Irrigation and Water Quality

1. Irrigation and Water Quality Section F SWES 316

4. Salt Moves to the Terminus of the Wetting Front!

7. Border/Flood Irrigation Furrow Irrigation Sprinkler Subsurface drip

9. Why Water Quality is Important • Effects on Soil Salinity • Effects on Soil Sodicity • Bicarbonate content • Toxic ion content • Effects on nutrients applied by fertigation

10. Irrigation and Water Quality • When soils are irrigated, the quality of the water used will eventually determine the salinity and sodicity of the soil: • Soils will be at least 1.5 times more saline than the water used to irrigate them, unless very high (>20%) leaching fractions are used. • The ESP (and SAR) of the soil will eventually equal the sodium adsorption ratio (SAR) of the water.

11. Irrigation Water Quality • Important Measurements: • Salinity (measured by EC) • Sodium (measured by SAR) • Potential toxic ions (Na, Cl, B) • Alkalinity or HCO3- SAR=

12. Irrigation Water Hazards • Interpreting Salinity • Remember that even with excellent management, soil EC will be at least 1.5X that of the water used (except in very coarse-textured soils or very high LF). • Crop tolerance varies, however • Interpreting sodicity - depends on clay content and salinity

13. Irrigation Water Hazards • Concentrations of toxic ions: • Cl- • Na+ • B (H3BO3) • Bicarbonate - precipitates Ca2+ from soils, makes Na problems worse.

14. General Guidelines

15. Specific Guidelines

16. Controlling Salinity in Irrigated Soils • The only way to control salinity in irrigated soils is to __________________________ _________________. • If irrigation water EC is <0.75 dS/m, no risk of salt buildup. Normally, the soil is at least 1.5X the salinity of the irrigation water used because ___________________________. leach adequate quantities of water through soil not all water added can leach

17. Leaching Requirement • Definition: The percentage of water (rain + irrigation) applied that must move below the root zone to control salt buildup. • Equation:

18. Notes on LR • The LR is the amount of total water that should be applied above crop water use. • Can be applied at every irrigation, or only periodically • The LR you calculate depends on assumptions. • This is a fairly crude method - also consider irrigation system characteristics. • LRs above 30% are not very practical.

19. Other Ways to Live with Salts • Keep soils moist - this keeps salt concentrations more dilute. May require frequent irrigation. Drip irrigation !!! • Plant seeds on the sides of sloping beds. • Salts move with water • Use plants that are salt-tolerant

20. Squash planted on sides of beds to avoid zone of highest salt.

21. Fun things you can do with EC • Estimate total dissolved solids (TDS) • EC (dS/m) x 640  TDS (ppm) • Estimate osmotic potential of soil solution • o (bars)  EC (dS/m) x (-0.36)

22. Controlling Sodium • The SAR describes the equilibrium relationship between Na, Ca, and Mg. It should be as low as possible. Over time, the ESP of the soil will equal the SAR value of the irrigation water. • In order to control Na, a source of ______ must be added to irrigation water. Ca2+

23. Irrigation Water Treatment (1) • Regular treatment of irrigation water can help prevent the formation of sodium problems. • Irrigation water can be regularly treated with gypsum to lower SAR of water. Typical rates: 100-300 lbs/acre-foot water (326,000 gallons) • There is no effective (economical) water treatment to counteract salinity.

24. Sodium level (SAR) What does gypsum do? Based on irrigation water analysis Poor soil structure Good soil structure

25. Soil Amendments and Water Treatments • Soil application of amendments are used for initial reclamation and long-term maintenance of soil quality. Rates are often large and based on economics. • Water treatments are generally intended to alter the chemistry of irrigation water so that no further degradation in soil quality will occur. Rates used for water treatment are usually small and based on solubility and stoichiometry.

26. Bicarbonate Hazard • Excess HCO3- causes precipitation of CaCO3 thus increasing the Na hazard (SAR) of irrigation water

27. Irrigation Water Treatment (2) • Carbonate (CO32-) and bicarbonate (HCO3-) in irrigation water are detrimental because they: • precipate Ca2+ from soil solution and • hasten replacement of Ca2+ with Na+ • Treatment of irrigation water with H2SO4 to a pH <6.0 will convert all CO32- and HCO3- to CO2.

28. Water Quality and Nutrient Management (1) • Ammonia VolatilizationNH4+ NH3 + H+ • Addition of NH4+ fertilizers in alkaline water will encourage this equilibrium toward the right. • Solution: acidify water first

29. Water Quality and Nutrient Management (2) • Adding NH3 to irrigation water (as a fertilizer) will raise water pH to 9-10. This will: • Cause NH3 volatilization • Remove Ca and Mg from water by precipitating them as carbonates • Can irreversibly plug irrigation systems • Solution: Acidify water first

30. Calcium Carbonate Precipitation 1. High bicarbonate Ca++ + 2HCO3- > CaCO3 + H2CO3 >H2O + CO2 2.Ammoniated water NH3 + H2O > NH4OH NH4OH > NH4 +OH OH- + HCO3 > CO3-2 +H2O Ca++ + CO3-2 > CaCO3(s)

32. Constant H2SO4 injection keeps water pH low and prevents formation of CaCO3 in the drip lines, and also dissolves some CaCO3 in the soil, helping to maintain high exchangeable Ca2+ and low exchangeable Na+.

34. Water Quality and Nutrient Management (3) • Many P fertilizers are not very soluble in water. • Adding them to irrigation water high in soluble Ca will lead to precipitation of Ca-phosphates • Loss of P • Plugging of irrigation system