Why Water Quality is Important Effects on Soil Salinity Effects on Soil Sodicity Bicarbonate content Toxic ion content Effects on nutrients applied by fertigation
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.
Irrigation Water Quality Important Measurements: Salinity (measured by EC) Sodium (measured by SAR) Potential toxic ions (Na, Cl, B) Alkalinity or HCO 3 - SAR=
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
Irrigation Water Hazards Concentrations of toxic ions: Cl - Na + B (H 3 BO 3 ) Bicarbonate - precipitates Ca 2+ from soils, makes Na problems worse.
General Guidelines Degree of Restriction on Use Potential ProblemUnitsNoneSlight to ModerateSevere pHNormal range 6.5 – 8.4 Salinity ECdS/m< – 3.0>3.0 TDSmg/L< – 2,000>2,000 Infiltration SAR=0-3 and EC w => – 0.2<0.2 SAR=3-6 and EC w => – 0.3<0.3 SAR=6-12 and EC w => – 0.5<0.5 SAR=12-20 and EC w => – 1.3<1.3 SAR=20-40 and EC w => – 2.9<2.9 Specific ion effects Sodium Surface irrigationSAR<33-9>9 Sprinkler irrigationmeq/L<3>3 Chloride Surface irrigationmeq/L< >10 Sprinkler irrigationmeq/L<3>3 Boronmg/L< – 3.0>3.0
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
Leaching Requirement Definition: The percentage of water (rain + irrigation) applied that must move below the root zone to control salt buildup. Equation:
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.
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
Squash planted on sides of beds to avoid zone of highest salt.
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)
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. Ca 2+
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: lbs/acre-foot water (326,000 gallons) There is no effective (economical) water treatment to counteract salinity.
What does gypsum do? Poor soil structure Good soil structure Sodium level (SAR) Based on irrigation water analysis
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.
Bicarbonate Hazard Excess HCO 3 - causes precipitation of CaCO 3 thus increasing the Na hazard (SAR) of irrigation water
Irrigation Water Treatment (2) Carbonate (CO 3 2- ) and bicarbonate (HCO 3 - ) in irrigation water are detrimental because they: precipate Ca 2+ from soil solution and hasten replacement of Ca 2+ with Na + Treatment of irrigation water with H 2 SO 4 to a pH <6.0 will convert all CO 3 2- and HCO 3 - to CO 2.
Water Quality and Nutrient Management (1) Ammonia Volatilization NH 4 + NH 3 + H + Addition of NH 4 + fertilizers in alkaline water will encourage this equilibrium toward the right. Solution: acidify water first Water pH % NH 3 volatilized
Water Quality and Nutrient Management (2) Adding NH 3 to irrigation water (as a fertilizer) will raise water pH to This will: Cause NH 3 volatilization Remove Ca and Mg from water by precipitating them as carbonates Can irreversibly plug irrigation systems Solution: Acidify water first
Calcium Carbonate Precipitation 1. High bicarbonate Ca HCO 3 - > CaCO 3 + H 2 CO 3 >H 2 O + CO 2 2.Ammoniated water NH 3 + H 2 O > NH 4 OH NH 4 OH > NH 4 + OH OH- + HCO 3 > CO H 2 O Ca ++ + CO 3 -2 > CaCO 3(s)
Constant H 2 SO 4 injection keeps water pH low and prevents formation of CaCO 3 in the drip lines, and also dissolves some CaCO 3 in the soil, helping to maintain high exchangeable Ca 2+ and low exchangeable Na +.
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