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Soil and Water Salinity

Soil and Water Salinity. Dissolved salts decrease the osmotic potential of soil water (which lowers the Total Soil Water Potential)

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Soil and Water Salinity

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  1. Soil and Water Salinity • Dissolved salts decrease the osmotic potential of soil water (which lowers the Total Soil Water Potential) • a decrease in soil solution potential increases the amount of energy which plants must expend to extract water from the soil (water flows from high to low potential) • As a result, respiration is increased and the growth and yield of most plants decline progressively as osmotic potential decreases. • Although most plants respond to salinity as a function of the total osmotic potential of soil water, some plants are susceptible to specific ion toxicity

  2. Saline and Sodic Soils:review the classification scheme for the various soils

  3. Electrical Conductivity – ability to conduct an electrical current through a material • Saline soils and salty water conduct electricity better than nonsaline soils or pure water. -- more dissolved ions in water = higher electrical conductivity. -- measure resistance to current and take reciprocal -- dS/m (SI units) or mmhos/cm (old units)

  4. Measurement of Salinity – TDS and EC TDS – Total dissolved solids • Cations + anions + anything <2 microns • Good quality water has <500 mg/L or ppm TDS • measure using gravimetry or EC • Evaporate water off and accurately weigh the residue • Problematic due to hydration and volatilization • EC (dS/m) x 640 ≈ TDS (mg/L) • TDS ‘meters’ are really EC meters with conversion factor

  5. Sodicity Measurement • Exchangeable sodium as a percent of the total CEC = “ESP” ESP = exchangeable Na X 100 units = cmolc/kg soil CEC (old units = meq/100g) The concentration of cations on the soil exchange phase Note change on HW 3, #8: calculate ESP using measured cations in your data (instead of SAR); then you can estimate SAR using Figure 9.22 nomogram

  6. Example for HW 3, # 8: • Using the measured cations in your soil (cmolc/kg), calculate the ESP Na+ = 2; Ca+2 = 4; Mg+2 = 3; H+ = 1; Al+3 = 1 CEC = sum of all exchangeable cations: CEC = 2 + 4 + 3 + 1 + 1 =11 cmolc/kg ESP = (Na/CEC) x 100 ESP = (2/11) x 100 = 18%

  7. Fig 9.22page 301 ESP = 18 so SAR ≈ 15 from fig below or SAR ≈ 16 using USSL nomogram

  8. Nomogram for estimating ESP to/from SAR (more accurate than textbook figure) (Handbook 60, U.S. Salinity Lab, 1954)

  9. Sodium Adsorption Ratio = “SAR” Sodicity Measurement SAR = [Na+] units = mmol/L [Ca+2 + Mg+2]½ SAR = [Na+] [Ca+2 + Mg+2]½ units = mmolc/L 2 (old units = meq/L) The concentration of cations in the soil solution

  10. Converting between various unitsmmol/L, mmolc/L, meq/L, mg/L • Use ion valence to go from mol/L to molc/L • Na+ has one charge, so 1 mol/L = 1 molc/L • Ca+2 has two charges, so 1 mol/L = 2 molc/L • 1 eq = 1 molc or 1 meq = 1 mmolc • 1 cmolc = 10 mmolc • Use atomic weights to go from moles to mass Na = 23 g/mol (or mg/mmol)

  11. Calculating solid concentration based on solution concentrations • Need to know how much solution (“extracting” or equilibrium volume) • Common to use 100 mL; or 1:5 soil:water ratios • When using sat’d paste extract, need to know SP • Need to know how much soil (preferably on a dry-weight basis) • Common to use 5 to 10 g soil • 1 kg = 1000 g • 1 L = 1000 mL • 1 ppm = 1 mg/L (solution) or 1 mg/kg (solid)

  12. Example • How many ppm (mg/L) of Ca+2 are in the sample given in class? (Ca = 40.7 meq/L) 40.7 meq/L x 1 mmol/2 mmolc x 1 mmolc/meq x 40.1 mg/mmol = 816 mg/L or ppm Ca

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