrient uptake and accumulation by plants is often reduced under

saline conditions as a result of competitive processes between the nutrient

and a major salt species. Although plants selectively absorb K



over Na



-induced K deficiencies can develop on crops under salinity stress by

Na salts (Janzen and Chang 1987). On the other hand, Cl



salts can reduce



uptake and accumulation in crops even though this effect may not

be growth-limiting (Munns and Termaat 1986).

Even under nonsaline conditions, significant economic losses of horti-

cultural crops have been linked to inadequate calcium (Ca

2

) nutrition

(Shear 1975). Many factors can influence the amount of plant-available

Ca. These include the total supply of Ca

2

, the nature of the counter-ions,

the pH of the substrate, and the ratio of Ca

2

to other cations in the irriga-

tion water (Grattan and Grieve 1999). Calcium-related disorders may

even occur in plants grown on substrates where the Ca

2

concentration

appears to be adequate (Pearson 1959; Bernstein 1975). Deficiency symp-

toms are generally caused by differences in Ca

2

partitioning to the grow-

ing regions of the plant. All plant parts—leaves, stems, flowers, fruits—

actively compete for the pool of available Ca

2

and each part influences

2

movement independently. Organs that are most actively transpiring

are those most apt to have the highest Ca

2

concentrations. In horticul-

tural plants whose marketable product consists primarily of large heads

enveloped by outer (“wrapper”) leaves [e.g., cabbage, lettuce, escarole, or

endive], excessive transpiration by the outer leaves diverts Ca

2

from the

rapidly growing meristematic tissue (Bangerth 1979). Calcium deficiency

appears as physiological disorders of the younger tissues: internal brown-

ing of cabbage and lettuce, blackheart of celery. Calcium deficiency disor-

ders may also occur in reproductive tissues and may reduce market qual-

ity: blossom-end rot of tomatoes, melons, and peppers; “soft-nose” of

mangoes and avocados; and cracking and “bitter pit” of apples. Arti-

chokes grown under arid but nonsaline conditions also exhibits Ca-defi-

ciency injury as necrosis of inner bracts. The incidence of the disorder

increased when salt-stress was imposed (Francois 1995).

Any hazard to horticultural crops that are susceptible to Ca-related dis-

orders in the absence of salinity becomes even greater under saline condi-

tions. As the salt concentration in the rootzone increases, the plant’s

428 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

requirement for Ca also increases (Bernstein 1975). At the same time, Ca

uptake from the substrate may be depressed because of ion interactions,

precipitation, and increases in ionic strength (Grattan and Grieve 1999).

Significant reduction in market quality and associated economic losses

occur when these susceptible crops are also challenged by salinity.

Sodium-induced Ca

2

deficiencies have been observed by many

researchers when the Na



/Ca

2

ratio in the solution, at a given salinity

level, increases above a threshold level. This is particularly true for many

crops in the grass family (e.g., corn, sorghum, rice, wheat, and barley) and

striking differences have been observed among species and cultivars. Cal-

cium deficiency may be related, at least in part, to the effect of Na



2

distribution within the plant. Some researchers found that Na



inhibits the radial movement of Ca

2

from the root epidermis to the root

xylem vessels (Lynch and Läuchli 1985), while others found that Ca

2

transport to meristematic regions and developing leaves was inhibited

(Maas and Grieve 1987; Grieve and Maas 1988). Salinity-induced Ca

2

deficiency has also been observed on crops from different families, such

as blossom-end rot in tomatoes and bell peppers and black heart in celery

(Geraldson 1957).

CROP RESPONSE TO SPECIFIC IONS AND ELEMENTS

In addition to osmotic effects that reduce plant biomass and yields and

salinity’s effect on mineral nutrition, specific ions (i.e., Na, Cl, and B) can

cause additional injury to the crops, causing further crop damage. These

specific ions will be discussed separately.

Sodium

Sodium is not considered an essential element for most crop plants, but

it does beneficially affect growth of some plants at concentrations below

the salt-tolerance threshold. At concentrations above the threshold, Na



can have both direct and indirect detrimental effects on plants. Direct

effects are caused by the accumulation of toxic levels of Na



and are gen-

erally limited to woody species. The ability of a plant to tolerate excessive

amounts of Na



varies widely among species and rootstocks. Na



injury

on avocados, citrus, and stone fruit is rather widespread and can occur at



concentrations as low as 5 mol m

3

in soil water. The symptoms may

not appear immediately after exposure to saline water, however. Initially,



is retained in the roots and lower trunk, but after 3 or 4 years the con-

version of sapwood to heartwood apparently releases the accumulated



, which is transported to the leaves and causes leaf burn (Bernstein

et al. 1956).

PLANT SALT TOLERANCE 429

Agricultural Salinity Assessment and Management

Indirect effects include both nutritional imbalance and impairment of

soil physical conditions. The nutritional effects of Na



are not simply

related to the exchangeable Na



percentage of soils but depend on the

concentrations of Na



, Ca

2

, and Mg

2

in the soil solution. In sodic, non-

saline soils, total soluble salt concentrations are low and, consequently,

2

and/or Mg

2

concentrations are often nutritionally inadequate. These

deficiencies, rather than Na



toxicity per se, are usually the primary cause

of poor plant growth among nonwoody species and, in many cases, woody

species as well. Furthermore, since Na



uptake by plants is strongly regu-

lated by Ca

2

in the soil solution, the presence of sufficient Ca

2

is essential

to prevent the accumulation of toxic levels of Na



. This is particularly

important with Na



-sensitive woody crops. As a general guide, Ca

2

and

2

concentrations in the soil solution above 1 mol m

3

each are nutri-

tionally adequate in nonsaline, sodic soils (Carter et al. 1979; Hanson 1983).

As the total salt concentration increases into the saline-sodic range,

2

concentrations become adequate for most plants and osmotic effects

begin to predominate. However, some species are susceptible to salinity-

induced Ca

2

deficiencies as previously indicated. Therefore, for most

crops species, rather than having tolerance limits for Na



per se, it would

be more valuable to list a favorable Na/Ca ratio or sodium adsorption

ratio (SAR), an approach used by Ayers and Westcot (1985).

