Post on 06-Feb-2016
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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
Saline and Sodic Soils:review the classification scheme for the various soils
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)
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
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
Example for HW 3, # 8:
• Using the measured cations in your soil (cmolc/kg), calculate the ESPNa+ = 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 100ESP = (2/11) x 100 = 18%
Fig 9.22page 301
ESP = 18 so SAR ≈ 15 from fig below or SAR ≈ 16 using USSL nomogram
Nomogram for estimating ESP to/from SAR (more accurate than textbook figure)(Handbook 60, U.S. Salinity Lab, 1954)
Sodicity Measurement
SAR = [Na+] [Ca+2 + Mg+2]½ units = mmolc/L
2 (old units = meq/L)
The concentration of cations in the soil solution
• Sodium Adsorption Ratio = “SAR”
SAR = [Na+] units = mmol/L [Ca+2 + Mg+2]½
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)
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)
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