Agriculture for Engineers 4 (3+1)

Lesson 13. Poor Quality of Irrigation Water and Management Practices

13.2 PROBLEMS WITH POOR QUALITY WATER
        Several soil and plant related problems arise due to use of poor quality water for irrigation.

13.2.1 Extraction of Water:

• If excess soluble salts of irrigation water accumulated in crop root zone, crop has difficulty in extracting enough water
• Root growth is also suppressed; increasing the difficulty of water uptake.
• Salinity stress in plants is often called physiological drought.
• Due to reduced uptake of water and other effects, yields are reduced.
• The reduction in yield due to salinity is more in warm climate than cool climate.

13.2.2 Soil permeability:

• Soil permeability is reduced due to the deflocculation effect of sodium.
• If permeability is reduced, infiltration of water into and through the soil is reduced.
• Adequate root penetration is inhibited due to the presence of impermeable soil layer caused by CaCO₃ and high exch.Na %
• Crusting of seed bed, Water logging, reduced oxygen and nutrient supply to the crops are the problems due to high sodium content relative of Ca & Mg.

13.2.3 Toxicity Symptoms:

• More uptake of B, Cl, Na, sulphate and bicarbonate by plant creates toxicity problems.
• Vegetative growth decrease as osmotic pressure of the soil solution increases.
• Reduction in growth takes place even without any external toxic symptoms.
• Increase in salinity, salt injury appears.
• Thick cuticle, waxy bloom and deep blue-green colour of leaves.
• At high salt levels, leaf burn appears in barley, sorghum and field beans.

13.2.4 Anatomical and Physiological Effects:

• Salinity reduces cell division, cell enlargement and protein synthesis. 
• It affects the structure and integrity of plant membranes and causes mitochondria and chloroplast to swell. 
• Sodium and chloride at toxic levels disrupt the structure of the protein molecules.
• High chloride content hinders the development of xylem tissue.

13.2.5 Nutritional Effects:

• Higher level of certain ions affect the absorption of other nutrient elements
• High concentration of sulphate reduces the uptake of calcium enhances the uptake of sodium. 
• This process causes high level of sodium in plants, thus causing sodium toxicity.
• High concentration of Ca reduces the uptake of K.
• High concentration of Mg induces Ca deficiency.

 13.2.6 Soil Microorganisms:

• NO₂ & NO₃ producing bacteria sensitive to high salt concentration than NH₄ producing bacteria.
• Azotobacter is resistance to salt concentration.

13.2.7 Other effects:

• Excessive vegetative growth, lodging, delayed crop maturity result due to excessive nitrogen in water.
• White and black deposit on soil due to high salt content and sodium and leaf burn due to using poor quality irrigation water in sprinkler irrigation are some of the problems.
• Tilth of the soil will be poor due to high exchangeable sodium percentage.
• Exchgeable Na tends to make moist soil impermeable to air and water & on drying soil becomes hard and difficult to work.
• The dense crusts formed interfere with germination and emergence of seedlings.
• Soluble carbonates are in water applied to soil in absence of Ca and Mg in soil, soil becomes alkaline & unfavorable.
• Na₂CO₃ in irrigation water is toxic to plants.

 13.3 Management level of the irrigation

13.3.1 Use of saline water

Even the waters containing high amount of dissolved salts has been used successfully in highly permeable sandy soils. Similarly, the waters showing considerable alkali hazards have also been used successfully on permeable soils or by addition of gypsum and FYM on semi-permeable soils. The high RSC content can be corrected by addition of gypsum and the water can be used for the purpose of irrigation. Attempts have also been made for using saline waters by diluting it with good quality water or by giving alternate irrigation with good and bad quality water.

     The CSSRI, Karnal has recommended following limits of EC for use of saline waters:

Tentative water quality ratings for Indian conditions

Soil groups	Soil texture	Clay content (%)	Crop tolerance to salinity	Upper permissible limits for water (EC in dS/m)
				No drainage limitation	Ground water level less than 1.51 m
Deep black and alluvial	Clayey	30	Fairly High	1.5 2.0	0.75 1.0
Alluvial	Clay loam	20-30	Fairly High	2.0 4.0	1.0 2.0
	Loam	10-20	Fairly High	4.0 6.0	2.0 3.0
	Sandy loam to sandy	10	Fairly High	6.0 8.0	3.0 4.0

             

The waters having high salinity can be used successfully in lighter textured soils by growing salt tolerant crops.

13.3.2 Leaching requirement (LR)

The leaching requirement may be defined as the fraction of the irrigation water that must be leached through the root zone to control soil salinity at any specific level.

The leaching requirement (LR) is simply the ration of the equivalent depth of the drainage water to the depth of irrigation water and may be expressed as a fraction or as per cent. Under the assumed conditions (uniform aerial application of irrigation water, no rainfall, no removal of salt in the harvested crop and no precipitation of soluble constituents in the soil), this ration is equal to the inverse ratio of the corresponding electrical conductivities as follows :

where;

LR	:	Leaching requirement expressed in percentage
Ddw	:	Depth of drainage water in cm
Diw	:	Depth of irrigation water in cm
ECiw	:	Electrical conductivity of the irrigation water in dS/m
ECdw	:	Electrical conductivity of the drainage water in dS/m