Sodic soil conditions affect almost all crops because of the deterioration

of soil physical conditions. Dispersion of soil aggregates in sodic soils

decreases soil permeability to water and air, thereby reducing plant

growth. Poorly structured soils also result in prolonged saturated envi-

ronments, encouraging root disease. Therefore, yield reductions in crops

that are not specifically sensitive to Na



generally reflect the combined

effects of nutritional problems and all problems associated with impaired

soil physical conditions.

Chloride

Chlorine is an essential micronutrient for plants but, unlike most

micronutrients, it is relatively nontoxic when supplied at low concentra-

tions sufficient only to meet plant requirements (Maas 1986). In fact, most

nonwoody crops are not specifically sensitive to Cl



even at higher con-

centrations. One exception to this generalization involves certain cultivars

of soybeans that tend to accumulate excessive and toxic amounts of Cl



(Abel and McKenzie 1964; Parker et al. 1983). Tolerant cultivars restrict



transport to the shoots. Many woody species are also susceptible to



toxicity, which varies among varieties and rootstocks within species.

As in soybeans, these differences usually reflect the plant’s ability to pre-

vent or retard Cl



translocation to the shoots or scions. Cooper (1951,

1961) found that the salt tolerance of avocados, grapefruits, and oranges is

430 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

closely related to the Cl



accumulation properties of the rootstocks. Similar

effects of rootstocks on salt accumulation and tolerance have been

reported for stonefruit (Bernstein et al. 1956) and nut trees (Ferguson et al.

2002). Large differences in the salt tolerance of grape varieties have been

linked with the Cl



-accumulating characteristics of different rootstocks

(Ehlig 1960; Sauer 1968; Bernstein et al. 1969; Groot Obbink and Alexan-

der 1973). By selecting rootstocks that exclude Cl



from the scions, this

problem can be avoided.

Table 13-4 lists the maximum Cl



concentrations permissible in the soil

water that do not cause leaf injury in selected fruitcrop cultivars and root-

stocks. In some cases, however, the osmotic threshold may be exceeded so

that yield is decreased without obvious injury. The list is by no means

complete, and most popular rootstocks are not listed because quantitative

data are not available.

The major detrimental effect of Cl



results from its contribution to the

overall osmotic stress. No comprehensive testing has been done to specif-

ically determine crop tolerances to Cl



salinity but, since most of the salt-

tolerance data were obtained in field plots salinized with Cl salts of Na



and Ca

2

, the data can be converted to express tolerances in terms of Cl



concentration. If Cl



is the predominant anion in the soil solution, then



concentration [Cl



], expressed in meq/L (mmolc/L) is approximately

10 times the EC expressed in dS/m (USSL 1954). Therefore, multiplying

the threshold values given in Tables 13-1 and 13-2 by 10 gives the maxi-

mum allowable Cl



concentration in mol m

3

in the saturated-soil extract

without a loss in yield. Dividing the slope by 10 estimates the percent

yield-potential decrease expected per each 1 mol m

3

increase in Cl



con-

centration above the threshold.

Boron

Boron (B) is an essential micronutrient for plants. The optimum con-

centration range of plant-available B, however, is very narrow for most

crops. Various criteria have been proposed to define levels that are neces-

sary for adequate B nutrition and yet low enough to avoid B toxicity

symptoms, plant injury, and subsequent yield reduction (Ayers and

Westcot 1985; Gupta et al. 1985; Keren and Bingham 1985). Boron defi-

ciency is more widespread than B toxicity, particularly in humid climates,

whereas excess B toxicity tends to be more of a concern in arid environ-

ments. Like salt tolerance, B tolerance fluctuates with climate, soil condi-

tions, and plant variety.

Much of the existing B tolerance data were obtained from experiments

conducted from 1930 through 1934 by Eaton (1944). These data provided

threshold tolerance limits for more than 40 different crops. While very

useful, Eaton’s experimental data cannot be fitted to any reliable

PLANT SALT TOLERANCE 431

Agricultural Salinity Assessment and Management

432 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

TABLE 13-4. Chloride Tolerance Limits of Some Fruit-Crop

Rootstocks and Cultivars

Maximum

Permissible



in Soil

Water without

Leaf Injury

Crop Rootstock or Cultivar (mol m

3

)

(1) (2) (3)

Rootstocks

Avocado West Indian 15

(Persea americana) Guatemalan 12

Mexican 10

Citrus Sunki mandarin, grapefruit 50

(Citrus sp.) Cleopatra mandarin, Rangpur lime 50

Sampson tangelo, rough lemon 30

Sour orange, Ponkan mandarin 30

Citrumelo 4475, trifoliate orange 20

Cuban shaddock, Calamondin 20

Sweet orange, Savage citrange 20

Rusk citrange, Troyer citrange 20

Grape Salt Creek, 1613-3 80

(Vitis sp.) Dog Ridge 60

Stone fruit Marianna 50

(Prunus sp.) Lovell, Shalil 20

Yunnan 15

Cultivars

Berries

Boysenberry 20

(Rubus sp.) Olallie blackberry 20

Indian Summer raspberry 10

Grape Thompson seedless, Perlette 40

(Vitis sp.) Cardinal, Black Rose 20

Strawberry Lassen 15

(Fragaria sp.) Shasta 10

For some crops, these concentrations may exceed the osmotic threshold and cause some

yield reduction.

Data available for one variety of each species only

Adapted from Maas and Grattan (1999).

Agricultural Salinity Assessment and Management

growth–response function for most crops. Nevertheless, his results are the

source of most of the threshold tolerance limits presented in Table 13-5.

Plant response to excess B can be described by the two-piece linear

response model (Bingham et al. 1985; Francois 1984, 1986, 1988, 1989,

1991, 1992). These data have provided threshold and slope parameters for

a limited number of crops and are included in Table 13-5. With few excep-

tions, the B tolerance data are based on crop responses to different B lev-

els in sand cultures. The thresholds indicate maximum permissible con-

centrations in the soil water that do not cause yield reductions. Some

crops, however, may exhibit leaf injury at low to moderate concentrations

without decreasing yield. For example, heavy leaf damage due to long-

term B accumulation in grapes had no effect on the commercial fruit yield

(Yermiyahu et al. 2006). Based on response to B, crops have been classified

in six groups, ranging from very sensitive to very tolerant. Like salt toler-

ance, B tolerance varies with climate, soil conditions, and crop cultivars;

therefore the data may not apply to all cultural conditions. Because differ-

ent rootstocks of citrus and stone fruits absorb B at different rates, that tol-

erance will likely be improved by using rootstocks that restrict B uptake.

