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Technical issues of salt-affected soils

Sodic soils

Saline soils have a high content of soluble salts, which often crystallise in the surface, forming a white crust. They also tend to remain wet after few days of rain. Patterns of plant growth in salt-affected fields are usually poor, as saline soils tend to inhibit germination and emergence of cereal grains. Herbaceous crops can appear bluish green under severe salt stress, while leaf-tip burn and die off of older leaves occur in cereals. As salts build up in the soil, or the period of waterlogging (excess of water) increases, the composition of plant species changes and vigour is reduced and surface soil conditions deteriorate.

Formerly, the exchangeable sodium percentage, which equals exchangeable sodium (meq/100 g soil) divided by the cation exchange capacity (meq/100 g soil) times 100, was the primary measure of sodicity. The test for exchangeable sodium percentage, however, has proved unreliable in soils containing soluble sodium silicate minerals or large amounts of sodium chloride

Because the suitability of a saline water for irrigation is so dependent upon the conditions of use, including crop, climate, soil, irrigation method and management practices, water quality classifications are not advised for assessing water suitability for irrigation. However, for the purpose of identifying the levels of water salinities for which these guidelines are intended, it is useful to give a classification scheme.

Such a classification is given in the below table in terms of total salt concentration, which is the major quality factor generally limiting the use of saline waters for crop production. Only very tolerant crops (hardly any conventional crops) can be successfully produced with waters that exceed about 10 dS/m in EC. Few generally-used irrigation waters exceed about 2 dS/m in EC. Many drainage waters, including shallow groundwaters underlying irrigated lands, fall in the range of 2-10 dS/m in EC. Such waters are in ample supply in many developed irrigated lands and have good potential for selected crop production, though they are often not used in this regard and are more typically discharged to better quality surface waters or to waste outlets. It is the use of such saline waters that is the major focus of these guidelines. Reuse of second-generation drainage waters for irrigation is also sometimes possible and useful, especially for purposes of reducing drainage volume in preparation for ultimate disposal or treatment. Such waters will generally have ECs in the range 10-25 dS/m. Thus, they too are considered in these guidelines, though to a much lesser degree because the "crops" that can be grown with them are atypical and much less experience exists upon which to base management recommendations and to develop guidelines. Very highly saline waters (25 - 45 dS/m in EC) and brine (> 45 dS/m in EC) are beyond the scope of these guidelines and their uses for crop production are therefore not discussed herein.

Sodic soils

Sodic soils have salts where sodium is predominant. The nature of salts present in sodic soils makes them alkaline. The excess sodium in the soil causes clay and organic matter to disperse, or separate, which in turn blocks the soil pores, resulting in poor water infiltration. A brownish-black crust sometimes forms on the surface due to dispersion of soil organic matter. Sodic soils also remain wet for a long time. Cultivation of plants is difficult in sodic soils due to high pH, poor drainage and toxic effects of sodium.  

Saline-sodic soils

Saline soils have a high content of soluble salts, like saline soils, but the salts are predominantly composed of sodium. They have intermediate properties and if not managed properly they can easily become sodic.

The presence of a permanent or seasonal high water table is often a sign of saline or sodic soils. Where salt-affected soils are suspected, confirmation of the type and extent of the problem should be obtained by analysis.

Classification of saline waters

Water class

Electrical conductivity dS/m

Salt concentration mg/l

Type of water

Non-saline

<0.7

<500

Drinking and irrigation water

Slightly saline

0.7 - 2

500-1500

Irrigation water

Moderately saline

2 - 10

1500-7000

Primary drainage water and groundwater

Highly saline

10-25

7000-15 000

Secondary drainage water and groundwater

Very highly saline

25 - 45

1 5 000-35 000

Very saline groundwater

Brine

>45

>45 000

S

Reaction

The numerical designation of reaction is expressed as pH. With this notation, pH 7 is neutral. Values lower than 7 indicate acidity; values higher, indicate alkalinity. Most soils range in pH from slightly less than 2.0 to slightly more than 11.0, although sulfuric acid forms and pH may decrease to below 2.0 when some naturally wet soils that contain sulfides are drained.

The descriptive terms to use for ranges in pH are as follows:

Term

Range

Ultra acid

< 3.5

Extremely acid

3.5 - 4.4

Very strongly acid

4.5 - 5.0

Strongly acid

5.1 - 5.5

Moderately acid

5.6 - 6.0

Slightly acid

6.1 - 6.5

Neutral

6.6 - 7.3

Slightly alkaline

7.4 - 7.8

Moderately alkaline

7.9 - 8.4

Strongly alkaline

8.5 - 9.0

Very strongly alkaline

> 9.0

Both colorimetric and electrometric methods are used for measuring pH. Colorimetric methods are simple and inexpensive. Reliable portable pH meters are available.