Course Content
Crop Production (Unit 6)
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ASRB NET / SRF / Ph.D. Agronomy

Water Quality in Agriculture

Water quality refers to the chemical, physical, and biological properties of irrigation water that affect soil health, crop growth, and productivity. Poor-quality water can lead to salinity, sodicity, toxicity, or health hazards.

Parameters of Irrigation Water Quality

(a) Salinity Hazard

  • Measured by Electrical Conductivity (EC) of water (dS/m).
  • High salt content reduces osmotic potential, affecting plant water uptake.

Water Class

EC (dS/m)

Suitability

Low salinity

< 0.75

Suitable for all crops, soils

Medium

0.75 – 3.0

Suitable for moderate salt-tolerant crops

High

3.0 – 7.5

Suitable only for salt-tolerant crops (barley, cotton, sorghum)

Very high

> 7.5

Not suitable without management

 

(b) Sodicity Hazard

(c) Specific Ion Toxicities

  • Chloride (Cl⁻) – > 4 meq/L toxic to sensitive crops (beans, citrus).
  • Boron (B) – essential micronutrient, but toxic > 2 ppm for sensitive crops.
  • Sulfates (SO₄²⁻) – usually safe, but excessive → salinity.

 

(d) pH of Irrigation Water

  • Normal range: 6.5 – 8.5.
  • Alkaline water (pH > 8.5) → usually sodic, poor infiltration.
  • Acidic water (pH < 6.0) → may contain toxic metals (Fe, Mn, Al).

 

(e) Other Parameters

  • Total Dissolved Solids (TDS) – < 450 ppm safe, > 2000 ppm unsafe.
  • Biological quality – wastewater/effluent irrigation may contain pathogens (E. coli).

 

Classification of Irrigation Water (USSL, 1954)

  • Based on EC and SAR, irrigation water is classified into 16 classes (C1S1 to C4S4).

Salinity Class

EC (dS/m)

Sodium Class

SAR

C1 = Low

< 0.25

S1 = Low

< 10

C2 = Medium

0.25–0.75

S2 = Medium

10–18

C3 = High

0.75–2.25

S3 = High

18–26

C4 = Very high

> 2.25

S4 = Very high

> 26

Example: C2S1 water → Medium salinity, low sodicity (generally suitable).

 

Management of Poor-Quality Water

(a) Saline Water (High EC)

  • Grow salt-tolerant crops (barley, cotton, sorghum).
  • Provide good drainage.
  • Leaching requirement (LR): apply extra water to flush salts below root zone.

(b) Sodic Water (High SAR/RSC)

  • Gypsum application – provides Ca²⁺ to replace Na⁺.
  • Use organic manures / green manures to improve soil structure.
  • Conjunctive use – blend sodic water with canal water.

(c) Alkaline Water (High pH)

  • Apply acid-forming fertilizers (ammonium sulfate).
  • Add sulfur or sulfuric acid to neutralize alkalinity.

(d) Wastewater / Effluents

  • Pre-treatment (settling, filtration).
  • Restricted to non-edible crops or trees.
  • Long-term use may cause heavy metal accumulation.

 

Crop Tolerance to Salinity

Tolerance

EC Threshold (dS/m)

Crops

Sensitive

< 2

Beans, citrus, onion, carrot

Moderately sensitive

2–4

Wheat, maize, soybean, groundnut

Moderately tolerant

4–6

Sorghum, sunflower, rice

Tolerant

6–10

Cotton, barley, sugar beet

 

Standards (India)

  • BIS (Bureau of Indian Standards) – irrigation water EC < 2.25 dS/m generally safe.
  • FAO Guidelines (Ayers & Westcot, 1985) – widely used globally for assessing irrigation water quality.

 

Conclusion

  • Water quality is as important as water quantity.
  • Parameters: EC (salinity), SAR & RSC (sodicity), pH, toxic ions.
  • Poor-quality water can still be used with proper management (gypsum, leaching, blending, tolerant crops).
  • Future: focus on monitoring, treatment, and conjunctive use for sustainable irrigation.

 

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