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1- BOOMING CEMENT INDUSTRY
2-
PAK-TURKISH ECONOMIC COOPERATION
3-
INCREASING MANGO EXPORTS
4-
KESC NEEDS REHABILITATION
5- SALINITY AND PLANT TOLERANCE

 

SALINITY AND PLANT TOLERANCE

 

Salinity tolerance is influenced by many plant, soil, and environmental factors

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By Dr. S.M. ALAM, NIA, Tandojam.
June 30  - July 6, 2003 

 

 

 

Soil salinity is a measure of the total amount of soluble salt in soil. As salinity levels increase, plants extract water less easily from soil, aggravating water stress conditions. High soil salinity can also cause nutrient imbalances, result in the accumulation of elements toxic to plants, and reduce water infiltration if the level of one salt element sodium is high. In many areas of the world, soil salinity is the factor limiting plant growth.

Salt-affected plants are stunted with dark green leaves which, in some cases, are thicker and more succulent than normal. In woody species, high soil salinity may lead to leaf burn and defoliation. High salinity causes alfalfa yield to decrease while the leaf-to-stem ratio increases, influencing forage quality. Grasses also appear dark green and stunted with leaf burn symptoms.

Salinity tolerance is influenced by many plant, soil, and environmental factors and their interrelationships. Generally, fruits, vegetables, and ornamentals are more salt sensitive than forage or field crops. In addition, certain varieties, cultivars, or rootstalks may tolerate higher salt levels than others. Plants are more sensitive to high salinity during seedling stages, immediately after transplanting, and when subject to other stresses (e.g., disease, insect, nutrient).

Climate and irrigation also influence salinity tolerance. As soil dries, salts become concentrated in the soil solution, increasing salt stress. Therefore, salt problems are more severe under hot, dry conditions than under cool, humid conditions. Increasing irrigation frequency and applying water in excess of plant demand may be required during hot, dry periods to minimize salinity stress.

SOURCES OF SOIL SALINITY: Salts are a common and necessary component of soil, and many salts (e.g., nitrates and potassium) are essential plant nutrients. Salts originate from mineral weathering, inorganic fertilizers, soil amendments (e.g., gypsum, composts and manures), and irrigation waters. An additional, important source of salts in many landscape soils comes from ice melters used on roads and sidewalks. The addition of virtually any soluble material will increase soil salinity. It is only when salts are present in relatively high amounts that plant growth is adversely affected.

SALINITY MEASUREMENT: Soil salinity is determined by measuring the electrical conductivity of solution extracted from a water-saturated soil paste. Salinity is abbreviated as ECe (Electrical Conductivity of the extract) with units of decisiemens per meter (dS/m) or millimhos per centimeter (mmhos/cm). Both are equivalent units of measurement and give the same numerical value.

MANAGING SOIL SALINITY: In principle, soil salinity is not difficult to manage. The first requisite for managing soil salinity is adequate drainage, either natural or man-made. Determine salinity level by collecting a representative soil sample to a 12 inch depth and having it analyzed by a lab. If the salinity level is too high for the desired vegetation (see attached tables), remove salts by leaching the soil with clean (low salt) water. Application of 6 inches of water will reduce salinity levels by approximately 50%, 12 inches of water will reduce salinity by approximately 80%, and 24 inches by approximately 90%. The manner in which water is applied is important. Water must drain through the soil rather than run off the surface. Internal drainage is imperative and may require deep tillage to break up any restrictive layer impeding water movement. Sprinkler irrigation systems generally allow better control of water application rates; however, flood irrigation can be used if sites are level and water application is controlled.

PLANT RESPONSES TO SOIL SALINITY: Table 1 describes general plant responses to different soil salinity ranges. Due to economic and/or environmental limitations (e.g., inadequate drainage), it may not be possible to leach salt from soil. In these situations, select plants which are tolerant of the salinity level in soil. Tables 2 through 8 describe the salt tolerances of common agricultural, horticultural, and ornamental plants. Tolerance values should be used as a guide when selecting vegetation. Varietal differences and environmental conditions may make plants more or less salt tolerant than indicated in the tables. For harvested crops, threshold values indicate soil salinity levels where plants begin to experience yield-reducing effects. Above the threshold, salinity levels associated with expected yield losses of 10%, 25% and 50% are indicated. Ornamental plants are grouped according to their relative salinity tolerance (low, medium or high) with ECe ranges indicated for each category. With the exception of turf, relatively little research has been done on landscape and ornamental plant salinity tolerance. Most research conducted on ornamentals has addressed tolerance to salt spray deposited on foliage. A high tolerance to salt spray, however, may indicate a high tolerance to salinity in the root zone.

 

 

TABLE 1: GENERAL GUIDELINES FOR PLANT RESPONSE TO SOIL SALINITY

Salinity (ECe, dS/m)

Plant response

0 to 2

mostly negligible

2 to 4

growth of sensitive plants may be restricted

4 to 8

growth of many plants is restricted

8 to 16

Only tolerant plants grow satisfactorily

above 16

Only a few very tolerant plants grow satisfactorily

 


 

TABLE 2. SALINITY TOLERANCE OF COMMON FIELD CROPS.

Crop

Threshold value

10%
yield
loss

25%
yield loss

50%
yield loss

ECe
(dS/m)

ECe (dS/m)

ECe (dS/m)

ECe (dS/m)

Barley

8.0

9.6

13.0

17.0

Beans (field)

1.0

1.5

2.3

3.6

Canola

2.5

3.9

6.0

9.5

Corn (grain)

2.7

3.7

6.0

7.0

Oats (grain)

5.2

6.7

9.0

12.8

Rye (gram)

5.9

7.7

12.1

16.5

Safflower

5.3

8.0

11.0

14.0

Sorghum

4.0

5.1

7.1

10.0

Sugarbeets

6.7

8.7

11.0

15.0

Sunflower

2.3

3.2

4.7

6.3

Triticale (grain)

6.1

8.1

12.0

14.2

Wheat

4.7

7.0

9.5

13.0