Criteria and their limits.   In irrigated agriculture, the quality of water used for irrigation should receive adequate attention. Irrigation water, regardless of its source, always contains some soluble salts in it. Apart from the total concentration of the dissolved salts, the concentration of some of the individual salts, and especially of those which are most harmful to crops, is important in determining the suitability of water for irrigation. The constituents usually determined by analysing irrigation water are the electrical conductivity for the total dissolved salts, soluble sodium percentage, sodium absorption ratio, the boron content, pH, cations, such as calcium, magnesium, sodium, potassium, and anions, such as carbonates, bicarbonates, sulphates, chlorides and nitrates. The salinity and sodium-hazard classes are shown in Table 6. The accepted limits for boron and residual carbonates are given in table 8.

Quality of water from different sources.   Water from the rivers which flows over salt-affected areas or in the deltaic regions has a greater concentration of salts sometimes as high as 7,500 ppm or even more. The quality of tank or lake water depends mainly on the soil salinity in the watershed areas and the aridity of the place.

The quality of water from ground-water resources, i.e. from shallow or deep wells, is generally poor under the situation of :
(i) High aridity - arid and semi-arid regions, with less than 45 cm of rainfall;
(ii) high water-table and water-logged conditions ; and
(iii) the vicinity of sea-water, as in the coastal region.

Ground-water in other areas is generally good. In saline regions, where the water from shallow aquifers is poor, the water from deeper strata may be good. Such water may be tapped by digging deep-tube wells.

Sodium class	SAR
S1 Low	Less than 10
S2 Moderate	10 to 15
S3 High	18 to 26
S4 Very high	More than 26

The continual use of poor-quality water for irrigation water can be improved by adding certain substances, such as sulphuric acid or sulpher to reduce the pH to the desired level. Gypsum, which is available commercially, can be applied to the soil or mixed with water where poor-quality water, containing a high concentration of sodium salts, is used for irrigation. The permeability of soils can be improved by incorporating organic matter into them to facilitate the leaching down of the salts beyond the root zone.

Choice of crops and varieties.   A crop or a variety which is more tolerant to salt should be selected for growing under the cropping pattern adopted. Crops have been broadly grouped as follows, according to their salt-tolerance limits.

Tolerant species (over 5,000 ppm salts): Barley, sugar-beet, date-palm, rape, kale, cotton, Rhodes grass, sesbania.

Semi-tolerant species (2,500 to 5,000 ppm salts): Rice, sorghum, maize, barley, sunflower, lucerne, berseem or guar, safflower, onion, spinach, lettuce, carrot, cluster-been, wheat, pearl-millet or bajra, grasses.

Sensitive species (below 2,500 ppm salts): Peas, cabbage, vetch, beans, gram, peaches, grapes, orange, grapefruit, potato, tomato.

The limits of relative tolerance of crops to boron are as under :
Tolerant species (2 to 4 ppm): Date-palm, sugar-cotton, lucerne, onion, turnip, cabbage, carrot and lettuce.

Semi-tolerant species (1 to 2 ppm): Wheat, maize, barley, cotton, sunflower, potato, tomato, peas, beans and sweet-potato.

Sensitive species (0.3 to 1.0 ppm): Apple, apricot, grape, orange, grapefruit, plum, pear, cherry and walnut.

Emergence from the soil, the seedling phase and flowering are three very critical stages in the growth of most of the field crops in the presence of salinity. Frequent irrigation and in amounts in excess of the storage capacity of a soil are essential for maintaining low moisture stress and to leach down the salts.

Row crops should be grown near the furrow bottom where salt concentration is low. The plant population should also be enhanced by increasing the seed-rate and by reducing the spacing to compensate for the poor performance of individual plants.

Water, if only saline, can be used by diluting it with good quality water. If it is a solid soil, there will be a permeability problem. It is necessary to replace the sodium by calcium by applying suitable ammendments and organic matter for allowing more water to penetrate the soil, so that salts can be easily leached down beyond the root-zone. Adequate arrangements for drainage should be made to lower the water-table and to maintain a satisfactory salt balance in the root-zone. Fertilizer releasing nitrogen slowly with a calcium base may be preferred and applied in split doses by placement.