A number of these rootstocks are listed in order of increasing B accumula-

tion in Table 13-6.

Francois and Clark (1979) examined the response of 25 ornamental

shrub species to irrigation with waters containing either high (7.5 mg/L)

or low (2.5 mg/L) B concentrations. Boron tolerance was based on growth

reduction and overall plant appearance. The salt tolerance of these species

had been established in an earlier study (Bernstein et al. 1972) and no cor-

relation was found between B tolerance and salinity tolerance of the

species tested.

Symptoms of boron toxicity. At the early stages, symptoms of salinity

and specific ion toxicities in plants are often difficult to distinguish from

each other. Foliage may be off-color green with yellowing of the leaf tips

or margins. This observation, however, is of little diagnostic value unless

accompanied by chemical analysis for specific ions in the tissue. As B in

the root environment increases, however, characteristic visual symptoms

are evident. Sharp boundaries often distinctly separate the affected and

the green unaffected tissues. Leaf margins become scorched and necrotic,

and finally the leaf drops prematurely.

Boron toxicity patterns are generally correlated with the venation of

the leaf in that chlorosis followed by necrosis appears first at the end of

the veins. Parallel-veined leaves (e.g., grasses, lilies) generally show

necrosis in leaf tips where the veins terminate. A similar pattern is found

in lanceolet leaves (e.g., stock, carnations) where the principal vein termi-

nates in the tip. In species of geranium or broccoli, for example, where

veins are of more radial distribution, B toxicity appears as an injured zone

PLANT SALT TOLERANCE 433

Agricultural Salinity Assessment and Management

TABLE 13-5. Boron Tolerance Limits for Agricultural Crops

Crop Boron-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name Based On: (g m

3

) (% per g m

3

) Rating

(1) (2) (3) (4) (5) (6)

Alfalfa Medicago sativa L. Shoot DW 4.0–6.0 T

Apricot Prunus armeniaca L. Leaf and stem injury 0.5–0.75 S

Artichoke, globe Cynara scolymus L. Laminae DW 2.0–4.0 MT

Artichoke, Jerusalem Helianthus tuberosus L. Whole plant DW 0.75–1.0 S

Asparagus Asparagus officinalis L. Shoot DW 10.0–15.0 VT

Avocado Persea americana Mill. Foliar injury 0.5–0.75 S

Barley Hordeum vulgare L. Grain yield 3.4 4.4 MT

Bean, kidney Phaseolus vulgaris L. Whole plant DW 0.75–1.0 S

Bean, lima Phaseolus lunatus L. Whole plant DW 0.75–1.0 S

Bean, mung Vigna radiata (L.) R. Wilcz. Shoot length 0.75–1.0 S

Bean, snap Phaseolus vulgaris L. Pod yield 1.0 12 S

Beet, red Beta vulgaris L. Root DW 4.0–6.0 T

Blackberry Rubus sp. L. Whole plant DW < 0.5 VS

Bluegrass, Kentucky Poa pratensis L. Leaf DW 2.0–4.0 MT

Broccoli Brassica oleracea L. (Botrytis group) Head FW 1.0 1.8 MS

Cabbage Brassica oleracea L. (Capitata group) Whole plant DW 2.0–4.0 MT

Carrot Daucus carota L. Root DW 1.0–2.0 MS

Cauliflower Brassica oleracea L. (Botrytis group) Curd FW 4.0 1.9 MT

Celery Apium graveolens L. var. dulce (Mill.) Pers. Petiole FW 9.8 3.2 VT

Cherry Prunus avium L. Whole plant DW 0.5–0.75 S

Clover, sweet Melilotus indica All. Whole plant DW 2.0–4.0 MT

Corn Zea mays L. Shoot DW 2.0–4.0 MT

434

Agricultural Salinity Assessment and Management

Cotton Gossypium hirsutum L. Boll DW 6.0–10.0 VT

Cowpea Vigna unguiculata (L.) Walp. Seed yield 2.5 12 MT

Cucumber Cucumis sativus L. Shoot DW 1.0–2.0 MS

Fig, Kadota Ficus carica L. Whole plant DW 0.5–0.75 S

Garlic Allium sativum L. Bulb yield 4.3 2.7 T

Grape Vitis vinifera L. Whole plant DW 0.5–0.75 S

Grapefruit Citrus x paradisi Macfady Foliar injury 0.5–0.75 S

Lemon Citrus limon (L.) Burm. f. Foliar injury, Plant DW < 0.5 VS

Lettuce Lactuca sativa L. Head FW 1.3 1.7 MS

Lupine Lupinus hartwegii Lindl. Whole plant DW 0.75–1.0 S

Muskmelon Cucumis melo L. (Reticulatus group) Shoot DW 2.0–4.0 MT

Mustard Brassica juncea Coss. Whole plant DW 2.0–4.0 MT

Oats Avena sativa L. Grain (immature) DW 2.0–4.0 MT

Onion Allium cepa L. Bulb yield 8.9 1.9 VT

Orange Citrus sinensis (L.) Osbeck Foliar injury 0.5–0.75 S

Parsley Petroselinum crispum Nym. Whole plant DW 4.0–6.0 T

Pea Pisum sativa L. Whole plant DW 1.0–2.0 MS

Peach Prunus persica (L.) Batsch. Whole plant DW 0.5–0.75 S

Peanut Arachis hypogaea L. Seed yield 0.75–1.0 S

Pecan Carya illinoinensis (Wangenh.) C. Koch Foliar injury 0.5–0.75 S

Pepper, red Capsicum annuum L. Fruit yield 1.0–2.0 MS

Persimmon Diospyros kaki L. f. Whole plant DW 0.5–0.75 S

Plum Prunus domestica L. Leaf and stem injury 0.5–0.75 S

Potato Solanum tuberosum L. Tuber DW 1.0–2.0 MS

Radish Raphanus sativus L. Root FW 1.0 1.4 MS

Sesame Sesamum indicum L. Foliar injury 0.75–1.0 S

Sorghum Sorghum bicolor (L.) Moench Grain yield 7.4 4.7 VT

Squash, scallop Cucurbita pepo L. var melopepo (L.) Alef. Fruit yield 4.9 9.8 T

Squash, winter Cucurbita moschata Poir Fruit yield 1.0 4.3 MS

435

(continued)

