Introduction

Introduction


  • Knowledge of irrigation water quality is critical to understanding management for long-term productivity.
  • Irrigation water quality is evaluated based upon total salt content, sodium and specific ion toxicities.
  • In many areas, irrigation water quality can influence crop productivity more than soil fertility, hybrid, weed control and other factors.
  • The concentration and composition of dissolved constituents in a water determines its quality. Quality of irrigation water is one of the main factor that affect the physical and chemical properties of the soil and ultimately, the crop growth.
  • The irrigation water must be free from excess soluble salts and chemical substances that may create soil quality problems such as salinity, sodicity, permeability and specfic ion toxicity.
  • The underground water remains in contact with various types of rocks and minerals which may be the source of its contamination.
  • There is a universal fact that as we increase the area under irrigation, means we are increasing the area under salinity problem. Thus, there are two problems.
      • Irrigation problem
      • Salt problem
  • The soluble salts contained in irrigation water has two problems or adverse effects
      • On the physico-chemical properties
      • On the metabolic activities of the crop plants.
Characteristics that determine quality:
  • The characteristics of an irrigation water that appears to be t important in detemining its quality are:
Total concentration of soluble salts
  • It is expressed in terms of electrical conductivity (EC). The soluble salts are SO4– and Cl– of Ca++, Mg++, Na+ and K+.
  • In general, irrigation water with EC values less than 750 micro-mhos/cm is safe for plant growth.
  • Under good drainage conditions, the crops can be grown successfully even at higher EC values. Thus both the drainage condition of the soil and salt tolerance of the crops is important in this case.
Sodium adsorption ratio (SAR)
  • This determines the alkali hazard of the irrigation water. As we know that alkali soils are formed by the accumulation of the exchangeable Na+.
  • Thus a poor quality irrigation water rich in Na + is responsible for alkalinity in the soil. The equation of SAR is given below:
  • SAR = Na+/ √Ca++ + Mg++/2
  • Thus higher the SAR of irrigation water, more harmful it is for the crops. Thus, low SAR is better (<10).
Boron content
  • The concentration of boron is safe in water when it is < 0.04 ppm. However, it is an important plant nutrient but very toxic when present in high concentration.
Bicarbonate content
  • Irrigation water containing high concentrations of bicarbonate ion (HCO3– ), there is tendency for calcium and magnesium to precipitate as carbonates.
  • This reaction does not go to under ordinary circumstances. Thus, the concentration of calcium and magnesium are reduced and relative proportion of sodium is increased which is harmful for plant growth.
  • The effect of its concentration on the quality of water is determined by the “residual sodium carbonate” or ‘ RSC’. The water containing RSC less than 1.25 meq L -1 is safe for plant growth.
      • RSC = (CO3-- + HCO3-) – (Ca++ + Mg++)
Specific ions
  • Chloride is a common ion in irrigation waters. Although chloride is essential to plants in very low amounts, it can cause toxicity to sensitive crops at high concentrations.
  • Like sodium, high chloride concentrations cause more problems when applied with sprinkler irrigation. Leaf burn under sprinkler from both sodium and chloride can be reduced by night time irrigation or application on cool, cloudy days.
  • Drop nozzles and drag hoses are also recommended when applying any saline irrigation water through a sprinkler system to avoid direct contact with leaf surfaces.



Chloride (ppm)

Effect on Crops

Below 70

Generally safe for all plants.

70-140

Sensitive plants show injury.

141-350

Moderately tolerant plants show injury.

Above 350

Can cause severe problems.

Chloride tolerance of selected crops. Listing in order of increasing tolerance: (low tolerance) dry bean, onion, carrot, lettuce, pepper, corn, potato, alfalfa, sudangrass, zucchini squash, wheat, sorghum, sugar beet, barley (high tolerance).


  • As with boron, sulfate in irrigation water has fertility benefits, and irrigation water often has enough sulfate for maximum production for most crops. Exceptions are sandy fields with <1 percent organic matter and <10 ppm SO4– S in irrigation water.
  • The nitrate ion often occurs at higher concentrations than ammonium in irrigation water.
  • Waters high in N can cause quality problems in crops such as barley and sugar beets and excessive vegetative growth in some vegetables. However, these problems can usually be overcome by good fertilizer and irrigation management.
  • Regardless of the crop, nitrate should be credited toward the fertilizer rate especially when the concentration exceeds 10 ppm NO3– N (45 ppm NO3¯).
  • Some important and widely acceptable ratings are given below. These should be taken as general guideline and necessary correction may be made depending upon the soil-crop situation.

Parameter

Permissible/ safe

Moderately safe

Moderately unsafe

Unsafe

RSC (meL-1)

< 1.25

-

1.25-2.50

>2.50

SAR

< 10

10-18

18-26

>26

B ( mg L-1)

< 2.0

2.0-2.5

2.5-3.0

>3.0

Cl ( mg L-1)

< 140

140-350

-

>350


Management of poor quality irrigation water
  • The poor quality irrigation water should be mixed with good quality water, if possible to dilute bad effect.
  • To remove the soluble salts, they may be removed by leaching through heavy irrigation.
  • Organic matter, if applied binds boron and make organo- metallic complex. Thus, toxic effect of boron will be reduced.
  • Application of gypsum will neutralize the bad effect of Na+.
Last modified: Thursday, 21 June 2012, 7:23 AM