Whatever may be the source of irrigation water viz., river, canal, tank, open well
or tube well, some soluble salts are always dissolved in it. The main soluble constituent in water are Ca, Mg, Na and K as cations and chloride, sulphate bicarbonate and carbonate as anions.
However ions of other elements such as lithium, silicon, bromine, iodine, copper, cobalt, fluorine, boron, titanium, vanadium, barium, arsenic, antimony, beryllium, chromium, manganese, lead, selenium phosphate and organic matter are also
present.
Among the soluble constituents, calcium, sodium, sulphate, bicarbonate and
boron are important in determining the quality of irrigation water and its suitability for irrigation purposes. However other factors such as soil texture, permeability, drainage, type of crop etc., are equally important in determining the suitability of irrigation water.
The following are the most common problems that result from using poor quality water.
1. Salinity
If the total quantity of salts in the irrigation water is high, the salts will
accumulate in the crop root zone and affect the crop growth and yield. Excess salt
condition reduces uptake of water due to high concentration of soil solution.
2. Permeability
Some specific salts reduce the rate of infiltration in to the soil profile
3. Toxicity
When certain constituents of water are taken up by plans which accumulates in
large quantities and results in plant toxicity and reduces yield.
4. Miscellaneous
Excessive Nitrogen in irrigation water causes excessive vegetative growth and
leads to lodging and delayed crop maturity. White deposits on fruits or leaves may occur
due to sprinkler irrigation with high bicarbonate water.
Classification of irrigation water quality
Quality of water | EC (m.mhos /cm) | pH |Na (%) | Cl (me/l) | SAR
Excellent | 0.5 |6.5 – 7.5 |30 | 2.5 |1.0
Good | 0.5 – 1.5 | 7.5 – 8.0 | 30 – 60 |2.5 – 5.0 |1.0 – 2.0
Fair |1.5 – 3.0 | 8.0 – 8.5 | 60 – 75 | 5.0 – 7.5 | 2.0 – 4.0
Poor |3.0 – 5.0 |8.5 – 9.0 |75 – 90 |7.5 – 10. |4.0 – 8.0
Very poor |5.0 – 6.0 | 9.0 – 10. |80 – 90 | 10.0 – 12.5 |8.0 – 15.0
Unsuitable |>6.0 |> 10 |>90 |>12.5 |>15
(SAR – Sodium Adsorption ratio)