Agricultural Salinity Assessment and Management

TABLE 13-5. Boron Tolerance Limits for Agricultural Crops (Continued)

Crop Boron-Tolerance Parameters

Common Botanical Tolerance Threshold

Slope

Name Name Based On: (g m

3

) (% per g m

3

) Rating

(1) (2) (3) (4) (5) (6)

Squash, zucchini Cucurbita pepo L. var melopepo (L.) Alef. Fruit yield 2.7 5.2 MT

Strawberry Fragaria sp. L. Whole plant DW 0.75–1.0 S

Sugar beet Beta vulgaris L. Storage root FW 4.9 4.1 T

Sunflower Helianthus annuus L. Seed yield 0.75–1.0 S

Sweet potato Ipomoea batatas (L.) Lam. Root DW 0.75–1.0 S

Tobacco Nicotiana tabacum L. Laminae DW 2.0–4.0 MT

Tomato Lycopersicon lycopersicum (L.) Fruit yield 5.7 3.4 T

Karst. ex Farw.

Turnip Brassica rapa L. (Rapifera group) Root DW 2.0–4.0 MT

Vetch, purple Vicia benghalensis L. Whole plant DW 4.0–6.0 T

Walnut Juglans regia L. Foliar injury 0.5–0.75 S

Wheat Triticum aestivum L. Grain yield 0.75–1.0 3.3 S

Maximum permissible concentration in soil water without yield reduction. Boron tolerances may vary, depending on climate, soil conditions, and

crop varieties.

The B tolerance ratings are based on the following threshold concentration ranges: 0.5 g m

;s3

very sensitive (VS); 0.5–1.0 sensitive (S); 1.0–2.0 mod-

erately sensitive (MS); 2.0–4.0 moderately tolerant (MT); 4.0–6.0 tolerant (T); and 6.0 very tolerant (VT).

Adapted from Maas and Grattan (1999).

436

Agricultural Salinity Assessment and Management

all around the margin. In leaves with a well-developed network of veins,

and with many veins ending in areas between principal side veins (ger-

beras, asters, eucalyptus, most citrus species), symptoms first develop as

interveinal yellow or red spots. As injury progresses, chlorosis spreads to

the margins (Oertli and Kohl 1961).

Other B toxicity symptoms commonly observed in landscape plants

include terminal twig dieback, necrotic leaf spots, abnormal leaf forms

PLANT SALT TOLERANCE 437

TABLE 13-6. Citrus and Stone-Fruit Rootstocks Ranked in Order of

Increasing Boron Accumulation and Transport to Scions

Common Name Botanical Name

Citrus

Alemow Citrus macrophylla

Gajanimma C. pennivesiculata or C. moi

Chinese box orange Severina buxifolia

Sour orange C. aurantium

Calamondin x Citrofortunella mitis

Sweet orange C. sinensis

Yuzu C. junos

Rough lemon C. limon

Grapefruit C. x paradisi

Rangpur lime C. x limonia

Troyer citrange x Citroncirus webberi

Savage citrange x Citroncirus webberi

Cleopatra mandarin C. areticulata

Rusk citrange x Citroncirus webberi

Sunki mandarin C. reticulata

Sweet lemon C. limon

Trifoliate orange Poncirus trifoliata

Citrumelo 4475 P. trifoliata x C. paradisi

Ponkan mandarin C. reticulata

Sampson tangelo C. x Tangelo

Cuban shaddock C. maxima

Sweet lime C. aurantiifolia

Stonefruit

Almond Prunus duclis

Myrobalan plum P. cerasifera

Apricot P. armeniaca

Marianna plum P. domestica

Shalil peach P. persica

Adapted from Maas and Grattan (1999).

Agricultural Salinity Assessment and Management

and texture, and bark cracking. Necrosis associated with B is often black

and sometimes red (e.g., eucalyptus) and is most severe on the older

foliage (Chapman 1966). Characteristic symptoms of B toxicity in stone-

fruit trees are reduction of flower bud formation, poor fruit set, and mal-

formed fruit exceptionally poor flavor (Johnson 1996). In citrus trees,

symptoms often progress from tip chlorosis and mottling to the formation

of tan-colored, resinous blisters on the underside of the leaves (Wutscher

and Smith 1996).

Boron toxicity and how it is expressed by plants is related to some

extent on the mobility of B in the plant. Although in most plant species B

is thought to be immobile, accumulating in the margins and tips of the

oldest leaves, B can be remobilized by some species (Brown and Shelp

1997). These B-mobile plants have high concentrations of polyols (sugar

alcohols) that bind with the B and allow it to be mobilized in the shoot.

Examples include almonds, apples, grapes, and most stonefruits. For

these crops, B concentrations are higher in younger tissue than in older

tissue, and injury is expressed in the young, developing tissue. This likely

explains symptoms, such as reduced bud formation and twig die-back.

Boron-immobile plants, such as pistachios, tomatoes, walnuts, and figs,

do not have high concentrations of polyols and the B concentrates in the

margins of older leaf tissue. Injury in these crops is expressed as the clas-

sical necrosis on leaf tips and margins.

Salinity–boron interactions. Because excess B often occurs in areas

with saline soils and waters, it is relevant to consider B uptake by plants

under saline conditions inasmuch as B toxicity may be confounded with

the associated problems of salt accumulation (Nicholaichuk et al. 1988).

Although plant response to high concentrations of B in the root media has

been extensively reviewed (Nable et al. 1997), the interactive effects of

salinity and B on plant performance have received less attention (Grieve

and Poss 2000; Alpaslan and Gunes 2001; Ben-Gal and Shani 2002; Diaz

and Grattan 2009; Edelstein et al. 2005; Tripler et al. 2007; Yermiyahu et al.

2007, 2008). Moreover, studies addressing the interaction of the dual

stresses on crop response reach widely different conclusions. Bingham

et al. (1987) reported that wheat shoot growth was influenced by each

stress independently but not by their interaction. Several studies have

shown that salt stress may increase B toxicity symptoms and reduce crop

yield (Aspaslan and Gunes 2001; Grieve and Poss 2000; Supanjani 2006;

Wimmer et al. 2003). Conversely, results of other studies suggest that

increased salinity may reduce B uptake and mitigate its toxic effects in

wheat (Holloway and Alston 1992), chickpeas (Yadav et al. 1989), melons

(Edelstein et al. 2005), and eucalyptus (Grattan et al. 1997; Marcar et al.

1999). Recent research at the USDA-ARS U.S. Salinity Laboratory shows

that there are complex interactions among salinity, B, and pH (Grieve et al.

438 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

2010; Smith et al. 2010a,b). These new findings suggest that more research

is needed to better understand the mechanisms of these interactions.

Selenium and other trace elements

Since the publication of ASA Monograph 38 Agricultural Drainage

(Skaggs and van Schilfgaarde 1999), environmental concerns related to

trace elements have added a new dimension to drainage management and

disposal. Certain irrigated soils derived from sedimentary rock materials

contain, or at least originally contained, high concentrations of trace ele-

ments that dissolve in the soil-water and move to the shallow groundwater.

Pratt and Suarez (1990) provide a table that lists the recommended maxi-

mum concentration of 15 trace elements in irrigation waters that provide

long-term protection of plants and animals. However, the recent concern

that these elements pose for irrigated agriculture is not so much their effects

on limiting production but rather the toxicological effects they can cause

when drainage effluents that contain them are used for irrigation or are

discharged into bodies of water. If such effluents are used to supplement

irrigation water supplies, certain trace elements may accumulate in the soil

and/or crop to levels that pose a health hazard to consumers. Molybdenum

(Mo) and selenium (Se) are readily absorbed by plants and can be toxic to

animals and humans (Page et al. 1990). If trace-element-tainted drainage

effluents are discharged into channels, lakes, ponds, estuaries, or other

bodies of water, there are ecological concerns that they may concentrate as

they move up the food chain, a process called biomagnification.

The composition of salts in the drainage effluent can influence the

uptake of certain trace elements by plants. Selenium, for example, is

found in soil solutions in California’s San Joaquin Valley, where it exists

together with high concentrations of sulfate. Uptake of both SeO

2

and

2

by plants is mediated by the same high-affinity enzyme, and the

anions compete for binding sites on this cell membrane carrier (Läuchli

1993). Plant accession of Se from a substrate high in sulfate will be signifi-

cantly lower than from a Cl system. Irrigation with drainage water domi-

nated by sulfate salts reduced selenate accumulation in vegetables (Burau

et al. 1991), wheat (Grieve et al. 1999), soybeans (Wang et al. 2005), and the

seed oil crop lesquerella (Grieve et al. 2001). In some areas where total soil

Se is high (5 mg/kg dry wt) and sulfate concentrations are much lower

than those in the San Joaquin Valley, plants can accumulate Se to phyto-

toxic levels, such as the case with wheat grown in an isolated area in the

Punjab state of India (G. S. Dhillon, personal communication, 2007).

Selenium accumulation by plants is also influenced by the irrigation

method. Although root uptake of Se is inhibited by the presence of sulfate

in the external media, a similar interaction apparently does not occur in leaf

tissue. Therefore, Se is readily taken up by leaves of forage forage Brassica

PLANT SALT TOLERANCE 439

Agricultural Salinity Assessment and Management

species (Suarez et al. 2003), Swiss chard, spinach, and soybeans (D. L.

Suarez, personal communication) sprinkler-irrigated with Se-containing,

sulfate-dominated saline waters. Studies conducted in sand tank cultures

have shown that sulfate salinity can also reduce Mo accumulation in

alfalfa shoots (Grattan et al. 2004b) but had the opposite effect on tall

wheatgrass (cv ‘Jose’) (Diaz and Grattan 2009).

PARAMETERS INFLUENCING PLANT RESPONSE TO

SALT STRESS

Although crop yields are a function of salt concentrations within the

rootzone, it must be recognized that this relationship is influenced by

interactions between salinity and various soil, water, and climatic condi-

tions. While other environmental stresses may limit crop yield, they

may increase, decrease, or have no effect on the apparent salt tolerance

of the crop. It is important, therefore, that the effects of any interacting

factor be compared on the basis of relative crop yield. Even though

expressing the yields on a relative basis minimizes large differences in

absolute yield from experiments conducted in different sites and condi-

tions, these factors can still affect the apparent salt tolerance expressed

on a relative basis.

Soil water content

Salt-affected crops often must contend with water deficits or excess as

well. Therefore, actual crop performance during the growing season is

related to how the plant responds to both salinity and water stress. In

flooded or poorly drained soils, the overall diffusion of oxygen to roots is

reduced, thereby limiting root respiration and plant growth (Sharpley

et al. 1992). When the rootzone is saturated with saline water, the com-

bined effects of salinity and oxygen deficiency can adversely affect seed

germination (Aceves-N et al. 1975), selective ion transport processes in

the plant (Drew et al. 1988; Barrett-Lennard 2003), and shoot growth

(Aubertin et al. 1968; Aragüés et al. 2004; Isidoro and Aragüés 2006).

Water deficit, at least to some degree, is practically unavoidable under

field conditions, since the soil-water content varies temporally and spatially

throughout the season. Exactly how the plant responds to the combina-

tion of stresses from salinity and water deficit remains unresolved (Meiri

1984). Obviously the combination of stresses is more damaging than

either one alone, but are they additive or antagonistic? Quantifying the

growth-limiting contribution of each is difficult, since both change over

time and space. Water-deficit stress may predominate in the upper root-

zone, while salt stress may predominate in the lower rootzone.

440 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

Wadleigh and Ayers (1945) first demonstrated that bean plants res -

ponded to the additive combination of water deficit and salt stress. How-

ever, Meiri (1984) concluded from data collected by Parra and Romero

(1980) that matric potential affected bean shoot growth more than did

osmotic potential. Thermodynamically, matric and osmotic components

are additive, but resistance to soil water flow must be considered. For

example, plant response to these stresses under conditions of low evapo-

rative demand is likely to be different than that observed under high

evaporative demand, since matric rather than osmotic potential domi-

nates control of water flow from soil to roots. The magnitude of the dif-

ference may be related to differences in evaporative demand and root-

length density.

Regardless of how plants respond to integrated stress, they presum-

ably do better when grown on saline soils if water-deficit stress is mini-

mized. However, increasing irrigation frequency does not necessarily

improve yields of salt-stressed crops (Bresler and Hoffman 1986; Shal-

hevet et al. 1982, 1986). Salt-stressed plants are smaller, grow slower than

non-salt-stressed plants, and require less water over a given time. Conse-

quently, salt-stressed plants deplete a smaller percentage of available soil

water than do nonsaline plants, so they are less responsive to frequent

irrigations. Therefore, increased irrigation frequency benefits salt-stressed

plants only when it reduces water stress; maintains the salt concentration

in the soil solution below growth-limiting levels; and does not contribute

to additional stresses, such as O

deficit or root disease (e.g., phytoph-

thora). As Wadleigh and Ayers (1945) concluded more than a half a cen-

tury ago, it is not that salt-stressed plants should necessarily be irrigated

more frequently, but rather that they should be irrigated at lower soil-

water depletion.

Salt composition. The composition of salts in water varies widely

across the globe. In most waters, the dominant cations are Na



, Ca

2

, and

2

, while the dominant anions are Cl



, SO

2

, and HCO



(Grattan and

Grieve 1999). Most horticultural crops are subjected to irrigation water or

soil solutions with Na



/(Na



 Ca

2

) in the range of 0.1 to 0.7, suggesting

that the composition of saline water employed in experimental studies

should reflect this ratio. Despite recommendations by early investigators

of plant salt tolerance that plants under salt stress require higher concen-

trations of Ca

2

than under nonsaline conditions (Hayward and Wadleigh

1949; Pearson 1959; Hayward and Bernstein 1958; Bernstein 1975), a high

percentage of salinity studies of agronomic and horticultural crops con-

tinue to be conducted with NaCl as the sole salinizing agent. The use of

this unrealistic salinizing composition may induce ion imbalances that

contribute to Na-induced Ca

2

deficiencies and Ca-related physiological

disorders in certain susceptible crops (Shear 1975; Maas and Grieve 1987;

PLANT SALT TOLERANCE 441

Agricultural Salinity Assessment and Management

Sonneveld 1988; Suarez and Grieve 1988). Furthermore, the use of single-

salt solutions in salt-tolerance experiments may result in misleading and

erroneous interpretations about plant response to salinity.

A similar argument can be offered for the anions. Although the major-

ity of salinity studies use Cl



as the sole salinizing anion, most soil solu-

tions contain a high proportion of SO

2

and HCO



. Plants can perform

equally well with moderate variations in the Cl



/SO

2

ratio, but at meas-

urably low ratios at the same salinity level, some plants perform better in

the sulfate-dominated solutions. Bicarbonate is somewhat different from



or SO

2

because it can be damaging under even mildly saline condi-

tions when it is the dominant anion. It is likely that more can be learned if

future salinity-nutrition studies, regardless of experimental scale or objec-

tives, are conducted with more realistic ion ratios.

Much of the salt-tolerance information has been derived from studies

of plant responses to Cl-dominated saline irrigation waters that typically

contain both NaCl and CaCl

. A few research teams have evaluated plant

salt tolerance by using irrigation waters prepared to simulate recycled or

saline waters typical of a specific location or site. Dutch growers fre-

quently employ solutions with compositions adjusted to the average salt

composition of surface waters in the western region of the Netherlands

(Bik 1980; Sonneveld 1988). Saline waters (EC  2.5 to 4.5 dS m

1

) from

local wells in Israel continue to be used successfully for cut-flower pro-

duction on more than 700 ha throughout the Negev Desert (Shillo et al.

2002). Arnold and coworkers (2003) demonstrated that recycled runoff

effluents from a nursery operation and water from a constructed wetland

were suitable for irrigating certain bedding and cut-flower crops. Irriga-

tion waters used in recent research at the U.S. Salinity Laboratory were

prepared to mimic waters available at three locations within California:

(1) Na



- and SO

2

-dominated drainage effluents present in the San

Joaquin Valley (Grattan et al. 2004a,b; Grieve et al. 2005; Skaggs et al.

2006a,b); (2) compositions of increasing salinity that would result from

concentration of Colorado River waters (Grieve et al. 2006); and (3) waters

affected by seawater intrusion along the California coastal areas (Carter

et al. 2005; Carter and Grieve 2008).

Soil biota

Full coverage of the interactions of salinity and soil flora and fauna is

clearly beyond the scope of this chapter. However, the importance of soil

organisms cannot be ignored. The use of controlled mycorrhization has

been shown to alleviate deleterious effects of salt stress and improve yields

of tomatoes (Al-Karaki 2006), lettuce (Ruiz-Lozano et al. 1996), sorghum

(Cho et al. 2006), and bananas (Yano-Melo et al. 2003). Rhizobium spp.,

which are integral to legume production, seem more salt-tolerant than

442 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

their host plants, but evidence indicates that nodulation and N

fixation by

some crops are impaired by salinity (Läuchli 1984). Growth of several

legumes was reduced more when grown symbiotically than with N fertil-

ization. Some investigators have suggested that mycorrhizal symbioses

improve the ability of some crops to tolerate salt by improving P nutrition

(Hirrel and Gerdemann 1980; Ojala et al. 1983; Poss et al. 1985) or by

enhancing K⬊Na status (Sannazzaro et al. 2006).

Although salinity does not specifically cause plant diseases, salt-

stressed plants may be predisposed to infection by soil pathogens. Salinity

has been reported to increase the incidence of phytophthora root rot in

chrysanthemums (MacDonald 1982), citrus (Blaker and MacDonald 1986),

chili peppers (Sanogo 2004), and tomatoes (Snapp et al. 1991); the colo-

nization of pistachios (Mohammadi et al. 2007) and olives (Levin et al.

2007) rootstocks by Verticillium dahlia; and the incidence and severity of

crown and root rot of tomatoes by Fusarium oxysporum (Triky-Dotan et al.

2005). The combined effects significantly reduced fruit size and yield of

tomatoes (Snapp et al. 1991). Wetter soil under salt-stunted plants may

contribute to increased susceptibility to fungal diseases. Inadequate

drainage could exacerbate this condition.

Soil fertility

In irrigated agriculture, fields are usually fertilized to achieve maxi-

mum productivity. Sometimes fertilizer applications are inadequate or

even omitted because of cost or availability. If crops are grown on low-

fertility soils, they may seem more salt-tolerant than those grown with

adequate fertility. The reason is that fertility, not salinity, is the primary

factor limiting plant growth. Proper fertilizer applications would increase

yields whether or not the soil was saline but proportionately more if it

were nonsaline. The results of Bernstein et al. (1974) indicate that the

effects of salinity and nutritional stresses tend to be additive, provided

that neither of these stresses are extreme. When yields are limited similarly

by salinity and infertility, the effects of decreasing salinity or increasing

fertility will give similar benefits. However, if yields are reduced much

more by one factor than the other, alleviating the most severe condition

will increase yield more than alleviating the less restrictive condition.

Therefore, one must be careful in interpreting salinity  fertility

studies in terms of whether fertilizer additions increase or decrease

crop salt tolerance. Response functions are based on relative crop yield

as salinity increases from non-growth-limiting to severely growth-

limiting levels (Maas and Grattan 1999). Although suboptimal soil fer-

tility may be the most growth-limiting factor at low salinity, salt stress

may be the most growth-limiting at higher salinity levels with the same

level of fertility (Grattan and Grieve 1994). Therefore, depending on the

PLANT SALT TOLERANCE 443

Agricultural Salinity Assessment and Management

severity of salt stress, fertilizer additions may increase or decrease crop

salt tolerance.

Although crop salt tolerance is expressed on a relative basis, actual

yields must be considered in evaluating the benefits of fertilizer. For

example, fertilizer additions may decrease crop salt tolerance, but it still

may be economically advantageous to fertilize if absolute yields are

increased. However, unless salinity causes specific nutritional imbal-

ances, fertilizer applications exceeding that required under nonsaline con-

ditions have rarely been beneficial in alleviating growth inhibition by

salinity. Most studies indicate that excess N, P, and K applications have

little effect or that they reduce salt tolerance (Grattan and Grieve 1994);

however, Ravikovitch and Yoles (1971) found that N, P, or both seemed to

increase the salt tolerance of millet and clover.

Reliable data on the salt tolerance of crops during emergence and

seedling growth are extremely limited (Maas and Grieve 1994). Although

salt stress may delay emergence, the final emergence percentage for most

crops is not affected if salt concentrations remain at or below the tolerance

threshold for mature yields. No systematic evaluation of the tolerance of

crop seedlings grown under actual or simulated field conditions has ever

been undertaken. Clearly, more research is needed to better understand

how crops respond to integrated stresses they encounter between germi-

nation and emergence.

Irrigation methods

The method of irrigation can affect the crop’s response to salinity. The

irrigation method (1) influences the salt distribution in the soil, (2) deter-

mines whether leaves will be subjected to wetting, and (3) determines the

ease at which high soil-water potentials can be achieved (Bernstein and

Francois 1973; Shalhevet 1984). Since irrigation methods that maintain a

higher soil-water potential reduce the time-averaged salt concentration in

the soil-water, they allow for optimal plant performance.

With pressurized systems, such as drip and sprinkler, small applica-

tions of water can be applied to fields uniformly, unlike surface irrigation

methods, such as furrow, basin, or flood. Surface irrigation systems require

some minimum quantity of water to enable uniform applications over the

field. This minimum quantity may be in excess of the yield-threshold soil-

water depletion, thereby resulting in unnecessary drainage losses. There-

fore, pressurized systems (sprinkler, drip, etc.) are more conducive for

light, uniform irrigations.

Although irrigating at lower soil-water depletion (i.e., higher matric

potential) may be desirable to maintain a favorable soil-water environ-

ment, use of sprinkler irrigation to achieve this creates an additional prob-

lem. Salts in the irrigation water can be readily absorbed by wetted foliage

444 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

and cause foliar injury. This subject will be addressed in more detail later

in this section.

In light of this discussion, it is not surprising that crop salt tolerance

has been found to vary under different irrigation methods (Bernstein and

Francois 1973; Bernstein and Francois 1975; Meiri et al. 1982), where crop

performance was best under drip irrigation and worst under sprinkler

irrigation.

Salt distribution patterns, such as those described, are related to the

combined effects of root water extraction patterns and the net direction of

water flow in the soil. Salt accumulation is lowest at the point in the soil

where irrigation water contacts the soil but increases in the direction of

soil water flow. Water moves in the direction where it is transpired or

evaporated, thereby concentrating salts in areas where it occurs (Kruse

et al. 1990). Salt accumulation patterns under furrow, sprinkler, drip, and

subsurface drip irrigation methods have been described by Oster et al.

(1984) and Wang et al. (2002). Subsurface drip irrigation practices can cre-

ate unique salt accumulation patterns where salts accumulate in the soil

above the drip line (Hanson et al. 2009). Plant roots encounter unexpected

salination when rain moves salts accumulated at the soil surface back into

the rootzone.

PLANT TOLERANCE TO SALINE SPRINKLING WATERS

Sprinkler-irrigated crops are subject to additional salt damage when

the foliage is wetted by saline water. Salts are directly accumulated by the

leaves and, as a result, some species become severely injured and lose

their leaves. Of course, sprinkler-irrigated crops are subject to injury from

both soil salinity and salt spray. Any genetically controlled mechanisms

that may have evolved in plants to restrict Na



and Cl



from the shoot

may become irrelevant under sprinkler irrigation. The degree of injury is

related to the salt concentration in the leaves, but weather conditions and

water stress can influence the onset of injury. For instance, leaves may

contain toxic levels of Na



or Cl



for several weeks without exhibiting

any injury symptoms, but the first hot, dry weather will cause severe leaf

necrosis. Consequently, there are no practical guidelines for correlating

foliar injury to salt concentrations in the leaves.

Obviously, saline irrigation water is best distributed through surface

distribution systems. However, if sprinkling with marginally saline water

cannot be avoided, several precautions should be considered (Maas 1986).

If possible, susceptible crops should be irrigated below the plant canopy

to eliminate or reduce wetting of the foliage. Since injury is related more

to the number of sprinklings than to their duration, infrequent, heavy irri-

gations would be preferable to frequent, light irrigations. Intermittent

PLANT SALT TOLERANCE 445

Agricultural Salinity Assessment and Management

wetting by slowly rotating sprinklers that allow drying between cycles

should be avoided. Lateral sprinkler systems might be moved downwind,

when possible, so that salts accumulated on the leaves from salt drift

would be washed off as the sprinkler moves past. Perhaps the best strat-

egy for minimizing foliar injury to plants is to irrigate at night when both

evaporation and salt absorption are reduced. Daytime sprinkling should

be avoided on hot, dry, windy days.

Sprinkling with low-salinity water for 3 to 5 minutes either prior to or

after sprinkler irrigations with saline water effectively reduced foliar salt

accumulation and injury in barley and corn (Aragüés et al. 1994; Benes

et al. 1996). These investigators concluded that much of the salt accumu-

lated by wetted leaves is absorbed during the first few minutes of irriga-

tion and also after sprinkling when the saline water evaporates and con-

centrates on the leaf surface. Sprinkling barley with 9.6 dS m

1

water, for

example, reduced grain yields by 58% compared to nonsprinkled plants,

but when saline-sprinkled plants received both pre- and post-washing

with nonsaline water, yields were reduced only 17% (Benes et al. 1996).

The soil surface was covered to shed the sprinkling waters in all cases.

Post-rinsing of soybean plants with nonsaline water prevented leaf injury

due to potentially toxic levels of Cl



(Wang et al. 2002; Grieve et al. 2003).

In this field trial, the soil surface was not covered; therefore, Cl



and other

ions were accumulated via both the root pathway and foliar absorption.

This information may be useful to growers who have access to and can

readily switch between sources of irrigation waters of different quality.

CONTROLLING SOIL SALINITY

Most of the crop salt-tolerance data provided in Tables 13-1 and 13-2

reflect how the plant responds to a relatively uniform soil-salinity pro-

file from the established seedling stage to harvest. Although useful, par-

ticularly for crop comparison purposes, field-grown crops respond to

salinity profiles that change over time, making relative yield predictions

understandably difficult. There are advantages, however, in imposing

water management practices that allow salinity profiles to change over

time, as opposed to maintaining relatively constant soil salinity profiles.

With controlled changes in soil salinity, crops with different tolerances

to salinity can be included within a crop rotation (Rhoades et al. 1988,

1989). Increases in soil salinity are also acceptable when the tolerance of

a crop increases within a season (Shennan et al. 1995; Steppuhn et al.

2009). Adequate control of soil salinity changes requires that the farmer

has access to multiple and dependable supplies of irrigation water

where at least one supply is of good quality. Within limits, farmers who

have irrigation water supplies of different qualities can use them alter-

446 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

nately (cyclically) in different years or at different times of the year, or

they can blend supplies to achieve a suitable quality water (Grattan and

Rhoades 1990).

Regardless of the irrigation water supplies and quality available to

the grower, irrigation practices must be managed to control soil salinity

within an acceptable level. This requires that a favorable salt balance be

attained. This does not suggest, however, that a calculated leaching

requirement must be achieved each irrigation. Leaching fractions (LFs)

often decrease as the season progresses. In fact, a reduced LF is a conse-

quence of a mature, deep-rooted crop actively growing in a soil with

low permeability during months of high evaporative demand. Pro-

longed periods of saturation required to achieve leaching could produce

anoxic conditions and encourage root disease. Nevertheless, a favorable

salt balance must be maintained, even if intermittent leaching (e.g., dur-

ing the winter, alternate years) is the only means to remove excess salts

from the soil.

A long-term salt balance can only be achieved at the farm scale if there

is adequate drainage beyond the rootzone. Crops grown in areas affected

by rising saline water tables are subjected to salination. Crop production

in these situations cannot be sustained indefinitely, since a long-term salt

balance cannot be achieved. Use of saline water to irrigate crops grown

in soils with high water tables accelerates the problem. Moreover, the

required leaching further raises the saline water table, thereby salinizing

the rootzone even more. This paradox can only be overcome by adequate

drainage and disposal, thereby ensuring that crop yields can be sus-

tained over the long term (van Schilfgaarde 1990).

SUMMARY

In making decisions about salinity management and the use of low-

salinity irrigation water, there are a number of variables that a grower may

consider. First is that the published threshold and slope values for various

crops represent statistical means, not absolute values, and actual crop tol-

erance falls within a range around these means. Crop selection should

therefore include consideration of the relative total production of a crop,

since a high-production crop may have a net economic yield high enough

to offset the effects of salinity stress. In addition, there are some potential

crop-specific benefits of high-salinity environments, such as increases in

sugars, total soluble solids, postharvest handling characteristics, and the

concentration of various flavonoids, ascorbates, tocopherols, carotinoids,

and lycopene. For ornamental species, the visible effects of salt stress may

also not affect the aesthetics of the plant, and salt stress effects may have

benefits, such as low growth and low water uptake. Salt-tolerant plants

PLANT SALT TOLERANCE 447

Agricultural Salinity Assessment and Management

(halophytes) may also be planted for a combination of their crop value and

their value in treating drainage and other wastewater.

In addition, there are plant-specific effects of high-salinity environ-

ments on nutrient uptake; for example, high sodium levels may inhibit

plant uptake of Ca

2

. The grower may wish to consider the plant-specific

effects of the specific salt composition of the field on the proposed crop

and adjust cropping accordingly. Finally, salt stress may be affected by

soil-water content, salt composition, soil biota, soil fertility, irrigation

method, and timing of irrigation. Management of these variables may

reduce the cumulative stresses on a crop and thus minimize the net

impact of salt stress.

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458 AGRICULTURAL SALINITY ASSESSMENT AND MANAGEMENT

Agricultural Salinity Assessment and Management

NOTATION

 soil salinity where crop yields are reduced 50%

EC  electrical conductivity of the irrigation water (EC

), the soil water

(EC

), and the saturated soil extract (EC

)

 yield threshold soil salinity (EC

) value (also referred to as the

yield threshold “A” coefficient in other chapters) above which

yields decline

p  coefficient that reflects the curve steepness and ST-index is salt-

tolerance index

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